EIGRP and Route redistribution

[SJ K]

Hi all,

While learning OSPF, it is said that OSFP area help to break down areas and thus keep the osfp topology and database and computation small.

Then it came to my mind that EIGRP can actually do such break down as well using AS number.

However, when comparing EIGRP and RIP, it is not mentioned that EIGRP is able to let you split up routing domains using AS number but RIP doesn't

e.g as below

Q1) is it that my understanding is wrong and it should not be setup that way ?

Q2) is there anyway we can redistribute routes from AS1 into AS2 and vice versa ? can route redistribution work across the same protocol ?

Q3) I came across this phrase "Redistribution may be one-way (from one domain to another but not vice-versa) or two-way (bi-directional)"

- why would someone want to redistribute routes 1 way ?  From A to B, in that case, B know how to get to A, but A doesn't know how to get to B.

Q4) When doing route redistribution between A and B, are we

a) redistributing the routes in the IOS routing table into the desired/designated routing protocol ? or

b) redistributing the routes in a routing protocol topology into the desired/designated routing protocol

Q5) Pardon me if I don't understand or cannot see the big picture, when we redistribute routes from 1 routing protocol into another, are we kind of "adding" routes/entries into  another routing protocol topology table; if there are changes to the routes in the topology table, re-computation on the topology table will have to be done.

It seems to me that route distribution will cause 1 routing domain route entries to flow into another routing domain - is my understanding correct ? Wouldn't that defeat the purpose of separating routing domain to keep topology table and route computation small ?

But again if we don't distribute routes into other routers/networks running different protocols, then routing to other network will not be possible.

Having route summarization might help, but in my example above, if there are discontinuous network within an AS, summarization will not be possible, isn't it ?

Q6) Can we also redistribute summary routes from 1 protocol to another ?

Regards,
Noob

[Bilal Nawaz · Accepted solution]

The definition of an autonomous system is a device or set of devices that fall under one administrative domain. So when we look at what autonomous system is, then we can see where that would apply in lab and real world scenario's. On your R3, it is between AS's 1 and 2, if you were to try and configure this you would not be able to achieve it, it's either in or out. EIGRP will only form neighbors with peers in the same AS. You will discover in your future studies that BGP allows for this, it's a tool used to peer two different AS's together.
 

In order for routes to be exchanged between AS1 and AS2 will have to have some sort of "break out" of eigrp - example could be to use RIP or OSPF between the AS's and then redistribute routes from one AS in to the other, using RIP / OSPF / BGP etc.. as a transit between AS's.

Yes you can redistribute across the same protocol, like OSPF process 1 between OSPF process 2. I cant think of how it would work with other routing protocols though.

Routes may be redistributed one way in an example scenario of 2 merging companies / peering with a 3rd party. Often used with NAT to cater for return traffic. But it's very rare to have incomplete routing end to end.

hth

Bilal

[Jon Marshall · Accepted solution]

q1) you don't usually use multiple ASs for EIGRP. The two main tools are -

1) summarisation - this is key for a large EIGRP network ie. you summarise between sites. If you don't then you can face the problem of a link going down in one part of your network and this has to be propagated to the rest of the network.

Summarisation helps to limit the query scope.

2) EIGRP stub which also helps to limit the query process for EIGRP

q2) yes you can ie. there is no restriction in redistributing between two EIGRP processes on the same router.

You would configure it as you would any redistribution and it would need to be two way redistribution on R3 for you to be able to ping between AS subnets.

q3) you may want one way redistribution for a number of reasons eg. with an MPLS setup you may redistribute the BGP learned routes into your IGP but you may use "network ..." statements under BGP or the aggregate-address command to advertise out the local subnets.

Redistributing static routes is another common example.

It really depends on the situation.

q4) redistribution is always done using routes from the IP routing table and not the protocols own internal database.

So if the route is not in the IP routing table it won't be redistributed.

q5) This really is a big picture thing as you say and depends entirely on the situation eg. as Bilal mentions a company merger may dictate this but also you often need to run multiple protocols eg. and IGP and BGP for various reasons and, as mentioned, redistributing statics is quite a common thing to do.

q6) yes if you want, there is nothing particularly special about summary routes.

Again as long as they are in the IP routing table they can be redistributed.

Jon

[SJ K]

Hi Jon, Bilal,

Thanks for the replies and the answers.

Hi Jon,

is it possible to redistribute summary route on a router with no summary routes in place ?

e.g. R1 has 4 subnets from EIGRP in the routing table

D 192.168.0.0/24

D 192.168.1.0/24

D 192.168.2.0/24

D 192.168.3.0/24

Is it possible to distribute this 4 routes into 192.168.0.0/22 and send it over to router running e.g. RIP ?

Regards,
Noob

[Jon Marshall · Accepted solution]

The short answer is I don't know for RIP but I am not aware that you can.

You can certainly do it with OSPF ie. you use the "summary-address ..." command on the ASBR when you redistribute to create the summary address.

If there is no specific command then what you can do is create a summary route to Null0 ie.

ip route 192.168.0.0 255.255.252.0 null0

and then redistribute static into the routing process.

This works fine as long as the router has the more specific /24s in it's IP routing table because the more specific match will always be used so traffic should be forwarded correctly.

Jon

[SJ K]

Duly noted.

Thanks Jon!