AS Number question

Hi there,

I have a very basic question regarding ASN.

For to router to exchange routes, they have to be in the same ASN right? But this depends on what routing protocol is used on link. Is that true?

Cheers to who will take the time to reply.

Glenus

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Comments

bighornsheep

Someone else can correct me if I am mistaken, but if two routers do not have a route that is in the same AS number, they must be on disjoint networks, so in order for them to advertise routes to each other, there must be intermediate routers that will receive the routes first.

This should be the case regardless of the routing protocol you use.

Does this make sense? Someone else can correct me if I am wrong..

networker050184

glenus wrote:

Hi there,

I have a very basic question regarding ASN.

For to router to exchange routes, they have to be in the same ASN right? But this depends on what routing protocol is used on link. Is that true?

Cheers to who will take the time to reply.

Glenus

Are you talking about the proccess ID and as number in ospf and eigrp? If that is what you are talking about it is locally significant in ospf, and in eigrp all routers need to use the same number to form neighbor relationship.

SanKuKaï

Thanks for your replies

If I understand well:

OSPF: process id -> does not have to match between connected routers but is locally important to distinguish between the processes.

IGRP and EIGRP: AS Number -> Must match between routers for them to exchange routes.

Am I right now?

networker050184

Sounds good to me!

rossonieri#1

NOPE - the only routing protocol that interconnect different ASN is external BGP.
other than that none will talking - eventhough they know their neighbor exist.

HTH.

rossonieri#1

glenus wrote:

OSPF: process id -> does not have to match between connected routers but is locally important to distinguish between the processes.

Am I right now?

maybe what you are trying to say is an area id.

Netstudent

rossonieri#1 wrote:

NOPE - the only routing protocol that interconnect different ASN is external BGP.
other than that none will talking - eventhough they know their neighbor exist.

HTH.

What about EIGRP and redistribution? That can interconnect different AS numbers.

rossonieri#1

netstudent,

nope - redistribution in EIGRP or the other was only intended for other AS/PID of other routing protocols.
you cant redistribute AS 100 from AS 101 in the same EIGRP right? they do know AS 100 and 101, but they cant exchange infos about their networks because it simply different administrative domain.

HTH.

networker050184

He is asking about IGP process id and as as in

ospf 100

eigrp 100

The number needs to match in eigrp but not ospf. He isn't asking about routing between AS as with BGP. Your going to confuse the guy.

rossonieri#1

glenus wrote:

Hi there,

I have a very basic question regarding ASN.

For to router to exchange routes, they have to be in the same ASN right? But this depends on what routing protocol is used on link. Is that true?

Cheers to who will take the time to reply.

Glenus

hi networker050184,

i'm wondering which part of this quote states IGP

if that what you mean that i confused the OP?

i'm just saying that - no, you cant interconnect different ASN without the use of eBGP.
not that i'm trying to confuse the OP.
and :

"the number needs to match in eigrp but not ospf" - i've never tried that, can it talk to other PID?

hmmm....

cheers.

networker050184

I beleive the peroson originally asking the question is refering to the AS as in studying for CCNA. You need to know that the proccess id is only localy significant in ospf, so yes you can use a different number on different routers. In igrp/eigrp the as number is not locally significant and needs to match to form neighbor relationship. As in routing between AS you do need to use eBGP, but that is beyond the scope of his study and the question he was asking.

Darthn3ss

networker050184 wrote:

He is asking about IGP process id and as as in

ospf 100

eigrp 100

The number needs to match in eigrp but not ospf. He isn't asking about routing between AS as with BGP. Your going to confuse the guy.

well

if you typed

router OSPF 100, that would specify the process number. you specify the area/ASN with the network statement.. with E/IGRP, you specify the AS when you type router (e)igrp 101... and no area in the network statement... i belive i'm correct, but who knows eh?

networker050184

You are correct.

Netstudent

well I still want to know why you can't route between EIGRP autonomous systems with redistribution.

networker050184

Not 100% but I believe you can with redistribute and route-maps commands. I have done it with ospf processes but never with eigrp. If I'm wrong, I'm sure someone will corect me!

Netstudent

those were my thoughts exactly. I read that straight from cisco's whitepages.

networker050184

I don't see why not, I think rossoneri#1 is thinking in terms of BGP not in terms of IGPs such as ospf and eigrp.

rossonieri#1

hi netstudent & networker050184,

think of this picture :

RTRA(EIGRP 100)

RTRB (EIGRP 101)

if you do redistribute this right away --> how does RTRB knows EIGRP100 if they have no connectivity at all.

yes - i'm not saying that you cant redistribute other AS, but you had to have an intermediary AS at first.

now look at this :

net A (EIGRP 100)

fa0/0 RTRA s0/0 (EIGRP 102) s0/0 RTRB fa0/0

(EIGRP 101)net B

that will do redistribute.

HTH.

networker050184

I don't belive you need an intermediary as. You just need one router in both as to do the redistribution on. You can do this with only two as.

Netstudent

rossonieri#1 wrote:

hi netstudent & networker050184,

think of this picture :

RTRA(EIGRP 100)
RTRB (EIGRP 101)

if you do redistribute this right away --> how does RTRB knows EIGRP100 if they have no connectivity at all.

yes - i'm not saying that you cant redistribute other AS, but you had to have an intermediary AS at first.

now look at this :

net A (EIGRP 100)
fa0/0 RTRA s0/0 (EIGRP 102) s0/0 RTRB fa0/0
(EIGRP 101)net B

that will do redistribute.

HTH.

Okay I kinda see where you are getting at. And as networker said, I don't think you need an intermediary AS, but rather a ASBR.

http://www.cisco.com/warp/public/103/eigrp-toc.html#twoeigrpauto

networker050184

Like I said one router in both as or commonly reffered to as ASBR.

rossonieri#1

i'm glad you guys see the point that you cant redistribute directly different AS --> especially IGP protocols.

"like" an ASBR in OSPF yes - but pls note that there is no ASBR in EIGRP.

but,

the networker picture above (from cisco).. using 3 routers (one having both AS 2000 and 1000), and my example uses 2 - utilizing EIGRP 102 (this is the "intermediary" AS - the serial interface) to redistribute their backend AS 100 and 101 - and AFAIK my example is the safest way to redistribute networks.

pretty much the same - different approach.

cheers.

networker050184

Yeah there are many approaches to any problem this one is no different. Just realize you don't need three AS to redistibut is all I was getting at.

Oh and the ASBR I know its for ospf areas, but you could use the terminology loosely to explain this situation.

EdTheLad

I'm confused what is this "process id" you guys are talking about? Aswell what exactly is an autonomous system and whats this word "redistrubution" that keeps coming up?
If i have 2 ASs connected via eigrp with AS 32 and the other 23 will they communicate in reverse since the AS numbers are reversed? It all gets so confusing.

networker050184

HA HA.... funny I'm sure you know more than me about the situation at hand!

gabrielbtoledo

I think rossonieri#1 is right.
If you take a look at cisco pic, you will see that is needed 3 routers. The middle router will redistribute routes from left to right and right to left. Just like as rossonieri#1 said in his hand-made picture.

networker050184

We are not talking about the number of routers. We are talking about the amount of AS you need.

gabrielbtoledo

I see, then you are right. No need to have a third AS.
Sorry about the confusion.

networker050184

Hey no problem we are here to learn!!!!!

rossonieri#1

hmmmm....

maybe you might want to check this :

http://forum.cisco.com/eforum/servlet/NetProf?page=netprof&forum=Network%20Infrastructure&topic=WAN%2C%20Routing%20and%20Switching&CommCmd=MB%3Fcmd%3Ddisplay_location%26location%3D.1ddf761f

not to make this a mess - i like to discuss and learn.

cheers.