From: Brian McGahan (bmcgahan@internetworkexpert.com)
Date: Sat Jul 03 2004 - 14:46:55 GMT-3
Jorge,
Is this an IE rack? If so the MTU has most likely been changed
because someone was practicing 802.1q tunneling. In the case of a .1q
tunnel the MTU must be increased to at least 1504 to account for the
second dot1q header that in prepended to the packet, which could cause a
packet that is already at MTU to exceed 1500 bytes.
The workarounds are either to change the system mtu back to 1500
and reload the switch (reload is required to apply the change) or to
issue the ip ospf mtu ignore command.
This problem also used to occur when combining the mixed media
modular token ring interfaces of the 2600/3600 with the fixed config
2500s. The 2500s had an MTU of 4000 something while the modular
interfaces had 8000 something. This also is a problem in IS-IS, as the
hello packet is padded to the MTU of the interface. This can be
disabled with the "no hello-padding" command under the IS-IS process.
HTH,
Brian McGahan, CCIE #8593
bmcgahan@internetworkexpert.com
Internetwork Expert, Inc.
http://www.InternetworkExpert.com
Toll Free: 877-224-8987 x 705
Outside US: 775-826-4344 x 705
> -----Original Message-----
> From: nobody@groupstudy.com [mailto:nobody@groupstudy.com] On Behalf
Of
> barrerj1@hotmail.com
> Sent: Saturday, July 03, 2004 11:46 AM
> To: 'Shibu Nair'
> Cc: ccielab@groupstudy.com
> Subject: RE: OSPF MTU problem
>
> Ok.
>
>
>
>
>
> I get it
>
>
>
>
>
> Thanks to all
>
>
>
> JB
>
>
>
> -----Original Message-----
> From: Shibu Nair [mailto:shinair@cisco.com]
> Sent: Saturday, July 03, 2004 10:37 AM
> To: barrerj1@hotmail.com
> Cc: ccielab@groupstudy.com
> Subject: Re: OSPF MTU problem
>
>
>
> I think you are using VLAN interface as the L3 interface on the switch
> side.
> When you use the vlan interface on the switchside, MTU size is little
> more than 1500.
> So manual setup the mtu as the same on both sides or "ip ospf
> mtu-ignore" interface
> command should solve the issue.
> Try out
> Regards
> Shibu
> At 06:26 AM 7/3/2004 -0500, barrerj1@hotmail.com wrote:
>
>
>
> Hi all!
>
>
>
> While doing an OSPF practice lab, I was encountered by the following
> problem.
>
>
>
> 2 routers connected to a 3550 and running OSPF between them, the state
> was stuck in EXSTART and couldn't complete the adjacency.
>
>
>
> I checked CCO and found that the problem was an MTU mismatched.
> However, the CCO doc points this problem to be a common when you
connect
> to another vendor's router. In my case all devices are CISCO.
>
>
>
>
>
>
>
>
> Neighbors Stuck in Exstart/Exchange State
>
>
> The problem occurs most frequently when attempting to run OSPF between
a
> Cisco router and another vendor's router. The problem occurs when the
> maximum transmission unit (MTU) settings for neighboring router
> interfaces don't match. If the router with the higher MTU sends a
packet
> larger that the MTU set on the neighboring router, the neighboring
> router ignores the packet.
>
>
http://www.cisco.com/en/US/tech/tk365/tk480/technologies_tech_note09186a
> 0080093f0d.shtml
>
>
>
>
>
>
>
> Can any one explain why this behavior as I did not change any of the
MTU
> configurations before the problem occurred?
>
>
>
>
>
> Thanks
>
> JB
>
>
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