BGP AS Migration

fgonzal · ‎09-27-2011

Hi all.

I've just run into the following problem when configuring Dual Autonomous System Support for BGP.

We handle 2 AS numbers that we intend to merge.

R1 has the following config:

router bgp 10

no bgp default ipv4-unicast

bgp log-neighbor-changes

neighbor 192.168.0.2 remote-as 10

!

address-family ipv4

neighbor 192.168.0.2 activate

no auto-summary

no synchronization

exit-address-family

R2 is located in a different AS (As20) and we try to pretend it is in AS10 ( from R1 perspective). R2 config is as follows:

router bgp 20

no bgp default ipv4-unicast

bgp log-neighbor-changes

neighbor 192.168.0.1 remote-as 10

neighbor 192.168.0.1 local-as 10 no-prepend replace-as

!

address-family ipv4

neighbor 192.168.0.1 activate

no auto-summary

no synchronization

exit-address-

It seems that BGP session is properly established with this config. However as soon as we add a network command to originate a given prefix the bgp session goes down. We see the following when running a debug ip bgp:

*Mar 1 00:08:30.643: %BGP-3-NOTIFICATION: received from neighbor 192.168.0.1 3/3 (update missing required attributes) 0 bytes

*Mar 1 00:08:30.643: BGP: 192.168.0.1 went from Established to Closing *Mar 1 00:08:30.643: %BGP-3-NOTIFICATION: received from neighbor 192.168.0.1 3/3 (update missing required attributes) 0 bytes
*Mar 1 00:08:30.643: BGP: 192.168.0.1 went from Established to Closing

Then the BGP session is established again but it goes down inmediately.....

I haven't a clue about where the problem might be. Any suggestion?

Thanks

Flo.

rsimoni · ‎09-27-2011

Hi Flo,

it sounds like a buggy behavior.

Can you tell which platforms/IOS your routers are?

Can you also add some more chatty debugs (on the routers closing the session).

deb ip bgp events

deb ip bgp updates

Riccardo

fgonzal · ‎09-28-2011

Hi Riccardo

Thanks for your comments.

I've just run the ip bgp events/updates debug and this is what I can see in both sides:

R1 ( the router that receives the update)

*********************************************************************************

*Mar 1 00:06:38.803: BGP: 192.168.0.2 Update missing attributes, flags 0x7

*Mar 1 00:06:38.807: BGP: 192.168.0.2 reset due to BGP Notification sent

*Mar 1 00:06:38.807: %BGP-5-ADJCHANGE: neighbor 192.168.0.2 Down BGP Notification sent

*Mar 1 00:06:38.807: %BGP-3-NOTIFICATION: sent to neighbor 192.168.0.2 3/3 (update missing required attributes) 0 bytes

R1# FFFF FFFF FFFF FFFF FFFF FFFF FFFF FFFF 0035 0200 0000 1940 0101 0040 0204 0201 000A 4003 04C0 A800 0280 0404 0000 0000 2002 0202 02

*Mar 1 00:06:38.927: BGP: service reset requests

R2(config)#router bgp 20

R2(config-router)#net

R2(config-router)#network 2.2.2.2 mask

R2(config-router)#network 2.2.2.2 mask 255.255.255.255

R2(config-router)#

*Mar 1 00:06:38.179: BGP(0): route 2.2.2.2/32 up

*Mar 1 00:06:38.179: BGP(0): nettable_walker 2.2.2.2/32 route sourced locally

*Mar 1 00:06:38.183: BGP(0): 192.168.0.1 send UPDATE (format) 2.2.2.2/32, next 192.168.0.2, metric 0, path Local

*Mar 1 00:06:38.251: %BGP-3-NOTIFICATION: received from neighbor 192.168.0.1 3/3 (update missing required attributes) 0 bytes

*Mar 1 00:06:38.255: BGP: 192.168.0.1 reset due to BGP Notification received

*Mar 1 00:06:38.255: %BGP-5-ADJCHANGE: neighbor 192.168.0.1 Down BGP Notification received

R2(config-router)#

*Mar 1 00:06:38.255: BGP: service reset requests

*Mar 1 00:06:38.259: BGP: service reset requests

R2(config-router)#

*Mar 1 00:06:39.675: BGP: Regular scanner event timer

*Mar 1 00:06:39.675: BGP: Performing BGP general scanning

*Mar 1 00:06:39.675: BGP(0): scanning IPv4 Unicast routing tables

*Mar 1 00:06:39.675: BGP(IPv4 Unicast): Performing BGP Nexthop scanning for general scan

*Mar 1 00:06:39.675: BGP(0): Future scanner version: 5, current scanner version: 4

*Mar 1 00:06:39.679: BGP(2): scanning VPNv4 Unicast routing tables

*Mar 1 00:06:39.679: BGP(VPNv4 Unicast): Performing BGP Nexthop scanning for general scan

*Mar 1 00:06:39.679: BGP(2): Future scanner version: 5, current scanner version: 4

*Mar 1 00:06:39.679: BGP(4): scanning IPv4 Multicast routing tables

*Mar 1 00:06:38.803: BGP: 192.168.0.2 Update missing attributes, flags 0x7
*Mar 1 00:06:38.807: BGP: 192.168.0.2 reset due to BGP Notification sent
*Mar 1 00:06:38.807: %BGP-5-ADJCHANGE: neighbor 192.168.0.2 Down BGP Notification sent
*Mar 1 00:06:38.807: %BGP-3-NOTIFICATION: sent to neighbor 192.168.0.2 3/3 (update missing required attributes) 0 bytes
R1# FFFF FFFF FFFF FFFF FFFF FFFF FFFF FFFF 0035 0200 0000 1940 0101 0040 0204 0201 000A 4003 04C0 A800 0280 0404 0000 0000 2002 0202 02
*Mar 1 00:06:38.927: BGP: service reset requests
*Mar 1 00:06:38.927: BGP: service reset requests

*********************************************************************************

R2 (router that is configured with the "local-as no-prepend replace-as"). As soon as I configured the network 2.2.2.2 mask 255.255.255.255 command to announce a loopback interface to the peer, everything is triggered. These are the debug messages:

***************************************************************************

R2(config)#router bgp 20
R2(config-router)#net
R2(config-router)#network 2.2.2.2 mask
R2(config-router)#network 2.2.2.2 mask 255.255.255.255
R2(config-router)#
*Mar 1 00:06:38.179: BGP(0): route 2.2.2.2/32 up
*Mar 1 00:06:38.179: BGP(0): nettable_walker 2.2.2.2/32 route sourced locally
*Mar 1 00:06:38.183: BGP(0): 192.168.0.1 send UPDATE (format) 2.2.2.2/32, next 192.168.0.2, metric 0, path Local
*Mar 1 00:06:38.251: %BGP-3-NOTIFICATION: received from neighbor 192.168.0.1 3/3 (update missing required attributes) 0 bytes
*Mar 1 00:06:38.255: BGP: 192.168.0.1 reset due to BGP Notification received
*Mar 1 00:06:38.255: %BGP-5-ADJCHANGE: neighbor 192.168.0.1 Down BGP Notification received
R2(config-router)#
*Mar 1 00:06:38.255: BGP: service reset requests
*Mar 1 00:06:38.259: BGP: service reset requests
*Mar 1 00:06:38.259: BGP: service reset requests
*Mar 1 00:06:38.259: BGP: service reset requests
R2(config-router)#
*Mar 1 00:06:39.675: BGP: Regular scanner event timer
*Mar 1 00:06:39.675: BGP: Performing BGP general scanning
*Mar 1 00:06:39.675: BGP(0): scanning IPv4 Unicast routing tables
*Mar 1 00:06:39.675: BGP(IPv4 Unicast): Performing BGP Nexthop scanning for general scan
*Mar 1 00:06:39.675: BGP(0): Future scanner version: 5, current scanner version: 4
*Mar 1 00:06:39.679: BGP(2): scanning VPNv4 Unicast routing tables
*Mar 1 00:06:39.679: BGP(VPNv4 Unicast): Performing BGP Nexthop scanning for general scan
*Mar 1 00:06:39.679: BGP(2): Future scanner version: 5, current scanner version: 4
*Mar 1 00:06:39.679: BGP(4): scanning IPv4 Multicast routing tables

***********************************************************************************

Regarding the IOS, I'm currently using Version 12.4(25d),

Cheers!

Flo.

fgonzal · ‎09-28-2011

I think I've just found out what the problem is. This feature is only available to eBGP and it is quite reasonable.

*****

This feature allows a router to appear to external peers as a member of another autonomous system for the purpose of autonomous system number migration.

*******

I've just configured it and it works. I think we can use a private AS and then we can remote the private as when advertising it to the internet.

Thanks for your help.

Flo.

rsimoni · ‎09-28-2011

indeed..and it makes perfect sense.

good that you spotted it

smitesh kharecha · ‎09-30-2011

local-as is the best and least complex command when migration of AS is concern.

Regards,
Smitesh