Routes that are learned from another routing protocol, between two processes of the same routing protocol, from static routes, or from a direct connection to the destination network can be redistributed into a routing domain. Routes can also be redistributed between IS-IS levels 1 and 2.
2
In contrast to metrics, which are used to determine the best path among multiple routes to the same destination discovered by the same routing protocol, administrative distances are used to determine the best path among multiple routes to the same destination discovered by different routing protocols.
3
A route to a destination within a routing domain with a higher administrative distance can be redistributed into a routing domain with a lower administrative distance. If that route is redistributed back into the higher-distance domain, packets might be misrouted into the lower-distance domain.
4
Redistributing variably subnetted destination addresses from a classless domain into a classful domain can cause problems. The classful domain might not be able to recognize all the subnets attempting to be redistributed from the classless domain.
5
OSPF and IS-IS understand the default metric. RIP, IGRP, and EIGRP do not.
6
The metric command assigns a metric to specific redistribution statements. The default-metric command assigns a metric to all redistribution commands that do not include the metric command.
7
Without the subnets keyword, only major network addresses that are not directly connected to the router will be redistributed.
8
A router that originates a summary route should use the null interface as the next hop of the summary route. Any packets that match the summary route, but for which there is no more-specific route to the packet's destination address, will be dropped. This prevents the router from forwarding "lost" packets.
