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Cisco Bug: CSCvv64633 - BGP: advertised community list is malformed due to GSHUT community

Last Modified

Oct 14, 2020

Products (1)

  • Cisco 2600 Series Multiservice Platforms

Known Affected Releases

15.2(4)S5.1 15.7(3)M 16.12.3

Description (partial)

Symptom:
BGP community list  is malformed in BGP update sent from a device (C1) to a peer (R4).
Correct community list is seen on the device itself.

How the device see the prefix:

C1#sh ip bgp 192.0.2.0
BGP routing table entry for 192.0.2.0/24, version 9
Paths: (2 available, best #2, table default)
  Advertised to update-groups:
     1          2         
  Refresh Epoch 1
  3
    10.0.13.3 from 10.0.13.3 (192.0.2.3)
      Origin incomplete, metric 0, localpref 100, valid, external
      Community: 3:1 3:2 3:3 3:4 3:5
      rx pathid: 0, tx pathid: 0
      Updated on Sep 8 2020 06:57:08 UTC
  Refresh Epoch 1
  2
    10.0.12.2 from 10.0.12.2 (192.0.2.2)
      Origin IGP, metric 0, localpref 100, weight 1000, valid, external, best
      Community: 2:1 2:2 2:3 gshut
      rx pathid: 0, tx pathid: 0x0
      Updated on Sep 8 2020 06:57:04 UTC

How the peer learns the prefix:
R4#sh ip bgp 192.0.2.0
BGP routing table entry for 192.0.2.0/24, version 14
Paths: (1 available, best #1, table default)
  Not advertised to any peer
  Refresh Epoch 1
  1 2
    10.0.14.1 from 10.0.14.1 (192.0.2.1)
      Origin IGP, localpref 100, valid, external, best
      Community: 3:1 3:2 3:3 3:4  <<<<
      rx pathid: 0, tx pathid: 0x0

Conditions:
1. The device learns a prefix from more than one peer (with communities).
2. One of the peers advertize gshut community.
3. Racing update seen from both peers.
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