Option B - back-to-back eBGP VPNv4

[kim]

Hi guys,

thanks for a great resource!

I'm looking at this document on the topic:

Cisco IOS-XR BGP with MPLS Designs 

It says in section 4 that in order to make the exchange (In image 3) work ASBR3 needs to have a dummy iBGP neighbor configured. The doc just says there in this instance is an issue with advertising the vpnv4 routes from ASBR3. It doesn't really explain why.

I realize that this is a corner case, since irl more routers would exist within the average AS.

I'm hoping someone can elaborate.

Appreciated, thanks.

Cheers

[Harold Ritter]

Hi @kim

I am not sure what the restriction was, but this restriction is not present in recent XR versions. I just ran a quick test and it worked like a charm without the dummy ibgp session on the middle ASBR.

Regards, 

[kim]

Hi @Harold Ritter,

thanks!

Well thats interesting, that an iBGP peer is needed to trick XR into passing eBGP routes - which version did you test on? Feels kind of like a bug i ran into once where the XE didn't populate forwarding tables if CDP wasnt turned on for a specific interface. I'll compare to my lab, just needed to reach a milestone in the workbook.

Cheers!

[Harold Ritter · Accepted solution]

Hi @kim ,

I ran the test with XRv 6.3.1 and it works like a charm.

Regards,

[kim]

Hey

Wow, okay - i'm running 6.5.3, promising - well now i cant wait to test it. The doc was updated june last year...

Thanks again!

Cheers

[Harold Ritter]

Hi @kim ,

Let us know if you run into issues or questions while testing.

Regards,

[kim]

Hey

Ok tested it, works perfectly here as well. Go figure. Wonder what section 4 was originally about.

Anyway, thanks guys 

Cheers

[MHM Cisco World]

I know this issue solved but I want to mention something here 
If I am right this B2B inter-AS "option-B" so 
there is two way the AS exchange label 
1- config the LDP under the interface connect both AS
2- config the BGP to exchange label <- are this what is ask for??

[kim]

Hey

Well even though it was solved, any input is appreciated!

This is option b though, Option B 

Option a is more like how you would build your bgp towards the end customer, just with the added label path maybe.

Cheers

[MHM Cisco World]

OK let me review my note within two days I will inform you what I get
""why we use dummy iBGP with Inter-AS op.B IOS XR ?""