BGP peer flaps across multilink PPP link

Unanswered Question

I have a router with 4 T1's in a multilink configuration that peers via BGP to another router (point-to-point circuits). The issue I am seeing is that the BGP peer flaps when one of the 4 links fails. Do I need to disable multilink fragmentation or something to prevent this? Here is the config:

interface Multilink1

description Qwest MPPP link

ip address 1.2.3.5 255.255.255.252

no ip redirects

no ip unreachables

no ip proxy-arp

ppp multilink

ppp multilink group 1

service-policy output voip

!

router bgp 2

no synchronization

bgp log-neighbor-changes

network 192.168.1.0

timers bgp 2 6

neighbor HQ peer-group

neighbor HQ remote-as 1

neighbor HQ password 7 <deleted>

neighbor HQ update-source Multilink1

neighbor HQ route-map noxit out

neighbor 1.2.3.4 peer-group HQ

no auto-summary

Thanks in advance if anyone has any ideas.

-Jim

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Laurent Aubert Thu, 12/18/2008 - 21:08

Hi Jim,

BGP flap may be due to your aggressive timers. Try with the default ones to see if you still have the issue.

Thanks

Laurent.

True, but I would not expect the BGP peer to go down when the multilink interface is still up on the other 3 interfaces. I see this in the log:

Dec 18 06:37:16.736: %LINK-3-UPDOWN: Interface Serial0/3/0, changed state to down

Dec 18 06:37:17.340: %BGP-3-NOTIFICATION: received from neighbor 1.2.3.4 4/0 (hold time expired) 0 bytes

Dec 18 06:37:17.340: %BGP-5-ADJCHANGE: neighbor 1.2.3.4 Down BGP Notification received

Dec 18 06:37:17.736: %LINEPROTO-5-UPDOWN: Line protocol on Interface Serial0/3/0, changed state to down

Dec 18 06:37:47.844: %BGP-5-ADJCHANGE: neighbor 1.2.3.4 Up

Laurent Aubert Sat, 12/20/2008 - 16:31

These logs clearly indicate the BGP peer reset the session because the hold down timer expired. It seems the hello were sent only on the serial that failed. The session came up again after the failed link was removed from the bundle.

Increasing this timers will improve the stability of the session in case of interface flapping in the bundle.

Thanks

Laurent.

scottlivingston Sun, 12/21/2008 - 14:04

I would have to dig up the exact config, but a number of years ago I bundled 4 T1's to an ISP and ran a multihop session between their loopback and mine. Never had a problem if one circuit bounced.

scott

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