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Clearing BGP Sessions

November 12, 2008

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

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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}

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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.

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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

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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

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One comment

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