Forces an MTU of regardless of what is negotiated by your router.
More info: MTU is negotiated upon connection. We usually like 1500 MTU, however, there can be situations where by forcing 1492 is required. For example, if your router negotiated 1500 initially but you make a connection to a server that want's to re-negigotiate a lower MTU it will try to di this using ICMP, but if your router has a miss-confuigured firewall that blocks ICMP this can't be re-negoiated and the connection to the server will have problems. This is seen when connecting to certain web sites, bank web sites and https sites. Forcing 1492 from the start means that all your traffic is in packets of 1492 bytes and so won't have this problem.
Modifies the MSS in TCP packets so that it does not exceed the MRU
When sending IP traffic, ignore the MRU we receive during PPP negotiation. This can save a LCP renegotiation (which has been known to help OBSD based PPP's)
Re-negotiates PPP LCP after acquiring the connection from BT (who may provide a wrong high MTU of 1500 on a 1492 link)
More info: During the ppp connection when your router initially syncs up and logs in to us, the ppp connection is passed via BT. Sometimes BT can change the MTU. With this option we will accept the PPP connection from BT, but will then re-negotiate the PPP connection with your router allowing the MTU to be reduced. On TalkTalk connections the MTU is always negotiated as 1492, you need to select LCPFix and an MTU 1500 to fix this (otherwise the LNS will use 1492 when re-negotiating).