MTU challenges on Cisco 7206VXR

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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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
>
> _______________________________________________
> cisco-bba mailing list
> cisco-bbapuck.nether.net
> ht
tps://puck.nether.net/mailman/listinfo/cisco-bba

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Ok, I agree.  So where are the problems coming from?

I do see that on the "root" ATM interface I have
the following entered:
	interface ATM4/0
	 mtu 1504
	 no ip address
	 no atm ilmi-keepalive
	!
And the PVCs are defined under a sub-interface as follows:
	interface ATM4/0.100 multipoint
	 pvc 0/261
	  encapsulation aal5mux ppp Virtual-Template1
	 !
	 pvc 0/262
	  encapsulation aal5snap
	  protocol pppoe
	 !
	 pvc 0/263
	  encapsulation aal5mux ppp Virtual-Template1
	 !

Is it possible that the PVC connections are inheriting the
1504 for the
ATM4/0 interface and that I should delete that line
altogether?

Frank

-----Original Message-----
From: Tom Storey [mailto:tomsnnap.net] 
Sent: Wednesday, March 12, 2008 10:01 PM
To: frnkblkiname.com
Cc: cisco-bbapuck.nether.net
Subject: Re: [cisco-bba] MTU challenges on Cisco 7206VXR

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
>
> _______________________________________________
> cisco-bba mailing list
> cisco-bbapuck.nether.net
> ht
tps://puck.nether.net/mailman/listinfo/cisco-bba


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