How to specify a router for BGP as end user of multiple ISPs

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May 27th, 2008
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we've had some issues with our relatively cheap but high performance ISP (i.e. lots of bandwidth at good price but not 100% reliable) and want to protect our service by getting a second lower speed circuit from a different provider. Whilst we can do clever multihoming and DNS to allow our service to switch providers, it converges too slowly... so...

We want to get some PI space, an AS number and put in place two routers to give resilience. Some people tell me each router should be at least a Cisco 7204VXR with 1GB of memory, but to me that seems overkill for what we want, or maybe I misunderstand, since by and large we'll only be announcing our own prefix, and accepting default routes from our ISPs (and, if there's a partial failure) tweaking preferences to force fail-over.

I've searched the forum but the search tools are crap and I couldn't find a discussion of how to size a router in this instance. Any ideas would be gratefully received.

thanks for reading this.

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Overall Rating: 4.3 (6 ratings)
arturo.guzman Tue, 05/27/2008 - 08:26
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If you will to advertice your own Public IP's you need to receibe all prefixes from your ISP and then you have a router with 1 Giga, this if you will connected to the backbone of your ISP's.

Else, if you don't to advertice your Ip's and your ISP advertice it you dont need it..

speculatrix Tue, 05/27/2008 - 08:33
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we do want to advertise our own block of IP addresses - we intend to get a /24 from RIPE for this purpose (I'm told a smaller block would be ignored by some ISPs), so I guess this means we DO need a 72xx VXR with 1GB memory.


droehsler Tue, 05/27/2008 - 08:35
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You do not need a 7200 to announce a single class C.


speculatrix Tue, 05/27/2008 - 08:44
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thanks, I didn't think so! our primary circuit is 50M, soon to be 100M, but even that shouldn't make a 28xx even break a sweat, should it!?!

droehsler Tue, 05/27/2008 - 08:54
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Look for the router performance guide on Cisco's website. 2851 can do 112 Mbps of CEF switching. 64 byte packet size.

The 3845 can do 256 Mbps.

Depending upon future needs you may want to consider a layer 3switch depending upon how you ISP delivers your bandwidth or a router and a layer 3 switch with iBGP between them.


speculatrix Tue, 05/27/2008 - 09:16
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thanks for that, I started looking at the 2821 and now the 2851; I might head to the 3845 so that we'd have plenty of room for growth, we're short of man-power more than we're short of cash, and would rather install the network and not have to worry about performance for quite a while!

thanks for your time


droehsler Tue, 05/27/2008 - 08:28
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If you are only accepting default routes you can use a 2800 series router without a problem. If you wish to take in the entire Internet routing table you will need a 3845 with like 512 memory.




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