BGP re-advertise

netstar-sg-service · ‎02-04-2009

Hello All,

I just configure the bgp after i set up the all routers i check the bgp status. Its look like this...

Router#sh ip bgp neighbors 192.168.20.2 received-routes

BGP table version is 7, local router ID is 192.168.20.1

Status codes: s suppressed, d damped, h history, * valid, > best, i - internal,

r RIB-failure, S Stale

Origin codes: i - IGP, e - EGP, ? - incomplete

Network Next Hop Metric LocPrf Weight Path

*> 172.168.10.0/24 192.168.20.2 0 65501 65500 i

*> 192.168.1.0 192.168.20.2 0 65501 65500 ?

*> 192.168.10.0/30 192.168.20.2 0 0 65501 ?

* 192.168.20.0/30 192.168.20.2 0 0 65501 ?

*> 192.168.30.0/30 192.168.20.2 0 0 65501 ?

Total number of prefixes 5

Router#sh ip bgp neighbors 192.168.20.2 advertised-routes

BGP table version is 7, local router ID is 192.168.20.1

Status codes: s suppressed, d damped, h history, * valid, > best, i - internal,

r RIB-failure, S Stale

Origin codes: i - IGP, e - EGP, ? - incomplete

Network Next Hop Metric LocPrf Weight Path

*> 10.10.10.0/24 0.0.0.0 0 32768 ?

*> 192.168.20.0/30 0.0.0.0 0 32768 ?

*> 172.168.10.0/24 192.168.20.2 0 65501 65500 i

*> 192.168.1.0 192.168.20.2 0 65501 65500 ?

*> 192.168.10.0/30 192.168.20.2 0 0 65501 ?

* 192.168.20.0/30 192.168.20.2 0 0 65501 ?

*> 192.168.30.0/30 192.168.20.2 0 0 65501 ?

Total number of prefixes 7

Here you see , the received routes from 192.168.20.2 are re-advertised to same 192.168.20.2 router(all most 5 prefixes) .

Could you please tel me this why this happened?

Thank you.

-Ananthan

uvaraj · ‎02-04-2009

Hello Ananthan,

I think you have advertised 192.168.20.0 network both at IGP and BGP, Correct me if i am wrong.

Uvaraj

netstar-sg-service · ‎02-05-2009

Hello Uvaraj,

The network 192.168.20.0 si WAN between AS 65501 and AS65502 (IP address 192.168.20.1)..

You see all other networks also re-advertised

172.168.10.0/24

192.168.1.0

192.168.10.0/30

192.168.30.0/30

But stil indicate that next hope as 192.168.20.2 (AS65501).Actually these networks received from AS 65501.

Thank you.

-Ananthan

Mohamed Sobair · ‎02-05-2009

Hi,

By default a BGP speaker will advertise any prefixes recieved by its neighbors besides by configuring (Network Command, Redistribution and Aggregation).

To avoid being a transit AS, you should always apply the appropriate filtering in place.

HTH

Mohamed

Harold Ritter · ‎02-05-2009

Mohamed,

A BGP router doesn't by default advertise a BGP prefix to the peer it has received it from if it selects this peer as its best path. This default behavior is somewhat changed by the explicit use of a peer-group or the dynamic use of an update-group.

Regards

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

Harold Ritter · ‎02-05-2009

Ananthan,

This is normal behavior if the peer is part of a peer-group. The local router receives the routes from the peer (192.168.20.2) and advertises them to the peer-group, which 192.168.20.2 is also part of. The update would be rejected by 192.168.20.2 because the AS path contains its own AS anyway.

Regards

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

Harold Ritter · ‎02-05-2009

Ananthan,

Just to add that this behavior might happen even without explicit peer-group. This is because of update-group feature. This feature dynamically regroup all neighbors with the same outbound routing policy for the purpose of reducing the number of updates we build. The BGP router builds the update once and send to all members of the update-group. You can see update-group members with the following command:

show ip bgp update-group

Regards

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

doraemonheng · ‎07-02-2009

Hi Hritter,

Great to found this article. We have facing similar issue.

Is there anyway to prevent/stop this? As we are advertising 900+ routes back to neighbor.

Thanks & Regards,

netstar-sg-service · ‎07-02-2009

Hi ,

You can filter like this...

Create a filter...

!

ip as-path access-list 5 permit ^$

!

apply under bgp

router bgp

neighbor xxxxxx filter-list 5 out

!

its allow only your networks...

Thank you.

-Ananthan

doraemonheng · ‎07-02-2009

Hi Ananthan,

As we have other ebgp peers, by using this method, it will caused this particular peer not receiving any other routes but only my AS routes right?

Thanks & Regards,

netstar-sg-service · ‎07-02-2009

Hi ,

Yes , your AS will advertise only its network advertisements..(Its mean its not advertise networks to eBGP peering, the routes learning from same eBGP(same AS) peering )

Thanks..

-Ananthan