If you are
using Nexus 5500 as a router, watch out for a routing anomaly. Otherwise, you
may spend a lot of time debugging, troubleshooting becomes even more challenging
when the other router is another vendor’s product.
In a typical
scenario when a router is attached to a pair of Nexus 5500, and OSPF adjacency is
to be established, the best practice is to run layer 3 Non VPC VLANs on a separate
link from VPC trunk, and enable peer-gateway. However, the standard practice will
result in OSPF adjacency established for directly connected Nexus 5k only, while
the remote Nexu 5k remain in EXSTART.
Turns out,
this is due to a little known Cisco
bug. As a result, Cisco clearly states “separate link for nonvpc vlans on
n5k is not supported”.
Currently
the fix release is still pending. So a workaround is required, which is to run
both VPC and non-VPC VLANs on VPC peer link.
Before
change:
5548-sw1# sh
ip ospf nei
OSPF Process ID 1 VRF default
Total number of neighbors: 2
Neighbor ID
Pri State Up
Time Address Interface
10.147.187.20 128 EXSTART/DR 00:10:14
10.147.187.20 Vlan110
10.147.254.161 129
FULL/BDR 00:10:12 10.147.187.19
Vlan110
Making
change (both Nexus 5k):
5548-sw1# conf t
Enter configuration commands, one per
line. End with CNTL/Z.
5548-sw1(config)# int po10
5548-sw1(config-if)# switchport trunk
allowed vlan 110
Must reset
OSPF on interface for change to take effect:
5548-sw1(config)# int vlan 110
5548-sw1(config-if)# ip ospf shut
5548-sw1(config-if)# no ip ospf shut
5548-sw1# sh ip ospf nei
OSPF Process ID 1 VRF default
Total number of neighbors: 2
Neighbor ID
Pri State Up
Time Address Interface
10.147.187.20 128 FULL/DR
00:00:02 10.147.187.20 Vlan110
10.147.254.161 129
FULL/BDR 00:00:02 10.147.187.19
Vlan110
