BGP Routing Tables Exceed 500K Entries, Older Routers Fail

Last August 12, the Border Gateway Protocol (BGP) routing tables exceeded 500K entries as more IPv4 classes were announced. Older router models were affected, most notably popular models like the Cisco Catalyst 6500 and 7600. The default value for IPv4 table size in these models is 512K, which is no longer enough to hold the global table size. The impact was felt globally as reports of disruptions and slower Internet speeds came in from the continental US and in the UK.

BGP is the routing protocol used to map the Internet. The BGP routing tables are typically kept in a specialized memory within a router called the Tertiary Content Addressable Memory (TCAM). Once there are more than 512K routes in the tables, older router models can no longer track these routes properly so they either crash or ignore the newly learned routes completely.

As early as May this year, Cisco had already warned its customers that this event was inevitable and that network administrators must start preparing workarounds or equipment replacement. The default value can actually be changed easily enough, but it would require rebooting the router. Maintenance windows were not done in time to prevent the disruptions.

The expanding BGP will continue to be a problem. As old routers are upgraded or replaced, experts warn users of more Internet blockages and slowdowns. In light of all this, calls for switching to IPv6 has also surfaced.

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