BGP neighbor flapping

[Rojer-bkk]

Hi,

I noticed the log neighbor up/down very often on CE router. I check the PE found log

: Receiving unsupported capability 65

Is related to neighbor flapping issue?

[Harish Balakrishnan]

Hello Rojer,

BGP Capability code 65 is the support for 4 octect BGP AS number feature. I dont think this could be your root cause for the

BGP flapping... please check how stable your CE-PE link is..

you can setup a IP SLA to monitor the other end of your link and see whether you are getting a drops over a period of time

regards

Harish

rate any helpful posts!

[Rojer-bkk]

Hi Harish,

Thanks for reply.

You mean if CE use 2-byte ASN and PE use 4-byte ASN, the BGP session shoudn't be created if i didn't change some configure on CE. Am i correct?

[Harish Balakrishnan]

Hello Rojer,

The adjacent router is letting you know that that router is 4 octect capable router. In your case is the neighbor ever came up ? it is up and running and exchanging routes but sometimes you are seeing flaps ?

what is the exact behaviour

regards

Harish

[Rojer-bkk]

Hello Harish,

Neighbor came up and down after mintues. I just check the bgp 4-ASN is not related this issue.

[Harish Balakrishnan]

Hello Rojer,

Ok.. when the neighbor is up, are getting the routes exchanged and communication happens ?

regards

Harish.

[Rojer-bkk]

Hello Harish,

Right. From log, it look like the link or BGP peer issue.

10:35:57.635: %BGP-5-ADJCHANGE: neighbor 10.10.10.1 Up

10:46:15.195: %BGP-5-ADJCHANGE: neighbor 10.10.10.1 Down BGP Notification sent

10:46:15.203: %BGP-3-NOTIFICATION: sent to neighbor 10.10.10.1 4/0 (hold time expired) 0 bytes

10:46:15.539: %BGP_SESSION-5-ADJCHANGE: neighbor 10.10.10.1 IPv4 Unicast topology base removed from session  BGP Notification sent

10:46:46.324: %BGP_SESSION-5-ADJCHANGE: neighbor 10.10.10.1 IPv4 Unicast topology base removed from session  Peer closed the session

10:46:57.547: %BGP-5-ADJCHANGE: neighbor 10.10.10.1 Up

[Harish Balakrishnan]

Hello Rojer,

Please setp up a simple ip sla as follows to check the basic reachability

ip sla 20

icmp-echo 10.10.10.1

timeout 300

frequency 30

ip sla schedule 20 life forever start-time now

verify as follows every time you get a flap and see any timeout for reachability

sh ip sla statistics 20

regards

Harish

[vinayk3]

Hi Roger ,

You can use the following steps.

1. Please check if your BGP neigbor fallping every 3 minutes if yes then it could be related to MTU or MSS value mismatch.

2. Check on both the ends the interface health, do you see any errors, interface resets or and reliability of the link.

thanks

Vinay

[Raju Sekharan]

Hi Rojer,

To check if capability negotiation is the reason for flap, try disabling capability negotiation

conf t

router bgp [as]

address-family ipv4 vrf xxxxx

neighbor x.x.x.x dont-capability-negotiate

Thanks

Raju