MTU value on WAN interface for PPPoE connections (UK)

[Scruffy]

In the UK the default MTU value from most ISP's is 1492 for FTTP & FTTC connections.

Do I need to tweak the MTU value on the WAN interface when using PPPoE to 1492 or will the fortigate cope with the lower value as I can see the default MTU is 1500. Or will I experience issues such as packet loss if I do not lower the value. 

TIA

[abarushka]

Hello,

Normally MTU should be automatically adjusted. You can verify by running "diag netlink interface list <PPPOE interface name>"

https://community.fortinet.com/t5/FortiGate/Technical-Tip-PPPoE-interface-configuration/ta-p/209005

[Scruffy]

thank you for the info however mine says  - 

f=wan family=00 type=1 index=5 mtu=1500 link=0 master=0

If I ping 4.2.2.1 -l 1464 -f

then I get

Pinging 4.2.2.1 with 1464 bytes of data:
Reply from 4.2.2.1: bytes=1464 time=21ms TTL=53

whereas ping 4.2.2.1 -l 1472 -f

Pinging 4.2.2.1 with 1472 bytes of data:
Packet needs to be fragmented but DF set.

So I am pretty sure the MTU value is set to 1492 on the ISP side so it does not look like it is auto adjusting?

[abarushka]

Hello,

Could you please double check whether you specified correct interface name? I suspect that you run "diag netlink interface list <WAN interface name>" instead "diag netlink interface list <PPPOE interface name>".

[Scruffy]

Hi

we have the WAN physical interface configured for PPPoE

[hbac]

Hi @Scruffy,

You can configure TCP MSS value by following this article: https://community.fortinet.com/t5/FortiGate/Technical-Tip-Setting-TCP-MSS-value/ta-p/194518

Regards, 

[Toshi_Esumi · Accepted solution]

The pppoe dynamic interface @abarushka is asking is not "wan" interface you configured PPPoE.
It should be named as like "pppx" in the routing table. Run a CLI command "get router info routing-table | grep 0.0.0.0" like below to find the name.

fg40f-utm (root) # get router info routing-t all | grep 0.0.0.0
S* 0.0.0.0/0 [1/0] via xx.xx.xx.xx, ppp3, [1/20]

Then check the MTU size it got like below:

fg40f-utm (root) # diag netlink interface list ppp3

if=ppp3 family=00 type=512 index=41 mtu=1492 link=22 master=0

Since it's a dynamic interface, you can't set MTU size manually.


Toshi