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Versatile Routing and Services with BGP: Understanding and Implementing BGP in SR-OS by Colin Bookham, Alcatel-Lucent

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Chapter 6

Reconvergence

The time to reconverge upon change and/or network failure has become an important factor in the delivery of business services. Historically, BGP was intentionally slow to reconverge as a way of avoiding route oscillation between Autonomous Systems. However, the requirement is now frequently to deliver sub-second reconvergence times because of the real-time nature of traffic being carried through Layer-3 and Layer-2 VPNs. This has meant changes not only in the speed with which BGP reacts to failure, but also changes in the protocol to allow for increased path visibility between BGP speakers. This chapter discusses some of the more notable changes.

Advertisement of Multiple Paths

Advertisement of Multiple Paths (ADD-PATH) is an extension to BGP that allows for advertisement of multiple paths for the same prefix/NLRI. This allows for upstream propagation without subsequent advertisements replacing earlier ones by an intervening BGP speaker's path-selection process. The purpose is to reduce route oscillations, enable load-balancing, and improve routing convergence by making an alternative path immediately available to a reconverging router.

To achieve this, each advertised path is identified by a 4-octet Path Identifier. To carry the Path Identifier in the UPDATE message, the NLRI encodings are extended by prepending the Path Identifier field. The combination of Path Identifier and prefix thereafter identifies a given path. Assignment of Path Identifier values ...

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