Route Redistribution

What is Redistribution
redistribution only utilize current IP routing tables, it absorbs route entries of one specific routing protocol (as well as connected subnet of the interface with the specific routing protocol enabled) and insert them into another routing protocol with default or customized metric.

issues with multipoint redistribution between same routing protocol
the router who interconnects two routing domains might receive 2 route paths (original and redistributed / external one) destined for the same internal subnet from the 2 routing domains respectively, that is called domain loop.
All IGP inherently prevents domain loop issues when redistributing between the same type of IGP.
RIP: External routes will be configured to use a larger metric value than any existing internal routes. both type of route have AD of 120.
OSPF: internal routes always have higher priority over any external E1 or E2 routes. However, both internal and external routes have AD of 110.
EIGRP: AD of Internal route is 90, while AD for external route is 170.

multipoint redistribution between two different routing protocols
suboptimal routing issue occurs only when redistribution between OSPF and RIP, as external OSPF route has lower AD than internal RIP ones. Solution to this issue is manipulating AD or using route filtering.

RIP distance ad-value
EIGRP distance eigrp internal-ad external-ad
OSPF distance ospf {external ad-value} {intra-area ad-value} {inter-area ad-value}
Per-route AD distance {new AD} {advertise route id} {wildcard mask} {acl number or name}







Redistribution into OSPF
redistribute protocol
{process-id | as-number} {metric xx} {metric-type xx} [match {internal | nssa-external | external 1 | external 2}] [tag xxx] [route-map xxxx] [subnet]
redistributed routes have metric of 20 by default.
If subnet not specified, only classful network (not auto-summarized one but the real one in the routing table) is redistributed.

Redistribution into EIGRP
redistribute protocol {process-id | as-number} {metric bw delay reliability load
mtu } [match {internal | nssa-external | external 1 | external 2}] [tag {tag-value}]
[route-map {name}]
NOTE: metric should be configured each time, otherwise, redistribution will fail. Or set the default metric using for each redistributed route by below command:
Route eigrp 1
   default-metric {bw (kbps) delay (tens-of-microseconds) reliability load MTU}