Re: ISIS L1 vs L2

Ok one more... Just stumbled across this. Great write-up that Ivan did (and
Marko commented) on this topic.

http://blog.ioshints.info/2011/11/multi-level-is-is-in-single-area-think.html
?m=1

Thanks, guys...

Sent from my iPhone

On Nov 28, 2012, at 12:22 PM, Ronnie Angello <ronnie.angello_at_gmail.com>
wrote:

> Reminds me of the drunken OSPF vs IS-IS discussion in Chicago after CCDE.
Remember that? ;)
>
> Sent from my iPhone
>
> On Nov 28, 2012, at 10:48 AM, Brian McGahan <bmcgahan_at_ine.com> wrote:
>
>> If youbre not going to grow to other areas then it doesnbt matter; flat
L1 everywhere is the same as flat L2 everywhere. For example think of it
within the scope of OSPF. If you have an OSPF network that is area 0
everywhere vs. a network that is area 1 everywhere, they will both have the
same operational behavior. All routers and links are in the same flooding
domain and failure domain. The problem comes in if you want to grow the
network to other areas. With OSPF flat area 1 wonbt work because you
canbt add other non-area 0 areas. The same is true with IS-IS.
>>
>> Most flat IS-IS networks use L2 everywhere, just in case there is a need to
add hierarchy later. But like I said if youbre never going to add hierarchy
in the future it doesnbt matter, just with L2 everywhere you have that
option down the road without having to do a major redesign.
>>
>> The only thing you want to make sure *not* to do is to run L1 and L2 on all
links. This would be the same as running both OSPF area 0 and area 1 on all
links, or running two separate OSPF processes that are enabled on all links.
The only thing L1 and L2 will do for you if you run it everywhere is to
increase the amount of memory and CPU resources that your routers need, and
potentially add to your convergence time when a failure event occurs.
>>
>> You may want to check out the book bOSPF and IS-IS: Choosing an IGP for
Large-Scale Networksb by Jeff Doyle for more info on the comparison of the
two.
>>
>>
>> HTH,
>>
>> Brian McGahan, CCIE #8593 (R&S/SP/Security)
>> bmcgahan_at_INE.com
>>
>> Internetwork Expert, Inc.
>> http://www.INE.com
>>
>>
>> From: Ronnie Angello [mailto:ronnie.angello_at_gmail.com]
>> Sent: Wednesday, November 28, 2012 9:34 AM
>> To: shiran guez
>> Cc: Routing Freak; Cisco certification; Brian McGahan; Narbik Kocharians;
Marko Milivojevic; Brian Dennis
>> Subject: Re: ISIS L1 vs L2
>>
>> So at least we agree don't do L1 only... I honestly haven't done much real
world IS-IS design, but my reference is Optimal Routing Design (Chapter 5,
page 190).
>>
>> Ronnie
>>
>> Sent from my iPhone
>>
>> On Nov 28, 2012, at 9:56 AM, shiran guez <shiranp3_at_gmail.com> wrote:
>>
>> L1/L2 everywhere is not making much sense as you will have redundant
database for both L1 and L2. normally Core you will work L2 edge to "stub" you
will do L1/L2 and stub networks you will set L1, L2 will provide you future
flexibility, even if you think that you will not expand or change, it is not a
good design to do a limit yourself from the start, it does not cost anything
to do it L2, but it will cost plenty if you will need to change Core in
future.
>>
>> my 2 cents :-)
>>
>>
>> On Wed, Nov 28, 2012 at 3:18 PM, Ronnie Angello <ronnie.angello_at_gmail.com>
wrote:
>> A single L1/L2 domain would be best as it provides flexibility... That way
you already have a contiguous L2 domain. If the network grows, it's easier to
add an L1 routing domain than it is to add an L2 routing domain. You just add
an L1 IS to the edge and push the L1 domain into the network...
>>
>> Ronnie
>>
>> Sent from my iPhone
>>
>> On Nov 27, 2012, at 10:24 PM, Routing Freak <routingfreak_at_gmail.com>
wrote:
>>
>> > Hi all
>> >
>> > In my customer core network where we had a heated about wither OSPF or
ISIS
>> > and finally ISIS won the race for the core IGP due to some business
>> > decision and now my problem here is that when i was designing the
network
>> > with one large ISIS area with all the linka s L1, everyone opposed me to
>> > not to configure L1 and go for L2.
>> >
>> > I know that L1 is within single area and L2 can be connected across
areas
>> > and also within a single area and it carries all the routes within L1
and
>> > L2.
>> >
>> > But in my design , i have single large area with all links as L1, what
is
>> > the problem in that, L1 or L2 it should be the same.
>> >
>> > I didnt understood what is the logic behind the fact that L1 should not
be
>> > used and L2 should be used.
>> >
>> > I thought may be when they are forming more areas, then l2 makes sense,
but
>> > they r not going to expand the site with another area, so why not going
for
>> > L1 will suffice the requirement .
>> >
>> > L1 is within one single area and doesnt know any other routes of other
area
>> >
>> > L2 router is one where all the areas merge and exchange routes in one
>> > separate area. It can be any area and not area 0 and just all the
routers
>> > in that particular area should be running L2 adjacency with each other.
>> >
>> >
>> > In one large area, which is better L1 or L2. Any ISIS Experts, be sure
to
>> > reply to this.
>> >
>> >
>> > Blogs and organic groups at http://www.ccie.net
>> >
>> > _______________________________________________________________________
>> > Subscription information may be found at:
>> > http://www.groupstudy.com/list/CCIELab.html
>>
>>
>> Blogs and organic groups at http://www.ccie.net
>>
>> _______________________________________________________________________
>> Subscription information may be found at:
>> http://www.groupstudy.com/list/CCIELab.html
>>
>>
>>
>>
>>
>>
>>
>>
>>
>>
>> --
>> Shiran Guez
>> MCSE CCNP NCE1 JNCIA-ENT JNCIS-ENT CCIE #20572
>> http://cciep3.blogspot.com
>> http://www.linkedin.com/in/cciep3
>> http://twitter.com/cciep3

Blogs and organic groups at http://www.ccie.net
Received on Wed Nov 28 2012 - 12:43:21 ART