CCDE – BGP Convergence
Introduction
This post will look at the steps involved in BGP convergence and how it interacts with IGP to converge.
Any network of scale will use route reflectors (RRs) so this post will focus on deployments with RRs. Networks running a full mesh will have all paths available which makes hot potato routing and fast convergence easily achievable, at the cost of scaling and management overhead. A combination of full mesh and RRs is also possible where one scenario would be to run a full mesh within a point of presence (PoP) and RRs within the pop, peering with central RRs.
BGP can be used for both internal (iBGP) and external (eBGP) peerings and convergence and timers differ depending if it’s internal or external peerings.
BGP is a path vector protocol which means that it behaves as a distance vector protocol where it can only advertise routes that are installed into the RIB. There is an exception to the rule when BGP selective route download (SRD) is used to not download routes to the RIB but still advertise the routes. BGP will by default only install one path into the RIB even if there are multiple equal candidates and it Continue reading



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