[j-nsp] LDP Max PDU Length

Saku Ytti saku+juniper-nsp at ytti.fi
Thu Mar 9 01:27:55 EST 2006

On (2006-03-08 18:08 -0500), Eric Van Tol wrote:

> I'm performing some MPLS testing between a J2300 and a Telco Systems
> TM-100 switch.  I'm attempting to get an LDP session established between
> the two devices and it's not working, failing on the Juniper with a "Bad
> PDU Length" message.  The Max PDU Length from the Juniper shows it to be
> a value of 4096.  The TM100 shows, in the LDP Initialization Message,
> that the Max PDU is 1440.  

 If your physical MTU's really are matching, you could try to decrease
JNPR's ISO/CLNS MTU (set int <int> unit <unit> family iso mtu <mtu>) to
match that of telcos MTU. It may be that telco isn't smart enough 
to increase ISO/CLNS MTU from 1497.

> It's my understanding from RFC3036 that the Max PDU is negotiated
> between the two neighbors and the smaller of the two is used.  However,
> it goes on to say that the receiving node can reject the PDU length if
> it is too small or too large.
> My questions:
> 1.  How exactly is the max PDU length determined - is it based of the
> MTU of the physical medium?
> 2.  If it's an arbitrary value, why wouldn't the value then be
> negotiated to 1440?
> 3.  What exactly determines that a received PDU length is
> "unacceptable"?

 debug, see what PDU length is reported in the TLV and see how long the
received PDU was.
 One thing I've noticed about telcos (not sure if it applies to TM-100) is
that they put each network address to it's own fragment, consequently PDU's
will be rather small, and would never go above 1500, so supporting higher
MTU wouldn't make sense for them.
 In typical CSCP/JNPR network you will not have any LSP's fragmented.

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