how routing should be propagated over iBGP between two EBGP routers

[davidxiayuan]

We currently have two routers receiving full Internet BGP table from 2 transit providers on both IPV4 and IPV6. they are running iBGP between each other, Just wondering how I should propagate the routing between the two routers via iBGP?

Option1. propagate the full internet BGP table received from transits between the two iBGP peers? So that, R1 and R2 will be providing a redundant path of each other when one of the router lost their transit providers?

Options2. propagate only default route between R1 and R2.

These two routers a ASR-1004 with 4B memeory on each. Could someone give some suggestions about how the routing should be propagated between R1 and R2 via IBGP? would option 1 or option2 better than the other one. or if there is some other option I should go for which is the best?

Thank you all in advance

[Giuseppe Larosa · Accepted solution]

Hello David,

with ASR 1004 with 4 GB route processor I would go for option 1 as it provides for each public IP prefix a chance to propagate best path to each other.

note that for a prefix R1-ISP1 prefix is considered best also on R2, R2 will not advertise the BGP advertisement received from ISP2 to R1.

However, if each ASR is eBGP multihomed with both ISP1 and ISP2 in normal conditions outbound traffic will never flow over ASR to ASR link.

if R1 eBGP session with ISP1 fails the iBGP learned route coming from ISP1 via R2 can reach R1 via the iBGP connection. (if ISP1 provides the best route to the prefix from R2 point of view)

In short option 1 provide routing optimization

with option 2 traffic would be send from R1 to R2 only if both eBGP sessions on R1 are failed as the default route is less specific then any existing route. Also with option 2 you need to take care to avoid to advertise the default route to your upstream providers.

Hope to help

Giuseppe

[davidxiayuan]

Thank you very much for your reply Giuseppe. This pretty much resolved my question, I wasn't sure about if it would be too much load to put on the router, If I propagate the full internet routing table again over the iBGP, would consume too much resource on the each of the router. and in case of routing re-calculating it would take too much cpu of the router.

[Kishore Chennupati]

Hi,

Are you planning to use both the upstream links on both the routers to the ISP's? I mean are you planning to use the routers in a HSRP fashion or not? As guiseppe righlty suggested that because R1 would learn the prefixes from both the ISP's incase of a failure traffic would never flow between R1 and R2. Only in case of both the links(peering) on the R1 to the ISP's go down Then it will start using R2 via iBGP.  Option1 would be a good one. simple and not need to tweak but again we need to your requitremnts as well. There must have been a reason why you decided to have 4 upstream links(of course redundancy but there could other elements like load balancing as wel)

HTH

Kishore