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Sunday, August 15, 2010

How do we failover from serial to Ethernet?

We have two sites connected via a Point to Point T1 link. In each side of the point to point link we have two different subnets (i.e. and We have a a full copy of each other on each side of the link (just in case it goes down). At both ends of the PTP link we have a cisco router. If we have both the serial and the ethernet interfaces plugged in at the same time, the router will always use the Ethernet interface, because it is a directly connected network. Can we set it up with two paths to the same place (ethernet and serial) and leave the ethernet connection disconnected until it needs to be up? In other words, our serial link goes down, so we plug in our ethernet interface to the standby network and bring it up. -- RTR ======T1/Ethernet====== RTR --

* If the network is as above, you do not need to disconnect the Ethernet cable. You can use static routes and force the traffic to go over T1 and use Ethernet only when T1 goes down:
ip route 192.168.x.0 254

* Now, since the ethernet metric (cost) is higher than the T1, it will be used only when T1 is down. Alternatively, you can also use a routing protocol and increase the cost of the Ethernet link. -- RTR -
Ethernet (100.1.1.x)-- RTR --

* If the direct link is like above, then yes, you can configure the routing protocol like RIP/EIGRP/OSPF and force the traffic to go through the other link.

* If it is directly connected network, then you cannot force the router to send the traffic over T1 link. What was suggested is possible if the router is routing over Ethernet link as well (Ethernet link should have address other than 192.168.x.x address).

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