12-18-2008 10:36 AM - edited 03-04-2019 12:45 AM
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
12-18-2008 09:08 PM
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.
12-19-2008 01:49 PM
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
12-20-2008 04:31 PM
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.
12-21-2008 02:04 PM
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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