In article , Barry Streets wrote: :On cisco router running IOS version 12.2 there is a route statement ip route :207.21.242.XXX 255.255.255.255 null0; what's the use of "null0"???
This question would be better put to comp.dcom.sys.cisco.
But to save you the trouble: the null0 interface means that the packet should be dropped. You could achieve the same effect by using an ACL (access control list) to deny the traffic, but routing is more easily handled in hardware than ACL's are. There can also be differences about whether the router send back an icmp denial packet, which can make the ip route version a better choice for dealing with forged DoS attacks.
:There are also several route statements that at the end of the route there :is a single digit, I assume that this is used to weight routing so that if a :primary route goes down it then uses the backup route.
The effect of adding the metric would depend on what routing protocol the router is using, and upon how the router has been configured to "inject" one kind of routing protocol into a different kind.
If you are using the RIP protocol, then a metric of 16 means that the packet should be dropped.
If you have multiple ip route statements that match the same destinations, then the metric might be used to control the ratios with which the router load-balances between the various possibilities.
If you have all routing protocols turned off and are using just plain static routing, then Yes, under IOS if there is a static route to an interface and the interface is detected as having gone down, then that route will be disabled and the route with the next lowest numbered metric would take effect. In most versions of IOS since about IOS 11, on most Cisco routers and "multilayer switches", if you have multiple routes matching the same destination and the metrics are the same, then by default IOS will do per-packet load sharing.