less than a /24 & BGP tricks

neal rauhauser nrauhauser at gmail.com
Tue Jun 30 08:54:29 CDT 2009


   I have a network with two upstreams that land in datacenters many miles
apart. The hardware involved is Cisco 7507s with RSP4s and VIP4-80. I've got
a curious problem which I hope others here have faced.

   A while ago we got a /28 from each provider and attached it to a
dedicated fast ethernet interface at each location. Inbound traffic arrives
normally and anything arriving on that port is policy routed to the upstream
that provided the prefix.

   This was all well and good when it was a little firewall with a Linux
machine  behind it being used to check latency and do other diagnostics, but
the sales people noticed it and have lined up a couple of opportunities to
sell a service that would depend on our being able to receive and send
traffic from blocks less than a /24.

   The policy routing works fine at low volume, but the RSP4 is rated to
only do four megabits and I know they're going to exceed that.

   I can terminate this subnet on another router, wire that device into the
7507 with a crossover, and establish a BGP session. I'm wondering if there
is a tidy way to set next hop in some fashion using route-maps such that all
the marking would be done on the auxillary machine and the traffic passing
through the 7507 would be CEF switched rather than process switched.






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