Clearing BGP Sessions

There are a couple ways, some more recommended than others, to reset or clear a BGP session between two neighbors.

The Cisco IOS Software Command Summary lists the following circumstances under which you must reset a BGP connection, for changes to take effect:

Additions or changes to BGP-related access lists
Changes to BGP-related weights
Changes to BGP-related distribution lists
Changes to the BGP-related timer’s specifications
Changes to the BGP administrative distance
Changes to BGP-related route maps

Traditional clearing of BGP session (aka Hard Reset)

Tears down the BGP session with all Neighbors, a specific neighbor or peer group.
A new session is re-established within 30-60 seconds, depending.
Processing of the full Internet table can take a really long time.
This is almost NEVER recommended on production networks, due the disruptive behaviour.

router#clear ip bgp {* | neighbor ip | peer-group}

Soft Reconfiguration (from IOS 11.2+)

Outbound Soft Reconfiguration

Re-sends your complete BGP Table,
Is not configurable and is enabled by default.

Inbound Soft Reconfiguration

Requires configuration beforehand.
Both neighbors need to support soft reset route refresh capability.
Stores complete BGP table of you neighbor in router memory.
Not a good idea on a peering router with full feed, due to the memory requirements.
Handy for testing purpose, but recommended only for testing.

This will resend your BGP table to the neighbor, soft-reconfig outbound
router#clear ip bgp {neighbor ip} soft out

This will enable soft-reconfiguration inbound for the neighbor,
This way all the routes are stored in memory before filters are applied.
router(config-router)#router bgp 12345 router(config-router)#neighbor {neighbor ip} soft-reconfig [inbound]

This takes all the routes in memory and re-applies your filters on those routes.
router#clear ip bgp {neighbor ip} soft in

Route Refresh (aka Soft Reset) (from IOS 12.1+)

Used to request a neighbor to resend routing information, without bringing a session down.
Useful after configuration changes to update BGP tables,
Route-refresh-capability must be supported by both neighbors.
Route-refresh-capability is negotiated upon BGP peer session establishment.
Requires no additional memory like Soft-Reconfiguration.
This is the PREFERRED way to have your updates applied.

router#clear ip bgp {* | neighbor ip | peer-group} in