Subject: [Isis-wg] L2 LSP IP Internal Reachability Information TLV contents

From: "Ken Larmer" <[email-masked]>
Date: Tue, 28 Aug 2001 14:24:29 -0400

Hi,

	I am a bit confused as to what exactly should be present in the L2 LSP "IP
Internal Reachability Information" TLV? RFC-1195 states the following:

5.2 Overview of IP-Specific Information for IS-IS

If included in level 2 LSPs, this field includes only entries reachable by
the router, which originates the LSP, either via one of its interfaces, or
indirectly via level 1 routing.

	We are conducting Interoperability tests on various Integrated IS-IS
implementations. I am seeing conflicting "IP Internal Reachability
Information" in the L2 LSPs being generated throughout the common L1/L2
Domain. To make matters worse, I am seeing conflicting contents when I do a
reboot versus a "clear isis" with the same exact configuration?  Sometimes
the L2 LSP contains ONLY directly attached subnets and other times it
contains the union of L1 areas?

	The reason I raise these issues is because there MAY be a conflict in the
protocol specifications, which affects calculation of routes through an IS
with an LSPDB overload condition?

Let me explain using the following configuration:

	IP Subnet (10.10.10.0) [A]<L1/L2>[B]<L1/L2>[C]<L1/L2>[D] IP subnet
(11.11.11.0)
                                                               ^
                                             LSPDB Overload condition

	All 4 IS routers (A,B,C,D) share a common area address (same L1 Sub-Domain)
and are also part of the same L2 Domain. IS-B has a LSPDB overload condition
and advertises this in it's L1/L2 LSP by setting the LSPDB overload bit. The
other routers receive these LSPs and process them properly; they calculate
routes to directly attached destinations but do not use IS-B as a transient
IS. The problem is that an IP host connected to subnet 11.11.11.0 can still
reach subnet 10.10.10.0. This is made possible because IS-B is advertising,
in it's L2 LSP, the contents specified above in RFC-1195 Section 5.2. As a
consequence, IS-C and IS-D calculate IP subnet 10.10.10.0 to be reachable
via L2 routing, because they think IS-B has a direct connection to this
subnet.

	Am I missing something fundamental here? Of course, if the above is the
case then it will cause LSPDB overload conditions to be ineffective, given a
similar configuration. This means the draft-ietf-isis-transient could also
be made ineffective? Is this an implementation problem, protocol specific
problem or just me being out to lunch?

Any assistance would be greatly appreciated!

Cheers,
Ken Larmer

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---

From: mike shand <[email-masked]>
Date: Wed, 29 Aug 2001 10:10:18 +0100

Ken,
         I'm confused. What do you mean by L1/L2 LSP? Do you mean the 
overload bit is set in BOTH L1 and L2? In which case other routers should 
NOT be seeing these reachable at L1 OR L2.

Or do you mean that it is set in L1 only, and because the information is 
present in L2 LSPs from B (which do not have ovl set), the subnets appear 
reachable (correctly) at L2, but not at L1?

Or do you mean something else entirely?

         Mike

---

From: "Ken Larmer" <[email-masked]>
Date: Wed, 29 Aug 2001 09:23:43 -0400

Hi Mike,

	Sorry, I should have made this a bit clearer. The OVL bit is being set in
both the L1 and L2 LSPs originated by IS-B. But because the IS-B's L2 LSP
contains all of the IP subnets within the L1 area (all IS routers are in the
same area), there exists a route to subnet 10.10.10.0 via L2 from IS-C and
IS-D.

	Say each link has a metric = 10. IS-B is advertising L2 reachability to IP
subnet 10.10.10.0 with a metric = 20.  IS-C and IS-D install an L2 IP route
in their routing table to IP subnet 10.10.10.0 via IS-B. IS-B possesses an
L1 IP route to IP subnet 10.10.10.0 via IS-A. So IS-B installs a L1 IP route
to 10.10.10.0 in its IP routing table. Conversely, IS-A possesses a L2 IP
route to IP subnet 11.11.11.0 via IS-B. IS-B possesses an L1 IP route to IP
subnet 11.11.11.0 via IS-C...

	Consequently, we possess an IP route from IS-D (subnet 11.11.11.0) to IS-A
(subnet 10.10.10.0), which uses the following path. Source = IS-D and
Destination = IS-A. IS-D forwards the IP packet to IS-C via L1, IS-C
forwards the IP packet to IS-B via L1 and then IS-B forwards the IP packet
to IS-A via L1.

	Mike, does the above scenario defeat the purpose of setting the LSPDB OVL
bit?

Cheers,
Ken Larmer

> -----Original Message-----
> From: [email-masked]
> [mailto:[email-masked]]On Behalf Of mike shand
> Sent: Wednesday, August 29, 2001 5:10 AM
> To: Ken Larmer
> Cc: Isis-Wg@Spider. Juniper. Net; Ken Larmer
> Subject: Re: [Isis-wg] L2 LSP IP Internal Reachability Information TLV
> contents
>
>
> Ken,