[cisco-bba] MTU challenges on Cisco 7206VXR

Robert Blayzor rblayzor.bulk at inoc.net
Thu Mar 13 08:38:52 EDT 2008

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
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