3560 and AS-Path Prepend issue

[Patrick Beaven]

I've got several switches and routers all running BGP. One of them "cisco 3560" switch is not responding as all the others do.

I typically like to insert an aspath prepend for all ibgp talkers to control traffic but this single switch is not advertising as it should.

Running 12.2 ip services code

setup

route-map PREPEND permit 10

set as-path 64601 64601

!

router bgp 64601

neighbor 13.14.12.181 next-hop-self

neighbor 13.14.12.181 remote-as 64601

neighbor 13.14.12.181 route-map PREPEND out

network 13.14.12.180 mask 255.255.255.252

redist static

redist connected

int loopback1 ip address 13.14.205.1 255.255.255.0

no auto-summary

What Am I Missing. Its sending the advertisements to .181 with no prepending?

Any Ideas?  I have several other routers working fine with this setup.

[Peter Paluch]

Hi Patrick,

To be honest, I have not tried to manipulate the AS_PATH on iBGP session which is exactly what you are trying to do here. Could this be the culprit? Please recall that under normal circumstances, the AS_PATH is modified only between eBGP neighbors. Is it possible that the other BGP routers you have are modifying the AS_PATH on eBGP sessions?

Best regards,

Peter

[Patrick Beaven]

I user prepending on IBGP all the time but most of my gear is not Cisco. I did also try setting to the ebgp peer it also has with no change. Its like its just ignoring the route-map. I have this same setup on some cisco routers with no issue. Just the 3560 switch. Any idea's?

[Peter Paluch]

Patrick,

I have just tested this on 2691 series 12.4(15)T13 Advanced IP Services IOSes, and I can confirm that in iBGP peerings, the AS_PATH modification in the outbound route-map does not apply, i.e. it is ignored just as you have experienced here. As soon as the peering is changed to eBGP, the AS_PATH modification works as specified in the route-map.

I am confused as to what configuration on Cisco devices actually worked for you - based on my experiments, iBGP peerings can not be forced to arbitrarily modify the AS_PATH attribute.

Best regards,

Peter

[Patrick Beaven]

I actually have several ibgp peers that Im injecting the prepend but they are not Cisco routers. I have a few older cisco routers that work fine with this setup. This is interesting, I will have to use localpref to change the routing path for those devices. I appreciate your looking into this.

Pat,

[Peter Paluch]

Hello Patrick,

You are welcome. I can see how this behavior may have changed between old and new IOS versions. In any case, I can see the logic with the current behavior - that in iBGP peerings, AS_PATH is not modified. I believe this is a grey area in BGP specification that is open to interpretation, and various vendors may implement this behavior differently.

Best regards,

Peter