The MPLS WG Archive

[Bob Thomas <rhthomas@cisco.com>]

Harikishan,

> Q1::
> 
> Section 3.4.5.1.2 LDP RFC 3036 states that, when a label mapping message =
> 
> is received with a pathvector TLV and loop is detected, the LSR should =
> transmit
>  a label release message carrying a Status TLV to the sending LSR to =
> signal
> "Loop Detected".
> 
>  But,the Label release message(3.5.11) does not include a status TLV in =
> it's
> optional List of TLVs? So, what is the way to inform downstream that the =
> 
> label release was a result of loop detected by the upstream??

Please refer to RFC3036, Section 3.4.6 - Status TLV, page 45:

   Note that use of the Status TLV is not limited to Notification
   messages.  A message other than a Notification message may carry a
   Status TLV as an Optional Parameter.  When a message other than a
   Notification carries a Status TLV the U-bit of the Status TLV should
   be set to 1 to indicate that the receiver should silently discard the
   TLV if unprepared to handle it.


> Q2::
> 
>     Check_Received_Attributes (A.2.6) algorithm is in such a way that =
> the=20
>  Path Vector TLV will be processed only when HopCount TLV is present=20
> in the Label Request/Mapping message. ( CRa.1)
> 
> Does that mean Loop Detection can be used only along with HopCount
> and it can't be used independently ( without HopCount )?? I know that

Yes.

Please refer to RFC 3035, Section 2.8 - Loop Detction, particularly
the bullets in Section 2.8.1 - Label Request Message, pages 24 and 25, and
Section 2.8.2 - Label Mapping Message, page 26:

   The rules that govern use of the Hop Count TLV in Label Request
   messages by LSR R when Loop Detection is enabled are the following:

   -  The Label Request message MUST include a Hop Count TLV.

and

   The rules that govern the use of the Hop Count TLV in Label Mapping
   messages sent by an LSR R when Loop Detection is enabled are the
   following:

   -  R MUST include a Hop Count TLV.


Bob



> sometimes (for ATM/FR LSRs) HopCount is a MUST in Label Request
> messages and HC may or may not be present in Label mapping messages
> based on the Merge type of the LSR.
> 
> The LDP MIB  "draft-ietf-mpls-ldp-mib-07.txt" has the following
> loop detection capabilities.
> " mplsLdpLsrLoopDetectionCapable OBJECT-TYPE
> 
> SYNTAX INTEGER {
> 
> none(1),
> 
> other(2),
> 
> hopCount(3),
> 
> pathVector(4),
> 
> hopCountAndPathVector(5)
> 
> }"
> 
> 
> 
> which implies that pathVector(4) should be configurable independent=20
> 
> of anyother loop detection mechanism ( like HopCount). This contradicts
> 
> with the description of Check_Received_Attributes (Cra.1) Algorithm=20
> 
> in RFC 3036 which states that PathVector TLV will not be processed
> 
> if there is no HopCount TLV in the received message.
> 
> 
> 
> *******
> 
> Thanks,
> HariKishan