Default route update on Nexus 7K

[cheery Tomato]

Default route update on Nexus 7K

As per the attached picture.

I have 4 Nexus 7k which are connected via eBGP.

Nexus 3 & 4 advertise the default route to the Nexus 1 & 2

Nexus 1 & 2 are also connected to Router 1 & 2

Router 2 is advertising its default routes into EIGRP

Nexus 1 & 2 have a default route via eBGP to Nexus 3 & 4 in the routing table

and a default route in the EIGRP topology table to Router 2

eBGP default route with AD 20

EIGRP default route with AD 170

The connection to Nexus 3 & 4 is cut

Now the default is to Router 2 via EIGRP

The eBGP connection to Nexus 3 & 4 is restored

However the eBGP default does not take over from the EIGRP default route in the IP routing table.

I had to clear the ip routing tables default route to get the eBGP defalt route to take over.

Has anyone seen this behaviour, this seems to be a bug to me.

I would expect the IP routing table to update when the better route is avilable via eBGP when the connection is restored.

As always any help is greatly appreciated.

[Francesco Molino]

Hi

Can you share the output of the following commands (in the scenario where default route is learned over eigrp and the ebgp isn't installed):
- sh ip bgp
- sh ip bgp 0.0.0.0

[cheery Tomato]

This is after the incident now but I am suseptable to it happening again.

So this is cut from my logs at the time when the incident occurred

!

Nexus3# sh ip bgp 0.0.0.0
BGP routing table information for VRF default, address family IPv4 Unicast
BGP routing table entry for 0.0.0.0/0, version 190443
Paths: (5 available, best #4)
Flags: (0x080012) on xmit-list, is in urib, is backup urib route

Path type: external, path is valid, not best reason: AS Path
AS-Path: 65530 64513 , path sourced external to AS
10.224.1.182 (metric 0) from 10.224.1.182 (10.224.1.182)
Origin IGP, MED not set, localpref 100, weight 0

Path type: internal, path is valid, not best reason: RR Cluster Length
AS-Path: NONE, path sourced internal to AS
10.8.255.35 (metric 130816) from 10.60.127.13 (10.60.127.13)
Origin IGP, MED 1707264, localpref 100, weight 0
Originator: 10.8.255.35 Cluster list: 10.60.127.13

Path type: internal, path is valid, not best reason: Neighbor Address
AS-Path: NONE, path sourced internal to AS
10.8.255.35 (metric 130816) from 10.224.16.2 (10.60.127.13)
Origin IGP, MED 1707264, localpref 100, weight 0
Originator: 10.8.255.35 Cluster list: 10.60.127.13

Advertised path-id 1
Path type: internal, path is valid, is best path
AS-Path: NONE, path sourced internal to AS
10.8.255.35 (metric 130816) from 10.8.255.35 (10.8.255.35)
Origin IGP, MED 1707264, localpref 100, weight 0

Path type: external, path is valid, not best reason: AS Path
AS-Path: 64513 , path sourced external to AS
10.225.0.69 (metric 0) from 10.225.0.69 (10.8.255.6)
Origin IGP, MED not set, localpref 100, weight 0

Path-id 1 advertised to peers:
10.60.127.13 10.60.127.16 10.224.1.182 10.224.16.2
10.225.0.69 10.231.1.13
!
Nexus3# sh ip route 0.0.0.0
IP Route Table for VRF "default"
'*' denotes best ucast next-hop
'**' denotes best mcast next-hop
'[x/y]' denotes [preference/metric]
'%<string>' in via output denotes VRF <string>

0.0.0.0/0, ubest/mbest: 1/0
*via 10.50.244.10, Eth3/18, [170/1707008], 2d03h, eigrp-1, external
via 10.8.255.35, [200/1707264], 01:24:39, bgp-64551, internal, tag 64551
!
Nexus3# clear ip route 0.0.0.0
Clearing 0.0.0.0/32
!
Nexus3# sh ip route 0.0.0.0
IP Route Table for VRF "default"
'*' denotes best ucast next-hop
'**' denotes best mcast next-hop
'[x/y]' denotes [preference/metric]
'%<string>' in via output denotes VRF <string>

0.0.0.0/0, ubest/mbest: 1/0
*via 10.225.0.69, [20/0], 00:04:32, bgp-64551, external, tag 64513
via 10.50.244.10, Eth3/18, [170/1707008], 00:01:29, eigrp-1, external

[Francesco Molino]

Can you run the sh ip bgp 0.0.0.0 when the ebgp route is installed. There's something that doesn't match your design and just want to validate please.

Also can you share the config of your bgp and eigrp config just in case.

[cheery Tomato · Accepted solution]

I added a local preference to route via the preffered path to fix the issue immediately.

Afterwards I found a BGP network 0.0.0.0 statement that then caused the default to originate on the offending router.

Removing this the issue was resolved for sure but have left the Local Preference in place anyway.

Cheers for the assistance.