Core Issue:
Routing policies for a peer include all the configurations such as route-map, distribute-list, prefix-list, and filter-list that may impact inbound or outbound routing table updates. Whenever there is a change in the routing policy, the BGP session must be cleared, or reset, for the new policy to take effect.
There are two types of reset, hard reset and soft reset.
Clearing a BGP session using a hard reset invalidates the cache and results in a negative impact on the operation of networks as the information in the cache becomes unavailable.
Soft reset is recommended because it allows routing tables to be reconfigured and activated without clearing the BGP session. Soft reset is done on a per-neighbor basis. There are two types of soft reset:
When soft reset is used to generate inbound updates from a neighbor, it is called dynamic inbound soft reset.
When soft reset is used to send a new set of updates to a neighbor, it is called outbound soft reset.
In order to perform a soft reset for inbound routing table updates issue the "neighbor soft-reconfiguration" command under BGP process
Resolution
Some times when route propagated in BGP doesn't exist in BGP table. BGP takes its own sweet time to converge. In this kind of scenario we need to clear advertised routes and in some cases we use the following command:
clear ip bgp *
Above command can impact to reset all neigbourship more over it like BGP will create routing table from scratch. This is as good as rebooting the router. To minimise impact we can use following command.
clear ip bgp A.B.C.D soft out –> When you make changes in outbound policy
and use
clear ip bgp A.B.C.D soft in –> When you make changes in inbound policy .
You can also clear BGP neighbor on address family base using following command structure
Router# clear bgp ipv4 ?
multicast Clear multicast address-family
mvpn Display BGP information for MVPN address family
unicast Clear unicast address-family
Router # clear bgp ipv6 ?
multicast Clear multicast address-family
mvpn Display BGP information for MVPN address family
unicast Clear unicast address-family
Example:
clear bgp ipv4 unicast 1.1.1.1 soft in
You can also verify the advertised routes with following command:
sh ip bgp neighbors A.B.C.D advertised-routes
For more information, refer to the When and how should I reset a BGP session? section of BGP: Frequently Asked Questions.
For details on the BGP Soft Reset Enhancement feature with Cisco IOS Software Release 12.0 and later, refer to BGP Soft Reset Enhancement.
