ibgp session flapping

[jonathan johnson]

Hi

I came across a strange issue today where a couple of ibgp sessions were stsying up for 3 minutes then dropping.

I had recently changed the lan mtu from 1500 to 9000 and all was ok until the bgp sessins had to re-establish, then when they did they flapped.  Initially i didnt think it was an mtu issue as i could ping using a 9000 byte packet between the hosts (setting the df flag) but when i changed the mtu back to 1500 it all worked fine.

I experimented and gradually raised the mtu until i got to 6486 then anything larger caused the flaps - this was occurring ona 7206 G1 and also a 7206 G2.  I'm fairly sure that they are running different IOS versions so it seems unlikely to be a bug, but you never know...

I tried disabling the bgp mtu path discovery, but it made no difference.

Has anyone come across this or has any suggestions on why it's occuring?

Thanks,

Jonno

[Reza Sharifi]

Hi,

Are you using loopback interfaces for this iBGP session?

HTH

[Julio E. Moisa]

Hi

If you are using the loopbacks for iBGP peering and these loopbacks are being advertised by BGP and other Routing protocol you will have flapping. Also you should use the same MTU between the BGP peerings. 

Could you please share the router configs?

Thank you 

[jonathan johnson]

Hi,

No, I'm not using loopbacks on this iBGP session - here's the config I have applied.  The ip ospf mtu-ignore is there so that OSPF stayed up while I messed around with the mtu sizes.

Router_1 (7206 G2)

interface GigabitEthernet0/2.700
encapsulation dot1Q 700
ip address 10.10.10.1 255.255.255.240
ip mtu 6486
ip ospf mtu-ignore
ip ospf cost 20
mpls ip

router bgp 65000
neighbor 10.10.10.8 remote-as 65000
neighbor 10.10.10.8 description BGP to 7206 G1
address-family ipv4
neighbor 10.10.10.8 activate
neighbor 10.10.10.8 send-community both
neighbor 10.10.10.8 send-label

Router_2 (7206 G1)

interface GigabitEthernet0/2.700
encapsulation dot1Q 70
ip address 10.10.10.8 255.255.255.240
ip mtu 6486
ip ospf cost 20
ip ospf mtu-ignore
mpls ip

router bgp 65000
neighbor 10.10.10.1 remote-as 65000
neighbor 10.10.10.1 description BGP to 7206 G2
address-family ipv4
neighbor 10.10.10.1 activate
neighbor 10.10.10.1 send-community both
neighbor 10.10.10.1 send-label

[Julio E. Moisa]

Hi

For testing remove mpls ip from the interfaces. Send-label is used between ISP (Inter AS)  to exchange label information where LDP is not passing. It will add BGP forwarding under the interfaces automatically. 

[Reza Sharifi]

Hi,

Can you try using a loopback interface and test the MTU again?

BTW, it is recommended and best practice to use a loopback interface for iBGP session.

HTH