Hi Everyone, in midnight I really confuse of myself saa. Hb z ai jcause the MTU inter-working ALU-Juniper.

This is the simple topology


I want to make ospf inter-working. Because MTU Problem, so the ospf never reach full state, and just stack in ExchangeStart.






This problem solving with tuning MTU in Alu and make it suitable with juniper (but if you want to change in juniper it's no problem). 


This is the formula from https://kb.juniper.net/InfoCenter/index?page=content&id=KB19382


In my Juniper I set mtu with 5000. so it's the max-frame-size for physical mtu in juniper. and the ospf IP MTU formula is

 IP-MTU = Media MTU - 14 DLC Header Bit











Event in omni we use vlan to interworking ALU it's doesn't matter. Because the traffic with tagging vlan out from omni will be un-tagged in ALU port. This is special condition if we don't use vlan tagging to inter-working Juniper and Omni. If this condition is happened. Use the formula:

 For 802.1q --> IP-MTU = Media MTU - 14 DLC Header Bit - 4 bit dot1q  
 For QinQ -----> IP-MTU = Media MTU - 14 DLC Header Bit - 8 bit qinq

So in Alu, I set OSPF Mtu (with null condition without tagging)

 ALU OSPF IP Mtu = 5000 - 14 = 4986

Then it's work

ALU Side verification:



Juniper Side verification:








The best practice with out calculation is:

Just looking for the mtu size in one router. In this case I use the juniper. Alu must follow juniper mtu.
So I try to looking ospf mtu interface in juniper




From this information, I know OSPF IP MTU Juniper use is 4986. So I try to config in Alu and make the OSPF IP MTU is equal with Juniper.



See You, I hope this can help your problem.