Can you explain to me how does default routing out of the exit interface work? for example ip route 0.0.0.0 0.0.0.0 f0/0. I know that if you use the next hop IP address in the default router configuration (ip route 0.0.0.0 0.0.0.0 192.168.1.1), router can resolve the next hop IP to the MAC Address to put in the data link frame as destination. How does router know what to put in data link frame as the destination address if it has no IP address to resolve? How does it work? Thanks a lot
It certainly does allow you to specify an Ethernet interface as the exit in a static route, including a static default route. There are several things that make this a problematic thing to do:
- it means that the router will send an ARP request for EVERY layer 3 destination address, so it is generating lots of traffic.
- it will only be successful in the next hop router has enabled proxy-arp, and increasingly some organizations regard proxy-arp as a security weakness and disable it. This means that now the success of your routing is dependent on something that you may not control.
- if the next hop router does enable proxy-arp then the MAC address gets added to the ARP table which contains ALL of the destination IP addresses to which the router has forwarded, so the ARP table gets very large, consuming memory and CPU cycles to maintain it.
So the best advice is that static routes specifying the exit interface are ok if the exit interface is some point to point link like HDLC, PPP, Frame Relay but otherwise it is much better to specify the nex hop address.