Weighting interfaces

Depends on which routing protocol you are using. I suspect you are using static routes, and if you are this is even simpler. You will have one static route pointing to the next hop on the FastEthernet, and another pointing to out the serial with a "weight" on the route. Assuming that

ip route 0.0.0.0 0.0.0.0 10.1.1.2 100 ip route 0.0.0.0 0.0.0.0 10.2.1.2 110

The last number is a "weight", and the one with the lowest number is used if a route to the next hop is up. This is called a floating static.

For the avoidance of doubt the "weight" Thrill5 refers to is called "administrative distance" by Cisco. The above will work if the FastEthernet 0/0 interface goes down (or you manually shut it down) when the high speed link is unavailable (you didn't say how temporary it's going to be). If the interface doesn't go down when the service is unavailable you have to do more complex things.

Sam

~ I have a Cisco 2651XM router with the standard FastEthernet interfaces ~ 0/0 and 0/1, four smart serial interfaces, and 16 Switch ports. ~ Currently I'm using one serial interface for a low data rate (256kbps ~ duplex) link. I will soon have a faster, but temporary (sometimes ~ I'll have it and sometimes I won't), connection to the outside world ~ that I want to hook up to FastEthernet 0/0. I want the serial ~ interface to be the default fall back connection. When the IP Satcom ~ modem (which provides the high speed solution) is plugged into FE 0/0 ~ and connects, I would like all computers connected to the Ethernet ~ Switch ports to utilize this high speed data link. ~ ~ The computers connected to the switch ports will have a default ~ gateway pointing to a VLAN interface that won't change regardless of ~ the external interface in use. ~ ~ Anyone know if there is a way to "weigh" the FastEthernet 0/0 ~ interface so that when it is up, it takes priority over the serial ~ interface? I don't have all of the equipment in to play with yet, but ~ I do have the router so I figured I'd go ahead and start stumbling ~ forward. ~

"Weighting" at the interface level won't help you here, because your Fa0/0 is apt to be up, even when the network path via your Satcom modem link is down.

Ideally, you would run a real routing protocol, and would receive a default route, or somesuch, from the Fa0/0 iff the outside world is reachable via the Satcom link.

Failing that, you would want to set up some kind of "ping-based routing" scheme, whereby the router can periodically ping some IP address behind the Satcom link, then bring up the serial link iff that address isn't reachable.

Aaron

Thanks to everyone for the replies. When the high speed link isn't available, it really isn't available. The modem will be physically removed and taken elsewhere, so having FE0/0 go administratively down should be automatic. I think the technique described by Thrill5 should work based on our setup. "Weight" was the only term that I could thing of and probably the reason I couldn't find much on Google. Thank for the link Aaron. This might be of help as we move forward with the network and things get more complex