So every time you make a policy change into your configuration, you somehow need to tell the router that this policy has been introduced or something has been changed. Now the way this is done is the BGP router is going to ask its peers to refresh what has happened. And you can do this in the Cisco IOS command line by saying "clear ip bgp <neighbor-address> in|out". You must not forget the "in" or the "out" direction because it will do a hard reset of the BGP session. BGP refresh was added later so this is an optional setting that needs to be negotiated during the BGP setup. Also, as mentioned earlier in a different video, if you enable "soft-reconfiguration" you're going to disable route refresh. Which means in some cases when you clear the BGP session even with the "out" direction specified, it's going to do a hard reset of the BGP session. Now different vendors have different ways of handling this and you need to read the documentation for your vendor. For example, Juniper tries to do automatic route refresh. However, if you change certain policy for a particular group of BGP peers, for example, you add a different address family, this means that this address family needs to be negotiated in a new BGP session and it's going to tear down all the BGP sessions not just for that individual neighbor, but for the entire group. If you want to have configuration that is applied to multiple groups, but does not affect how BGP on Junos does the updates to the neighbor, you need to use the general group statement up inside the configuration, rather than the group statement that is inside the policy BGP. Please make sure that you read the documentation for whichever BGP vendor that you're using to understand how route refresh is handled. The reason this is important is once your BGP is removed from the internet routing table, it may take some time before other routers accept your prefix to be inside their BGP table. This is damping that is done by other BGP peers. It can take anywhere between a few minutes to a couple of hours depending on how frequent this has been removed and added, and how far away from this particular neighbor you are. So for Cisco IOS you can say "clear ip bgp", and then an <address>, and then "in" or "out". The address can be an IP address of the peer whether IPv4, IPv6, it can be the "*" which means all peers. You can specify the ASN which means all peers in this AS. You can say "external" which means all external peers and then you can specify a peer-group and a name, which means all peers in a peer-group. Similarly, other vendors will have different ways you can group which peers you want to clear. In any situation, make sure you remember to have the direction that you want clear specified at the end.
© Produced by Philip Smith and the Network Startup Resource Center, through the University of Oregon.
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