iBGP within the same AS number

[Brian Langham]

I have a 7600 router peering with two asr 9001's withing the same location. It seems I am receiving full routes from one ASR and partial routes from the other. This is maxing out the limit of hardware switching and using software switching, which brings about high cpu usage. Being new to BGP, how do I only send default routes to the 7600 from the 9001's, i.e. a default route or just specify a next hop?

[Jon Marshall]

Assuming each ASR is receiving a default route from it's upstream BGP peer then - 

"ip prefix-list <name> permit 0.0.0.0/0" 

then if you control the ASR's you can simply apply that prefix list directly to the neighbor eg. on each ASR under your BGP configuration - 

"neighbor <7600 IP address> prefix-list <name> out" 

if you do not control the ASRs then on the 7600 under the BGP configuration - 

"neighbor <ASR1 IP address> prefix-list <name> in" 

"neighbor <ASR2 IP address> prefix-list <name> in" 

but if you control from ASRs it is better there as you don't need to send the routes in the first place. 

Apply to one of the neighbors first to make sure it does as you expect. 

If you are not receiving a default then you would need to originate one, and there are a number of ways to do this but probably the easiest for you would on each ASR under the BGP configuration - 

"neighbor <7600 IP address> default-originate" 

note that will advertise a default route regardless so even if the link to your upstream provider went down the ASR will still advertise the default to the 7600 so it could still send traffic to that ASR. 

This may or may not be an issue as your ASRs may be directly connected to each other and exchanging BGP routes so there will still be a path out. 

Jon

[Brian Langham]

That is good information. I would like to add that each ASR is peered with an upstream provider that is delivering full routes. Also, if I didn't mention, a BGP peer with each other (ASR). All under the same AS number

[cid:image001.png@01D3B062.167227E0][cid:image002.png@01D3B062.167227E0][cid:image003.png@01D3B062.167227E0]ASR1

[cid:image004.png@01D3B062.167227E0] 7609

ASR2

[Brian Langham]

When adding default-originate to ASR, how long until "prefixes received"will drop from table on 7600?

[Jon Marshall]

Sorry didn't make it clear. 

If you are passing on a default route learnt from your provider then you use the prefix list example and only the default route will be advertised. 

If you are originating a default on each ASR because they are not receiving default routes from the provider you would still need to use a prefix list to stop advertising any of the BGP routes to the 7600. 

If you are receiving defaults from providers much easier and more effective to just pass them on to the 7600. 

Jon

[Brian Langham]

If you are originating a default on each ASR because they are not receiving default routes from the provider you would still need to use a prefix list to stop advertising any of the BGP routes to the 7600. 

The above statement is true. The ASR's are receiving full routes from the upstreams. I don't want to pass the full routes from the ASR's to the 7600 due to CEF and route limits. So you're saying i need to create a prefix-list and add to the specific neighbors?

[Jon Marshall · Accepted solution]

I understand the ASR's are receiving full routes from the upstream providers and you don't want these sent to the 7600. 

The decision as to which solution to use depends on whether the default route is included in the routes received from your upstream providers. 

If it is then just use a prefix list as already suggested. 

If it isn't then you need to generate a default from each ASR to the 7600 but also use a prefix list to stop any of the other routes being advertised. 

Jon