From: ccie2be (ccie2be@nyc.rr.com)
Date: Sun Nov 16 2003 - 11:55:38 GMT-3
Thanks Alexei. I think that now all the pieces are in place. Let me just
make sure I have everything correct in my mind.
By default, the Cat 3550 uses an MTU size of 1504 bytes. This is to allow
dot1q, which needs the 4 extra bytes, to work properly.
If, however, dot1q isn't used, the default MTU of 1504 can be changed on a
system wide basis to another value with the command, CAT(config)#system mtu
<size>, but the MTU can't be changed for a particular interface.
If dot1q is required and the Cat 3550 is running OSPF, then these are your
options:
1) Configure an MTU size of 1504 on the router connected to the Cat 3550
2) Configure the interface command, IP OSPF IGNORE-MTU on either the Cat
3550 or the directly connected router
Please feel free to correct any mistate or misunderstanding I have or
comment on what I've said.
dt
----- Original Message -----
From: "asadovnikov" <asadovnikov@comcast.net>
To: "'seonghui'" <seonghui@vads.com>; "'ccie2be'" <ccie2be@nyc.rr.com>;
<ccielab@groupstudy.com>
Sent: Sunday, November 16, 2003 12:10 AM
Subject: RE: OSPF - MTU mismatch
> The system MTU is not stored in vlan.dat file, but in env file. This is a
> system wide setting used mostly to allow for extra 4 bytes required by
dot1q
> tunnel. The default is 1500, but in the LABs where you need to configure
> dot1q tunnel one wuld need to change MTU to 1504. Then you may need to
> adjust IP MTU on non-tunnel interfaces of 3550 down to 1500, and when you
> move on to another LAB remember to change system mtu back to 1500.
>
> Best commands to use are:
> CAT#show system mtu
> CAT(config)#system mtu <size>
>
> Best regards,
> Alexei
>
> -----Original Message-----
> From: nobody@groupstudy.com [mailto:nobody@groupstudy.com] On Behalf Of
> seonghui
> Sent: Saturday, November 15, 2003 1:43 PM
> To: 'ccie2be'; ccielab@groupstudy.com
> 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_note09186a0080
> 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,
> > > >
> > > > _________________________________________________________________
> > > > MSN Messenger: converse com os seus amigos online.
> > > > http://messenger.msn.com.br
> > > >
> > > >
> _______________________________________________________________________
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