PPPoE MTU 1500 vs MSS TCP

[fegauthier]

Hi guys!

I have PPPoE Connection and I want to know if there’s a best way to configure it.

It’s working this way :

set interfaces ethernet eth0 mtu '1508'
set interfaces ethernet eth0 vif 35 mtu '1508'
set interfaces pppoe pppoe0 mtu '1500'

It also working this way :

set interfaces ethernet eth0 mtu '1500'
set interfaces ethernet eth0 vif 35 mtu '1500'
set interfaces pppoe pppoe0 mtu '1492'
set interfaces pppoe pppoe0 policy route 'MSS'

set interfaces bridge br0 policy route 'MSS'

set policy route MSS description 'TCP MSS clamping for PPPoE'
set policy route MSS rule 5 protocol 'tcp'
set policy route MSS rule 5 set tcp-mss '1452'
set policy route MSS rule 5 tcp flags 'SYN'

Which way is better and why?

[jack9603301]

The first method may cause the actual transmission network to be abnormal because the MSS value is not set (for example, you can ping, but you cannot view the web page)

[fegauthier]

With the first method, I can view web pages too.

The results seems to be the same, but I want to know what is the difference if these two works.

[jack9603301]

I have encountered such a problem once, and I can’t access the Internet. Just follow the TCP MSS suggestions from the community.

[Dmitry]

Hello @jack9603301 and @fegauthier.
In this case, the preferred method setup PPPoE client MTU to 1500 and face/vif MTU to 1508. One more important thing, your ISP should accept/allow jumbo frames and allow on PPPoE BRAS (Broadband Remote Access Server) MTU 1500

The second way more universal and has some overhead.

[fegauthier]

Thanks for your info! @Dmitry