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Cisco Bug: CSCuw54969 - XR BGP default-originate behavior

Last Modified

Sep 18, 2018

Products (1)

  • Cisco Carrier Routing System

Known Affected Releases

5.1.1.BASE

Description (partial)

Symptom:
In XR, when there's both default-originate config and a default
prefix in the BGP table, the current behavior is that the latest trigger
takes effect i.e. if the default route is learnt newly, we advertise out that
route and similarly, if default-originate config is applied newly, we
advertise out the locally originated default route.

However if you config default-originate with its own policy
(i.e. default-originate policy, not neighbor outboud policy),
then the locally originate default route is advertised out periodically
(every minute).

Additionally to that behavior, the output of "show bgp neighbor X.X.X.X
advertised-routes" always display "LOCAL" origin, no matter if the device is sending the learned prefix or the locally originated default route.

Conditions:
We have the following topology:

AS 7018                   AS 30036                  AS 7018
11.11.11.2         11.11.11.1    12.249.52.14    12.249.52.13
7604-A   ———————————————  ASR9001-I ——————————————— 6504-A (this device simulates 2 BGP neighbors)
                                 12.249.52.18    12.249.52.17

6504-A is sending a default route to ASR9001-I device:


RP/0/RSP0/CPU0:ASR9001-I#sh bgp ipv4 u 0.0.0.0/0
Mon Oct  5 21:55:30.170 UTC
BGP routing table entry for 0.0.0.0/0
Versions:
  Process           bRIB/RIB  SendTblVer
  Speaker                123         123
Last Modified: Oct  5 18:01:41.554 for 03:53:48
Paths: (2 available, best #1)
  Advertised to update-groups (with more than one peer):
    0.1 
  Path #1: Received by speaker 0
  Advertised to update-groups (with more than one peer):
    0.1 
  7018
    12.249.52.13 from 12.249.52.13 (192.168.1.37)
      Origin IGP, metric 0, localpref 100, valid, external, best, group-best, multipath, import-candidate
      Received Path ID 0, Local Path ID 1, version 123
      Origin-AS validity: not-found
  Path #2: Received by speaker 0
  Not advertised to any peer
  7018
    12.249.52.17 from 12.249.52.17 (192.168.1.37)
      Origin IGP, metric 0, localpref 100, valid, external, multipath, import-candidate
      Received Path ID 0, Local Path ID 0, version 0
      Origin-AS validity: not-found
	  
	  
If we add the default-originate to the 11.11.11.2 neighbor configuration, we can see that the ASR9001-I is sending his locally originated default route to the 7604-A device:

RP/0/RSP0/CPU0:ASR9001-I#sh bgp neighbor 11.11.11.2 advertised-routes 
Mon Oct  5 22:08:24.849 UTC
Network            Next Hop        From            AS Path
0.0.0.0/0          0.0.0.0         Local           i

Processed 1 prefixes, 1 paths

7606-A#sh ip bgp ipv4 u 
BGP table version is 2, local router ID is 11.11.11.2
Status codes: s suppressed, d damped, h history, * valid, > best, i - internal,
              r RIB-failure, S Stale, m multipath, b backup-path, x best-external, f RT-Filter, a additional-path
Origin codes: i - IGP, e - EGP, ? - incomplete

   Network          Next Hop            Metric LocPrf Weight Path
r> 0.0.0.0          11.11.11.1                             0 30036 i


According to the XR BGP default-originate behavior, if we flap the 12.249.52.13 adjacency, his default route will be newest than the locally originated default route, then the ASR9001-I will advertise to 7604-A the default route from 12.249.52.13, but the output of "show bgp neighbor X.X.X.X advertised-routes" always display "LOCAL" origin, no matter if the device is sending the learned prefix or the locally originated default route.


7606-A# <<<< With debug ip bgp updates enabled and restarting the adjacency between ASR9001-I and 12.249.52.13 device:

*Oct  5 17:39:17.047: BGP(0): 11.11.11.1 rcv UPDATE w/ attr: nexthop 11.11.11.1, origin i, originator 0.0.0.0, merged path 30036 7018, AS_PATH , community , extended community , SSA attribute 	<<<< ASR9001-I is sending the default route learned from 12.249.52.13 (with AS 7018)
*Oct  5 17:39:17.047: BGPSSA ssacount is 0
*Oct  5 17:39:17.047: BGP(0): 11.11.11.1 rcv UPDATE about 0.0.0.0/0 -- DENIED due to: AS-PATH contains our own AS;
*Oct  5 17:39:17.367: BGP(0): no valid path for 0.0.0.0/0
*Oct  5 17:39:17.367: BGP: topo global:IPv4 Unicast:base Remove_fwdroute for 0.0.0.0/0
7606-A#


But on the ASR9001-I, the "show bgp neighbor X.X.X.X advertised-routes" always display "LOCAL" origin, no matter if the device is sending the learned prefix or the locally originated default route:


RP/0/RSP0/CPU0:ASR9001-I#sh bgp neighbor 11.11.11.2 advertised-routes 
Mon Oct  5 22:15:16.246 UTC
Network            Next Hop        From            AS Path
0.0.0.0/0          0.0.0.0         Local           i

Processed 1 prefixes, 1 paths
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