EIGRP with two AS numbers

[wmcmurry]

Are there any issues with running two seperate EIGRP Autonomous System numbers within the same router ?

What about using the same network (ie 10.75.188.0) in each AS number ?

[vmiller]

For Eigrp, the number after router eigrp is not an AS, its a process id. If you run 2 different process ids, they will only communicate if you redistribute.

WHat problem are you trying to solve ?

[Kevin Dorrell]

I'm sorry, but I disagree. For EIGRP, the number is an AS number. It appears in the protocol. In fact, routers will not talk routers in a different AS. But it is true that the two instances will not talk to each other unless you do specific redistribution.

OTOH, in OSPF, the number is a local process number, and does not appear in the protocol. In OSPF, you can have all you routers running different process numbers, and they will still communicate, as long as the other parameters are correct.

Now comes the question I did not know how to answer: can you have a network in two AS's at the same time? Sorry, I don't know that. I can say that in OSPF, an interface can only be owned by one process, and indeed can only be in one area. But I'm not sure about EIGRP.

Kevin Dorrell

Luxembourg

[wmcmurry]

I have a LAN that is part of a large corporate WAN. All routers on the WAN run the same EIGRP AS. I would like to load balance on redundant links to a seperate facility and not propogate the redundant routes to the corporate WAN.

I thought that I could start a second EIGRP AS on the four routers that would accomplish this load sharing with CEF.

Would there be any issues with having the same networks (ie 10.75.188.0 and 10.75.191.0) in both instances of EIGRP?

[Kevin Dorrell]

How many of the four routers carry links to the corporate network? I suppose it is only one or two of them. If that is the case, then, yes, it does make sense to run two autonomous systems: one for your internal network, and one for your link(s) to corporate.

Define an AS for your internal network, and put all your internal links into it. On the one or two routers that connect to corporate, and only on those links, define the corporate EIGRP AS. Then redistribute routes from each AS to the other. This will allow you to manipulate the routing and load balancing within your internal network without upsetting the routing to and from corporate.

That is in principle. To see whether such a design is feasible, or even desirable, I think I would need to know a lot more about your network topology. Then again, my experience of EIGRP design is quite limited, and you would do well to get a second opinion. ;o)

Kevin Dorrell

Luxembourg

[asd_asd_22 · Accepted solution]

there is no issues in using two AS numbers within the same router but the you have to redistribute them if you want to connect the two AS, and you must take care to the CPU and memory because your are running two different processes and this will consume most of your CPU and memory.

and you can't use the same network in each AS because the network command only chooses the interface that will participate in the EIGRP process(send and receive routing upadtes) and the interface can reside only in one AS.

[wmcmurry]

So if I were to be running two AS's and each had the network 10.75.188.0 this would cause routing issues ?

[ggarlington]

By creating two EIGRP AS you are creating two seperate networks. You cannot route between to seperate networks that use the same IP address. How would the router make a decision on which route is correct?

[asd_asd_22]

yes, putting the network in 2 AS will cause a routing issue.

[ariesc_33]

How about creating a subinterface or a secondary ip address on the name interface both on a different AS, with a different subnet, is it possible?

[nstringf]

What problem are you trying to solve with two EIGRP AS numbers? If you are trying to decrease your network boundary to prevent SIAs, this doesn't work. Also, if you have the same network in both ASes then the last/first receive or lowest AS route will be picked for the routing table depending on the IOS version. This may not be the desired operation.