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RE: OSPF - MTU mismatch posted 11/17/2003
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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_note09186a0080
> 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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