"Redistribute" terminology confusion

[vv0bbLeS]

Hello all,

In learning about route redistribution, I'm wanting to be sure I'm thinking about it correctly. To me, the word "redistribute" would imply some kind of "outbound/push" operation in normal vernacular (i.e. "distribute" = "send to"). Yet when I input the statement on a router, it seems like in actuality the "redistribute" statement is performing an "inbound/pull" operation.

For example, if I entered this config on a router (assume both eigrp and bgp are running):

router eigrp 100

    redistribute bgp 200

To me, what I'm saying is "redistribute FROM bgp 200" i.e. import routes FROM bgp 200 (instead of the normal use of "distribute" meaning send TO something).

Am I thinking about this correctly in that "redistribute" is an inbound/pull operation? Or should I be thinking about it as "bgp 200 redistribute TO eigrp 100" (which is confusing even to say).

[balaji.bandi]

In order to learn the route from BGP, you like to redistribute to EIGRP you use above mentioned syntax

if you like EIGRP route to BGP below syntax.

router bgp 200
 redistribute eigrp 100

[pmckenzie]

Think of like the gathering of stray routes from a different protocol. You want to combine them with the ones you already have in your EIGRP routing table.

Are you also thinking about the metrics required to redistribute into EIGRP

by default BGP will be given an infinite cost.

Have a look at this great example

https://www.kwtrain.com/blog/route-redistribution-part-1 

[horia.gunica · Accepted solution]

Hello!

It can be confusing and different vendors do it in different ways .

For Cisco - you need to think inside the routing protocol you are configuring - let's take your example :

router eigrp 100 -> I am under EIGRP AS 100

    redistribute bgp 200 -> In the EIGRP AS 100 process - take whatever routes are learned via the BGP 200 process and distribute them to my neighbors .

So - in a sense - you are "distributing" the routes to your neighbors - but you are first taking them from the routing protocol you are stating (bgp AS 200 in this case) .

I hope that clears it up - this is how I "view" it and it keeps things clear for me.

Best regards!

Horia

[Giuseppe Larosa]

Hello @vv0bbLeS ,

yes the redistribute command is applied within the Receiving protocol process and not on the "sending process"/ source of the routes.

You will need to provide a seed metric for redistributing into EIGRP you can use the default metric command for this.

EIGRP metric is composite and it cannot re-use the metric of the source protocol ( in case of BGP the MED attribute can be even not set).

router eigrp 100

redistribute bgp 200

default-metric 10000 100 255 1 1500

! here the default metric is used to give a starting value for redistributed routes so that they can be injected in EIGRP database

The second important note after the need for a seed metric is that you can redistribute only BGP prefixes that are installed in the local IP routing table as BGP route (or are actually connected).

Just to make an example an iBGP route with AD 200 by default may be present in the BGP table but not installed because a better route with lower AD is provided by an IGP ( EIGRP 100 itself for example).

Hope to help

Giuseppe

[vv0bbLeS]

Thank you all for your replies! I understand the concept of redistribution, I just wanted to make sure I was thinking about the "terminology" correctly, which you all have confirmed I am.

Thanks to @horia.gunica for pointing out the "redistribute" term applies to the current routing process sending routes to its neighbors, i.e. the fact that the current routing process itself (i.e. eigrp) will in fact be the one redistributing the routes, but only AFTER first PULLING them in from the protocol named in the command (i.e. bgp).

And thanks to @Giuseppe Larosa  for the tip on the routes having to already be in the global routing table to be redistributed. Very helpful!