6

To give you an idea of what is happening, I've two eBGP peers with no routes advertised. The capabilities are zero and there is no NLRI. The BGP is established between the eBGP peers.

Next, I ask one of the peers to advertise IPv4 routes by adding the routes. Immediately, this BGP router goes from ESTABLISHED -> ACTIVE -> ESTABLISHED.

Is it necessary to tear down the TCP session and start again with the new capabilities in the OPEN message? I see the same in the pcap. Can we not have the UPDATE message notify the other peer about the new capabilities?

Thank you.

  • @BVH. It is not necessary to tear down an established BGP session to advertise an additional route(s). How are you "adding IPv4 routes" to be advertised? Can you share your relevant BGP configuration as well as any static routes or additional routing protocol(s) in use on this device? – one.time Jun 26 '14 at 19:22
  • In other words if the capabilities of a peer changes it has to reset TCP session. – BHV Jun 27 '14 at 9:30
  • What bgp implementation? Please share your config – Mike Pennington Jun 27 '14 at 10:18
  • Looks to me like the peer doesn't have soft reconfiguration enabled but in order for soft reconfiguration to work, both peers need to support it. If you are advertising no capabilities, the other end doesn't know if you support route-refresh so it flaps the session after they change their filters. – GeorgeB Jun 12 '15 at 16:53
  • Did any answer help you? if so, you should accept the answer so that the question doesn't keep popping up forever, looking for an answer. Alternatively, you could provide and accept your own answer. – Ron Maupin Aug 10 '17 at 4:36
5

This is for Cisco but I would imagine it applies to others as well:

Whenever there is a change in the routing policy, the BGP session must be reset (cleared) for the new policy to take effect and the routing table to be reconfigured. Using a hard reset to clear a BGP session causes cache invalidation and results in a negative impact on network operation.

Soft reset is recommended because it allows routing table policies to be reconfigured and activated without clearing the BGP session. Soft reset is done on a per-neighbor basis. Soft resets can be inbound or outbound:

But the same document also says:

This feature provides an additional method for soft reset that allows the dynamic exchange of route refresh requests and routing information between BGP routers, and the subsequent re-advertisement of the respective outbound routing table. Soft reset using the route refresh capability does not require preconfiguration and consumes no additional memory resources.

To use this new method, both BGP peers must support the soft route refresh capability, which is advertised in the OPEN message sent when a peer sends its routing table update.

Since you say you see no capabilities, there is no "route refresh" capability being advertised so there is no choice but to reset the BGP session in order to effect the change in routing policy.

References:

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