Subject: [Isis-wg] Doubt regarding IP Internal Reach TLV

From: "R Selvaraj" <[email-masked]>
Date: Wed, 23 May 2001 00:43:04 -0700

Hi,
Let me resend my last doubt. Actually, I would like to know wheather a L12 router shall include its local L2 circits IP adress in its L1 LSP's Internal Reach TLV and vice versa for its L2 LSP.

Thanks,
Selva.

>From: "R Selvaraj" <[email-masked]>
>To: [email-masked]
>SUBJECTDate: Mon, 21 May 2001 01:36:12 -0700
>
>Hi,
>Section 5.3.4 of RFC 1195 states that IP addresses within the routing domain reachable directly via one or more interfaces on the router should be encoded in the L1 LSP's IP Internal reachability TLV. reachability . Here, do we need to consider circuit level also. I mean, a L12 router while encoding its L1 LSP's IP Inernal Reachability TLV should it encode only all of its L1 circit's IP address and should not consider its L2 circuit's IP address at all.
>
>I found some implementation, for L12 router, not encodeing L2 circuit's IP address in its L1 LSP's IP Internal Reach TLV.
>
>It'll be helpful if I get clarified.
>
>Thanks,
>Selva.
>
>
>
>
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---

From: "Ken Larmer" <[email-masked]>
Date: Wed, 23 May 2001 09:53:19 -0400

Hi Selva,

	My inclination would be to only place L1 routes into L1 LSPs and L2 routes
into L2 LSPs.

However, I just conducted some testing on a well-known platform and the
results are a bit puzzling? If I have 2 circuits, both set to circuit-type
L1/L2, one circuit has adjacencyUsage = L1/L2 (matching area) and the other
with adjacencyUsage = L2-Only (no matching area), I get the following
results:

L1 LSP IP Internal Reachability Information = IP subnets from both circuits
L2 LSP IP Internal Reachability Information = IP subnets from both circuits

	When I change the circuit which does not have a matching area to
"circuit-type = L2-Only", so that I now have two circuits with
adjacencyUsage = L1/L2 (matching area) and adjacencyUsage = L2-Only
(non-matching area and circuit-type = L2-Only), I get the following results:

L1 LSP IP Internal Reachability Information = IP subnet from circuit with
adjacencyUsage = L1/L2
L2 LSP IP Internal Reachability Information = IP subnets from both circuits

????

Cheers,
Ken

-----Original Message-----
From: [email-masked]
[mailto:[email-masked]]On Behalf Of R Selvaraj
Sent: Wednesday, May 23, 2001 3:43 AM
To: [email-masked]
Subject: [Isis-wg] Doubt regarding IP Internal Reach TLV

Hi,
Let me resend my last doubt. Actually, I would like to know wheather a L12
router shall include its local L2 circits IP adress in its L1 LSP's Internal
Reach TLV and vice versa for its L2 LSP.

Thanks,
Selva.

>Section 5.3.4 of RFC 1195 states that IP addresses within the routing
domain reachable directly via one or more interfaces on the router should be
encoded in the L1 LSP's IP Internal reachability TLV. reachability . Here,
do we need to consider circuit level also. I mean, a L12 router while
encoding its L1 LSP's IP Inernal Reachability TLV should it encode only all
of its L1 circit's IP address and should not consider its L2 circuit's IP
address at all.
>
>I found some implementation, for L12 router, not encodeing L2 circuit's IP
address in its L1 LSP's IP Internal Reach TLV.

---

From: Henk Smit <[email-masked]>
Date: Wed, 23 May 2001 11:09:10 -0700 (PDT)

I haven't checked rfc1195, but the following has always been
 my understanding.

> Let me resend my last doubt.
> Actually, I would like to know wheather a L12 router shall include its
> local L2 circits IP adress in its L1 LSP's Internal Reach TLV

  No.
 Routers inside an area should only know about "things" that are living
 inside that area. "Things" include routers, adjacencies, IP prefixes.
 If a L1L2 router has prefix-X configured on a L2-only circuit, then
 IMHO that prefix-X is not part of the L1 area, and thus the L1L2 router
 should not advertise prefix-X in its L1 LSP.
  Prefix-X will be reachable because the L1L2 router has the "attached bit"
 set in its L1 LSP. (Unless this router can not reach other areas, then
 it doesn't set the attached-bit, and L1-only routers can not reach prefix-X.
 Too bad, such is life).

  If a L1L2 router implements the extensions from RFC2966, then a L1L2
 router is allowed to advertise all or any of the IP prefixes it has learned
 via L2 router. This includes directly connected prefixes configured on
 L2-only circuits.

> and vice versa for its L2 LSP.

  A L1L2 router is responsable for reachability from the L2 backbone to all
 IP prefixes that live inside its L1 area. Therefor a L1L2 router must
 advertise all IP prefixes that live in its L1 area, inside its L2 LSP.
 IMHO this includes all IP prefixes configured on directly connected
 L1-only circuits.

  Of course when you configure summary-addresses, individual L1 prefixes
 might not be advertised in the L2 LSP, because they are covered by the
 summary address.

  So the "problem" that you see is that a L1L2 router advertises the
 IP prefixes configured on L2-only circuits in its L1 LSP ? Well, IMHO
 that would not be necessary. But it wouldn't hurt eiter. (Except for
 the fact that the L1 LSP would be a little bigger. No big deal).
 The benefit of doing this would be the case I pointed out above.
 If the L1L2 router looses contact to other areas, it will unset the
 attached bit in its L1 LSP. Then L1-only routers would not be able
 to reach the IP prefixes configured on the L2-only circuits of the
 L1L2 router. If a router always advertises the prefixes configured
 on its L2-only circuits in its L1 LSP, then this problem will not
 occur.

    Hope this helps,

         henk.

---

From: "Omer Sali" <[email-masked]>
Date: Thu, 24 May 2001 09:36:50 +0300

Hi Ken,

The IP Internal Reachability Information includes all the subnets ISIS knows
from all the interfaces it runs on. The RFC says nothing about a condition
like: "advertise only IP's of interfaces you have an adjacency over them".
For example, in section 1.3 of RFC 1195 you can find:
"Specifically, zero or more [IP address, subnet mask, metric] combinations
may be included in each Link State Packet. Each level 1 router is manually
configured with the [IP address, subnet mask, metric] combinations which are
reachable on each interface. "

So, in the first case you have discribed you had ISIS running on the
interface at level 1 so you saw the IP address advertised in the L1 LSP
(although you had no adjacency on L1). In the second case ISIS did not ran
on L1 so the router did not advertise it in its L1 LSPs.

By the way, most implementations allow you to run ISIS on an interface in
"passive mode" saying: "don't exchange packets (don't raise adjacencies) but
advertise the IP of the interface as IP Internal Reachability".

Hope it helps,

Omer