Stéphane Lee

Once in a while (once in a blue moon?), I write a technical work-related post. The lingo may be hard to catch but a knowledge of networking will help. We have exceeded 240,000 routes since October and it is not getting lower. In comparison, there were less than 100k routes in 2000 and less that 175k in 2005.

A route is the advertisement of a subnet (or prefix) in BGP (border gateway protocol). The subnet can be as large as a /8 (4.0.0.0/8 for example) or small as a /24. Anything smaller than is generally filtered out. An ISP uses BGP to announce its subnets with other ISPs so that the IP addresses become reachable (pingable). The Internet is a network of ISPs connected together and communicating with BGP. An ISP can choose to announce their subnets in several /24 or aggregate them as a single route (announce one /20 instead of 16 x /24). Instead of having 244k routes today, we would have only 160k if they were aggregated properly.

Cisco uses a technique known as TCAM: ternary content addressable memory. TCAM is like RAM but very high performance and very expensive. The routing table is loaded in the TCAM. The router queries the TCAM with an IP address and it gets back the available routes. This process is very fast (in nanoseconds). The Sup720 supervisor engine in the Catalyst 6500 series is very popular. The Catalyst 6500 series has a good port density. The 6509 model has 9 slots and in one slot, you can add a blade with 48 1Gbps Ethernet RJ45 ports. For the Sup720, there is a hardware limit of 239k entries (IPv4). The default config is 192k. When the limit is exceeded, the routing is done by the software and this is much slower.

The list price for the recommended upgrade (1000k limit) is $40k but you can get a used one for $12-14k. If you do not want to upgrade, you can filter out the /24s and add a default route. This will cut the routes in half and save you some cash.