Subject: Re: [Isis-wg] RE: Regarding draft-ietf-isis-wg-mib-06.txt

From: Jeff Learman <[email-masked]>
Date: Fri, 21 Dec 2001 13:21:19 -0500

I'm surprised that the default for level would be L1+L2.  If
most ISIS routers are deployed as L1+L2 routers, then either
they're only used in small domains or else they're not being
deployed properly.  I would expect the typical router to be
L1, with L1+L2 routers in much smaller quantities.  It's
supposed to be hierarchical and scalable, after all.

Even if today, ISIS routers tend to be L1+L2, does that mean
we should make it the default?  Shouldn't we be encouraging
the proper, hierarchical deployment?  Or am I missing something?

More minor comments below.

At 12:20 PM 12/21/2001, Jeff Parker wrote in response to Vishwas Manral:
[snip]
>> 3. Shouldn't isisSysMaxLSPGenInt be less than MaxAge(1200). 
>> May be we could
>> define a texual convention for this and declare for both 
>> isisSysMaxLSPGenInt
>> and isisMinLSPGenInt.
>
>While MaxAge is specified in the protocol, there are vendors
>who do not restrict the lifetime of the LSPs to this value.

Furthermore, MaxAge is a very reasonable default setting for this
parameter.  Increasing it increases the load, but with the advantage
only that certain relatively rare problems are corrected sooner.

>> 4. isisSysMaxAreaAddress shouldnt a value of 0 be accepted too. It is
>> regarded to be equiv to 3 right?
>
>In a PDU on the wire MaxArea 0 means 3.  
>I see no reason to preserve this behavior in the MIB.  

It would be reasonable if we wanted a way to force 0 or 3 to be used
in the PDUs.  This would only be valuable because of a problem in
some router, I suppose.  I don't feel strongly on this, just my $0.02.

Regards,
Jeff

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---

From: Naiming Shen <[email-masked]>
Date: Fri, 21 Dec 2001 16:12:56 -0800

]
 ]I'm surprised that the default for level would be L1+L2.  If
 ]most ISIS routers are deployed as L1+L2 routers, then either
 ]they're only used in small domains or else they're not being
 ]deployed properly.  I would expect the typical router to be
 ]L1, with L1+L2 routers in much smaller quantities.  It's
 ]supposed to be hierarchical and scalable, after all.
 ]
 ]Even if today, ISIS routers tend to be L1+L2, does that mean
 ]we should make it the default?  Shouldn't we be encouraging
 ]the proper, hierarchical deployment?  Or am I missing something?
 ]

I don't think the issue is the number of routers in which level
determines the default setting, or the default setting means people
actually runs isis in both level-1/level-2. The reason some vendors
(correctly) use level-1/level-2 as default is because a user can
drop the router in any topology which will work.

If the L1/L2 router is in pure L1 area, then it behaves like a L1
router since no neighbor will talk with it with L2 packets. If the
L1/L2 router is in the "border" position, then its perfect. If the
L1/L2 router is in a pure L2 domain, then it only works like a L2
router.

There is a rare pathological case if the default is L1 for IS-IS
and no one bothers to change it: all the routers in the network are
in one area and running L1/L2 mode, if the user forgets to change the
level setting and drop this new L1 router in the middle, it will think
all the neighbors as gateways and it may blackhole the traffic. The
default of L1/L2 comes handy in this case.

thanks.
- Naiming

---

From: Jeff Learman <[email-masked]>
Date: Sat, 22 Dec 2001 12:37:15 -0500

I believe that defaults should be optimized for when one is deploying
a large number of systems, rather than just one.  But minimizing the
settings that must be made when deploying a single router in an
existing network is also reasonable.

I think it's more important to make it easy to put together networks
that are configured correctly than to make it easy for networks to work
even when misconfigured, though, so I still recommend L1 as the default.

In any case, though, will this draft really affect how any vendor's
implementation will default?  Perhaps the default should be unspecified,
since there are good arguments to go either way.  (On the other hand,
this IS a standards group!)

Jeff

As usual, please note that I don't represent Cisco.