The MPLS-OPS Archive

[Robert Raszuk <raszuk@cisco.com>]

Pls send me offline your PE's config.

Rgs,
R.

> "Loureiro, Rodrigo - (Bra)" wrote:
> 
> Hi Robert,
> 
> Thanks for response again.
> 
> I made some labs again, in order to simulate the problem and what i
> can say is: PE routers simply does not fragment the packets greater
> than the interface MTU. I tested with POS, ATM and FastEthernet and,
> with or wothout "tag mtu" command enabled, the PE router does not
> fragment the packets prior to sending them to P router. If you disable
> MPLS in the interface, it works perfectly.
> 
> It seems that layer 3 driver are being by-passed by MPLS forwarding
> mechanism. I don´t kow if it make sense but, in fact, packets are not
> being fragmented by layer 3. Have you ever seen a problem like this ?
> 
> Regards,
> --
> 
> Rodrigo Loureiro
> 
> -----Mensagem original-----
> De: Robert Raszuk [mailto:raszuk@cisco.com]
> Enviada em: sexta-feira, 19 de outubro de 2001 15:47
> Para: Loureiro, Rodrigo - (Bra)
> Cc: 'Alan Hannan'; mpls-ops@mplsrc.com
> Assunto: Re: RES: Problems w/ MTU
> 
> <MPLS@UU.NET removed>
> 
> The most likely you need to tell PE1 to fragment by configuring it :).
> 
> Try to set "tag mtu 1500" on your POS int of PE1.
> 
> R.
> 
> > "Loureiro, Rodrigo - (Bra)" wrote:
> >
> > Hi Alan,
> >
> > Thanks for response.
> >
> > I´m not sure that PE routers will fragment the packets. The real
> proof
> > of this is that the maximum ICMP packet that i could transfer
> between
> > PE1 and PE2, without set the DF bit, was a 1506 bytes packet (1510
> > maximum feasible ethernet frame minus 4 label bytes). If
> fragmentation
> > were being done in the PE1 POS, i should be able to ping PE2 with
> any
> > packet length, once POS is configured with a 1500 bytes MTU. Also, i
> 
> > enabled a debug and i could see that packets greater than 1500 bytes
> 
> > were not being fragmented.
> >
> > My doubt is: PE1 wants to send an IP packet to PE2, and this packet
> is
> > being generated by PE1. Once PE1 has an entry for PE2 in its LFIB,
> > should it respect the IP MTU and fragment the packet prior to label
> > it, or should it ignore the IP MTU, as P routers do w/ pure label
> > switching ?
> >
> > Regards,
> > --
> >
> > Rodrigo
> >
> > -----Mensagem original-----
> > De: Alan Hannan [mailto:alan@routingloop.com]
> > Enviada em: sexta-feira, 19 de outubro de 2001 13:00
> > Para: Loureiro, Rodrigo - (Bra)
> > Cc: mpls@UU.NET; mpls-ops@mplsrc.com
> > Assunto: Re: Problems w/ MTU
> >
> >   This is a known issue, operationally.
> >
> >   You must set the MTU of your Layer 3 transmittal driver to be
> small
> >   enough to allow needed Label Stacking.  I believe we set the MTU
> of
> >   much ethernet to 1490 or so to allow this, which also wreaks
> >   cosmetic habit with certain syslog-alarm-happy boxes...
> >
> >   In practice, most POS MTUs are 4470, not 1500, FWIW.
> >
> >   The router should fragment, and it will, but the switch will drop.
> 
> >
> >   -alan
> >
> > Thus spake Loureiro, Rodrigo - (Bra) (rodrigo.loureiro@attla.com)
> >  on or about Fri, Oct 19, 2001 at 02:32:59PM -0300:
> > > Hi all,
> > >
> > > I am investigating a problem with packet forwarding/switching in a
> 
> > MPLS
> > > scope regarding maximum transfer unit, and i would like to share
> my
> > doubts
> > > with you.
> > >
> > > Imagine a four routers setup: PE1, P1, P2 and PE2. PE1 is
> connected
> > to P1
> > > via POS, P1 is connected to P2 via ATM and P2 is connected to PE2
> > via a
> > > FastEthernet cross-connection. Imagine also that POS interface is
> > configured
> > > with a 1500 bytes MTU, ATM with 4470 bytes and FastEthernet with
> > 1500 bytes
> > > (consider this hardware is capable to generate Ethernet frames up
> to
> > 1510
> > > bytes of payload for labeling purposes).
> > >
> > > Now, consider that PE1 and PE2 has a MP-BGP session directly
> between
> > them.
> > > Considering that BGP is capable to generate messages up to 4096
> > bytes (RFC
> > > 1771), we should guarantee that packets greater than 1500 bytes
> sent
> > from
> > > PE1 toward PE2 are being fragmented. Instead of this, the packets
> > will be
> > > discarded in the FastEthernet connection between P2 and PE2, once
> > MPLS will
> > > not  fragment the packets.
> > >
> > > I made a test with ICMP from PE1 to PE2, generating packets
> greater
> > than
> > > 1500 bytes, and i could see that packets were not being fragmented
> 
> > by PE1.
> > > So, one question arises: What should be the correct behavior for
> IP
> > packets
> > > originated inside the router toward another router in a MPLS
> > environment:
> > > should it follow the interface MTU and fragment the packet or
> should
> > it
> > > ignore it and label switch the packet regardless of its lengh ?
> > >
> > >
> > > Regards,
> > > --
> > >
> > > Rodrigo Loureiro
> > > AT&T LA - Network Engineer

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