The MPLS-OPS Archive

[CEW CEW <cewsg@yahoo.com>]

Hi...

since we are discussing about mtu, i did some test a
few days back and a little bit puzzled about the
behavior.

[r1]----[r2P]-----[r3P]-----[r4P]

tag switching is enabled between r2P, r3P and r4P.
all links are ethernet link.

I then do a ping sweep from r1 with mtu (1480 to 1540)
pinging
to r4P loopback address. All the ping packet can 
go through without a problem. Looks like the router
r2P is fragmenting the packet. I then do a ping with
DF bit set and the ping stops at mtu 1496, which make
sense becos 4 bytes of label are added when r2P send
the packet out. i then put a "tag-switching mtu 1526"
command on all tag switching interfaces. With DF bit
set, the ping stops at mtu 1500.

My questions are : 
(1) Do we need tag-swithing mtu command at all ?
(2) what is the significance of the value used
in the tag-switching mtu command.

Thanks
Eng Wee


--- rogerw@nordlink.com wrote:
> Hugo, I may be jumping into a comleted thread, but I
> will.
> 
> Every label on an MPLS packet adds 32 bits to the
> length of the packet. I can think of a possible
> stack of three labels(MPLS VPN, TE Tunnel, and
> hop-to-hop MPLS) and that would add 96 bits. If you
> are running Ethernet for instance, you could be over
> the standard 1500 and switches and routers that are
> not MPLS-aware would think these are jumbo frames
> and therefore out of spec, and delete them.
> 
> I hope that helps
> 
> Roger Williams
> 
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