[cisco-bba] MTU challenges on Cisco 7206VXR

[Tom Storey]

ATM has an MTU of around 4470 buytes, fast ethernet is 1500 tops.  
Gigabit ethernet can be upto ~9000 bytes if jumbo frames are enabled.

At the most you should only need to shave 8 bytes off of your MTU for  
PPPoE to account for additional PPPoE headers.

ATM, whether you run PPPoA or PPPoEoA, should be less likely to  
experience MTU issues. PPPoEoE over gigabit ethernet should not  
require any MTU adjustments, but over fast ethernet you should drop  
your MTU to 1492 at both ends.

Cheers,
Tom

On 13/03/2008, at 6:43 AM, Frank Bulk - iNAME wrote:

> We're mostly PPPoA over an OC-3, but we do have some PPPoE over the  
> OC-3,
> and about ~150 customers using PPPoE coming in on NPE-400's Ethernet  
> port.
> We have a Cisco 7206VXR with 12.2(26).
>
> We've had a few reports about the last two years, as we added PPPoE  
> support,
> that large packets don't get through.  The answer was always to  
> lower the
> MTU to 1400 on their CPE (mostly SonicWall, some WatchGuard) and the  
> problem
> was solved.
>
> We have an existing customer moving from PPPoA to PPPoE coming in on  
> the
> OC-3with a new consultant and he's pretty sure that "it should just  
> work".
> What's odd is that the ~150 PPPoE customers coming in over the  
> Ethernet port
> and we haven't had any PPPoE issues.  So I'm guessing something may be
> configured on the ATM interface that's not an issue on the Ethernet
> interface.
>
> Has anyone run into this problem, and if so, is there something I  
> can do on
> our BRAS to resolve this?
>
> Regards,
>
> Frank
>
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