Description

This article describes the issue of the IPv6 BGP peering on the lt-0/0/0 interface flapping.

Symptoms

  • The IPv6 BGP peering on the lt-0/0/0 interface is flapping.
  • The BGP neighborship gets created and some routes are populated.
  •  However, after about 10-20 seconds, BGP flaps. The number of expected routes is much lower than expected.

Solution

The default MTU value of lt-0/0/0 inet 6 is unlimited. This cause issues, when the router sends too many updates in a single packet.

When looking at the details of BGP peering on the lt-0/0/0 interface , check the MTU value on inet6 . By default, the MTU setting is set to unlimited . This can cause BGP peering to be created and then fail (due to timeout).  You may see some routes being built; but they will fail.

To resolve this issue, manually set the MTU to 1500. When this is set, the BGP peering will stabilize. To confirm this, check the status of lt-0/0/0 : 

> show interfaces lt-0/0/0.0 extensive
   Logical interface lt-0/0/0.0 (Index 74) (SNMP ifIndex 501) (Generation 146)
     [snip]
     Protocol inet, MTU: 1500, Generation: 169, Route table: 0
       Flags: Sendbcast-pkt-to-re
       Addresses, Flags: Is-Preferred Is-Primary
         Destination: 10.221.153.124/30, Local: 10.221.153.125,
         Broadcast: 10.221.153.125, Generation: 192
     Protocol inet6, MTU: *Unlimited*, Generation: 170, Route table: 0
       Flags: User-MTU
       Addresses, Flags: Is-Preferred Is-Primary
         Destination: 2001:7a40:0:17::/64, Local: 2001:7a40:0:17::1
     Generation: 214
       Addresses, Flags: Is-Preferred
         Destination: fe80::/64, Local: fe80::fac0:100:de:2500
     Generation: 216
The resolution is the set interface lt-0/0/0 unit 0 family inet6 mtu 1500 configuration. The lt-0/0/0.0 interface will now show the following information: 
> show interfaces lt-0/0/0.0 extensive
   Logical interface lt-0/0/0.0 (Index 74) (SNMP ifIndex 501) (Generation 146)
     [snip]
     Protocol inet, MTU: 1500, Generation: 169, Route table: 0
       Flags: Sendbcast-pkt-to-re
       Addresses, Flags: Is-Preferred Is-Primary
         Destination: 10.221.153.124/30, Local: 10.221.153.125,
         Broadcast: 10.221.153.127, Generation: 192
     Protocol inet6, MTU: *1500*, Generation: 170, Route table: 0
       Flags: User-MTU
       Addresses, Flags: Is-Preferred Is-Primary
         Destination: 2001:7a40:0:17::/64, Local: 2001:7a40:0:17::1
     Generation: 214
       Addresses, Flags: Is-Preferred
         Destination: fe80::/64, Local: fe80::fac0:100:de:2500
     Generation: 216