3.3.h Implement, optimize and troubleshoot redistribution between any routing protocol

3.3.h Implement, optimize and troubleshoot redistribution between any routing
protocol
Route Redistribution

Internetworks may be comprised of a variety of routing protocols. As such, redistribution may be required for inter-communication between these disparate protocols. A requirement of redistribution is a seed metric which in the case of directly connected networks is ordinarily provided by the interface, as in EIGRP below:

dsw1#sh int f0/1

FastEthernet0/1 is up, line protocol is up (connected)

Hardware is Fast Ethernet, address is 0016.479e.4542 (bia 0016.479e.4542)

Internet address is 10.1.4.6/30

MTU 1536 bytes, BW 100000 Kbit, DLY 100 usec,

reliability 255/255, txload 1/255, rxload 1/255

With redistribution, however, there is no directly connected interface to supply the metric. OSPF provides a default, whereas RIP and EIGRP are ambiguous:

Protocol Default Seed Metric

OSPF 20; except BGP, which is 1

IS-IS 0

RIP Infinity

IGRP/EIGRP Infinity

When redistributing into RIP or EIGRP a seed metric must be provided, unlike OSPF which provides a default. However, for effective redistribution into OSPF the subnets keyword is used to gather routes described by VLSM, or only classful networks will be included in the redistribution.

dsw1(config)#router ospf 1

dsw1(config-router)#redistribute eigrp 1

% Only classful networks will be redistributed

dsw1(config-router)#redistribute eigrp 1 subnets

Into RIP redistribution command options:

redistribute protocol [process-id] [match route-type]

[metric metric-value] [route-map map-tag]

Into EIGRP command options:

redistribute protocol [process-id] [match {internal | external 1 | external 2}]

[metric metric-value] [route-map map-tag]

Into OSPF and all non-protocol specific command options:

redistributeprotocol [process-id] {level-1 | level-1-2 | level-2} [autonomous-system-number] [metric {metric-value | transparent}] [metric-typetype-value] [match {internal | external1 | external2}] [tagtag-value] [route-mapmap-tag] [subnets] [nssa-only]

One way redistribution

Typically a default route is offered to the edge protocol whereas the core network has all edge routes redistributed into its core routing protocol.

Two way redistribution

As the name implies all routes are redistributed both ways. In a multi-area, multi-protocol topology this can present the unwanted side effect of routing loops and route feedback. A routing loop may occur if a packet becomes endlessly routed through the topology. Route feedback may occur if redistributed route information gets redistributed back to the originator.