5.3.2.1. Peer Tracking

When peer tracking is enabled on a session the neighbor IP address is tracked in the routing table; if a failure occurs and there is no longer any IP route matching the neighbor address or else if the longest prefix match (LPM) route is rejected by the configurable peer-tracking-policy then after a 1 second delay the session is taken down. By default peer-tracking is disabled on all sessions. The default peer-tracking policy allows any type of route to match the neighbor IP address except aggregate routes and LDP shortcut routes.

Peer tracking was introduced when BFD was not yet supported for peer failure detection. Now that BFD is available peer-tracking has less value and is used less often.

Note: Peer tracking should be used with caution. Peer tracking can tear a session down even if the loss of connectivity turns out to be short-lived — for example while the IGP protocol is re-converging. Next-hop tracking, which is always enabled, handles such temporary connectivity issues much more effectively.

5.3.2.2. Bidirectional Forwarding Detection (BFD)

SR OS also supports the option to setup an async-mode BFD session to a BGP neighbor so that failure of the BFD session can trigger immediate teardown of the BGP session. When BFD is enabled on a BGP session a 1-hop or multi-hop BFD session is setup to the neighbor IP address and the BFD parameters come from the BFD configuration of the interface associated with the local-address; for multi-hop sessions this is typically the system interface. With a 10 ms transmit interval and a multiplier of 3 BFD can detect a peer failure in a period of time as short of 30 ms.

5.3.2.3. Fast External Failover

Fast external failover applies only to single-hop EBGP sessions. When fast external failover is enabled on a single-hop EBGP session and the interface associated with the session goes down the BGP session is immediately taken down as well, even if other mechanisms such as the hold-timer have not yet indicated a failure.

5.3.3.1. BGP Graceful Restart

Some BGP routers do not have redundant control plane processor modules or else do not support BGP HA with the same quality or coverage as 7450 ESS, 7750 SR, or 7950 XRS routers. When dealing with such routers or certain error conditions BGP graceful restart is a good option for minimizing the network disruption caused by a control plane reset. BGP graceful restart assumes that the router restarting its BGP sessions has the ability/architecture to continue packet forwarding throughout the control plane reset. If this is the case then the peers of the restarting router act as helpers and “hide” the control plane reset from the rest of the network so that forwarding can continue uninterrupted. Forwarding based on stale routes and hiding the “staleness” from other routers is considered acceptable because the duration of the control plane outage is expected to be relatively short (a few minutes). In order for BGP graceful restart to be used on a session, both routers must advertise the BGP graceful restart capability during the OPEN message exchange; see the section titled BGP Advertisement for more details.

BGP graceful restart is enabled on one or more BGP sessions by configuring the graceful-restart command in the global, group or neighbor context. The command causes the GR capability to be advertised and enables helper mode support for IPv4 (AFI1, SAFI1), IPv6 (AFI 2, SAFI1), VPN-IPv4 and VPN-IPv6 routes. The GR capability advertised by a router does not list the supported AFI/SAFI unless enable-notification is configured.

Helper mode is activated when one of the following events affects an Established session:

As soon as the failure is detected, the helping 7450 ESS, 7750 SR, or 7950 XRS router marks the received IPv4, IPv6, VPN-IPv4 and VPN-IPv6 routes from the peer as ‘stale’ and starts a restart timer. (As noted above, the ‘stale’ state is not factored into the BGP decision process and not made visible to other routers in the network.) The restart timer derives its initial value from the Restart Time carried in the peer’s last GR capability. (The default advertised Restart Time is 300 seconds, but this can be changed using the restart-time command.) When the restart timer expires helping stops if the session has not yet re-established. If the session is re-established before the restart timer expires and the new GR capability from the restarting router indicates that forwarding state was preserved then helping continues and the peers exchange routes per the normal procedure. When each router has advertised all its routes for a particular address family it sends an End-of-RIB marker (EOR) for the address family. The EOR is a minimal UPDATE message with no reachable or unreachable NLRI for the AFI/SAFI. When the helping router receives an EOR it deletes all remaining stale routes of the AFI/SAFI that were not refreshed in the most recent set of UPDATE messages; there is an upper limit on the amount of time that routes can remain stale (before being deleted if they were not refreshed) and this is configurable using the stale-routes-time.

Note: If there is a second reset before GR has successfully completed, the router will always abort the GR helper process, regardless of the failure trigger.

5.3.4.1. TCP MD5 Authentication

The operation of a network can be compromised if an unauthorized system is able to form or hijack a BGP session and inject control packets by falsely representing itself as a valid neighbor. This risk can be mitigated by enabling TCP MD5 authentication on one or more of the sessions. When TCP MD5 authentication is enabled on a session every TCP segment exchanged with the peer includes a TCP option (19) containing a 16-byte MD5 digest of the segment (more specifically the TCP/IP pseudo-header, TCP header and TCP data). The MD5 digest is generated and validated using an authentication key that must be known to both sides. If the received digest value is different from the locally computed one then the TCP segment is dropped, thereby protecting the router from spoofed TCP segments.

5.3.4.2. TTL Security Mechanism

The TTL security mechanism (GTSM) relies on a simple concept to protect BGP infrastructure from spoofed IP packets. It recognizes the fact that the vast majority of EBGP sessions are established between directly-connected routers and therefore the IP TTL values in packets belonging to these sessions should have predictable values. If an incoming packet does not have the expected IP TTL value it is possible that it is coming from an unauthorized and potentially harmful source.

TTL security is enabled using the ttl-security command. This command requires a minimum TTL value to be specified. When TTL security is enabled on a BGP session the IP TTL values in packets that are supposedly coming from the peer are compared (in hardware) to the configured minimum value and if there is a discrepancy the packet is discarded and a log is generated. TTL security is used most often on single-hop EBGP sessions but it can be used on multihop EBGP and IBGP sessions as well.

To enable TTL security on a single-hop EBGP session, configure ttl-security and multihop to a value of 255. To enable TTL security on a multihop EBGP session, configure ttl-security and multihop to match the expected TTL of (255 - hop count). The TTL value for both EBGP peers must be manually configured to the same value, as there is no TTL negotiation.

Note: IP packets sent to an IBGP peer are originated with an IP TTL value of 64. IP packets to an EBGP peer are originated with an IP TTL value of 1, except if multihop is configured; in that case, the TTL value is taken from the multihop command.

5.5.1.1. Changing the Autonomous System Number

If the AS number is changed at the router level (config>router) the new AS number is not used until the BGP instance is restarted either by administratively disabling and enabling the BGP instance or by rebooting the system with the new configuration.

On the other hand if the AS number is changed in the BGP configuration (config>router>bgp) the effects are as follows:

5.5.1.2. Changing a Confederation Number

Changing the a confederation value on an active BGP instance will not restart the protocol. The change will take affect when the BGP protocol is (re) initialized.

5.5.1.3. BGP Advertisement

BGP advertisement allows a BGP router to indicate to a peer, using the optional parameter, the features that it supports so that they can coordinate and use only the features that both support. Each capability in the optional parameter is TLV-encoded with a unique type code. SR OS supports the following capability codes:

5.5.4.1. UPDATE Message Error Handling

The approach to handling Update message errors has evolved in the past couple of years. The original BGP protocol specification called for all UPDATE message errors to be handled the same way — send a NOTIFICATION to the peer and immediately close the BGP session. This error handling approach was motivated by the goal to ensure protocol “correctness” above all else. But it ignored several important points:

In recognition of these points and the general trend towards more flexibility in BGP error handling SR OS supports a BGP configuration option called update-fault-tolerance that allows the operator to decide whether the router should apply new or legacy error handling procedures to UPDATE message errors. If update-fault-tolerance is configured then non-critical errors as described above are handled using the “treat-as-withdraw” or “attribute-discard” approaches to error handling; these approaches do not cause a session reset. If update-fault-tolerance is not configured then legacy procedures continue to apply and all errors (critical and non-critical) trigger a session a reset.

5.6.2.1. AS Override

The AS Override feature can be used in VPRN scenarios where a customer is running BGP as the PE-CE protocol and some or all of the CE locations are in the same Autonomous System (AS). With normal BGP, two sites in the same AS would not be able to reach each other directly since there is an apparent loop in the AS Path.

When as-override is configured on a PE-CE EBGP session the PE rewrites the customer ASN in the AS Path with the VPRN AS number as the route is advertised to the CE.

5.6.2.2. Using Local AS for ASN Migration

The description in the previous section does fully explain the reasons for using local-as. This BGP feature facilitates the process of changing the ASN of all the routers in a network from one number to another. This may be necessary if one network operator merges with or acquires another network operator and the two BGP networks must be consolidated into one Autonomous System.

For example suppose the operator of the ASN 64500 network merges with the operator of the ASN 64501 network and the new merged entity decides to renumber ASN 64501 routers as ASN 64500 routers so that they the entire network can be managed as one Autonomous System. The migration can be carried out using the following sequence of steps:

This migration procedure has several advantages. First, customers, settlement-free peers and transit providers of the previous ASN 64501 network still perceive that they are peering with ASN 64501 and can delay switching to ASN 64500 until the time is convenient for them. Second, the AS path lengths of the routes exchanged with the EBGP peers are unchanged from before so that best path selections are preserved.

5.6.2.3. 4-Octet Autonomous System Numbers

When BGP was developed, it was assumed that 16-bit (2-octet) ASNs would be sufficient for global Internet routing. In theory a 16-bit ASN allows for 65536 unique autonomous systems but some of the values are reserved (0 and 64000-65535). Of the assignable space less than 10% remains available. When a new AS number is needed it is now simpler to obtain a 4-octet AS number. 4-octet AS numbers have been available since 2006. A 32-bit (4-octet) ASN allows for 4,294,967,296 unique values (some of which are again, reserved).

When 4-octet AS numbers became available it was recognized that not all routers would immediately support the ability to parse 4-octet AS numbers in BGP messages so two optional transitive attributes called AS4_PATH and AS4_AGGREGATOR were introduced to allow a gradual migration.

A BGP router that supports 4-octet AS numbers advertises this capability in its OPEN message; the capability information includes the AS number of the sending BGP router, encoded using 4 bytes (recall the ASN field in the OPEN message is limited to 2 bytes). By default, OPEN messages sent by 7450, 7750, or 7950 routers always include the 4-octet ASN capability but this can changed using the disable-4byte-asn command.

If a BGP router and its peer have both announced the 4-octet ASN capability then the AS numbers in the AS_PATH and AGGREGATOR attributes are always encoded as 4-byte values in the UPDATE messages they send to each other. These UPDATE messages should not contain the AS4_PATH and AS4_AGGREGATOR path attributes.

If one of the routers involved in a session announces the 4-octet ASN capability and the other one does not then the AS numbers in the AS_PATH and AGGREGATOR attributes are encoded as 2-byte values in the UPDATE messages they send to each other.

When a 7450, 7750, or 7950 router advertises a route to a peer that did not announce the 4-octet ASN capability:

When a 7450, 7750, or 7950 router receives a route with an AS4_PATH attribute it attempts to reconstruct the full AS path from the AS4_PATH and AS_PATH attributes, regardless of whether disable-4byte-asn is configured or not. The reconstructed path is the AS path displayed in BGP show commands. If the length of the received AS4_PATH is N and the length of the received AS_PATH is N+t then the reconstructed AS path contains the t leading elements of the AS_PATH followed by all the elements in the AS4_PATH.

5.6.3.1. Next-Hop IPv4 Address Family over IPv6

For IBGP sessions, next-hop information is taken from the system interface. If the system interface does not have an IPv4 address configured, no next-hop will be populated without a routing policy applied to the BGP session, and BGP NLRI messages is not sent for the IPv4 address family. The use of an export policy allows the operator to configure next-hop information explicitly.

For EBGP sessions, the next-hop information must be taken from an export routing policy that explicitly sets the next-hop based on operator configuration. If the export policy is not set, the BGP NLRI messages are not sent for the IPv4 address family due to no next_hop.

5.6.3.2. Next-Hop VPN-IPv4 Address Family over IPv6

For IBGP and EBGP sessions, next-hop information is specified as the system IP address.

5.6.3.3. Next-Hop VPN-IPv6 Address Family over IPv6

For IBGP sessions, the next-hop information is specified as the system IP address encoded as an IPv4-mapped-IPv6 address.

For EBGP sessions, the next-hop information is specified as the system IP address encoded as an IPv4-mapped-IPv6 address, by the way of an export policy configured by the user.

5.6.3.4. Next-Hop Resolution

For a BGP router to use a BGP route for forwarding it must know how to reach the BGP next-hop of the route. The process of determining the local interface or tunnel that should be used to reach the BGP next-hop is called next-hop resolution. The BGP next-hop resolution process depends on the type of route (the AFI/SAFI) and various configuration settings. The SR OS details are explained below:

5.6.3.5. Next-Hop Tracking

In SR OS next-hop resolution is not a one-time event. If the IP route or tunnel that was used to resolve a BGP next-hop is withdrawn due to a failure or configuration change an attempt is made to re-resolve the BGP next-hop using the next-best route or tunnel. If there are no more eligible routes or tunnels to resolve the BGP next-hop then the BGP next-hop becomes unresolved. The continual process of monitoring and reacting to resolving route/tunnel changes is called next-hop tracking. In SR OS next-hop tracking is completely event driven as opposed to timer driven; this provides the best possible convergence performance.

5.6.3.6. Next-Hop Indirection

SR OS supports next-hop indirection for most types of BGP routes. Next-hop indirection means BGP next-hops are logically separated from resolved next-hops in the forwarding plane (IOMs). This separation allows routes that share the same BGP next-hops to be grouped so that when there is a change to the way a BGP next-hop is resolved only one forwarding plane update is needed, as opposed to one update for every route in the group. The convergence time after the next-hop resolution change is uniform and not linear with the number of prefixes; in other words the next-hop indirection is a technology that supports prefix independent convergence (PIC). SR OS uses next-hop indirection whenever possible; there is no option to disable the functionality.

5.6.3.7. Using Multiple Address Families over IPv6 BGP Sessions

To ease transition to IPv6 and the deployment of IPv6 into service provider environments, SR OS permits the transport of the following address families over an IPv6 transported BGP session (a BGP session where both neighbors are configured and transported over IPv6):

As the IPv4, VPN-IPv4 and VPN-IPv6 address families require an IPv4 NEXT_HOP address to be present in the BGP NLRI messaging, the following approaches are taken in SR OS:

5.6.4.1. Deterministic MED

Deterministic MED is an optional enhancement to the BGP decision process that causes BGP to groups paths that are equal up to the MED comparison step based on the neighbor AS. BGP compares the best path from each group to arrive at the overall best path. This change to the BGP decision process makes best path selection completely deterministic in all cases. Without deterministic-med, the overall best path selection is sometimes dependent on the order of route arrival because of the rule that MED cannot be compared in routes from different neighbor AS.

Note: When BGP routes are leaked into a target BGP RIB, they are not grouped (in a deterministic MED context) with routes learned by that target RIB, even if the neighbor AS happens to be the same.

5.7.1.1. BGP Import Policies

The import command is used to apply one or more policies (up to 15) to a neighbor, group or to the entire BGP context. The import command that is most-specific to a peer is the one that is applied. An import policy command applied at the neighbor level takes precedence over the same command applied at the group or global level. An import policy command applied at the group level takes precedence over the same command specified on the global level. The import policies applied at different levels are not cumulative. The policies listed in an import command are evaluated in the order in which they are specified.

Note: The import command can reference a policy before it has been created (as a policy-statement).

When an IP route is rejected by an import policy it is still maintained in the RIB-IN so that a policy change can be made later on without requiring the peer to re-send all its RIB-OUT routes. This is sometimes called soft reconfiguration inbound and requires no special configuration in SR OS.

When a VPN route is rejected by an import policy or not imported by any services it is deleted from the RIB-IN. For VPN-IPv4 and VPN-IPv6 routes this behavior can be changed by configuring the mp-bgp-keep command; this option maintains rejected VPN-IP routes in the RIB-IN so that a Route Refresh message does not have to be issued when there is an import policy change.

5.7.2.1. BGP Decision Process

When a BGP router has multiple paths in its LOC-RIB for the same NLRI, its BGP decision process is responsible for deciding which path is the best. The best path can be used by the local router and advertised to other BGP peers.

On 7450 ESS, 7750 SR, and 7950 XRS routers, the BGP decision process orders received paths based on the following sequence of comparisons. If there is a tie between paths at any step, BGP proceeds to the next step.

5.7.2.2. BGP Route Installation in the Route Table

Each BGP RIB holding routes (unlabeled IPv4, labeled-unicast IPv4, unlabeled IPv6, labeled-unicast IPv6) submits its best path for each prefix to the common IP route table, unless disable-route-table-install is configured. It is up to the route table to choose the single best of these paths for forwarding to each IP prefix destination. The route table chooses the route by using the BGP decision process. The default preference for BGP routes submitted by the label-IPv4 and label-IPv6 RIBs (which appear in the route table and FIB as having a BGP-LABEL protocol type) can be modified using the label-preference command. The default preference for BGP routes submitted by the unlabeled IPv4 and IPv6 RIBs can be modified by using the preference command.

Note: Consider configuring the disable-route-table-install command on control-plane route reflectors that are not involved in packet forwarding (i.e. that do not modify the BGP NEXT_HOP); this improves the performance and scalability of such route reflectors.

If a BGP RIB has multiple BGP paths (LOC-RIB routes) for the same IPv4 or IPv6 prefix that qualify as the best path up to a certain point in the comparison process, then a certain number of these multipaths can be submitted to the common IP route table. This is called BGP Multipath and it must be explicitly enabled using the multipath command. The multipath command specifies the maximum number (64) of BGP paths, including the overall best path, that each BGP RIB can submit to the route table for any particular IPv4 or IPv6 prefix. If ECMP, with a limit of n, is enabled in the base router instance, then up to n paths are selected for installation in the IP FIB. In the datapath, traffic matching the IP route is load-shared across the ECMP next-hops based on a per-packet hash calculation.

By default the hashing is not sticky, meaning that when one or more of the equal-cost BGP next-hops fail, all traffic flows matching the route are potentially moved to new BGP next-hops. If required, a BGP route can be marked (using the sticky-ecmp action in route policies) for sticky ECMP behavior so that BGP next-hop failures are handled by moving only the affected traffic flows to the remaining next-hops as evenly as possible.

In the route table, a BGP path to an IPv4 or IPv6 prefix is a candidate for installation as an ECMP next-hop (subject to the path limits of the multipath and ecmp commands) only if it meets all of the following criteria:

SR OS also supports a feature called IBGP-Multipath. In some topologies a BGP next-hop is resolved by an IP route (for example a static, OSPF or IS-IS route) that itself has multiple ECMP next-hops. When ibgp-multipath is not configured only one of these ECMP next-hops is programmed as a next-hop of the BGP route in the IOM. But when ibgp-multipath is configured the IOM attempts to use all of the ECMP next-hops of the resolving route in forwarding.

Although the name of the ibgp-multipath command implies that it is specific to IBGP-learned routes this is not the case; it applies to routes learned from any multi-hop BGP session including routes learned from multi-hop EBGP peers.

Note: BGP-Multipath and IBGP-Multipath are not mutually exclusive and work together. BGP-Multipath enables ECMP load-sharing across different BGP next-hops (corresponding to different LOC-RIB routes) and IBGP-Multipath enables ECMP load-sharing across different IP next-hops of IP routes that resolve the BGP next-hops.

The final point about IBGP-Multipath is that it does not control load-sharing of traffic towards a BGP next-hop that is resolved by a tunnel, such as the case when dealing with BGP shortcuts or labeled routes (VPN-IP, label-IPv4, label-IPv6). When a BGP next-hop is resolved by a tunnel that supports ECMP the load-sharing of traffic across the ECMP next-hops of the tunnel is automatic.

Note: At the current time SR OS does not support direct resolution of a BGP next-hop to multiple RSVP-TE tunnels. However a BGP next-hop can be resolved by multiple LDP ECMP next-hops that each correspond to a separate LDP-over-RSVP tunnel. It is also possible for a BGP next-hop to be resolved by an IGP shortcut route that has multiple RSVP-TE tunnels as its ECMP next-hops.

5.7.2.3. Weighted ECMP for BGP Routes

In some cases, the ECMP BGP next-hops of an IP route correspond to paths with very different bandwidths and it makes sense for the ECMP load-balancing algorithm to distribute traffic across the BGP next-hops in proportion to their relative bandwidths. The bandwidth associated with a path can be signaled to other BGP routers by including a Link Bandwidth Extended Community in the BGP route. The Link Bandwidth Extended Community is optional and non-transitive and encodes an autonomous system (AS) number and a bandwidth.

In SR OS, a Link Bandwidth Extended Community can be added to an IPv4, IPv6, label-IPv4, label-IPv6, VPN-IPv4, or VPN-IPv6 route using either route policies or the ebgp-link-bandwidth command. The ebgp-link-bandwidth command is supported in BGP group and neighbor configuration contexts and automatically adds (on import) a Link Bandwidth Extended Community to received routes from single-hop (directly connected) EBGP peers. When a route is advertised to an EBGP peer, the Link Bandwidth Extended Community, if present, is always removed. The Link Bandwidth Extended Community associated with a BGP route can be displayed using the show router bgp routes commands; for the bandwidth value, the system automatically converts the binary value in the extended community to a decimal number in units of Mbps (1000000 bit/s).

7450, 7750, and 7950 routers automatically perform weighted ECMP for an IP BGP route when the ingress card is FP2 or better and all the ECMP BGP next-hops of the route include a Link Bandwidth Extended Community. The relative weight of traffic sent to each BGP next-hop is visible in the output of the show router route-table extensive and show router fib extensive commands.

Weighted ECMP across the BGP next-hops of an IP BGP route is supported in combination with ECMP at the level of the route or tunnel that resolves one or more of the ECMP BGP next-hops. This ECMP at the resolving level can also be weighted ECMP when the following conditions all apply:

5.7.2.4. BGP Route Installation in the Tunnel Table

If the best BGP path for a /32 IPv4 prefix is a label-IPv4 route (AFI 1, SAFI 4), and if it has the numerically lowest preference value among all routes (regardless of protocol) for the /32 IPv4 prefix, and if disable-route-table-install is not configured, the label-IPv4 route is automatically added, as a BGP tunnel entry, to the tunnel table. In SR OS the tunnel-table is used to resolve a BGP next-hop to a tunnel when required by the configuration or the type of route (see the section titled Next-Hop Resolution for many of these details). BGP tunnels play a key role in the following solutions:

BGP tunnels have a preference of 10 in the tunnel table, compared to 9 for LDP tunnels and 7 for RSVP tunnels, so if the router configuration allows all types of tunnels to resolve a BGP next-hop an RSVP LSP is preferred over an LDP tunnel and an LDP tunnel is preferred over a BGP tunnel.

If multipath and ecmp are configured appropriately a BGP tunnel can be installed in the tunnel table with multiple ECMP next-hops, each one corresponding to a path through a different BGP next-hop; the multipath selection process outlined in the previous section (BGP Route Installation in the Route Table) also applies to this case.

5.7.2.5. BGP Fast Reroute

BGP fast reroute is a feature that brings together indirection techniques in the forwarding plane and pre-computation of BGP backup paths in the control plane to support fast reroute of BGP traffic around unreachable/failed BGP next-hops. BGP fast reroute is supported with IPv4, label-IPv4, IPv6, label-IPv6, VPN-IPv4 and VPN-IPv6 routes. The scenarios supported by the base router BGP context are outlined in Table 45.

Refer to the VPRN section of the Layer 3 Services Guide for more information about BGP fast reroute information specific to IP VPNs.

5.7.2.6. QoS Policy Propagation via BGP (QPPB)

QPPB is a feature that allows different QoS values (forwarding class and optionally priority) to be associated with different IPv4 and IPv6 BGP LOC-RIB routes based on BGP import policy processing. This is done so that when traffic arrives on a QPPB-enabled IP interface and its source or destination IP address matches a BGP route with QoS information the packet is handled according to the QoS of the matching route. SR OS supports QPPB on the following types of interfaces:

QPPB is enabled on an interface using the qos-route-lookup command. There are separate commands for IPv4 and IPv6 so that QPPB can be enabled in one mode (source or destination or none) for IPv4 packets arriving on the interface and a different mode (source or destination or none) for IPv6 packets arriving on the interface.

Note: Source-based QPPB is not supported on subscriber interfaces.

Different LOC-RIB routes for the same IP prefix may be associated with different QPPB information. If these LOC-RIB routes are combined in support of ECMP or BGP fast reroute then the QPPB information becomes next-hop specific. This means that in destination QPPB mode the QoS assigned to a packet depends on the BGP next-hop that is selected for that particular packet by the ECMP hash or fast reroute algorithm. In source QPPB mode the QoS assigned to a packet comes from the first BGP next-hop of the IP route matching the source address.

5.7.2.7. BGP Policy Accounting

Policy accounting is a feature that allows different accounting classes to be associated with IPv4 and IPv6 BGP LOC-RIB routes based on BGP import policy processing. This is done so that per-accounting-class traffic statistics can be collected on policy accounting-enabled interfaces of the router. Policy accounting interfaces are only supported on IOM3 or better cards. The following types of interfaces are supported:

Policy accounting is enabled on an interface using the policy-accounting command. The name of a policy accounting template must be specified. Each policy accounting template contains a list of source classes and destination classes. Routers support up to 255 different source classes and up to 255 different destination classes. Each source class is identified by an index number (1-255) and each destination class is identified by an index number (1-255). The policy accounting template tells the IOM what accounting classes to collect stats for on a policy accounting interface. SR OS supports up to 1024 different templates, depending on the chassis type.

Note: Policy accounting templates containing one or more source class identifiers cannot be applied to subscriber interfaces.

Through policy mechanisms a LOC-RIB route for an IP prefix can have a source class index (1-25), a destination class index (1-255) or both. When an ingress packet on a policy-accounting enabled interface [I1] is forwarded by the IOM and its destination address matches a BGP route with a destination class index [D], and [D] is listed in the relevant policy accounting template, packets-forwarded and IP-bytes-forwarded counters for [D] on interface [I1] are incremented accordingly. Similarly, when an ingress packet on a policy-accounting enabled interface [I2] is forwarded by the IOM and its source address matches a BGP route with a source class index [S], and [S] is listed in the relevant policy accounting template, the packets-forwarded and IP-bytes-forwarded counters for [S] on interface [I2] are incremented accordingly.

It is possible that different LOC-RIB routes for the same IP prefix are associated with different accounting class information. If these LOC-RIB routes are combined in support of ECMP or BGP fast reroute then the destination-class of a packet depends on the BGP next-hop that is selected for that particular packet by the ECMP hash or fast reroute algorithm. If the source address of a packet matches a route with multiple BGP next-hops its source-class is derived from the first BGP next-hop of the matching route.

5.7.2.8. Route Flap Damping (RFD)

Route flap damping is a mechanism supported by 7450, 7750, and 7950 routers, as well as other BGP routers, that was designed to help improve the stability of Internet routing by mitigating the impact of route flaps. Route flaps describe a situation where a router alternately advertises a route as reachable and then unreachable or as reachable through one path and then another path in rapid succession. Route flaps can result from hardware errors, software errors, configuration errors, unreliable links, etc. However not all perceived route flaps represent a true problem; when a best path is withdrawn the next-best path may not be immediately known and may trigger a number of intermediate best path selections (and corresponding advertisements) before it is found. These intermediate best path selections may travel at different speeds through different routers due to the effect of the min-route-advertisement interval (MRAI) and other factors. RFD does not handle this type of situation particularly well and for this and other reasons many Internet service providers do not use RFD.

In SR OS route flap damping is configurable; by default it is disabled. It can be enabled on EBGP and confed-EBGP sessions by including the damping command in their group or neighbor configuration. The damping command has no effect on IBGP sessions. When a route of any type (any AFI/SAFI) is received on a non-IBGP session that has damping enabled:

5.7.3.1. BGP Export Policies

The export command is used to apply one or more policies (up to 15) to a neighbor, group or to the entire BGP context. The export command that is most-specific to a peer is the one that is applied. An export policy command applied at the neighbor level takes precedence over the same command applied at the group or global level. An export policy command applied at the group level takes precedence over the same command specified on the global level. The export policies applied at different levels are not cumulative. The policies listed in an export command are evaluated in the order in which they are specified.

Note: The export command can reference a policy before it has been created (as a policy-statement).

The most common uses for BGP export policies are as follows:

5.7.3.2. Outbound Route Filtering (ORF)

Outbound route filtering (ORF) is a mechanism that allows one router, the ORF-sending router to signal to a peer, the ORF-receiving router, a set of route filtering rules (ORF entries) that the ORF-receiving router should apply to its route advertisements towards the ORF-sending router. The ORF entries are encoded in Route Refresh messages.

The use of ORF on a session must be negotiated —i.e. both routers must advertise the ORF capability in their Open messages. The ORF capability describes the address families that support ORF, and for each address family, the ORF types that are supported and the ability to send/receive each type. 7450, 7750, and 7950 routers support ORF type 3, which is ORF based on Extended Communities. It is supported for only the following address families:

In SR OS the send/receive capability for ORF type 3 is configurable (with the send-orf and accept-orf commands) but the setting applies to all supported address families.

SR OS support for ORF type 3 allows a PE router that imports VPN routes with a particular set of Route Target Extended Communities to indicate to a peer (for example a route reflector) that it only wants to receive VPN routes that contain one or more of these Extended Communities. When the PE router wants to inform its peer about a new RT Extended Community it sends a Route Refresh message to the peer containing an ORF type 3 entry instructing the peer to add a permit entry for the 8-byte extended community value. When the PE router wants to inform its peer about a RT Extended Community that is no longer needed it sends a Route Refresh message to the peer containing an ORF type 3 entry instructing the peer to remove the permit entry for the 8-byte extended community value.

In SR OS the type-3 ORF entries that are sent to a peer can be generated dynamically (if no Route Target Extended Communities are specified with the send-orf command) or else specified statically. Dynamically generated ORF entries are based on the route targets that are imported by all locally-configured VPRNs.

A router that has installed ORF entries received from a peer can still apply BGP export policies to the session. If the evaluation of a BGP export policy results in a reject action for a VPN route that matches a permit ORF entry the route is not advertised — i.e. the export policy has the final word.

Note: The SR OS implementation of ORF filtering is very efficient. It takes less time to filter a large number of VPN routes with ORF than it does to reject non-matching VPN routes using a conventional BGP export policy.

Despite the advantages of ORF compared to manually configured BGP export policies a better technology, when it comes to dynamic filtering based on Route Target Extended Communities, is RT Constraint. RT Constraint is discussed further in the next section.

5.7.3.3. RT Constrained Route Distribution

RT constrained route distribution, or RT-constrain for short, is a mechanism that allows a router to advertise to certain peers a special type of MP-BGP route called an RTC route; the associated AFI is 1 and the SAFI is 132. The NLRI of an RTC route encodes an Origin AS and a Route Target Extended Community with prefix-type encoding (i.e. there is a prefix-length and “host” bits after the prefix-length are set to zero). A peer receiving RTC routes does not advertise VPN routes to the RTC-sending router unless they contain a Route Target Extended Community that matches one of the received RTC routes. As with any other type of BGP route RTC routes are propagated loop-free throughout and between Autonomous Systems. If there are multiple RTC routes for the same NLRI the BGP decision process selects one as the best path. The propagation of the best path installs RIB-OUT filter rules as it is travels from one router to the next and this process creates an optimal VPN route distribution tree rooted at the source of the RTC route.

Note: RT-constrain and Extended Community-based ORF are similar to the extent that they both allow a router to signal to a peer the Route Target Extended Communities they want to receive in VPN routes from that peer. But RT-constrain has distinct advantages over Extended Community-based ORF: it is more widely supported, it is simpler to configure, and its distribution scope is not limited to a direct peer.

In SR OS the capability to exchange RTC routes is advertised when the route-target keyword is added to the relevant family command. RT-constrain is supported on EBGP and IBGP sessions of the base router instance. On any particular session either ORF or RT-constrain may be used but not both; if RT-constrain is configured the ORF capability is not announced to the peer.

When RT-constrain has been negotiated with one or more peers SR OS automatically originates and advertises to these peers one /96 RTC route (the origin AS and Route Target Extended Community are fully specified) for every route target imported by a locally-configured VPRN or BGP-based L2 VPN; this includes MVPN-specific route targets.

SR OS also supports a group/neighbor level default-route-target command that causes routers to generate and send a 0:0:0/0 default RTC route to one or more peers. Sending the default RTC route to a peer conveys a request to receive all VPN routes from that peer. The default-route-target command is typically configured on sessions that a route reflector has with its PE clients. A received default RTC route is never propagated to other routers.

The advertisement of RTC routes by a route reflector follows special rules that are described in RFC 4684. These rules are needed to ensure that RTC routes for the same NLRI that are originated by different PE routers in the same Autonomous System are properly distributed within the AS.

When a BGP session comes up, and RT-constrain is enabled on the session (both peers advertised the MP-BGP capability), routers delay sending any VPN-IPv4 and VPN-IPv6 routes until either the session has been up for 60 seconds or the End-of-RIB marker is received for the RT-constrain address family. When the VPN-IPv4 and VPN-IPv6 routes are sent they are filtered to include only those with a Route Target Extended Community that matches an RTC route from the peer. VPN-IP routes matching an RTC route originated in the local AS are advertised to any IBGP peer that advertises a valid path for the RTC NLRI — i.e. route distribution is not constrained to only the IBGP peer advertising the best path. On the other hand VPN-IP routes matching an RTC route originated outside the local AS are only advertised to the EBGP or IBGP peer that advertises the best path.

Note: SR OS does not support an equivalent of BGP-Multipath for RT-Constrain routes. There is no way to distribute VPN routes across more than one ‘almost’ equal set of inter-AS paths.

On 7450, 7750, and 7950 routers, received RTC routes have no effect on the advertisement on MVPN-IPv4, MVPN-IPv6 and L2-VPN routes.

5.7.3.4. Min Route Advertisement Interval (MRAI)

According to the BGP standard (RFC 4271) a BGP router should not send updated reachability information for an NLRI to a BGP peer until a certain period of time, called the Min Route Advertisement Interval, has elapsed since the last update. The RFC suggests the MRAI should be configurable per peer but does not propose a specific algorithm and therefore MRAI implementation details vary from one router operating system to another.

In SR OS the MRAI is configurable, on a per-session basis, using the min-route-advertisement command. The min-route-advertisement command can be configured with any value between 1 and 255 seconds and the setting applies to all address families. The default value is 30 seconds, regardless of the session type (EBGP or IBGP). When all RIB-OUT routes have been sent to a peer the MRAI timer associated with that session is started and when it expires the RIB-OUT changes that have accumulated while the timer was running trigger the sending of a new set of UPDATE messages to the peer.

It may be important to send UPDATE messages that advertise new NLRI reachability information more frequently for some address families than others. SR OS offers a rapid-update command that overrides the peer-level min-route-advertisement time and applies the minimum setting (0 seconds) to routes belonging to specific address families; routes of other address families continue to be advertised according to the session-level MRAI setting. The address families that can be configured with rapid-update support are:

In many cases the default MRAI is appropriate for all address families (or at least those not included in the above list) when it applies to UPDATE messages that advertise reachable NLRI but it is less than ideal for UPDATE messages that advertise unreachable NLRI (route withdrawals). Fast re-convergence after some types of failures requires route withdrawals to propagate to other routers are quickly as possible so that they can calculate and start using new best paths and this is impeded by the effect of the MRAI timer at each router hop. SR OS provides a solution for this problem by supporting a configuration command called rapid-withdrawal. When rapid-withdrawal is configured UPDATE messages containing withdrawn NLRI are sent immediately to a peer — without waiting for the MRAI timer to expire. UPDATE messages containing reachable NLRI continue to wait for the MRAI timer to expire, and this timer remains governed by the min-route-advertisement time or the rapid-update command, if it applies. When rapid-withdrawal is enabled it applies to all address families.

5.7.3.5. Advertise-Inactive

BGP does not allow a route to be advertised unless it is the best path in the RIB and an export policy allows the advertisement.

In some cases it may be useful to advertise the best BGP path to peers despite the fact that is inactive —i.e. because there are one or more lower-preference non-BGP routes to the same destination and one of these other routes is the active route. One way SR OS supports this flexibility is using the advertise-inactive command; other methods include Best-External and Add-Paths.

As a global BGP configuration option the advertise-inactive command applies to all IPv4, IPv6, label-IPv4, and label-IPv6 routes and all sessions that advertise these routes. When the command is configured and the best BGP path is inactive it is automatically advertised to every peer unless rejected by a BGP export policy.

5.7.3.6. Best-External

Best-External is a BGP enhancement that allows a BGP speaker to advertise to its IBGP peers its best “external” route for a prefix/NLRI when its best overall route for the prefix/NLRI is an “internal” route. This is not possible in a normal BGP configuration because the base BGP specification prevents a BGP speaker from advertising a non-best route for a destination.

In certain topologies Best-External can improve convergence times, reduce route oscillation and allow better loadsharing. This is achieved because routers internal to the AS have knowledge of more exit paths from the AS. Enabling Add-Paths on border routers of the AS can achieve a similar result but Add-Paths introduces NLRI format changes that must be supported by BGP peers of the border router and therefore has more interoperability constraints than Best-External (which requires no messaging changes).

Best-External is supported in the base router BGP context. (A related feature is also supported in VPRNs; consult the Services Guide for more details.) It is configured using the advertise-external command, which provides IPv4, label-IPv4, IPv6, and label-IPv6 as options.

The advertisement rules when advertise-external is enabled can be summarized as follows:

5.7.3.7. Add-Paths

Add-Paths is a BGP enhancement that allows a BGP router to advertise multiple distinct paths for the same prefix/NLRI. This provides a number of potential benefits, including reduced routing churn, faster convergence, and better loadsharing.

In order for a router to receive multiple paths per NLRI from a peer, for a particular address family, the peer must announce the BGP capability to send multiple paths for the address family and the local router must announce the BGP capability to receive multiple paths for the address family. When the Add-Path capability has been negotiated this way all advertisements and withdrawals of NLRI by the peer must include a path identifier. The path identifier has no significance to the receiving router. If the combination of NLRI and path identifier in an advertisement from a peer is unique (does not match an existing route in the RIB-IN from that peer) then the route is added to the RIB-IN. If the combination of NLRI and path identifier in a received advertisement is the same as an existing route in the RIB-IN from the peer then the new route replaces the existing one. If the combination of NLRI and path identifier in a received withdrawal matches an existing route in the RIB-IN from the peer then that route is removed from the RIB-IN.

An UPDATE message carrying an IPv4 NLRI with a path identifier is shown in Figure 28.

Figure 28:  BGP Update Message with Path Identifier for IPv4 NLRI 

Add-Paths is only supported by the base router BGP instance and the EBGP and IBGP sessions it forms with other Add-Paths-capable peers. The ability to send and receive multiple paths per prefix is configurable per family, with the supported options being:

5.7.3.8. Split-Horizon

Split-horizon refers to the action taken by a router to avoid advertising a route back to the peer from which it was received. By default SR OS applies split-horizon behavior only to routes received from IBGP non-client peers, and split-horizon only works for routes to non-imported routes within a RIB. This split-horizon functionality, which can never be disabled, prevents a route learned from a non-client IBGP peer to be advertised to the sending peer or any other non-client peer.

To apply split-horizon behavior to routes learned from RR clients, confed-EBGP peers or (non-confed) EBGP peers the split-horizon command must be configured in the appropriate contexts; it is supported at the global BGP, group and neighbor levels. When split-horizon is enabled on these types of sessions it only prevents the advertisement of a route back to its originating peer; for example SR OS does not prevent the advertisement of a route learned from one EBGP peer back to different EBGP peer in the same neighbor AS.

5.8.1.1. BGP Routes

The user enables the resolution of IPv4 prefixes using tunnels to BGP next-hops in TTM with the following command:

The shortcut-tunnel and family nodes are contexts to configure the binding of BGP unlabeled routes to tunnels.

The default resolution of a BGP unlabeled route is performed in RTM. The user must configure the resolution option to enable resolution to tunnels in TTM. If the resolution option is explicitly set to disabled, the binding to tunnel is removed and resolution resumes in RTM to IP next-hops.

If resolution is set to any, any supported tunnel type in BGP shortcut context will be selected following TTM preference. If one or more explicit tunnel types are specified using the resolution-filter option, then only these tunnel types will be selected again following the TTM preference.

The following tunnel types are supported in a BGP shortcut context and in order of preference: RSVP, LDP, Segment Routing (SR), and BGP.

The user must set resolution to filter to activate the list of tunnel-types configured under resolution-filter.

If disallow-igp is enabled, the BGP route will not be activated using IP next-hops in RTM if no tunnel next-hops are found in TTM.

5.8.1.2. BGP Labeled Routes

The user enables the resolution of RFC 3107 BGP label route prefixes using tunnels to BGP next-hops in TTM with the following CLI syntax:

The label-route-transport-tunnel and family CLI nodes are contexts used to configure the binding of IPv4 or IPv6 BGP labeled routes to tunnels.

The label-route-transport-tunnel command provides a separate control for the different families of RFC 3107 BGP label routes: IPv4 routes, IPv6 routes (label-IPv6) inter-AS option B vpn-ipv4 and vpn-ipv6 routes at ASBR.

By default, IPv4 label routes, IPv6 label routes (label-IPv6) and inter-AS option B VPN label routes resolve to LDP without the user needing to enter this command.

If the resolution option is explicitly set to disabled, the default binding to LDP tunnel resumes. If resolution is set to any, any supported tunnel type in BGP label route context will be selected following TTM preference.

The following tunnel types are supported in a BGP label route context and in order of preference: RSVP, LDP, segment routing OSPF, and segment routing IS-IS.

If one or more explicit tunnel types are specified using the resolution-filter option, then only these tunnel types will be selected again following the TTM preference.

The user must set resolution to filter to activate the list of tunnel-types configured under resolution-filter.

5.8.1.3. VPN-IPv4 and VPN-IPv6 Routes

The user enables the resolution of vpn-ipv4 and vpn-ipv6 prefixes using tunnels to MP-BGP peers with the following CLI syntax:

The same auto-bind command is supported with BGP EVPN service:

The auto-bind-tunnel node is simply a context to configure the binding of VPRN or BGP EVPN routes to tunnels. The user must configure the resolution option to enable auto-bind resolution to tunnels in TTM. If the resolution option is explicitly set to disabled, the auto-binding to tunnel is removed.

If resolution is set to any, any supported tunnel type in VPRN or BGP EVPN context will be selected following TTM preference. If one or more explicit tunnel types are specified using the resolution-filter option, then only these tunnel types will be selected again following the TTM preference.

The following tunnel types are supported in a VPRN or BGP EVPN context in order of preference: RSVP, LDP, Segment Routing (SR), and GRE.

The user must set resolution to filter to activate the list of tunnel-types configured under resolution-filter.

When an explicit SDP to a BGP next-hop is configured in a VPRN or BGP EVPN service (config>service>vprn>spoke-sdp), it overrides the auto-bind-tunnel selection for that BGP next-hop only. There is no support for reverting automatically to the auto-bind-tunnel selection if the explicit SDP goes down. The user must delete the explicit spoke-SDP in the VPRN or BGP EVPN service context to resume using the auto-bind-tunnel selection for the BGP next-hop.

5.8.2.1. Validating Received Flow Routes

IPv4 and IPv6 flow routes received from a BGP peer must be validated before they can be installed as filter entries. A flow route is considered invalid if:

After received flow routes are validated they are processed by the relevant import policies, if applicable.

5.8.2.2. Using Flow Routes to Create Dynamic Filter Entries

When the base router BGP instance receives an IPv4 or IPv6 flow route and that route is valid/best, the system attempts to construct an IPv4 or IPv6 filter entry from the NLRI contents and the actions encoded in the UPDATE message. If successful, the filter entry is added to the system-created “fSpec-0” IPv4 embedded filter or to the “fSpec-0” IPv6 embedded filter. These embedded filters can be inserted into configured IPv4 and IPv6 filter policies that are applied to ingress traffic on a selected set of the base router IP interfaces. These interfaces can include network interfaces, IES SAP interfaces, and IES spoke SDP interfaces.

Similarly, filter entries can be added to system-created “fSpec-$vprnId” embedded filters for use with VPRN interfaces.

When flowspec rules are embedded into a user-defined filter policy, the insertion point of the rules is configurable through the offset parameter of the embed-filter command. The offset is 0 by default, meaning that the flowspec rules are evaluated after all other rules.

5.8.3.1. TTL Propagation for RFC 3107 Label Route at Ingress LER

For IPv4 and IPv6 packets forwarded using a RFC 3107 label route in the global routing instance, including label-IPv6, the following command specified with the all value enables TTL propagation from the IP header into all labels in the transport label stack:

The none value reverts to the default mode which disables TTL propagation from the IP header to the labels in the transport label stack.

These commands do not have a no version.

Note: The TTL of the IP packet is always propagated into the RFC 3107 label itself. The commands only control the propagation into the transport labels, for example, the labels of the RSVP or LDP LSP which the BGP label route resolves to and which are pushed on top of the BGP label.

Note: If the BGP peer advertised the implicit-null label value for the BGP label route, the TTL propagation will not follow the configuration described, but will follow the configuration to which the BGP label route resolves:

This feature does not impact packets forwarded over BGP shortcuts. The ingress LER operates in uniform mode by default and can be changed into pipe mode using the configuration of TTL propagation for RSVP or LDP LSP shortcut.

5.8.3.2. TTL Propagation for RFC 3107 Label Routes at LSR

This feature configures the TTL propagation for transit packets at a router acting as an LSR for a BGP label route.

When an LSR swaps the BGP label for a IPv4 prefix packet, thus acting as a ABR, ASBR, or data-path Route-Reflector (RR) in the base routing instance, or swaps the BGP label for a vpn-IPv4 or vpn-IPv6 prefix packet, thus acting as an inter-AS Option B VPRN ASBR or VPRN data path Route-Reflector (RR), the all value of the following command enables TTL propagation of the decremented TTL of the swapped BGP label into all LDP or RSVP transport labels.

When an LSR swaps a label or stitches a label, it always writes the decremented TTL value into the outgoing swapped or stitched label. What the above CLI controls is whether this decremented TTL value is also propagated to the transport label stack pushed on top of the swapped or stitched label.

The none value reverts to the default mode which disables TTL propagation. This changes the existing default behavior which propagates the TTL to the transport label stack. When a customer upgrades, the new default becomes in effect. The above commands do not have a no version.

The following describes the behavior of LSR TTL propagation in a number of other use cases and indicates if the above CLI command applies or not:

5.10.1.1. BGP Defaults

The following list summarizes the BGP configuration defaults:

5.10.1.2. BGP MIB Notes

The router implementation of the RFC 1657 MIB variables listed in Table 49 differs from the IETF MIB specification.

If SNMP is used to set a value of X to the MIB variable in Table 49, there are three possible results:

When the value set using SNMP is within the IETF allowed values and outside the SR OS values as specified in Table 49 and Table 50, a log message is generated.

The log messages that display are similar to the following log messages:

Sample Log Message for setting bgpPeerMinRouteAdvertisementInterval to 256

Sample Log Message for setting bgpPeerMinRouteAdvertisementInterval to 1