Clarence Filsfils, Cisco Systems
It is common to characterize any BGP-related routing convergence as hopelessly slow due to the linear relationship between the number of impacted prefixes (in the 500k range in early 2007 counting internet and vpn routes) and the number of convergence operation (bestpath, RIB and FIB update, transmission or reception of withdraw/update).
The objective of this paper is to demonstrate that this belief is wrong for any failures occurring within the network of a service provider or on peering links with redundantly-connected peers. This covers the vast majority (if not all) of business models involving BGP convergence requirements.
Thanks to BGP Prefix Independent Convergence (BGP PIC), an alternative path to the existing BGP next-hop is enabled at IGP convergence time (modify) in the core scenario, while in the edge scenario, the deletion of the IGP path to a BGP next-hop triggers an immediate and prefix-independent rerouting of the dependent BGP destinations via an alternate BGP next-hop.
Aside the obvious convergence gains, BGP PIC and its underlying hierarchical FIB organization bring significant scaling and robustness gains to router architecture.
The first section defines the problem and introduces concepts such as RIB, FIB, recursion, dependency, flattened and hierarchical FIB organization. The second section explains why modern high-end router design invests in more complex and expensive packet lookup engine to support hierarchical FIB databases. This allows for significant gains in scaling, robustness and routing convergence (BGP PIC Core). The next section generalizes the hierarchical FIB structure and introduces the concepts of shared BGP path-lists and loadbalancing FIB entries. BGP PIC edge is then defined both for the multipath and unipath BGP policies. The next section describes the BGP control Plane reaction to the core and edge failures, how it is automatically serialized with IGP convergence and how the later BGP control-plane induced FIB modifications reconciliate with the BGP-PIC-modified FIB in a lossless manner. We then review the vast applicability for BGP PIC behavior and report detailed lab measurement based on a commercially-available product. We finish with a conclusion.
Bio:
Clarence Filsfils is a Cisco Distinguished Engineer. He has been playing a key role in engineering, marketing and deploying the Quality of Service and Fast Routing Convergence technology at Cisco Systems.
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