bgp question

Answered Question
Jul 2nd, 2007

Hi All,

Looking for some decode assist on some output from a show bgp command... we are setting communities on this route, which you see in the bgp topology table. My question is... on the routes that show received-only, the community isn't being set. Does anyone know the reason why it shows up like this? Are the top 2 entries for the same route, one before filtering and one after?

Thanks, Lisa.... output below.

bermudadigital1-devonshire-bmu#show ip bgp 208.75.200.28

BGP routing table entry for 208.75.200.28/32, version 28567

Paths: (4 available, best #1, table Default-IP-Routing-Table)

Advertised to update-groups:

26559

208.75.200.11 from 208.75.200.11 (208.75.200.11)

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

Community: 26559:110

26559, (received-only)

208.75.200.11 from 208.75.200.11 (208.75.200.11)

Origin IGP, metric 0, localpref 100, valid, external

26559

208.75.200.12 from 208.75.200.12 (208.75.200.12)

Origin IGP, metric 0, localpref 100, valid, external

Community: 26559:110

26559, (received-only)

208.75.200.12 from 208.75.200.12 (208.75.200.12)

Origin IGP, metric 0, localpref 100, valid, external

I have this problem too.
0 votes
Correct Answer by Harold Ritter about 9 years 5 months ago

Lisa,

This is precisely it.

Regards,

Correct Answer by Harold Ritter about 9 years 5 months ago

Lisa,

It is normal behavior that the received-only routes do not carry the community that you set locally.

As Edison mentioned, the received-only routes are a direct effect of the "soft-reconfiguration inbound". These routes are kept intact as they are received from the neighbor hence why you do not see the community being set on them. Clearing the BGP neighbor will not change anything.

Hope this helps,

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royalblues Mon, 07/02/2007 - 07:29

Lisa,

If you are setting the communities on this router, you will be able to see the community only on the router to which it is being advertised and not on the router setting it

Narayan

lgontarsk Mon, 07/02/2007 - 07:35

In this case the output showing the communities being set is the router that is setting it.... and the communities show up in half of the topology listings.

The thing I'm curious about is why they show up in 1/2 of the listings, and why the received-only is showing up.

Thanks!!

Lisa G

lgontarsk Mon, 07/02/2007 - 07:58

We're receiving the bgp topology entries from 2 ebgp neighbors:

IBGP: 172.31.194.49 4 14323 25714 15937 28611 0 0 2d21h 1095

IBGP: 172.31.194.57 4 14323 26434 15840 28611 0 0 2d08h 1095

EBGP: 208.75.200.11 4 26559 15915 15968 28611 0 0 1w4d 4

EBGP: 208.75.200.12 4 26559 15907 15968 28611 0 0 1w4d 4

we are receiving 4 routes from each neighbor, and the prior output was from one of the received routes. So it looks to me like we should have one advertisement recieved from each ebgp peer, totaling 2 ... yet in the topolgoy for BGP we see 4 possibles. That's why we speculated that we see each 2x.

What other ouptut would you like?

Thanks!!

Edison Ortiz Mon, 07/02/2007 - 08:02

Do you have "soft-reconfiguration inbound" for the neighbor sending that information ? If so, that's the reason the routes are still stored in your BGP table.

Do a 'clear ip bgp * in' during off-hours in order to remove these routes.

lgontarsk Mon, 07/02/2007 - 08:09

Hi Edison, Yes, you are right... there's soft reconfiguration inbound.

I had just cleared all bgp neighbors soft in & out before taking this snapshot. I'll do it tonight though and will see if it alters anything.

Thanks!

Correct Answer
Harold Ritter Mon, 07/02/2007 - 08:27

Lisa,

It is normal behavior that the received-only routes do not carry the community that you set locally.

As Edison mentioned, the received-only routes are a direct effect of the "soft-reconfiguration inbound". These routes are kept intact as they are received from the neighbor hence why you do not see the community being set on them. Clearing the BGP neighbor will not change anything.

Hope this helps,

lgontarsk Mon, 07/02/2007 - 09:44

Thanks!

So basically in this output, each advertisement is seen once pre- filtering & policy-making alterations, and once after making policy-altering changes to the routes?

Thanks... Lisa

Advertised to update-groups:

2

26559

208.75.200.11 from 208.75.200.11 (208.75.200.11)

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

Community: 26559:60

26559, (received-only)

208.75.200.11 from 208.75.200.11 (208.75.200.11)

Origin IGP, metric 0, localpref 100, valid, external

26559

208.75.200.12 from 208.75.200.12 (208.75.200.12)

Origin IGP, metric 0, localpref 100, valid, external

Community: 26559:60

26559, (received-only)

208.75.200.12 from 208.75.200.12 (208.75.200.12)

Origin IGP, metric 0, localpref 100, valid, external

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