The MPLS-OPS Archive

[CEW CEW <cewsg@yahoo.com>]

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 ?

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

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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