Routing Questions

[azev]

Just want to double check with some of the routing expert here in this forums;

If a router received a packet destined to a network that is not in the routing table, and no gateway of last resort is configured, the packet will be sent to null interface and dropped.

is this the right statement ?

Since static route have administrative distance of 1;
Does cisco router always check its dynamic routing table before it's static routing ?
what is the order that the router go through when deciding which routing entry in its routing table ?


Thx

 

[spidey07]

It is not sent to the null int. It is just dropped and an ICMP destination unreachable is sent to the sender. Null0 is used to prevent routing loops or as a place holder, it is sometimes installed automatically for summary routes in link state protocols. You shouldn't have to worry about null0 unless you're trying to do something funky.

The concept of static vs dynamic routing is all a function of administrative distance. There is only one routing table and this includes all routes.

The decision order for installing a route into the table is if there is an exact same route learned from different methods and the same prefix length the lower admin distance is installed into the table. If different prefix lengths then ever route/prefix is installed. The actual routing of a packet always chooses the longest prefix length.