Pat,
Maybe I'm not explaining myself well enough
You have a branch router and a router at HQ. My understanding, taking these 2 sites alone, is that you run eigrp between these two locations. Assuming that you're advertising every subnet between the two locations, then a default route would be needed on the branch router pointing to HQ (assuming that you are providing an egress point for internet access for the remote branch). But all of the other routes are eigrp learned, so for the failover to happen, eigrp is smart enough to know that the link it was using to get to the HQ site is now down and it will have to route over the T1. The default route that you'll install on the remote site though would point toward your ds3 circuit and then you would do a floating route for the same route pointing to the t1. If the ds3 were to go down, all of your routes that were eigrp learned and the floating default will traverse the t1 now:
R1 = branch
R2 = HQ
R1 ----- R2
R1#sh run | in ip route
ip route 0.0.0.0 0.0.0.0 172.12.0.2
ip route 0.0.0.0 0.0.0.0 192.168.12.2 254 <--- Floating default route
R1#sh run | s router eigrp
router eigrp 100
network 172.12.0.1 0.0.0.0
network 192.168.12.1 0.0.0.0
no auto-summary
Below is the topology table. Notice I have 2 paths for each route. S0/1 is set at 45Mb (DS3) and S0/0 is 1544 (T1) S0/1 is the successor at a FD of 696832 for 2.2.2.0/24 (meaning it wins the right to be in the routing table):
R1#sh ip eigrp top
IP-EIGRP Topology Table for AS(100)/ID(192.168.12.1)
Codes: P - Passive, A - Active, U - Update, Q - Query, R - Reply,
r - reply Status, s - sia Status
P 2.2.2.0/24, 1 successors, FD is 696832
via 172.12.0.2 (696832/128256), Serial0/1
via 192.168.12.2 (2297856/128256), Serial0/0
P 22.22.22.0/24, 1 successors, FD is 696832
via 172.12.0.2 (696832/128256), Serial0/1
via 192.168.12.2 (2297856/128256), Serial0/0
The routing table on R1. Notice there is only 1 route pointing to s0/1 (DS3) because of it being the successor. Also notice the static route for the default pointing to 172.12.0.2: (The 2 bolded routes are the ones that we'll focus on.)
C 192.168.12.0/24 is directly connected, Serial0/0
2.0.0.0/24 is subnetted, 1 subnets
D 2.2.2.0 [90/696832] via 172.12.0.2, 00:07:48, Serial0/1
172.12.0.0/24 is subnetted, 1 subnets
C 172.12.0.0 is directly connected, Serial0/1
22.0.0.0/24 is subnetted, 1 subnets
D 22.22.22.0 [90/696832] via 172.12.0.2, 00:07:48, Serial0/1
S* 0.0.0.0/0 [1/0] via 172.12.0.2
I'll shut the DS3 interface to simulate a failure:
R1(config)#int s0/1
R1(config-if)#shut
R1(config-if)#
*Mar 1 00:15:35.979: %DUAL-5-NBRCHANGE: IP-EIGRP(0) 100: Neighbor 172.12.0.2 (Serial0/1) is down: interface down
R1(config-if)#
*Mar 1 00:15:37.959: %LINK-5-CHANGED: Interface Serial0/1, changed state to administratively down
*Mar 1 00:15:38.959: %LINEPROTO-5-UPDOWN: Line protocol on Interface Serial0/1, changed state to down
R1(config-if)#
Now look at the routing table:
R1#sh ip route
Gateway of last resort is 192.168.12.2 to network 0.0.0.0
C 192.168.12.0/24 is directly connected, Serial0/0
2.0.0.0/24 is subnetted, 1 subnets
D 2.2.2.0 [90/2297856] via 192.168.12.2, 00:00:55, Serial0/0
22.0.0.0/24 is subnetted, 1 subnets
D 22.22.22.0 [90/2297856] via 192.168.12.2, 00:00:55, Serial0/0
S* 0.0.0.0/0 [254/0] via 192.168.12.2
So when the interface goes down, the floating default will get put in the routing table because the next hop that it was originally pointing to is no longer available due to the interface that it would use to get to it being down. Also notice that 2.2.2.0 is now pointing to the other interface s0/0 (T1). If you look at the topology table for eigrp now, you'll see that the original route is no longer in the table:
P 2.2.2.0/24, 1 successors, FD is 696832
via 192.168.12.2 (2297856/128256), Serial0/0
P 22.22.22.0/24, 1 successors, FD is 696832
via 192.168.12.2 (2297856/128256), Serial0/0
P 192.168.12.0/24, 1 successors, FD is 2169856
via Connected, Serial0/0
But, when I bring that interface back up it will get reentered into the table and it's preferred again:
R1#sh ip eigrp top
IP-EIGRP Topology Table for AS(100)/ID(192.168.12.1)
Codes: P - Passive, A - Active, U - Update, Q - Query, R - Reply,
r - reply Status, s - sia Status
P 2.2.2.0/24, 1 successors, FD is 696832
via 172.12.0.2 (696832/128256), Serial0/1
via 192.168.12.2 (2297856/128256), Serial0/0
P 22.22.22.0/24, 1 successors, FD is 696832
via 172.12.0.2 (696832/128256), Serial0/1
via 192.168.12.2 (2297856/128256), Serial0/0
Oh, and just to show you the floating route concept, you'll also notice that since I brought that interface back up, the floating default that we had is now no longer the default route; it's the original one that we had at first pointing to 172.12.0.2 because of its lower AD (1 vs 254):
R1#sh ip route | inc 0.0.0.0
Gateway of last resort is 172.12.0.2 to network 0.0.0.0
S* 0.0.0.0/0 [1/0] via 172.12.0.2
HTH,
John
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