Redistribution of Routes-- Foggy Concept

[mrashby]

All,

I have been trying to wrap my brain around route redistribution but every time I think I have it down it just doesn't seem to work right when I do it. Can someone provide some more info on the subject so I can try to get a better understanding of what it does and why would I use it. I am going to attach a piece of my config from one of my routers that is redistributing routes can someone explain what it is doing just to verify. Thanks for any help.

router eigrp 5

redistribute bgp 00000

network 10.0.0.0

default-metric 4500 2000 255 1 1500

auto-summary

no eigrp log-neighbor-changes

!

router bgp 00000

bgp log-neighbor-changes

network 10.0.0.0 mask 255.255.255.0

redistribute eigrp 1 metric 1000

neighbor 170.x.x.x remote-as 12345

no auto-summary

[Georg Pauwen]

Hello,

in your example, you are mutually redistributing EIGRP and BGP. Since you are using the ´auto-summary´ command under your EIGRP process, a classfull summary of 10.0.0.0/8 will be generated and redistributed as well.

Identical routes received from your eBGP neighbor will be preferred over EIGRP routes, due to administrative distance (which 20 for EBGP, compared te 90 for EIGRP amd 170 for external EIGRP).

Is there anything specific you are running into or need clarification on ?

Regards,

GP

[Richard Burts · Accepted solution]

Mario

The general explanation for redistribution is that there may be some routing information that you have learned from one source (such as BGP in your case) and you want to advertise that information to parts of your network that do not run that protocol. So you redistribute that routing information. The essence of redistribution is that you allow a routing protocol (like EIGRP in your case) to learn information from some other source (like BGP in your case) and to advertise that learned information.

Many routing protocols distinguish internal routes (where the original source of the information was within the protocol) and external routes (where the original source of the information was outside the protocol - which is redistribution).

So in the example you posted you have EIGRP 5 and under that protocol you have redistribute BGP. What this does is to enable EIGRP 5 to learn information that originated in BGP.

There are several challenges when routing protocols redistribute. One of those challenges is that the routing protocol metrics used by one protocol are frequently not understood by the other protocol. We get around this by defining default-metric. So under EIGRP 5 you have default-metric 4500 2000 255 1 1500 which defines the metric parameters which EIGRP will use for any redistributed route.

Your example also enables redistribution under BGP. This redistribution will allow the BGP process to learn routes originally learned by EIGRP. One detail here is that the BGP redistribution specifies EIGRP 1 while the EIGRP definition uses EIGRP 5. So unless this example also has a process for EIGRP 1 the redistribution under BGP would not work as configured. I suspect that the explanation for this is that you have changed some parts of the configuration for posting to the forum (which is generally a good idea) and that you accidentally introduced this mis-match.

HTH

Rick