By setting the MTU on the physical interface that the PPPoE "dialler" uses to 1508 with the line
mtu 1508
You have the "headroom" on the ethernet port to do
pppoe-client ppp-max-payload 1500
The max-payload 1500 instructs the PPPoE client to negotiate a connection with a MTU 1500. This then provides an Internet connection with an MTU that matches Ethernet.
If the physical interface has an MTU of 1500 (The Ethernet default), then as you lose 8 to PPPoE encapsulation, you end up with an MTU of 1492.
Having an MTU that is smaller than 1500 can cause some applications to exhibit "interesting" behavior, especially if the application needs to run through a VPN client that itself encapsulates other packets with further overhead.
I have the 1812 router connected to the VDSL modem so the PPPoE side gets a dynamic address and then what would usually be the LAN side I have the /29 static block which connects to my ASA 5505 firewall.
Prior to the change, if I used the Cisco Anyconnect client to connect to a Cisco SSL VPN "server" (such an ASA), then it would frequently reconnect during the initial connection and drop out when in use, now it works perfectly 🙂
