1 On the Aggregatability of Router Forwarding Tables Author: Xin Zhao, Yaoqing Liu, Lan Wang and Beichuan Zhang Publisher: IEEE INFOCOM 2010 Presenter: Li-Hsien, Hsu Data: 9/28/2011

I. Introduction Two types of tables used by routers: RIB(Routing Information Base) for routing FIB(Forwarding Information Base) for forwarding FIB is derived from RIB. FIB usually uses high performance memory, which is more expensive and more difficult to scale. Therefore, their size is a more immediate concern to ISPs and vendors. 2

I. Introduction 3 Routing Scalability Problem RIB growth => FIB growth FIB growth: A high priority concern (From: bgp.potaroo.net)

FIB Aggregation(FA) What is FA? Within one router, combines multiple RIB entries with the same next hop into one. FA pros and cons − Purely local no change to routing protocol − No impact on packet forwarding − Compatible with other proposed routing scalability solution(IPv6) − But extra CPU processing time 4

Forwarding Correctness Strong forwarding correctness − Longest match before/after aggregation ends up with the same for all prefixes Weak forwarding correctness − Prefixes with Non-NULL nexthops, the same − Prefixes with NULL nexthops, might routable after aggregation − extra routable space 5

FIB Aggregation Techniques & Algorithm Filled nodes are extra routable space introduced by the aggregation. 4A, 4B. 6

Updates Handling Full aggregation per update is costly Significant computation overhead Three approaches to handle routing changes to keep computation overhead low: Operators choose an appropriate level of aggregation. Incrementally update the aggregated FIB Minimize computation, not the table size Re-run full FIB aggregation periodically The trigger can be a timer, a threshold on FIB size, and/or current router CPU load 7

Evaluation Data Source − BGP routing tables and updates from RouteView Project Evaluation Platform and implementation − Commodity PC, single thread process − Algorithms implemented in C without optimization 8

Table Size after FA 9 RouteViews Oregon tables on Each level reduces FIB size more. Level-1 30%~50%, Level-4 60%~90%

Table Size Over Time Median of table size ratio, 2001~2008 An overall slightly decreasing trend(, suggesting that the FIB has become more amenable to aggregation over the years.) 10

What does the ratio mean? If Level-4 applied, router deployed in 2000 can still be used today

Computation Time Computing time only takes tens to several hundreds milliseconds 12

Updates Process Among all the updates, 2,914,020 of them cause changes to unaggregated FIB. 13 ABA/BCB/CDD/7,254,478

Periodical Re-Aggregation With threshold 150,000, on average the FIB needs to be re-aggregated every 5 days 14

Conclusion The table size can be reduced by 30-70%, which translates to 2-8 years extra router lifetime The computation overhead is small and can be controlled by incremental update handling plus periodic re-aggregation. 15

Reference