From: asadovnikov (asadovnikov@comcast.net)
Date: Sun Nov 16 2003 - 02:10:53 GMT-3
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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