My ISP supports RFC 4638 on my VDSL connection and requests me to set an MTU of 1508 on the VLAN interface and an MTU of 1500 on the PPPoE interface. I used this Cisco document and made the following change to my config:
interface Ethernet0.6
encapsulation dot1Q 6
pppoe enable group global
pppoe-client dial-pool-number 1
pppoe-client ppp-max-payload 1508
The ppp-max-payload command does something funny to my Dialer config as it adds the following line item:
interface Dialer1
mtu 1508
IMO this should be 1500 (b/c of the 8 bytes overhead). During the PPP negotiation I get the following handshake:
*Jul 19 11:25:00.749: %DIALER-6-BIND: Interface Vi2 bound to profile Di1
*Jul 19 11:25:00.757: %LINK-3-UPDOWN: Interface Virtual-Access2, changed state to up
*Jul 19 11:25:00.757: Vi2 PPP: Sending cstate UP notification
*Jul 19 11:25:00.757: Vi2 PPP: Processing CstateUp message
*Jul 19 11:25:00.757: PPP: Alloc Context [1C360B4]
*Jul 19 11:25:00.757: ppp7 PPP: Phase is ESTABLISHING
*Jul 19 11:25:00.757: Vi2 PPP: Using dialer call direction
*Jul 19 11:25:00.757: Vi2 PPP: Treating connection as a callout
*Jul 19 11:25:00.757: Vi2 PPP: Session handle[28000007] Session id[7]
*Jul 19 11:25:00.757: Vi2 LCP: Event[OPEN] State[Initial to Starting]
*Jul 19 11:25:00.757: Vi2 PPP: No remote authentication for call-out
*Jul 19 11:25:00.757: Vi2 LCP: O CONFREQ [Starting] id 1 len 14
*Jul 19 11:25:00.757: Vi2 LCP: MRU 1508 (0x010405E4)
*Jul 19 11:25:00.757: Vi2 LCP: MagicNumber 0x44F6D48D (0x050644F6D48D)
*Jul 19 11:25:00.757: Vi2 LCP: Event[UP] State[Starting to REQsent]
*Jul 19 11:25:00.773: Vi2 LCP: I CONFREQ [REQsent] id 183 len 19
*Jul 19 11:25:00.773: Vi2 LCP: MRU 1500 (0x010405DC)
*Jul 19 11:25:00.773: Vi2 LCP: AuthProto CHAP (0x0305C22305)
*Jul 19 11:25:00.773: Vi2 LCP: MagicNumber 0x7A9FF786 (0x05067A9FF786)
*Jul 19 11:25:00.773: Vi2 LCP: O CONFACK [REQsent] id 183 len 19
*Jul 19 11:25:00.773: Vi2 LCP: MRU 1500 (0x010405DC)
*Jul 19 11:25:00.773: Vi2 LCP: AuthProto CHAP (0x0305C22305)
*Jul 19 11:25:00.773: Vi2 LCP: MagicNumber 0x7A9FF786 (0x05067A9FF786)
*Jul 19 11:25:00.773: Vi2 LCP: Event[Receive ConfReq+] State[REQsent to ACKsent]
*Jul 19 11:25:00.773: Vi2 LCP: I CONFACK [ACKsent] id 1 len 14
*Jul 19 11:25:00.773: Vi2 LCP: MRU 1508 (0x010405E4)
*Jul 19 11:25:00.773: Vi2 LCP: MagicNumber 0x44F6D48D (0x050644F6D48D)
*Jul 19 11:25:00.773: Vi2 LCP: Event[Receive ConfAck] State[ACKsent to Open]
*Jul 19 11:25:00.785: Vi2 PPP: Phase is AUTHENTICATING, by the peer
*Jul 19 11:25:00.785: Vi2 LCP: State is Open
So it seems the remote side is requesting 1500 but the Cisco insists on 1508. It's not possible to reconfigure the Dialer interface to 1500, it then defaults to the MAX value of 1492 and you'll have to reload to correct this.
As the default MTU is 1500 this doesn't cause any practical problems but I think the behavior is weird. Am I missing something or is this a bug? I'm running Version 15.8(3)M4 on a Cisco 897VA-B.
