PPP and frame relay allow for a Layer 3 interface to be established without knowing in advance the next-hop on the other side of a serial link. In this case, you can specify a Serial Layer 2 interface or a GRE tunnel as a next-hop instead of providing a specific next hop IP address. This is equivalent to specifying the node on the other side of the serial link as the next hop when its IP address is unknown. The via interface option is configured by specifying the type and the number of the Serial interface using the ip route command.
Note:
The interface used in the via route must have an IP address attached to it.
For example, the command ip route 193.168.10.0 24 serial 3/1:1 creates a static route to the network 193.168.10.0 24 via the Serial 3/1:1 interface.
A static route can have both via interface and IP addressed next hops, with a maximum of three next-hops. If such a combination is required, separate ip route commands should be used for the via interface static route and the IP addressed next hop routes. Also, if more than one via interface next hop is required, each must be configured by separate ip route commands.
Note:
You cannot define a static route through the FastEthernet Interface unless the interface was previously configured to use PPPoE encapsulation or was configured as a DHCP Client. See Configuring PPPoE and DHCP client configuration.
