An E-series router running an unpatched release of JUNOSe (see Fixed Release below) may see BGP sessions flapping constantly after a certain update has been received. The E-series router is the one terminating the session and is sending a NOTIFICATION with Code 3 (Update Message Error) and Subcode 9 (error with optional attribute).

Here is an example of a message logged on a peer router running JUNOS when receiving this notification:

This can be confirmed on the E-series router itself by changing the log severity for category bgpMessages to warning:

ERX(config)# log severity warning bgpMessages

Which would then uncover the following two warning messages after a flap:

As detailed in the message above, this is due to an incoming UPDATE including a new AS4_PATH attribute (introduced to support 4-byte ASNs, per RFC 4893) containing an AS_CONFED_SEQUENCE or an AS_CONFED_SET, which is considered illegal, as per RFC 4893.

This defect is being tracked via CQ 88706.

Resolved in the release(s) indicated in the Fixed Release field below.

Workaround:

In order to stop the BGP session flaps, a per-neighbor configuration option exists that will cause JUNOSe to ignore any illegal or incorrectly formatted attributes:

ERX(config)# router bgp <AS#>
ERX(config-router)# neighbor <x.x.x.x> lenient

Provided log category bgpMessages has been set to log severity warning by configuring:

ERX(config)# log severity warning bgpMessages

A message will still be logged when an illegal attribute is received:

Fixed Releases:

JUNOSe 8-1-4p0-4, 8-2-4p0-7, 9-0-2p0-1, 9-1-2p0-1, 9-2-1p0-1, 9-3-0p0-1, 10-0-0

2020-02-27: Archived.