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BGP on a Cat3750-G


We are considering running BGP between two ISPs on a cat 3750-G. This would be to control both inbound and outbound routing to our site. Before you say it, I know this isn't the best idea in the world but it's what we've got. Hopefully by the end of the year or by Q2 2007 we'll be able to get a nice strong router.

The switch seems to have plenty of memory:

cisco WS-C3750G-48TS (PowerPC405) processor (revision C0) with 118784K/12280K bytes of meory.

...And BGP is supported (EMI image).

The SDM 'routing' template only provides for 11K routes. I know there are far more routes than this so the extra will be loaded in standard memory. How badly will this affect the switch? Currently the switch has a single vlan with all hosts, so no internal routing is necessary, but the volume between those hosts will be in the 5 - 8 Gbs range. Is the CPU strong wnough to handle the internal switching AND a large BGP config or will it suffer and die as the routing table approaches 100K routes? Or provider is doing this with a few carriers on a 7200 with less memory than this switch. (but of course it's a router)

We're looking at a low volume of routed traffic initially, <100Mbs, but that will ramp up very fast and could easily hit 1+ Gbs by the end of the year, probably quite a bit more.

Any help you could provide would be greatly appreciated. As I said, I know this is not the best idea on the planet, but unless it's just not feasible convincing management otherwise is proving to be difficult.




Re: BGP on a Cat3750-G

Full-routing table is now climbing towards 150K routes, not just 100K. But even 100K would be already above limit of 3750 - if I remember correct, it can support maximum 20K routes.

Besides limitations imposed by the platform itself, there is also memory - for today's full table you need at least 256MB of RAM (based on live c7200 router with single peer, but large IGP and 'soft-reconfig inbound' enabled).

Do you really need full table? you could ask both providers to limit specific routes to their direct customers only + default (you'd have to prefer one of them).

Forwarding performance of your cat seems to be sufficient for your requirements.

Community Member

Re: BGP on a Cat3750-G

The 3750-12S with 'sdm prefer routing aggregate' can handle 20K routes. The 3750-48G can only do 'routing desktop', which limits it's Tcam routing entries to 11K. I didn't realize that the full routing table required 256MB memory, very good to know. We'll obviously have to limit the inbound routes if we go forward with this. A smaller table should be okay, the majority of our traffic should come from the states for the first few months, after that we'll be able to upgrade to equipment better suited to this.

Thanks for the reply.


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