For more details, please see ourCookie Policy.

Ethernet Switches & Routers

New Contributor
Posts: 4
Registered: ‎07-17-2016
Accepted Solution

MTU interoperability MLXe and Juniper MX960

[ Edited ]

Hi All,


I found a lot of useful information here in the forum but unfortunately nothing with regard a specific question.

For testing purpose I need to connect a MX960 Juniper with a MLXe (point to point ).

IPv4 and ISIS,BGP must be configured on the link. Ethernet is the encapsulation I am gonna to use.


Question: How to choose the correct MTU value to have ISIS up and running?



The fact is:


Based on the  ISO/IEC 10598 standard that all intermediary systems (IS) must be capable of receiving LSPs of the size of 1492 octets.

The maximum payload size of a standard Ethernet frame is 1500 byte.

IS-IS implementations retained the 1492 bytes LSP length specification, and pads the IS-IS IIHs up to that size and beyond. To do so, they use one or several Padding TLVs (TLV 8). 

It allows IS-IS to detect if both end of an adjacency comply with the minimum MTU or LSP length.

Juniper Junos  always pads IIHs up to 1492 bytes, despite the interface type.


What about the MLX (I have  MLXe 4-slot, line card BR-MLX-10Gx8-X)?


More, I do not want to leave the default setting  because of future implementation of different protocols on this link (which will add overhead).

Idea is to use Jumbo Frame but as you already know, each vendor is using a different approach in calculation the MTU (Brocade: IP MTU for JumboFrame = MTU Media - (L2 Header - CRC) = 9000 - (14 - 4) = 8992) and this number is different from the way how Juniper is calculating the MTU -> IP MTU = Media MTU - L2 Header = 9000 - 14 = 8986.


What is your opinion/experience on this topic if any?


Thanks all.


New Contributor
Posts: 4
Registered: ‎07-17-2016

Re: MTU interoperability MLX and Juniper MX960

[ Edited ]

My bad: I just figured out that for the MLXe, by default, the media MTU for 10Gb port is 9216 bytes (I think the maximum), on the contrary for Juniper MX960 default for the same media type is 1514 bytes and the maximum is 9192 bytes.


Idea now should be to find a common and unique (the same ?) MTU value valid for both devices  but I have the feeling that if I set the maximum MTU value for Juniper as well, may be the ISIS and the BGP on this point to point link will work. Why this? ISIS (extended) and BGP are based on TCP/IP/Ethernet and:


"TCP/IP/Ethernet : There is a setup process for each session between sender and receiver where devices exchange their maximum MSS (Maximum Segment Size) value. The lowest value MSS determines the MSS used for that session, and the subsequent MTU and frame size." (ref:


so I am assuming that in my specific case, the lowest MTU size will be choosen, and assuming I set the MTU to the maximum value on both end, I shoudl have no problem in creating the IS-IS and BGP peering.


Does it make sense to you?










Former Brocadian
Posts: 28
Registered: ‎07-18-2016

Re: MTU interoperability MLX and Juniper MX960



You are correct that the MTU will take the lowest value on the point to point link. So, you will need to set the MTU to the same value on both sides. Setting it to 9192 on both sides will reslove your mtu issue.



Bill Hadley

Any and all information provided by me is for entertainment value and should not be relied upon as a guaranteed solution or warranty of merchantability. All systems and all networks are different and unique. If you have a concern about data loss, or network disconnection, please open a TAC service request for service through Brocade, or through your OEM equipment provider. If this provided you with a solution to this issue, Please mark it with the button at the bottom "Accept as solution".

Join the Community

Get quick and easy access to valuable resource designed to help you manage your Brocade Network.