Subject: [mpls] A question about Hop count TLV?

From: Mach Chen <[email-masked]>
Date: Thu, 06 Sep 2007 18:03:04 +0800

Hi MPLSers,

I was confused by some description about Hop count TLV in
RFC3036/rfc3036bis.

Section 2.8.1. Label Request Message, says:
 
      -  The Label Request message MUST include a Hop Count TLV.
Section 2.8.2. Label Mapping Message, says:
   -  R MUST include a Hop Count TLV.
 
Section 3.4.4. Hop Count TLV, says:
   The Hop Count TLV appears as an optional field in messages that setup
LSPs.  It calculates the number of LSR hops along an LSP as the  LSP is
being setup.

If My understanding is correct, the Hop Count TLV will only be carried in
Label Request Message and Label Mapping Message, so the question is how to
understand the Hop count TLV is "optional" and "MUST" be included in Label
Request/Mapping message? Thanks!

Best regards,

Mach Chen

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---

From: "Bob Thomas \(rhthomas\)" <[email-masked]>
Date: Thu, 6 Sep 2007 07:37:49 -0400

Hi Mach,
 
Section 2.8 describes the LDP loop detection mechanism which requires
use of the Hop Count TLV.  The Hop Count TLV is optional when LDP loop
detection is not in use.
 
Bob

________________________________

	From: Mach Chen [mailto:[email-masked]] 
	Sent: Thursday, September 06, 2007 6:03 AM
	To: [email-masked]
	Subject: [mpls] A question about Hop count TLV?

	Hi MPLSers,

	 
	 
	Section 3.4.4. Hop Count TLV, says:
	   The Hop Count TLV appears as an optional field in messages
that setup LSPs.  It calculates the number of LSR hops along an LSP as
the  LSP is being setup.

	If My understanding is correct, the Hop Count TLV will only be
carried in Label Request Message and Label Mapping Message, so the
question is how to understand the Hop count TLV is "optional" and "MUST"
be included in Label Request/Mapping message? Thanks!

	Best regards,

	Mach Chen

---

From: Mach Chen <[email-masked]>
Date: Fri, 07 Sep 2007 09:16:23 +0800

Hi Bob,

But if a LSR enable loop detection, but its peer may not support Loop
Detection (I found some vendor’s implement does not support Loop
Detection), means the peer does not recognize the Hop Count TLV,  such
condition will result in failing to setup LSPs between such two LSRS. Is it
reasonable?  Since no LSP will be setup between such two LSRs,  there is no
need to establish a LDP session between such two LSRs. Am I right ?

Why not a LSR negotiates the Hop Count TLV “capability” with its peer,
just like Path Vector TLV does (D bit)? Or Is there should be a mechanism
that could help a LSR  to know its peers whether support Loop detection
during session establishment ?  

Mach

  _____  

发件人: Bob Thomas (rhthomas) [mailto:[email-masked]] 
发送时间: 2007年9月6日 19:38
收件人: Mach Chen; [email-masked]
主题: RE: [mpls] A question about Hop count TLV?

Hi Mach,

Section 2.8 describes the LDP loop detection mechanism which requires use of
the Hop Count TLV.  The Hop Count TLV is optional when LDP loop detection is
not in use.

Bob

  _____  

Hi MPLSers,

 

If My understanding is correct, the Hop Count TLV will only be carried in
Label Request Message and Label Mapping Message, so the question is how to
understand the Hop count TLV is “optional” and “MUST” be included in
Label Request/Mapping message? Thanks!

Best regards,

Mach Chen