OSPF Process ID

[Sagar4]

I want to know everything about OSPF process ID.

[balaji.bandi · Accepted solution]

The OSPF process-id is a numeric value local to the router. It does not have to match process-ids on other routers.

It is possible to run multiple OSPF processes on the same router, but is not recommended as it creates multiple database instances that add extra overhead to the router.

You can refer below Cisco OSPF guide :

https://www.cisco.com/c/en/us/support/docs/ip/open-shortest-path-first-ospf/7039-1.html

[M02@rt37]

Hello @Sagar4 

The OSPF process ID is a locally significant identifier used to distinguish between multiple OSPF processes running on the same router. 

To go further, the OSPF process ID is a local parameter used on a router to identify different OSPF processes running on that router. It helps distinguish between multiple instances of OSPF on the same device.

Routers fo not need to have the same OSPF process ID. The OSPF process ID is a numeric value ranging from 1 to more than 4,000,000,000. 

Note that changing the OSPF process ID requires restarting the OSPF process. This can impact OSPF neighborships and routing convergence. Do that the during maintenance windows and under control.

Other thread : https://community.cisco.com/t5/switching/ospf-process-id/td-p/1405396

[MHM Cisco World]

Opsf process ID'

One router have two ospf with different process ID then this router have two separate ospf database' and each have it spf.

This router need to redistrubte prefix between two ospf process to advertise prefix to other router.

Why we use two ospf process in one router? To overriding some ospf path selection criteria

MHM

[Joseph W. Doherty]

In addition to what @balaji.bandi and M02@rt37 already described, from the local router's perspective, a the OSPF process ID also is an OSPF AS.  OSPF routers only share LSAs with other OSPF routers within the same process ID.

For example:

RTR1 OSPF process ID #1 <AS X> OSPF process ID #1 RTR2 OSPF process ID #2 <AS Y> OSPF process ID #1 RTR3

RTR1 and RTR2 share an OSPF AS "X"

RTR2 and RTR3 share an OSPF AS "Y"

RTR process ID do not need to agree for the same OSPF AS

RTR2 needs to use two different OSPF process IDs, to keep the ASs separate (but its routing table will show both ASs routes, not true for RTR1 or RTR3 unless you do redistribution on RTR2)

If the above is a bit confusing, it's very similar too:

RTR1 EIGRP ID #1 <AS X> EIGRP ID #1 RTR2 OSPF process ID #2 <AS Y> OSPF process ID #1 RTR3

But we're using the same IGP, OSPF, more than once on RTR2, rather than two different IGPs.

If you wonder why have two OSPF ASs, possible for scaling or setting an administrative boundary.