From: Volkov Dmitry (dmitry.volkov@rogers.com)
Date: Sun Nov 16 2003 - 15:27:55 GMT-3
Alexei said "to set IP MTU":
It can be configured on L3 interfaces (including SVI):
3550(config-if)#ip mtu ?
<68-1000000> MTU (bytes)
Dmitry
> -----Original Message-----
> From: nobody@groupstudy.com [mailto:nobody@groupstudy.com]On
> Behalf Of ccie2be
> Sent: Sunday, November 16, 2003 1:13 PM
> To: asadovnikov; 'seonghui'; ccielab@groupstudy.com
> Subject: Re: OSPF - MTU mismatch
>
>
> Thanks for all your feedback. It's been exceptionally helpful.
>
> Please see Comments in-line - you seem to contradict yourself
> at the end
> when you talk about the solution to the problem of running
> both OSPF and
> 802.1q.
>
>
> ----- Original Message -----
> From: "asadovnikov" <asadovnikov@comcast.net>
> To: "'ccie2be'" <ccie2be@nyc.rr.com>; "'seonghui'"
> <seonghui@vads.com>;
> <ccielab@groupstudy.com>
> Sent: Sunday, November 16, 2003 12:35 PM
> Subject: RE: OSPF - MTU mismatch
>
>
> DT,
>
> You are welcome.
>
> Good write up, but let me try to append it a little:
>
> * By default 3550 uses system MTU of 1500.
>
> Although there were rumors of one of 3550 IOS version doing 1504 by
> default,
> I did not ever observed it myself.
>
> My recommendation is to upgrade 3550s to 12.1.13 or later,
> which I know
> will use default MTU of 1500.
>
> * Larger MTU is needed to provide full size frame transport
> over dot1q
> tunnel.
>
> Extra dot1q tag inserted by dot1q-in-dot1q encapsulation
> requires frame
> size
> to grow by 4 bytes, this is why default MTU size needs to
> be increased.
>
> 1504 should be used in case of dot1q tunnels. If dot1q
> tunnels are not
> in
> use stick to 1500 for system MTU value.
>
> * Value of system MTU can be changed with:
> CAT(config)#system mtu 1504
> or viewed with:
> CAT#show system mtu
>
> Note that show command will display currently active
> setting. If you
> change
> it you will have to reboot for new setting to take effect.
>
> * System MTU setting is not part of running/startup configuration or
> vlan.dat
> file. The value of system MTU is stored in env_vars and
> stored value can
>
> be viewed with:
> CAT#more env_vars
> . . .
> SYSTEM_MTU=1504
> If nothing is in the file - the default setting will be in effect
>
> Unlike an output of "show system mtu" which shows what
> currently active
> and
> then displays what will happen after next reboot, the
> "more env_vars"
> will
> show only what will happen after next reboot and not what
> is currently
> active.
>
> I do not recommend to use the file display for examining system mtu
> setting.
>
> * Subsequently to where the value of system MTU is held, if
> you remove
> "env_vars"
> file and reboot the box it will go to default.
>
> So if you renting equipment or something you may start by:
> wr erase
> del vlan.dat
> del env_vars
> reload
> to wipe your 3550 clean
>
> * If you have to run dot1q tunnel and hence run system MTU
> of 1504, and run
> routing protocol such as OSPF on the same 3550 you will
> face an issue of
> OSPF MTU mismatch, as 3550 will be attempting to use 1504 while a
> neighboring router will be using 1500.
>
> There are number of ways to solve it but by far my
> personal preference is
> to set IP MTU to 1500 on all 3550's L3 interfaces (such as
> interfaces
> configured
>
> ********************************************************
> This solution can't work based on what you said earlier
> because there's no
> command to set the mtu on an individual interface (L3 or L2)
> but if you set
> the mtu to 1500 on all interfaces, then you have the problem
> with dot.1q
> tunnels. Did I miss something or did you mis-speak?
> ********************************************************
>
> with an IP address). Note that in unlikely case of suing
> GRE on 3550
> tunnel
> interfaces would need different treatment.
>
> Best regards,
> Alexei
>
>
> -----Original Message-----
> From: ccie2be [mailto:ccie2be@nyc.rr.com]
> Sent: Sunday, November 16, 2003 9:56 AM
> To: asadovnikov; 'seonghui'; ccielab@groupstudy.com
> Subject: Re: OSPF - MTU mismatch
>
>
> 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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