The MPLS-OPS Archive

[Rajiv Asati <rajiva@cisco.com>]

At 11:50 AM 7/24/2002, CEW CEW wrote:
>Hi Rajiv,
>
>Thanks for the clarification,
>
>[r1]----[r2P]-----[r3P]-----[r4P]
>
>r2P is a 7206 with IOS 12.1.18ST, LC=PA-8E
>r3P=r4P is a 7507 with IOS 12.1.10 LC=non VIP based
>AUI ethernet
>
>So are u saying that for certain card that does not
>support jumbo frame, the packet will be dropped unless
>u use the tag-switching mtu command ?

Nope. What I am saying is that the "mpls mtu <>" command is not dependent 
up on the jumbo frame.
Some of the interfaces (ethernet chipsets actually) are capable of handling 
more than the designated IP MTU size, and by having "mpls mtu <>" 
configured gives a way to send a frame larger than 1500 bytes. I guess the 
intention of this command is to still send 1500 bytes of IP data in MPLS 
enabled interface. Of course, the total frame size on the wire may exceed 
1518 bytes by 4 or 8 bytes ( 1 or 2 labels) in this case. Of course, this 
is not really targeting the jumbo frame.

Also, if the other end is the switch, then it is required to have baby 
giant support. Baby giant is nothing but the acronym for the tagged packets 
which are slightly larger than 1518 bytes. See

http://www.cisco.com/warp/public/473/148.html


> > See the difference in the outputs of "sh int <>" and
> > "sh tag int <> de" and
> > notice the MTU size.
>
>Oh, i didn't noticed this.
>So, is there a way using show command for e.g, to find
>out which LC/platform really need the tag-switching
>mtu command ?

Not that I am aware of.
Well, I would prefer to do "MTU discovery" and adjust accordingly.

>THis is not clearly documented in CCO and is rather confusing at times.

I will look for a good CCO website, but in the mean time, you may want to 
read Jim Guichard's book on MPLS VPN Architecture. The book explains MTU 
issues in MPLS network quite nicely.

Rajiv


>Rgds
>Eng Wee
>
>
>
>
>
>--- Rajiv Asati <rajiva@cisco.com> wrote:
> > Eng Wee,
> >
> > Please see inline..
> >
> > At 10:00 PM 7/23/2002, CEW CEW wrote:
> > >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 ?
> >
> > Depends. See below.
> >
> > >(2) what is the significance of the value used
> > >in the tag-switching mtu command.
> >
> > This is specific to the MPLS packets.
> > This command however assumes that the outgoing
> > interface is capable of
> > transmitting a packet bigger than the maximum"IP
> > MTU" size. Depending upon
> > the platform and the LC, this assumption may or may
> > not be true.
> >
> > See the difference in the outputs of "sh int <>" and
> > "sh tag int <> de" and
> > notice the MTU size.
> >
> > I would be interested to know the platform, LC and
> > IOS version you had
> > tried with.
> >
> > Rajiv
> >
> >
> > >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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