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RE: OSPF - MTU mismatch posted 11/17/2003
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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@xxxxxxxxxxxxxx [mailto:nobody@xxxxxxxxxxxxxx] On 
>Behalf Of asadovnikov
>Sent: Monday, November 17, 2003 12:32 AM
>To: 'ccie2be'; ccielab@xxxxxxxxxxxxxx
>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@xxxxxxxxxx] 
>Sent: Sunday, November 16, 2003 2:45 PM
>To: asadovnikov; ccielab@xxxxxxxxxxxxxx
>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@xxxxxxxxxxx>
>To: "'ccie2be'" <ccie2be@xxxxxxxxxx>; <ccielab@xxxxxxxxxxxxxx>
>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@xxxxxxxxxx]
>Sent: Sunday, November 16, 2003 1:13 PM
>To: asadovnikov; 'seonghui'; ccielab@xxxxxxxxxxxxxx
>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@xxxxxxxxxxx>
>To: "'ccie2be'" <ccie2be@xxxxxxxxxx>; "'seonghui'" <seonghui@xxxxxxxx>;
><ccielab@xxxxxxxxxxxxxx>
>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@xxxxxxxxxx]
>Sent: Sunday, November 16, 2003 9:56 AM
>To: asadovnikov; 'seonghui'; ccielab@xxxxxxxxxxxxxx
>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@xxxxxxxxxxx>
>To: "'seonghui'" <seonghui@xxxxxxxx>; "'ccie2be'" <ccie2be@xxxxxxxxxx>;
><ccielab@xxxxxxxxxxxxxx>
>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@xxxxxxxxxxxxxx [mailto:nobody@xxxxxxxxxxxxxx] On Behalf 
>> Of seonghui
>> Sent: Saturday, November 15, 2003 1:43 PM
>> To: 'ccie2be'; ccielab@xxxxxxxxxxxxxx
>> 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@xxxxxxxxxx]
>> Sent: Sunday, November 16, 2003 2:35 AM
>> To: matijevi@xxxxxxxxxxxxx; seonghui; 'Silvio Nunes';
>ccielab@xxxxxxxxxxxxxx
>> 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@xxxxxxxxxxxxx>
>> To: "ccie2be" <ccie2be@xxxxxxxxxx>; "seonghui" <seonghui@xxxxxxxx>;
>"'Silvio
>> Nunes'" <silvio_98@xxxxxxxxxxx>; <ccielab@xxxxxxxxxxxxxx>
>> 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@xxxxxxxxxx>
>> > To: "seonghui" <seonghui@xxxxxxxx>; "'Silvio Nunes'" 
>> > <silvio_98@xxxxxxxxxxx>; <ccielab@xxxxxxxxxxxxxx>
>> > 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@xxxxxxxx>
>> > > To: "'Silvio Nunes'" <silvio_98@xxxxxxxxxxx>; 
>> > > <ccielab@xxxxxxxxxxxxxx>
>> > > 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@xxxxxxxxxxxxxx [mailto:nobody@xxxxxxxxxxxxxx]On 
>> > > > Behalf Of Silvio Nunes
>> > > > Sent: Saturday, November 15, 2003 11:06 PM
>> > > > To: ccielab@xxxxxxxxxxxxxx
>> > > > 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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