[cisco-bba] MTU challenges on Cisco 7206VXR

Frank Bulk - iNAME frnkblk at iname.com
Wed Mar 12 23:38:58 EDT 2008

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?


-----Original Message-----
From: Tom Storey [mailto:tom at snnap.net] 
Sent: Wednesday, March 12, 2008 10:01 PM
To: frnkblk at iname.com
Cc: cisco-bba at puck.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.


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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