Route Redistribution Notes

Route Redistribution Overview

=============================

Redistribution occurs from the routing table not the routing database

When redistributing protocol X into Y, take

-routes in the routing table via protocol X

-connected interfaces running protocol X

Route advertisement rules

-RIP vs EIGRP vs OSPF vs BGP

Connected Redistribution

=====================

Implicitly occurs for connected links running the redistributed protocol

Additional connected links can explicitly included or excluded

-redistribute connected [metric] ][route-map]

-overrides implicit redistribution

How IOS chooses Path

=======================

Routing database chooses one or more candidate paths

-EIGRP via DUAL, OSPF via SPF, etc

-Load-balancing possible via maximum-paths

IF multiple equal matches between protocols

-choose the lower AD

Install results in RIB and/or FIB

Administrative Distance Values

======================

0 : connected

1 : static

5 : EIGRP summary

20 : External BGP

90 : Interal EIGRP

110 : OSPF
115 : IS-IS
120 : RIP

160 : ODR

170 : External EIGRP

200 : Internal BGP

255 : Infinite

RIP Redistribution

=================

Doesn’t differentiate between internal and external routes

-AD of 120 for all routes

No default seed metric

-redistribute [protocol] metric [hops]

-default-metric [hops]

EIGRP redistribution

===================

AD of 170 for external EIGRP

-helps to automatically prevent route feedback

Uses router-id for loop prevention

No default seed metric unless EIGRP to EIGRP

-redistribute [protocol] metric [bw] [delay] [load] [relaibility] [mtu]

-default-metric [“”]

OSPF redistribution

==================

AD of 110 for all OSPF routes

Uses Router-ID for flooding loop prevention

Default seed metric 20 and metric-type E2/N2

OSpf path selection preference

-E1>E2>N1>N2

-E1 & N1 vs E2 & N2 metrics

BGP redistribution

=================

Uses ORIGIN code incomplete (?)

Normal EBGP and IBGP loop prevention

IGP to BGP

-denies ospf external by default

.redistribute ospf [pid] match internal external

BGP to IGP

-EBGP routes allowed, iBGP routes denied by default

.bgp redistribute-internal

.legacy synchronization rule

.can cause routing loop