Question Pertaining To Clear BGP

[carringtonmc]

I had a question about the clear BGP router command. If you have two AS's talking to one another, and your receive utilization drops to zero, and this issue is resolved by clearing BGP, what would some reasons be as to 'why' this command fixed the problem? Any feedback would be greatly appreciated.

[carringtonmc]

Let me rephrase:

What does the command "clear BGP" do, and what are its purposes of using it?

[dhananjoy chowdhury]

suppose myRrouter has neighborship with 2 other routers A.B.C.1 and X.Y.Z.1

<>----------------<>

Now if I do some config changes on myRouter like add or modify BGP access lists, weight, distance, version, timers, I need to clear the BGP connections for the modifications to take effect.

clear ip bgp X.Y.Z.1

>>> If you want to reset the neighborship with router X.Y.Z.1, then use

clear ip bgp *

>> this will reset all current BGP sessions

But using clear ip bgp * can impact all other links as BGP will create routing table from scratch.

This is same as rebooting the router. So we go for Soft reconfiguration which lets you to generate

inbound updates from a neighbor, change and activate BGP policies without clearing the BGP session.

clear ip bgp A.B.C.D soft in

>>When you make changes in inbound policy and to initiate inbound soft reconfiguration

clear ip bgp A.B.C.D soft out

>> When you make changes in outbound policy and to to initiate outbound soft reconfiguration

[Edison Ortiz]

BGP Session Reset

Whenever there is a change in the routing policy due to a configuration change, BGP peering sessions must be reset using the clear ip bgp command. Cisco IOS software support the following three mechanisms to reset BGP peering sessions:

•Hard reset - hard reset tears down the specified peering sessions including the TCP connection and deletes routes coming from the specified peer.

•Soft reset - soft reset uses stored prefix information to reconfigure and activate BGP routing tables without tearing down existing peering sessions. Soft reconfiguration uses stored update information, at the cost of additional memory for storing the updates, to allow you to apply new BGP policy without disrupting the network. Soft reconfiguration can be configured for inbound or outbound sessions.

•Dynamic inbound soft reset - The route refresh capability, as defined in RFC 2918, allows the local router to reset inbound routing tables dynamically by exchanging route refresh requests to supporting peers. The route refresh capability does not store update information locally for non disruptive policy changes. It instead relies on dynamic exchange with supporting peers. Route refresh must first be advertised through BGP capability negotiation between peers. All BGP routers must support the route refresh capability.

To determine if a BGP router supports this capability, use the show ip bgp neighbors command. The following message is displayed in the output when the router supports the route refresh capability:

Received route refresh capability from peer.

http://www.cisco.com/en/US/docs/ios/iproute/configuration/guide/irp_bgp_basic_net_ps6350_TSD_Products_Configuration_Guide_Chapter.html#wp1054155

HTH,

__

Edison.