Description

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
* 10.1.1.1/32            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
* 10.1.1.1/32            Self                 3000               ?

 

Solution

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.

 

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 .

 

Modification History

2023-04-08: Minor non-technical updates

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