Cisco BGP Flapping

[Russell Stamey]

I am a little confused with how to determine what the cause of these problems.  If I get the following logs (I am getting many of, up and downs), would the problem be a connection on the local router or the neighbor router?

%LINK-3-UPDOWN: Interface Serial0/1/0, changed state to down

%BGP-5-ADJCHANGE: neighbor 192.168.67.221 Down Interface flap

[Richard Burts]

Russel

From the little information that you have given us it appears that neighbor 192.168.67.221 is reached from the router by interface serial0/1/0. The first log message indicates that the serial interface changed state to down. When the interface is down it is no longer possible to reach the neighbor and the BGP neighbor goes down.

Why the neighbor goes down is a pretty simple question - it goes down because the path to the neighbor has gone down.

Why the interface went down is a good question but we do not have enough information to say much about that.

HTH

Rick

[Russell Stamey]

I guess my real question is, how do I determine which router is causing the problem?

If the link goes down on a neighbor router, will the local router show a "link updown" message as well, or will it just show a "BGP Flap" message?

It seems like alot of the times that the router gives us the "link updown" message when the problem is on the carrier side of the circuit.

[Nataliya Omelyanyuk]

Hi, Russel

%LINK-3-UPDOWN: error message indicates that some physical layer issue exists between your and neighborinfg device.

If your BGP peer (carrier device) is directly connected to your device and no any intermediate devices are in between, this message received on your device without any action from you side could indicate issue on carrier device.

If you have some intermediate devices between yours and carrier, BGP session flap could indicate some path problem between devices, but further troubleshooting needed to determine real cause.

BR,

Nataliya.

[Richard Burts]

Russel

Whether your interface will generate an error message about link down when the link goes down on a neighbor router depends on the type of interface, and sometimes also depends on how the interface is configured. If the interface is Ethernet/FastEthernet etc most of the time your interface would not go link down if the problem were on the neighbor router (there are a few circumstances where your interface might also go down but most of the time it would not). If the interface is a serial interface then it depends somewhat on how the interface is configured and is used. If it is a simple point to point serial (using HDLC or PPP encapsulation) then usually your interface will go link down if the neighbor goes down. But if your serial interface is something like Frame Relay then probably it would not go down if the neighbor goes down.

Since it appears that you are connected to the BGP neighbor by a point to point serial it will be difficult to tell from the log messages whether the problem is on your end or the neighbor end.

HTH

Rick