multiple ospf processes, single router-id

[hrtendrup]

In the past, different ospf processes on the same router would have required different router-ids:

v 15.5 - IOS classic:
Router(config-router)#do sho run | sec router ospf
router ospf 101
router-id 10.0.0.1
network 10.0.0.1 0.0.0.0 area 1
Router(config-router)#
Router(config-router)#
Router(config-router)#router ospf 102 vrf other
Router(config-router)#router
*Mar 14 03:28:08.312: %OSPF-4-NORTRID: OSPF process 102 failed to allocate unique router-id and cannot start
Router(config-router)#router-id 10.0.0.1
% OSPF: router-id 10.0.0.1 in use by ospf process 101
Router(config-router)#

in newer IOS-XE, the router doesn't seem to care if you try to assign the same RID to different processes:

v 17.12
router-iosxe# show run | sec router ospf
router ospf 100 vrf aci
router-id 192.168.78.1
passive-interface Loopback99
network 172.21.0.0 0.0.255.255 area 4
router ospf 101
router-id 192.168.78.1
area 0.0.0.1 nssa default-information-originate
network 169.254.100.40 0.0.0.7 area 0.0.0.1
router-iosxe#
router-iosxe# !! both ospf processes are up and routing traffic - fat, dumb, and happy
router-iosxe#show ip ospf | in ID
Routing Process "ospf 101" with ID 192.168.78.1
Routing Process "ospf 100" with ID 192.168.78.1

Does anyone know when this changed? Was it a change when Cisco started to transition to IOSXE? In theory, the RID being the same wouldn't matter if the 2 routing domains never overlapped or touched (excluding redistribution/route leaking), but Cisco used to prevent this, so I'm just curious about when/why this changed.

[Georg Pauwen]

Hello,

I think you are on the right track. As far as I recall, in order to improve the scalability and flexibility of IOS-XE for modern networks, especially with the growing adoption of SDN, network virtualization, and multi-tenancy features (like VRFs), that restriction was relaxed. You may have different virtualized routing instances that don’t interact with each other directly, and having the same router-ID in such cases does not create a conflict.

[M02@rt37]

Hello @hrtendrup 

To go further, the shift happened when cidco started decoupling OSPF processes within the FIB and RIB, allowing independent routing instances per VRF without enforcing unique router ids. This aligns with cisco’s broader Linux-based ios-xe design, where routing processes are more isolated than in the older monolithic classic ios.