I have redundant DS3 circuits between two sites (see diagram). Each site (A & B) has redundant switches and routers. Certain hosts have redundant NICs (with an internal failover mechanism).
I'm using EIGRP on all routers. HSRP is configured such that router "1" at each site is the respective LAN's default gateway. This causes traffic from LAN A to LAN B (and vice-versa) to prefer DS3 #1 between routers A1 and B1. By the way, DS3 #2 does handle some other traffic. So, it's not sitting idle.
Recently switch B1 failed (a UPS died). I would have expected router B1 to immediately begin routing traffic destined for LAN B out the interface connected to router B2. But it took a good deal of time to converge.
Upon investigation I realized the FD of router B1's original route to switch B1 is the exact same metric as the AD from router B2. So, the route advertised through B2 was not considered a feasible successor. And, B1 went active on the route for LAN B instead of immediatly using the route through B2.
I can overcome this by lowering the delay on router B2's interface to switch B2. That will lower the AD to B1 such that B1 considers the route a feasible successor.
The only thing I don't like is that it's a one way street. In other words, router B1 will have two successors for LAN B, but router B2 will still only have one successor. I'd like the redundancy to be a bit more symmetrical. Any suggestions?