Various OSPF Process

Answered Question
Apr 23rd, 2009
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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

Correct Answer by Giuseppe Larosa about 8 years 1 month ago

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


Correct Answer by Harold Ritter about 8 years 1 month ago

Pedro,


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


Regards

Correct Answer by Jon Marshall about 8 years 1 month ago

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

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Correct Answer
Jon Marshall Thu, 04/23/2009 - 05:41
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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 Thu, 04/23/2009 - 10:50
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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 Thu, 04/23/2009 - 11:42
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Laurent, so in that case, the router installs both routes?

Correct Answer
Harold Ritter Thu, 04/23/2009 - 11:49
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Pedro,


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


Regards

Correct Answer
Giuseppe Larosa Thu, 04/23/2009 - 11:55
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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


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