two ospf processes and network

Answered Question
Aug 15th, 2009
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Hi every body

Let say we have router running two ospf processes, ospf 1 and ospf 2.


Router is receiving update about a network 10.0.0.0 with the same cost, from both processes.


Will router install the route to 10.0.0.0 from both processes in its routing table ?


Thanks and have a nice weekend.

Correct Answer by Lucien Avramov about 7 years 9 months ago

As soon as there will be a bouncing of the ospf process, the router-id will be used to determine which one is prefered.


After a clear ip ospf process, it seems to use the routing id to decide the preferred route of what I can see: the highest routing id.


Correct Answer by Lucien Avramov about 7 years 9 months ago

two exact same networks can not coexist in the routing table, there will be an election process if the cost is the same, and I believe that is with the routing-id value.


If you want to receive updates from both, you need to be more specific in the network, for example, you can send a /27 or /32 instead and that will show in the routing table. If at least one is more specific it will work:


R5#sho ip route ospf

10.0.0.0/8 is variably subnetted, 2 subnets, 2 masks

O IA 10.0.0.0/24 [110/2] via 1.1.1.6, 00:00:09, FastEthernet0/0

O 10.0.0.4/32 [110/2] via 2.2.2.4, 00:00:09, FastEthernet0/1



Or if both are more specific as well:


R5#sho ip route ospf

10.0.0.0/32 is subnetted, 2 subnets

O IA 10.0.0.6 [110/2] via 1.1.1.6, 00:00:12, FastEthernet0/0

O 10.0.0.4 [110/2] via 2.2.2.4, 00:00:22, FastEthernet0/1



Here i used 2 ospf process and both in different areas, hen you see the IA for one route.


Only 1 route installed for the /24:

R5#show ip route ospf

10.0.0.0/24 is subnetted, 1 subnets

O 10.0.0.0 [110/2] via 2.2.2.4, 00:00:20, FastEthernet0/1




Correct Answer by Jon Marshall about 7 years 9 months ago

Sarah


No it won't install both routes, it will only install one of them and it will usually be the one it receives first.


Jon

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Correct Answer
Jon Marshall Sat, 08/15/2009 - 09:04
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Sarah


No it won't install both routes, it will only install one of them and it will usually be the one it receives first.


Jon

Correct Answer
Lucien Avramov Sat, 08/15/2009 - 09:25
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As soon as there will be a bouncing of the ospf process, the router-id will be used to determine which one is prefered.


After a clear ip ospf process, it seems to use the routing id to decide the preferred route of what I can see: the highest routing id.


Giuseppe Larosa Sun, 08/16/2009 - 12:04
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Hello Lucien,


I agre with Jon first process that converges will have its route installed in the routing table the OSPF process-id should play no role here.


different ospf processes are treated as "ships in the night" and no OSPF type of route is considered by the IP router maintaner process only AD and metric.


Hope to help

Giuseppe


Lucien Avramov Sun, 08/16/2009 - 12:40
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I said routing id not process id. Play in the lab with 3 routers and you will see it.

Correct Answer
Lucien Avramov Sat, 08/15/2009 - 09:20
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two exact same networks can not coexist in the routing table, there will be an election process if the cost is the same, and I believe that is with the routing-id value.


If you want to receive updates from both, you need to be more specific in the network, for example, you can send a /27 or /32 instead and that will show in the routing table. If at least one is more specific it will work:


R5#sho ip route ospf

10.0.0.0/8 is variably subnetted, 2 subnets, 2 masks

O IA 10.0.0.0/24 [110/2] via 1.1.1.6, 00:00:09, FastEthernet0/0

O 10.0.0.4/32 [110/2] via 2.2.2.4, 00:00:09, FastEthernet0/1



Or if both are more specific as well:


R5#sho ip route ospf

10.0.0.0/32 is subnetted, 2 subnets

O IA 10.0.0.6 [110/2] via 1.1.1.6, 00:00:12, FastEthernet0/0

O 10.0.0.4 [110/2] via 2.2.2.4, 00:00:22, FastEthernet0/1



Here i used 2 ospf process and both in different areas, hen you see the IA for one route.


Only 1 route installed for the /24:

R5#show ip route ospf

10.0.0.0/24 is subnetted, 1 subnets

O 10.0.0.0 [110/2] via 2.2.2.4, 00:00:20, FastEthernet0/1




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