I am learning topic of backup link. But something I am not understand. To my knowledge there is 3 method for backup link:
1. Backup interface. 2. Dialer group 3. Ospf demand circuit.
My question is even the backup link (assume a BRI interface) goes up,the routing table must have the routes to redirect the traffic to the backup link. so the backup interface must also "join" the routing protocol. For example, add the interface network to ospf or rip, add a static route "0.0.0.0 0.0.0.0 backup-interface". At this point if the primary link down, the backup interface route will be in the routing table for the backup interface "join" the routing protocol,and the traffic will be direct to the backup interface. But that's no need for the backup method mention above and just use the routing protocol is enough, so I think the backup interface is not needed (except ospf demand circuit) although use it may be get the backup interface a bit faster ( the backup interface will immediately up when the primary link down). Can anybody give me some idea? Thank You!
The job of backup interface command is to track the layer 1 and 2 of primary interface. While routing protocol on primary and backup links works at layer 3. Backup command will put the backup interface in standby mode. So the moment ios discovers primary interface down/down or up/down, it will activate the backup interface. So Using routing protocol only will not do the job as both operates at different layer.
So you need backup interface command and have both primary and backup interface under any routing protocol for effective routing.
Anyway here is the link which explains all three backup schemes (backup interface, floating static and dialer watch) with pros and cons in detail.
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