I swapped out my single Fortigate 100D at home a while back for a cluster of two in active/passive, as part of this migration, that I have written about before I needed to terminate any DHCP or PPPoE interfaces on a different piece of kit than the clustered firewalls.

I have had this in the lab for a while on a Cisco 2811 router set up pretty much exactly like I had in the previous article.

However, it came to my attention that OpenReach support RFC4638 (Mini Jumbo Frames) on their WAN, so I felt compelled to remove a few lines of config from my router to clean it up and gain whatever marginal benefit an extra 8 bytes of frame size will get me.

The current config looked like this (the parts that matter anyway):

interface FastEthernet0/0 description FG_side ip address my.public.ip.address 255.255.255.248 duplex auto speed 100 ! interface FastEthernet0/1 description WAN_side no ip address duplex auto speed auto pppoe enable group global pppoe-client dial-pool-number 1 ! interface Dialer1 ip unnumbered FastEthernet0/0 ip mtu 1492 encapsulation ppp ip tcp adjust-mss 1452 dialer pool 1 dialer idle-timeout 0 dialer-group 1 ppp authentication chap pap callin ppp chap hostname USERNAME HERE ppp chap password 7 PASSWORD ppp pap sent-username USERNAMEHERE password 7 PASSWORD no cdp enable !

The RFC allows for you to send a standard 1500 byte ethernet frame over the WAN – so we need to increase the MTU on the WAN side interface and tell ppp to negotiate a MRU size of 1500 as it is larger than the 1492 standard:

interface fa 0/1 mtu 1508 pppoe-client ppp-max-payload 1500

And we can also now remove ip tcp adjust-mss and ip mtu from the dialler as no frames will need their size change when going over wan:

interface Dial 1 no ip mtu 1492 no ip tcp adjust-mss 1452

You can see from the ping below running during my change that we are now able to ping google.com at a 1472 (accoung for 28 byte overhead) MTU.

Props to this thread, without it I wouldn’t have known OpenReach implemented this feature.

Why not follow @mylesagray on Twitter for more like this!

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