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[Junos] 4-byte private AS number removed from AS path after upgrade to Junos OS 13.3R6

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Article ID: KB32357 KB Last Updated: 20 Jul 2018Version: 1.0
Summary:

When a device is upgraded to Junos OS 13.3R6, the 4-byte private AS number [those in the range 4200000000 - 4294967294 (inclusive)] is removed from the Autonomous System (AS) path.

This article provides the reason for this and suggests a solution.

Symptoms:

When router products are upgraded to Junos OS release 13.3R6, the original AS path is changed and the 4-byte (32-bit) private AS number is removed as shown in the following example. This change applies to the AS numbers in the range 4200000000 - 4294967294 (inclusive).

The original AS path before upgrade to Junos OS 13.3R6:

root@lab> show route advertising-protocol bgp 172.16.1.2

vrfa.inet.0: 13 destinations, 15 routes (13 active, 0 holddown, 2 hidden)
Prefix                  Nexthop              MED     Lclpref    AS path
* 1.1.1.1/32            Self                 3000               4200000001 ?  

The AS path after upgrade to Junos OS release 13.3R6:
root@lab> show route advertising-protocol bgp 172.16.1.2

vrfa.inet.0: 13 destinations, 17 routes (13 active, 0 holddown, 2 hidden)
Prefix                  Nexthop              MED     Lclpref    AS path
* 1.1.1.1/32            Self                 3000               ?

 

Cause:

This is because in Junos OS release 13.3R6, these private AS numbers are recognized as private and removed if the remove-private knob is configured. In Junos OS releases prior to 13.3R6 (between 9.1R1 and 13.3R6), the private 4-byte AS number is supported but is not recognized as “private.”

Therefore, if the AS path has a 4-byte AS number and the remove-private knob is configured for the Border Gateway Protocol (BGP) peers, the AS number is removed from the path in all Junos OS releases starting with release 13.3R6.

Note

  • Junos OS release 13.3R6 provides support for RFC 6996, which has defined the AS number range 4200000000 - 4294967294 (inclusive) as private, as specified in PR915235.

  • Junos OS releases 14.2 and later contain changes for RFC 6996, RFC 7300 (last-as reservation), and draft-idr-as0-06. (treat AS 0 as withdraw).

  • Junos OS releases 14.1 and earlier contain changes for RFC 6996 and draft-idr-as0-06 (because 7300 is not implemented, both the 2-byte last-as and the 4-byte last-as are considered private AS). These values should, therefore, be removed from service before upgrading to release 14.2.

 

Solution:

This is expected behavior and as per design.

However, if the private AS number is required and should not be removed during upgrade, the workaround is to delete the remove-private knob.

For more details about the remove-private knob, refer to BGP Feature Guide > remove-private.

 

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