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After a while I decided to change the MD5 key. The session with the new key came up and looked fine, but the old session didn't close properly. Notice the close is initiated from the Juniper side, and the first packet from the Cisco side is now sent with an invalid MD5 digest - it turns out that the packet is actually sent with an MD5 digest based on the *new* key:

This is a feature, and hopefully Juniper will also add the same feature soon. The feature allows you to change the MD5 key without flapping the BGP session which is very important on large peering sessions. There is no requirement that MD5 keys must remain constant throughout an entire TCP session, or to terminate a TCP session when the key changes. As long as both sides agree, the key can be changed at any time including in the middle of a TCP session. New packets after the key change are sent with message digests based on the new key.

But as long as the session *is* reset anyway, the current situation is extremely confusing - the log messages (on both Cisco and Juniper) give no indication that the invalid key in question is for an *old* BGP session, no longer active! Steinar Haug, Nethelp consulting, sthaug@nethelp.no