From: Jonathan V Hays (jhays@jtan.com)
Date: Mon Nov 17 2003 - 09:55:59 GMT-3
It is worth reminding folks that on the Catalyst 3550, the "ip mtu"
command is available only on layer 3 interfaces (SVI and switched). You
can also configure the VLAN itself with the "mtu" command. See below for
examples:
NO GLOBAL COMMAND:
CAT1#conf t
Enter configuration commands, one per line. End with CNTL/Z.
CAT1(config)#ip m?
mrm mroute msdp multicast multicast-routing
CAT1(config)#vlan 20
CAT1(config-vlan)#ip m?
% Unrecognized command
CAT1(config-vlan)#m?
media mtu
SVI HAS IT:
CAT1(config-vlan)#exit
CAT1(config)#int vlan 20
CAT1(config-if)#ip m?
mask-reply mrm mroute-cache mtu multicast
SWITCHED INTERFACE HAS IT:
CAT1(config-if)#exit
CAT1(config)#int fa0/1
CAT1(config-if)#no switchport
CAT1(config-if)#ip m?
mask-reply mrm mroute-cache mtu multicast
CAT1(config-if)#ip m
>-----Original Message-----
>From: nobody@groupstudy.com [mailto:nobody@groupstudy.com] On
>Behalf Of asadovnikov
>Sent: Monday, November 17, 2003 12:32 AM
>To: 'ccie2be'; ccielab@groupstudy.com
>Subject: RE: OSPF - MTU mismatch
>
>
>You are most welcome. This is not a Cat 3550 specific
>command, it is just
>an IOS command which works on every platform.
>
>Best regards,
>Alexei
>
>-----Original Message-----
>From: ccie2be [mailto:ccie2be@nyc.rr.com]
>Sent: Sunday, November 16, 2003 2:45 PM
>To: asadovnikov; ccielab@groupstudy.com
>Subject: Re: OSPF - MTU mismatch
>
>
>Aha, so that's how that works. Thanks, this is good stuff to
>know. How did
>find that command? I looked in the latest command reference,
>Oct 2003, and
>the CAT(config-if)#ip mtu 1500 command isn't even listed.
>
>Does it show up when you do a CAT(config-if)#ip ?
>
>Again, thanks for sharing your knowledge with us. dt
>
>
>----- Original Message -----
>From: "asadovnikov" <asadovnikov@comcast.net>
>To: "'ccie2be'" <ccie2be@nyc.rr.com>; <ccielab@groupstudy.com>
>Sent: Sunday, November 16, 2003 1:21 PM
>Subject: RE: OSPF - MTU mismatch
>
>
>You can however set IP MTU, and this is all you need:
>CAT(config)#int vlan
>10 CAT(config-if)#ip mtu 1500
>
> CAT#show int vlan 10 | i MTU
> MTU 1504 bytes, BW 1000000 Kbit, DLY 10 usec,
> CAT#show ip int vlan 10 | i MTU
> MTU is 1500 bytes
>
>As you can see the MTU stays at 1504, while IP MTU is 1500,
>and this is what
>OSPF will use.
>
>Best regards,
>Alexei
>
>-----Original Message-----
>From: ccie2be [mailto:ccie2be@nyc.rr.com]
>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_n
>ote09186a0080
>> 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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