OSPF Costing, Fast Ethernet interfaces and input/output counters

Answered Question
Jul 11th, 2007

Dear all,

I have 2 ethernet interfaces on a Cisco 3640 with 1 interface operating as backup with the OSPF costing configured as such. Both ethernet interfaces are plugged into some 3560 switches but there is no ospf cost configured on either ethernet there. When I look at the interfaces on the roouter, I can see traffic coming on on the backup router ethernet interface on the 3640 but leaving the primary ethernet interface on the router.

FA0/0 on Cisco 3640 = Backup

FA3/0 on Cisco 3640 = Primary

Does anyone have any ideas?

This router is runnign 12.1(15) release IOS.

Thanks in advance

Mary

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Correct Answer by Harold Ritter about 9 years 6 months ago

Only the cost of the outgoint interface is taken in consideration by the SPF. Since no cost are configured on the FastEthernet interfaces of the 3560, it sees both path as equal and uses per-destination load-balancing (src and dst IP) by default.

You can use the "show ip cef exact-route " to validate, which link will be taken by a given flow from the 3560 to the 3640.

Hope this helps,

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Edison Ortiz Wed, 07/11/2007 - 09:28

Can you verify the OSPF cost for the traffic coming via the backup interface ? If it's the same or lower than the primary interface, then you need to address this issue from the device sending the traffic to you.

Correct Answer
Harold Ritter Wed, 07/11/2007 - 11:03

Only the cost of the outgoint interface is taken in consideration by the SPF. Since no cost are configured on the FastEthernet interfaces of the 3560, it sees both path as equal and uses per-destination load-balancing (src and dst IP) by default.

You can use the "show ip cef exact-route " to validate, which link will be taken by a given flow from the 3560 to the 3640.

Hope this helps,

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