From: Brian McGahan (bmcgahan@internetworkexpert.com)
Date: Sat Nov 15 2003 - 21:58:44 GMT-3
The reason is because of 802.1q tunnels. Much like MPLS, .1q
tunneling adds an additional header on to the frame as it's transiting
the provider network. This external tag is used to determine which
locally significant VLAN the packet should belong to. When this
additional VLAN header of 4 bytes is added, a packet can have a max size
of 1504 bytes. Instead of fragmenting the packets at layer 2, they just
bumped up the MTU a bit. Cisco engineering at it's finest ;)
HTH,
Brian McGahan, CCIE #8593
bmcgahan@internetworkexpert.com
Internetwork Expert, Inc.
http://www.InternetworkExpert.com
Toll Free: 877-224-8987
Direct: 708-362-1418 (Outside the US and Canada)
> -----Original Message-----
> From: nobody@groupstudy.com [mailto:nobody@groupstudy.com] On Behalf
Of
> ccie2be
> Sent: Saturday, November 15, 2003 11:00 AM
> To: seonghui@vads.com; ccielab@groupstudy.com
> Subject: Re: OSPF - MTU mismatch
>
> Hmmm, that's very interesting. Very strange but interesting. Anyone
know
> why that is?
>
> ----- Original Message -----
> From: "seonghui" <seonghui@vads.com>
> To: "'ccie2be'" <ccie2be@nyc.rr.com>; <ccielab@groupstudy.com>
> Sent: Saturday, November 15, 2003 1:43 PM
> Subject: RE: OSPF - MTU mismatch
>
>
> > Hi ccie2b,
> >
> > i believe the default MTU for cat3550 is 1500. If you do a delete
> vlan.dat,
> > MTU size will change to 1504. I had this problem when i was
preparing
> for
> my
> > lab exam. Thanks to scott and matijevic for helping me to solve the
> problem.
> >
> >
> > -----Original Message-----
> > From: ccie2be [mailto:ccie2be@nyc.rr.com]
> > Sent: Sunday, November 16, 2003 2:35 AM
> > To: matijevi@bellsouth.net; seonghui; 'Silvio Nunes';
> > ccielab@groupstudy.com
> > Subject: Re: OSPF - MTU mismatch
> >
> >
> > Thanks for getting back to me on this. However, I was more concerned
> with
> > the fact that by default, the MTU size on the Cat 3550 is different
than
> the
> > default MTU size on Cisco routers. I was aware that the MTU size on
> OSPF
> > neighbors need to match for an adjacency to form as the link you
> provided
> > proved. However, the example, in the link shows that on one router
the
> MTU
> > size had been manually changed from it's default setting.
> >
> > In the originial question, there was a problem because the default
MTU
> on
> > the Cat is different than the default MTU size on the router.
> >
> > This leads to the obvious question of why the Cat 3550 has a
different
> > default MTU size than Cisco routers?
> >
> > Another question is what other potential problems lay in wait
because of
> > this different default MTU size?
> >
> > I would think that if Cisco decided to have a different default MTU
size
> on
> > the Cat 3550 compared with it's routers, there's probably a reason
for
> that
> > and since this will cause problems, at least with OSPF, this
difference
> > would be well documented. Furthermore, I would expect that Cisco
would
> > highlight those features and functions that depended on the MTU size
> > matching between the Cat 3550 and routers, since those features
won't
> work
> > if the MTU size isn't manually changed.
> >
> > Form a lab point of view, I just think it would be good to have a
list
> of
> > those features/functions that require the MTU size to be manually
> changed
> so
> > that if something isn't working one would know to wonder if this
problem
> is
> > a result of a MTU mismatch.
> > ----- Original Message -----
> > From: <matijevi@bellsouth.net>
> > To: "ccie2be" <ccie2be@nyc.rr.com>; "seonghui" <seonghui@vads.com>;
> "'Silvio
> > Nunes'" <silvio_98@hotmail.com>; <ccielab@groupstudy.com>
> > Sent: Saturday, November 15, 2003 12:55 PM
> > Subject: Re: OSPF - MTU mismatch
> >
> >
> > > Hello CCIE2be,
> > > This is documented on Cisco's web site, I also believe it is
> documented
> in
> > > Doyle vol1 book. This problem has also been discussed before on
> > Groupstudy,
> > > search the archives and I am sure you can find it. Here is the
link on
> > > cisco's website:
> > >
> >
>
http://www.cisco.com/en/US/tech/tk365/tk480/technologies_tech_note09186a
00
> 80
> > 093f0d.shtml
> > > Sincerely,
> > > Matijevic
> > > ----- Original Message -----
> > > From: "ccie2be" <ccie2be@nyc.rr.com>
> > > To: "seonghui" <seonghui@vads.com>; "'Silvio Nunes'"
> > > <silvio_98@hotmail.com>; <ccielab@groupstudy.com>
> > > Sent: Saturday, November 15, 2003 12:37 PM
> > > Subject: Re: OSPF - MTU mismatch
> > >
> > >
> > > > I've checked the 3550 documentation and didn't see anything
> regarding
> > the
> > > > need to change MTU for OSPF or any other routing protocol for
> > adjacencies
> > > to
> > > > properly form. Is this problem specific MTU size documented
> anywhere
> or
> > > is
> > > > this something that you had best just know.
> > > >
> > > > Thanks
> > > >
> > > > ----- Original Message -----
> > > > From: "seonghui" <seonghui@vads.com>
> > > > To: "'Silvio Nunes'" <silvio_98@hotmail.com>;
> <ccielab@groupstudy.com>
> > > > Sent: Saturday, November 15, 2003 11:24 AM
> > > > Subject: RE: OSPF - MTU mismatch
> > > >
> > > >
> > > > > global command --> system MTU 1500
> > > > >
> > > > > follow by a reload
> > > > >
> > > > > -----Original Message-----
> > > > > From: nobody@groupstudy.com [mailto:nobody@groupstudy.com]On
> Behalf
> Of
> > > > > Silvio Nunes
> > > > > Sent: Saturday, November 15, 2003 11:06 PM
> > > > > To: ccielab@groupstudy.com
> > > > > Subject: OSPF - MTU mismatch
> > > > >
> > > > >
> > > > > Group,
> > > > >
> > > > > When I try to establish an relationship between Interface
Vlan1
> (Cat
> > > 3550)
> > > > > and Ethernet0 (R1) in OSPF, the relationship can not be form
> because
> > the
> > > > MTU
> > > > > is different:
> > > > > Interface Vlan1 - MTU Size 1504 bytes
> > > > > Ethernet 0 - MTU Size 1500 butes
> > > > >
> > > > > When I put the command IP OSPF IGNORE-MTU for each interface,
> > everything
> > > > > works fine.
> > > > >
> > > > > Is there another way to do the same thing ?
> > > > >
> > > > > Considering that there is not the command MTU xx for
interface
> Vlan..
> > > > >
> > > > > Regards,
> > > > >
> > > > >
This archive was generated by hypermail 2.1.4 : Fri Dec 12 2003 - 12:29:12 GMT-3