[cisco-bba] MTU challenges on Cisco 7206VXR

Frank Bulk frnkblk at iname.com
Thu Mar 13 15:01:42 EDT 2008

Thanks for the feedback.  So, do you think that 'mtu 1504' is a valid
configuration item and I should leave that alone?  I would prefer not to
drop 8 bytes from my PPPoA customers if removing the 'mtu 1504' entry would
solve the problem.


-----Original Message-----
From: Robert Blayzor [mailto:rblayzor.bulk at inoc.net] 
Sent: Thursday, March 13, 2008 7:39 AM
To: frnkblk at iname.com
Cc: cisco-bba at puck.nether.net
Subject: Re: [cisco-bba] MTU challenges on Cisco 7206VXR

Frank Bulk - iNAME wrote:
> 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?

I'd leave the MTU at default on the ATM interface and change the MTU on
your virtual-template to 1492.  We have tens of thousands of
DSL/Ethernet subs using PPPoE/oA with no MTU problems at all.  Usually
the device terminating the PPPoX is smart enough to know to adjust the
MTU... some not so much.

There are some devices in which you have to clamp the mss because the
client can't figure it out for some reason... however, that should be
few and far between.

Robert Blayzor
rblayzor at inoc.net

 > SELECT * FROM users WHERE clue > 0
0 rows returned

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