Why does PPPoE Dialer MTU affects IP assignment from ISP?

RouterOS Beginner Basics
1

Hi everyone,

OK, this is not really a Mikrotik problem. But I don’t know where else to ask. Actually it’s not really a problem but just seem so strange.

I’ve just subscribed to my ISP’s 5 static public IP plan. I’ve been assigned 210.135.232.88/29. I connect to my ISP using PPPoE on VLAN500.

If I configure the Mikrotik’s PPPoE dialer with MTU and MRU as 1492, pppoe-out1 is assigned 210.135.232.89/32 and I get my /29 static IPs.

If I configure the PPPoE dialer with no MTU nor MRU set, so Mikrotik is using default of 1480, I get a random dynamic IP as if I was on a dynamic IP plan.

Anyone know why that is? Why would MTU and MRU affect this?

2

Do a packet capture of the connections as the PPPoE is initiating in both circumstances and start a conversation with your ISP.