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[MX] GR and LLGR capability and compatibility changes after 15.1 release

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Article ID: KB35114 KB Last Updated: 04 Apr 2020Version: 1.0
Summary:

This article explains the capability and compatibility changes for graceful restart (GR) and long-lived graceful restart (LLGR) post 15.1 release.

Symptoms:

Some users may experience BGP peering issues specific to GR, restart timer, etc. after upgrading to Junos 15.1 and above.

Solution:

Beginning from 15.1 release, LLGR is enabled by default. If the remote side does not support it, then disabling LLGR is a viable workaround. It should not affect regular GR.

Restart timer showing "0x0000: 4078" is an expected change:

The BGP GR Restart Flag and Restart Time together is 16bit value. The first 4 bits are used for Flags and the remaining 12 bits is to encode time. The value observed is 0x4078. This value is decoded as 0100(4) + 0000 0111 1000(120). R bit is the First bit, which is not set in the above case.

The Second bit is the Notification bit, which is another feature. To disable this feature, use the following hidden knob:

set protocols bgp graceful-restart "disable-notification-flag"

Behavioral difference post upgrade:

The behavior is the same for older GR, but LLGR helper mode capability is sent out by default in the open message. If a Customer Edge (CE) router is non-confirm and decides to flap a session instead of ignore the capability it does not understand, then it will flap regardless of whether LLGR is configured. Note that this is not caused by Junos. It is the CE router’s incapability. To resolve this, manually disable the LLGR helper mode with the following command:

set protocols bgp graceful-restart long-lived receiver disable

Restart Flags:

According to RFC4724 - Graceful Restart Mechanism for BGP:

"Restart Flags:

This field contains bit flags related to restart.

0 1 2 3
+-+-+-+-+
|R|Resv.|
+-+-+-+-+


The most significant bit is defined as the Restart State (R) bit, which can be used to avoid possible deadlock caused by waiting for the End-of-RIB marker when multiple BGP speakers peering with each other restart.  When set (value 1), this bit indicates that the BGP speaker has restarted, and its peer MUST NOT wait for the End-of-RIB marker from the speaker before advertising routing information to the speaker.

The remaining bits are reserved and MUST be set to zero by the sender and ignored by the receiver."

According to the Notification Message support for BGP Graceful Restart:

"The second most significant bit ("N") is defined as the BGP Graceful Notification bit, which is used to indicate Graceful Restart support for BGP NOTIFICATION messages.  A BGP speaker indicates support for the procedures of this document, by advertising a Graceful Restart Capability with its Graceful NOTIFICATION bit set (value 1).  This also implies support for the format for a BGP NOTIFICATION Cease message defined in [RFC4486 ]

Notification Flag:

This field contains bit flags relating to restart."

0 1 2 3
+-+-+-+-+
|R|N|   |
+-+-+-+-+

Restart Time:

According to RFC4724 - Graceful Restart Mechanism for BGP:

"Restart Time:

This is the estimated time (in seconds) it will take for the BGP session to be re-established after a restart.  This can be used to speed up routing convergence by its peer in case that the BGP speaker does not come back after a restart."

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