Subject: [Isis-wg] Protocols Supported field in pseudonode LSP

From: "Chan, Chung Ming (Adtech ENG)" <[email-masked]>
Date: Sun, 25 Nov 2001 13:15:08 -1000

Hi,

I found out that in one vendor's pseudonode LSPs, there is no "Protocols
Supported" field included; but it includes the field in the non-pseudonode
LSPs.

Shall the Protocols Supported field MUST be included in pseudonode LSP
according to RFC 1195?

Not sure if there is any reason for that particular vender for doing it.

Could someone please verify this ?

RFC 1195 Section 3.1, paragraph 3:
"IP-capable (i.e., all IP-only and dual) routers need to know what network
layer protocols are sup-ported
by other routers in their area. This information is made available by
inclusion of a "proto-cols
supported" field in all IS-IS Hello and Link State Packets. This field makes
use of the NL-PID
(Network Layer Protocol Identifier), which is a one-octet value assigned by
ISO to identify
network level protocols. NLPID values have been assigned to ISO 8473 and to
IP."

Thanks,

Ming Chan
_______________________________________________
Isis-wg mailing list  -  [email-masked]
http://external.juniper.net/mailman/listinfo/isis-wg

---

From: Tony Li <[email-masked]>
Date: Sun, 25 Nov 2001 17:06:48 -0800

| I found out that in one vendor's pseudonode LSPs, there is no "Protocols
 | Supported" field included; but it includes the field in the non-pseudonode
 | LSPs.
 | 
 | Shall the Protocols Supported field MUST be included in pseudonode LSP
 | according to RFC 1195?
 | 
 | Not sure if there is any reason for that particular vender for doing it.
 | 
 | Could someone please verify this ?
 | 
 | RFC 1195 Section 3.1, paragraph 3:
 | "IP-capable (i.e., all IP-only and dual) routers need to know what network
 | layer protocols are sup-ported
 | by other routers in their area. This information is made available by
 | inclusion of a "proto-cols
 | supported" field in all IS-IS Hello and Link State Packets. This field makes
 | use of the NL-PID
 | (Network Layer Protocol Identifier), which is a one-octet value assigned by
 | ISO to identify
 | network level protocols. NLPID values have been assigned to ISO 8473 and to
 | IP."

Interesting question.

Well, the pseudonode LSP is really there to represent the broadcast medium,
not any particular router.  And since the broadcast medium is presumably
protocol independent, it doesn't seem like it actually conveys any useful
information.

Thus, IMHO, it would make sense if implementations ignored the Procotols
Supported TLV in a pseudonode LSP and not require it be present.

Tony

---

From: Jeff Learman <[email-masked]>
Date: Sun, 25 Nov 2001 21:20:50 -0500

I agree with Tony.  There are some additional things to consider,
though.

In accordance with RFC 1195, an implementation doesn't need
to consult an LSP's supported protocols option when running SPF
except for links to end systems, and a pseudonode LSP should have
only links between intermediate systems.  (Rather than relying on
the protos supported option, RFC 1195 relies on the deployment
rules.)

My understanding is that some routers do take the protos-supported
option when running SPF.  This allows some relaxation of the deployment
rules.  There is no interoperability problem, because any strict
implementation won't work anyway, in situations where this modification
helps.

However, in networks where this modification is being used, and
mixed networks don't meet the deployment requirements in 1195,
will it be important to make sure the designated router is a dual
router?  I can't find anything in 1195 that says to check protos-
supported when bringing up an adjacency.  If for any reason a
system would not form an adjacency with a router with which it
shared no protocols, that would cause a problem in the pseudonode
LSPs.  Hopefully this isn't a problem.

Regards,
Jeff

PS:  I don't work in IS-IS at Cisco.  My comments do not necessarily
reflect Cisco's opinions and I have no detailed knowledge of the Cisco
IS-IS implementation.