Tag Archives: 3.7i

3.7.i Describe BGP fast convergence features

3.7.i [i] Prefix independent convergence

The BGP Prefix Independent Convergence (PIC) improves convergence after a network failure. This convergence is applicable to both core and edge failures on IP and MPLS networks. You can use this feature to create and store an alternate path in the routing information base (RIB), forwarding information base (FIB) and the Cisco Express Forwarding (CEF). When a failure is detected, the alternate path immediately takes over, enabling fast failover.

These are the benefits of the feature:

● An alternate path for failover allows faster restoration of connectivity.

● Reduced traffic loss.

● Constant convergence time so that the switching time is the same for all prefixes.

Adam, Paul (2014-07-12). All-in-One CCIE V5 Written Exam Guide (Kindle Locations 4146-4147).  . Kindle Edition.

http://www.ietf.org/proceedings/85/slides/slides-85-rtgwg-10.pdf

3.7.i Describe BGP fast convergence features

3.7.i [ii] Add-path

The BGP Additional Paths feature allows the advertisement of multiple paths through the same peering session for the same prefix without the new paths implicitly replacing any previous paths. This behavior promotes path diversity and reduces multi-exit discriminator (MED) oscillations.

BGP routers and route reflectors (RRs) propagate only their best path over their sessions. The advertisement of a prefix replaces the previous announcement of that prefix (this behavior is known as an implicit withdraw). The implicit withdraw can achieve better scaling, but at the cost of path diversity.

Path hiding can prevent efficient use of BGP multipath, prevent hitless planned maintenance, and can lead to MED oscillations and suboptimal hot-potato routing. Upon nexthop failures, path hiding also inhibits fast and local recovery because the network has to wait for BGP control plane convergence to restore traffic. The BGP Additional Paths feature provides a generic way of offering path diversity; the Best External or Best Internal features offer path diversity only in limited scenarios.

The BGP Additional Paths feature provides a way for multiple paths for the same prefix to be advertised without the new paths implicitly replacing the previous paths. Thus, path diversity is achieved instead of path hiding.

Adam, Paul (2014-07-12). All-in-One CCIE V5 Written Exam Guide (Kindle Locations 4158-4159).  . Kindle Edition.

http://blog.ine.com/2010/11/22/understanding-bgp-convergence/

3.7.i Describe BGP fast convergence features

  • 3.7.i [iii] Next-hop address tracking

The BGP Support for Next-Hop Address Tracking feature is enabled by default when a supporting Cisco IOS software image is installed. Border Gateway Protocol (BGP) next-hop address tracking is event driven. BGP prefixes are automatically tracked as peering sessions are established. Next-hop changes are rapidly reported to the BGP routing process as they are updated in the Routing Information Base (RIB). This optimization improves overall BGP convergence by reducing the response time to next-hop changes for routes installed in the RIB. When a best-path calculation is run in between BGP scanner cycles, only next-hop changes are tracked and processed.

You can use bgp nexthop command to configure next-hop address tracking.

Adam, Paul (2014-07-12). All-in-One CCIE V5 Written Exam Guide (Kindle Locations 4161-4167).  . Kindle Edition.

http://www.cisco.com/c/en/us/td/docs/ios-xml/ios/iproute_bgp/configuration/15-mt/irg-15-mt-book/irg-nexthop-track.pdf