IS-IS Question

Just to state ifs theres one section in the BSCI exams which puts me to sleep its the ISIS section, how mind numbingly boring!!

Anyway my question is about the DIS.
In a lan enviroment i.e. broadcast network a DIS is elected i.e. basically the same as a DR in ospf, but in OSPF all DRother routers only form adjacencies with the DR where as in ISIS all routers on the Lan form adjacencies with each other and the DIS.

What exacly do they mean by this?

Instead of flooding and database synchronization taking place over n x (n-1) adjacencies, the pseudonode enables the reduction in flooding and database synchronization to occur only over the adjacencies formed with the pseudonode.

So what is the point of the other routers forming adjacencies with each other?In ospf you have the DR to decrease the number of adjacencies in ISIS you have a DIS to reduce the flooding.
So in ISIS the adjacencies dont exchange LSPs? if so what is the point in having an adjacency?

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Comments

darkuser

isis was the precursor to ospf and was designed to route clns.
it was modified to route ip.

http://www3.ietf.org/proceedings/05aug/RFCs/rfc1195.txt

EdTheLad

Please, why the rfc link?My question was specific.
If you cant answer the question why bother posting?,i have plenty of reading material but if i cant find the answer their i ask a specific question on the forum.

I know about IP being transported in the TLVs of the LSP, my question is why form adjacencies with all other routers? when you can form an adjacency with the DIS which will represent the network.The DIS exchanges all LSPs, it multicasts a CSNP every 10 seconds to neighbors and if they dont have a specific LSP they send a PSNP requesting it.Where in this senario does the adjacency with other neighbor routers on this Lan segment get involved.

Yankee

Ed blasts people with questions and people with answers while rambling on with his own responses. Persona non grata in my book
Yankee

EdTheLad

No i just want my questions answered, i make my questions as direct as possible and its a pain in the arse when someone responds with a link.
A forum to me is about discussion, i ask a question and i hope somebody who understands the topic answers.There's afew people on here with high opinions of themselves and i'm just not interested in that rubbish, i'm looking for information here and giving what info i have to others.
Anyway if you read what i have written above you will see what i understand
and what i dont.So if you understand how ISIS really works and didnt just learn the basic concept to pass the exam you should be able to answer the question, if not i'm still searching for my answer.

cisco_trouble

Ok bro, calm down. We all know the feeling. Ok, well i havent really gotten into ISIS as yet, and i take it you havent passed the exam either and need an answer. Lets look at it logically...if OSPF was developed using ISIS, then it should have similar characteristics.
Routers using link state routing protocols maintain a picture of the entire network, and are aware of outages several hops away. The router can then use this knowledge to determine the best route for traffic, and can avoid sending data across the network if that network is down. So from what im getting from you is that you a bit confuzzled to why have "full-mesh" adjacencies?
Just discuss what you know..and maybe we can help by giving u the clue

forbesl

ed_the_lad wrote:

There's afew people on here with high opinions of themselves......

Look inward, my son....

darkuser

ed_the_lad wrote:

Please, why the rfc link?My question was specific.
If you cant answer the question why bother posting?,i have plenty of reading material but if i cant find the answer their i ask a specific question on the forum.

I know about IP being transported in the TLVs of the LSP, my question is why form adjacencies with all other routers?

simply because it was a bad design and was improved or made legacy
by ospf dr's and bdr's
isis is hardly used exept in the routing core's of at&t.
honestly I don't think you need to get that deep to pass the ccnp.
you're not that important to get that pissy when your overcomlicated questions don't get answered ..... maybe it's not worth the time.
read and learn young jedi.

and a complimentary link ...
http://www.cisco.com/univercd/cc/td/doc/product/software/ios113ed/113ed_cr/np1_c/1cisis.htm

forbesl

darkuser wrote:

read and learn young jedi.

Would he be even considered a young jedi, or would he still be considered a padawan?

Webmaster

I'm sorry to see you haven't received more helpful responses on your imo valid question. I'd love to see more of those type of questions in the forums. You obviously did some reseach yourself already, and I don't think a link that explains the commands will help you understand that bit of theory.

It's been a while since I prepared for this exam, and IS-IS is one of those things you want to forget as soon as possible and use the valuable memory space for more common technologies, so forgive me if I'm wrong. I think the adjacencies have to be formed with all other routers in the LAN before the DIS can be elected, and after the DIS is elected and the psuedonode created and the adjancencies with the DIS are formed, they are maintained in case the DIS fails and a new one has to be elected.

darkuser is right about IS-IS's popularity, it is hardly used. After you passed the exam you want to forget about it IS-IS will also be removed from the CCIE R&S exam on Jan 1, 2006, which is usually an indication it will dissappear from the CCNP exams as well.

Here's another link

Understanding IS-IS Pseudonode LSP:

www.cisco.com/warp/public/97/DIS-LSP-1.html

if you understand how ISIS really works

You seem to understand the 'how', especially in this case I wouldn't worry to much about the 'why'.

2lazybutsmart

Ummm... there's something I've been wanting to say a few times on issues like this and I think it's appropriate to say it now:

If someone asks a question, you either answer it or don't. You don't have to show the person that his/her question is unworthy of your 'professional' time. I find that very 'unprofessional' for professionals to do.

No matter how much knowledge a person amasses, the rules of receiving knowledge and those of transmitting them remain the same.

--The seeker of knowledge must ask questions in a polite manner (which I suppose was the case here) and should not make fuss over the answer he/she gets (

). Nobody's bound to answer your questions, so whether you get lots of info, of otherwise, you just have to take it. And say thanks

--The givers of knowledge must always look at the seekers in a humble way. They should try to answer their questions in a simple manner and show them that they're not being left out because they know so little or whatever. Those of knowledge should never forget that there was a time when they were simply ignorant about whatever it is a person’s asking them about now.

Just my 2 cents,

2lbs.

p.s. a touch of mysticism…. that won’t hurt will it?

janmike

Well said, 2lbs.

Webmaster

janmike wrote:

Well said, 2lbs.

I 2nd that.

EdTheLad

I agree with you 2lbs, but i will explain why i responded the way i did.
When somebody responds to a question with a link and makes no attempt
to answer a specific question, this can potentially kill the question. It's not a problem if the link actual has the information regarding the question but when its a specific question in which the writer shows he has knowledge of the material, this can have a negative effect on the post.
Generally when i browse the forum and i see a response to a question which has a link, if the inital poster doesnt request more info i'm unlikely to respond as i think they have received what they needed.So after trying to show i have an understanding of the topic to get a response of an RFC is bad manners in my book! Anyway the end.

Now back to my inital question.
In IS-IS the DIS is responsible for maintaining consistency of the neighbours databases by using the CSNP, it also generates the LSP for the
network i.e. the psuedonode LSP. Maintaining consistency involves multicasting the CSNP every 10 sec and if a neighbor doesnt have an LSP
which is in the CSNP list it will request it using PSNP from the DIS.
Now the reason for the adjacency is because the operation is a little different than OSPF, when a router receives a new LSP from another interface it will multicast the LSP to all neighbors on the braodcast network including the DIS,so basically new updates are send to all adjacent neighbors.If for some reason a neighbor didnt receive an update the DIS
is there to resolve the problem.
In OSPF the DR has more responsibility as it manages the flow of LSAs.