BGP PIC/External Feature

[Sherif Atef Ahmed Ismail]

Dears

Would like your assistance please regarding "BGP External feature"

http://www.cisco.com/en/US/docs/ios-xml/ios/iproute_bgp/configuration/15-mt/irg-best-external.pdf

As I understand for below scenario, PE-2 will advertize its backup best external path to its peers consequently peers will install this route as a backup path allowing fast convergence

Now my question

1- In case of of primary link failure as shown in figure why PE-3 will send traffic to PE-1 and not PE-2 directly cause as I understood PE-2 has advertized its backup path to all peers ?

maybe cause PE-3 is not configured with " bgp additional-paths install"  or "bgp advertise-best-external", correct ?

2- As a result of the failure PE-1 should send withdraw message to all peers to notify them with failure .. To my knowledge PE-3 should then remove affected route from its routing table then computes BGP path process to figure out the new path ... Till this happens I believe there will be outage in network

If I am correct, then this feature is of no great benefit cause yes it solved the issue at PE1 but other routers will not send traffic at all till they converge !

Am I missing something ??

Many Thanks

Sherif Ismail

[Sherif Atef Ahmed Ismail]

Any ideas

[Sherif Atef Ahmed Ismail]

Well I knew the answer to first question

Most probably both PEs are configured with same RD so when P (most probably acting as RR here) advertize the route to remote clients it only advertzies the best route

Now remaining the answer to 2nd question

Regards

Sherif Ismail

[Vivek Ruhil]

Hi

If I understand this correctly, then the PE1 installs two routes, one that it receives from the CE1 and the other from the PE2 and marks that route as the best external. Now, once the PE1-CE1 link goes down PE1 already has a second best path available and the traffic coming from PE3 goes to PE1 and then gets forwarded to PE2. No need for PE3 to wait for the withdraw msg from PE1 or for it to converge..

[Sherif Atef Ahmed Ismail]

Hi Vivek

Thanks for your reply ... But what confuses me as I understand PE-1 should send to PE-3 withdraw message otherwise PE-3 would never update its RIB .. So till PE-3 converges there will be outage in network considering that all traffic is coming from CE behind PE-3

Accordingly I beleive example mentioned in document just explain the concept however in live networks PE-3 should have both routes (by configuring primary/backup PEs to have different RDs) to avoid this situation

What do you think

Regards

Sherif Ismail

[Vivek Ruhil]

Hi Sherif

You are correct, I think its a combination of features implemented for example I think we would have BGP PIC also configured on PE3 and it would have both the routes in the routing table.

So, why BGP External ?

I belive this is to ensure that unless PE1 sends out a withdraw message PE3 will keep sending the information to PE1 and to reduce outage PE1 has another route as "best-external" in its routing table.

So, truely, its a combination of features that are deployed within this network. I hope that clarifies the requirement.

[Sherif Atef Ahmed Ismail]

Many Thanks :)


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