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JUNOS Cookbook by Aviva Garrett

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13.3. Diagnosing TCP Session Problems

Problem

You want to figure out why the BGP session is not being established.

Solution

Start by looking at the current state of the TCP sessions on the router:

	aviva@RouterF> show system connections extensive

Also look at the information in the system logging files:

	aviva@RouterF> show log messages

Check that the TCP session can pass Internet control packets:

	aviva@RouterF> ping tos 0xc0 RouterD

Discussion

When two BGP peers have a problem establishing a BGP session, one of the first indications is that you see BGP hold-time expired error messages on the routers in the router's system logging files. You also see that the State field in the show bgp neighbor command output is not Established and that the State field in the show bgp summary command is Active or Connect, indicating that the BGP session is not established.

The hold-time expired errors usually occur because the TCP session between a pair of peers cannot effectively transmit data between the routers, not because of a problem with BGP itself. When the TCP session doesn't work properly, the BGP session times out, and BGP signals the problem by sending hold-time expired messages and generating a BGP Notification message to the remote peer. Notification messages are logged at the system logging severity level warning.

Some of the most frequent causes of hold-time expired errors are MTU issues on a directly connected link, issues related to forwarding of Internet control packets, and IGP failures ...

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