The MPLS-OPS Archive

[Robert Raszuk <raszuk@cisco.com>]

Krzysiek,

No. The MTU in the output of "show mpls forwarding-table X.X.X.X.
detail" has nothing to do with MTU :). The term "MTU" there is a
misnomer, it should be actually a maximum receive unit.

If the "MTU=" there is xyz then it implies that a labeled packet of size
xyz bytes can be *received* by the router and sent out on interface
indiacted without needing to fragment it. the MTU printed out depends on
each prefix and could have different values depending on how many labels
are pushed/popped.

It also has nothing to do with LDP. LDP at least for now does not
advertise MTU :). I am assuming you are talking about extensions
proposed in: draft-black-ldp-mtu-extensions-00.txt right ?

R.


> Krzysztof Szarkowicz wrote:
> 
> Check what MTU size is advertised by your LDP protocol (show mpls forwarding-table X.X.X.X. detail)
> 
> Regards,
> Krzysztof Szarkowicz
> 
> ----- Original Message -----
> From: "Donn Lasher" <dlasher@clearskynet.net>
> To: <mpls-ops@mplsrc.com>
> Sent: Thursday, February 21, 2002 8:26 PM
> Subject: MPLS & Path MTU problems
> 
> >
> > I've read all the MPLS-OPS archives on this issue, and tried the fixes, and
> > I'm still seeing a problem.
> >
> > two 3620's running 12.2.6, hooked to a 3524 switch. Multiple MPLS/Dot1Q
> > vlan's off both routers. When traffic passing through these routers from a
> > customer to a site blocking PMTU discovery(nice long list any more), it all
> > the traffice appears to the customer to be blackholed.
> >
> > ip icmp debug shows:
> >
> > Feb 20 22:39:11 X.X.X.X 5813: 3w5d: ICMP: dst (X.X.X.X) frag. needed and DF
> > set unreachable sent to 216.221.138.92
> > Feb 20 22:39:12 X.X.X.X 5815: 3w5d: ICMP: dst (X.X.X.X) frag. needed and DF
> > set unreachable sent to 216.221.138.92
> >
> > At the moment I'm applying a route-map to clear df bit on my NSP connected
> > router, but that's a hack. I'd really like to find and solve the real issue.
> >
> > Ethernet MTU is 1500. tag-switching MTU unset (defaulting to 1500)
> > tag-switching ip set on all interfaces
> >
> > I've tried setting the tag-switching mtu to 1540, making sure the switch
> > was set mtu 2018, didn't make any difference.
> >
> > Thanks in advance for any help.
> >
> >
> >
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