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
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,