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OSPF and Secondary Subnets

We use OSPF as our interior routing protocol and BGP as our exterior routing protocol. The core router is a Cisco 7507 with IOS 12.2.7a . So far, all OSPF neighbors have been on the same area and on the same subnet as the primary address of the router's fast ethernet interface. A subnet was added recently where the only connectivity is via a secondary address on the router's fast ethernet interface. There will be an OSPF neighbor on that subnet.

From what I read on the Cisco site, secondary addresses on router interfaces are considered stub networks so no OSPF hello packets are sent on those networks. Does this mean that ISL or 802.1q subinterfaces would be required to redistribute OSPF information to neighbors on multiple subnets via a single router interface ? If so, should Etherchannel (port-channel) be used for the subinterfaces to simplify adding more fast ethernet interfaces in the future ? The core switch is a Catalyst 6509 with native IOS and it supports channel-groups.



Re: OSPF and Secondary Subnets

I assume you are using a layer 2 6509...with 7507 being the external router. Since you have added a new subnet, as secondary, ospf wont send hellos on stub networks such as the secondary network. You can create two subinterfaces, on the fast ethernet, interface, and also create two vlans, one each for each subnet, and make the 7507 route between these vlans. You need to enable ISL trunking or dot1q between the 6500 and 7500. You dont need port channel for this. You can add as many subinterfaces as you want (practically speaking, limited by memory of router). Once this is over, you can add the new neighbor to the newly created vlan, and things would be good to go.

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