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New Member

BGP question

Hi,

Hopefully someone can clear this up. We have two 7204 routers, one connected to one peer and the other to another. Each has an eBGP session with it respective peer and there is an iBGP session between the two routers. One of the routers is configured to append our AS to incoming and outgoing updates to try and force as much traffic as possible down the primary link. The problem (or there may not be one) is that on the secondary router I get multiple routes for IP's as expected with the iBGP route being the preferred route. The only thing is the primary router only has a single eBGP route from the external peer that it is configured with. It does not show to other route which I had thought it should learn over iBGP. Is this normal behavior or should both routers have all entries in their BGP tables?

Router 1

scocore1#sh ip bgp 212.38.184.100

BGP routing table entry for 212.38.160.0/19, version 8001518

Paths: (1 available, best #1, table Default-IP-Routing-Table)

Advertised to non peer-group peers:

62.128.192.162

6067 6461 12708

195.97.229.205 from 195.97.229.205 (195.97.192.14)

Origin IGP, localpref 100, valid, external, best

Community: 670

Router 2

scocore2#sh ip bgp 212.38.184.100

BGP routing table entry for 212.38.160.0/19, version 115537

Paths: (3 available, best #3, table Default-IP-Routing-Table)

Advertised to non peer-group peers:

217.205.95.21

20860 20860 20860 4589 6461 12708

217.205.95.21 from 217.205.95.21 (212.134.11.190)

Origin IGP, localpref 100, valid, external

4589 6461 12708, (received-only)

217.205.95.21 from 217.205.95.21 (212.134.11.190)

Origin IGP, localpref 100, valid, external

6067 6461 12708

62.128.192.161 from 62.128.192.161 (195.97.229.206)

Origin IGP, metric 0, localpref 100, valid, internal, best

Hopefully someone can shed some light on this.

Thanks,

Richard

1 ACCEPTED SOLUTION

Accepted Solutions
Gold

Re: BGP question

Yes, failover would work as you expect.

:-)

Russ

4 REPLIES
Gold

Re: BGP question

So, to make certain I understand:

A---B---C---D

A is running eBGP to B, B is running iBGP to C, and C is running eBGP to D, correct? You have the routes weightes so the routes through C to D are preferred over the routes through B to A. If this is all correct, what you are seeing is normal operation.

If B is using the iBGP route through C as its best, it will split horizon it's eBGP learned route from A, and not advertise it to C. It won't advertise a route along the path it's using to reach that destination.

Hope that helps...

:-)

Russ.W

Cisco Employee

Re: BGP question

This is normal behavior. Bear in mind that a BGP router only advertises its best path. Since router 2 considers the path received from router 1 has being its best path, it doesn't advertise it back to router 1.

Hope this helps,

Harold Ritter
Sr. Technical Leader
CCIE 4168 (R&S, SP)
harold@cisco.com
México móvil: +52 1 55 8312 4915
Cisco México
Paseo de la Reforma 222
Piso 19
Cuauhtémoc, Juárez
Ciudad de México, 06600
México
New Member

Re: BGP question

Hi,

I take it by this that if router B using the previous diagram gets an update that removes the route for 212.38.184.100 from its eBGP from A then iBGP will update router B to route out via router C and then onto D.

Gold

Re: BGP question

Yes, failover would work as you expect.

:-)

Russ

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