routing between AS's

sputnic68

I want to route between two AS's internally in a router. Both AS's are running EIGRP. How would I accomplis this?

keenon

if i'm correct the middle AS should pass the AS routes to each side its connected.. kinda like ospf areas on each side and then (not use if a redistrubue is needed )its been a while but i will be into it very soon again.

kapfel

If your eigrp AS numbers are different you will have to manually redistribute them into one another otherwise.

Yankee

This is more complicated than one would initially think. No redistribution is necessary as the router will accept routes from both ASs (if configured in both). Regardless of AS it determines which path it prefers and inserts that one in its routing table. If each AS has a route to the same network the router only keeps one in its routing table (think it is the lower AS, but can't remember for sure). Because EIGRP advertises from its routing table and not the topology table. If the two ASs had a route to the same network, this router now will only advertise for that network in its routing table and connectivity for the other AS will be lost.
I know I haven't explained this well (short on time), but I ran into this on a production network then had to lab it up to figure out why some connectivity was lost.

Yankee

sputnic68

Thanks for the info guys! I'll get back with more information of what we want to accomplish.

darkuser

yes ... yankee rules
just configure two eigrp process on the router
on for each unique process id.

and now a customary link ....
http://www.cisco.com/univercd/cc/td/doc/product/software/ios122/122cgcr/fipr_c/ipcprt2/1cfeigrp.htm

Yankee

EIGRP is evolving quickly, so stuff I knew yesterday may not be true soon. I haven't had a chance to investigate but I heard there have been a bunch of new features added in the last week that give EIGRP some BGP like powers.

You can never stop learning in this dang profession...

Yankee

darkuser

wow bgp is like my favorite routing protocol and all.
(being from a heavy isp background)

now I have a reason to love eigrp except for being the second son of satan !!!?

Yankee

EIGRP is a very good routing protocol. Though I say that because it is the one I had learn best as it was in place on the corporate network when I arrived. Wish I knew more about BGP....

Yankee

darkuser

i love eigrp .... igrp was and IS EVIL !!!!

Yankee

What I said earlier may no longer be true. Did a quick lab of this and the routing table now knows if a network lives in 2 ASs and seems to properly advertise the network to both ASs. My latest test was using 12.3 something IOS so I am guessing the issue got corrected.

Yankee

keenon

be careful with 12.3 ios i have seen alot of "special issues " with them on a few different platforms and software versions.. so it sounds as if i was correct

darkuser

ugh

forbesl

darkuser wrote:

ugh

I'd say that too, if I had a bar of soap stuck in my mouth...

mattyboy

Not sure if you got to the bottom of this. But you will need to configure the router to redistribute between two different EIGRP AS's. The router treats the two processes as they are configured; two completely different routing processes, effectively splitting the router's processing of these. There is no automatic redistribution.

You need to remember that we are talking AS's here and not Areas like OSPF has.

Below is a link showing ways of doing the redistribution

http://www.cisco.com/warp/public/105/redist.html

hope this helps

Yankee

the real bottom line is if you want to share routes between two ASs then you shouldn't have two ASs... make it one AS

Yankee

mattyboy

Agreed.

Although I can imagine this scenario may have come from two large networks being joined together and a quick fix was needed to get them talking. Would have taken a lot of work changing all the routers into the same AS. I've been there before

Yankee

My experience with merging networks due to acquisitions is that there is usually address overlap so a simple redistribution usually ain't possible. Did do one where their network was messed up enough we kept them outside of us with only two entry points for redundancy and used limited redistribution for necessary connectivity. Luckily we recently sold them to some other suckers

Yankee