Enabling Mini Jumbo Frames (RFC4638) on OpenReach FTTC

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.

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