Does "different OSPF domain" = different OSPF process on same router?

[a1111]

Hello,

Can someone please help me understand this?

On page 246/1074 of the PDF (or 201 on the printed page) of the ENCOR OCG, there's the following:

"External routes are routes learned from outside the OSPF domain but injected into an OSPF domain through redistribution. External OSPF routes can come from a different OSPF domain or from a different routing protocol."

What does "different OSPF domain" mean? Is it the same as different OSPF process on same router? So if a router has two different OSPF processes running, and it redistributes routes from one to the other, then that is an example of an external route? 

Thank you.

[MHM Cisco World · Accepted solution]

Yes domain is same as process' 

Router with two process meaning it have two domain and you need redistrubte one process to other 

MHM

[MHM Cisco World]

I need to mention to you that there is some issue have multi process of ospf 

Check below link

https://www.cisco.com/c/en/us/support/docs/ip/open-shortest-path-first-ospf/4170-ospfprocesses.html

MHM

[MHM Cisco World]

then that is an example of an external route? 

External prefix in ospf any prefix 

Redistrubte from 

Eigrp

Bgp (ibgp/ebgp)

Static 

Connect 

Other ospf process 

All above appear ad external and ypu can see it by 

Show ip ospf database external 

MHM

[Richard Burts · Accepted solution]

The OP asks "What does "different OSPF domain" mean? Is it the same as different OSPF process on same router? So if a router has two different OSPF processes running, and it redistributes routes from one to the other, then that is an example of an external route?". In this context yes each running OSPF process is its own domain. And yes if  it redistributes routes from one to the other that would be an example of an external route.

[Joseph W. Doherty]

Nice reference.

In the section, Keep Different OSPF Domains Separate , mention is made to use BGP to avoid one OSPF domain's instability impacting another domain's when doing redistribution.  This is true.  However, mention isn't made that when you redistribute between OSPF domains, at least all you attributes are the same as the same routing protocol is being used.  So, it's a bit of a judgement call whether something like BGP should be used, for that purpose, alone.

[David Ruess · Accepted solution]

Hello,

It "can" be. Yes, if you configure 2 OSPF process on the same router the router will see those as separate routing domains and will not mix the routes until you mutually redistribute. This is only valid if 2 processes are configured on the same router as processes are locally significant so 2 processes on separate routers have no functional different and OSPF will treat it as a single routing domain when forming neighbor adjacencies and exchange routes like normal.

However, in the context of the OCG you're reading it likely means like another OSPF process owned by another entity being redistributed into yours. But I'd have to see the explanation in question.

Hope this helps

-David

[Joseph W. Doherty]

As others have already noted, multiple OSPF processes, on the same router (or L3 switch), would be different OSPF domains.

Basically, an OSPF domain is where you don't need to explicitly manual redistribute.  Or, there's a chain of OSPF neighbors sharing topology.

Something to keep in mind, OSPF process numbers don't need to match between routers.  So you could have, between two OSPF routers:

R1 OSPF process 10 (domain X) R2 OSPF process 10
R1 OSPF process 20 (domain Y) R2 OSPF process 20
R1 OSPF process 30 (domain Z) R2 OSPF process 30

or

R1 OSPF process 10 (domain X) R2 OSPF process 20
R1 OSPF process 20 (domain Y) R2 OSPF process 10
R1 OSPF process 30 (domain Z) R2 OSPF process 90

The latter, above, tends to be very confusing, so we normally use the same process number between routers.

In either of the above, without using VRFs, the router would route packets between the 3 domains, without redistribution.

for either:

R1 OSPF process 10 (domain X) R2 OSPF process 10
R1 OSPF process 20 (domain Y) R3 OSPF process 20
R1 OSPF process 30 (domain Z) R4 OSPF process 30

or

R1 OSPF process 10 (domain X) R2 OSPF process 20
R1 OSPF process 20 (domain Y) R3 OSPF process 10
R1 OSPF process 30 (domain Z) R4 OSPF process 90

Routers 2, 3 and 4 only know of the routes within one domain, unless on R1, you do redistribution.