Hi, Iam having 100Mbps metro ethernet link and 2Mbps serial link terminated in same router.Iam using 100Mb link as primary route and give serial link as second metric.My problem is when ethernet link goes down second serial route should come up,but it is not coming.Interestngly if give serial as primary and ethernet as secondary when serial fails,ethernet route coming up and running.
To be sure that we are talking about the same thing: Do I understand that you have set a default route on the serial link, and that that default route is a higher metric (lower priority) than the ethernet link? The arrangement commonly known as a "floating static route" ?
If so, then the success of the arrangement depends upon the router being able to determine that the other link is in fact down. Unfortunately, it is not rare that when a WAN link goes down functionally, that the first-hop link to the ISP stays up, so the hardware does not know that the link is down to remove it from the routing table. The next time you have the difficulty, check to see that the ethernet link shows up as being down in "show interfaces": the problem could be further down the line instead.
If that is your problem, then newer IOS's provide a ping-based method of tracking whether a link is -usefully- up (that is, traffic is able to get to the other end and return.) I can never remember the name of the feature though.