Various OSPF Process

[p.diaz]

Hi.

I need to clarify the concept of configuring various OSPF process. When I cofigure two or more OSPF process I start various instances of SPF algorithm? or the same instance runs to all the process? They are completely independently at all? Thank you

[Jon Marshall · Accepted solution]

Pedro

Each OSPF process will have it's own LSA's and run it's own SPF calculation and the demand on the router will increase.

Jon

[Laurent Aubert]

Hi,

Another thing to consider is OSPF route type selection algorithm doesn't apply across several OSPF processes.

The router sees them as two different routing protocols using the same Administrative Distance.

So if you receive first an LSA Type-5 for a prefix from one process, it will be installed in the RIB. If later you receive a LSA Type-3 for the same prefix from the other process, the RIB will not be updated.

HTH

Laurent.

[p.diaz]

Laurent, so in that case, the router installs both routes?

[Harold Ritter · Accepted solution]

Pedro,

Assuming both processes are using the administrative distance (110), the first process to install its route in the RIB will win.

Regards

[Giuseppe Larosa · Accepted solution]

Hello Pedro,

they are completely independent as explained by Jon.

It is a "ships in the nigh" approach.

The two ospf processes can be in competition in providing a route for a given prefix and as explained by Laurent the decision taken from the IP routing table mantainer is not based on OSPF route type but only on timing.

To handle this kind of scenario with multiple OSPF processes and some level of prefix overlapping you can use the distance ospf command under router ospf to differentiate AD for internal, inter-Area and external routes

distance ospf {external dist1 | inter-area dist2 | intra-area dist3}

Also OSPF default routes are affected by this kind of competition between processes.

see

http://www.cisco.com/en/US/docs/ios/iproute/command/reference/irp_osp1.html#wp1013195

Hope to help

Giuseppe