Re: Jumbo frames - tag swithing mtu

Cew

From previous exchange on this mailing list (guess by R. Razuk) .

U need the "MTU" and "MPls MTU" to be config on the interface.

The MTU decide max length of unlabeled packet .

the MPls MTU decide the max length of labeled packet .

so it is expected that the ping will stop at 1500 as it is the max MTU

despite that U have set the "mpls mtu" to 1526 .

"Mpls MTU" = 1526 will allow u to pack a label stack of depth 6 for

an IPv4 packet of length 1500 .

Brgds

>From: CEW CEW

>To: rogerw@nordlink.com, mpls-ops@mplsrc.com

>Subject: Re: [MPLS-OPS]: Jumbo frames - tag swithing mtu

>Date: Tue, 23 Jul 2002 19:00:42 -0700 (PDT)

>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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Re: Jumbo frames - tag swithing mtu