How is the Jumbo frame MTU calculated in Arista vs. Cisco vs. Juniper?
Max MTU in Arista -> 9214 vs. Cisco -> 9216 vs. Juniper -> 9192
They should all be the same frame size on the wire, right? (it’s a bit confusing that they are displayed differently in their respective OS…)
EOS> show interfaces ethernet 1 | grep MTU
IP MTU 9214 bytes , BW 10000000 kbit
EOS> show interfaces ethernet 1 l2 mtu
Port MTU Config MTU Status
———- ———- ———-
JUNOS> show interfaces xe-0/1/8 | grep MTU
Link-level type: Ethernet, MTU: 9192, MRU: 0, LAN-PHY mode, Speed: 10Gbps,
Protocol inet, MTU: 9178
Protocol multiservice, MTU: Unlimited
I can be wrong
Point 1 : there is no standard regarding what is the size of a Jumbo frame (Layer 2). Arista, Cisco, Juniper …. are saying it is around 9k
Point 2 : We can define the MTU from a Layer 2 perspective or a Layer 3 perspective
From you example you have 9214 and 9234
9214 is the IP MTU and 9234 is the Layer 2 MTU. It make sense because the IP header is 20 bytes.
There doesn’t appear to be a defined standard to use when Jumbo Frames are discussed.
Here’s a link to an IETF submission regarding the use of Jumbo Frames at IXPs.
You can see in section 1.2 that ”Jumbo Frames are considered to be Ethernet frames that can carry an IP payload greater than 1,500 bytes.”
Here’s a link to a discussion back in 2011 regarding what MTU size to use at IXPs.
In my experience (disclaimer; YMMV), the only time the exact MTU value matters and must match is when running OSPF. As John Moy points out in his book OSPF Anatomy of an Internet Routing Protocol, they found that during the development of OSPF the various flavors of transport (ex. TokenRing, FDDI) was causing them problems during the Database Description exchange. So they designed OSPF to detect and avoid links having MTU mismatches.
I hope that helps,
tl;dr : Arista==Cisco==(Juniper – all overhead)
I think the confusion comes in the vendor’s choice in assumptions for displaying Maximum Transmission Unit (MTU). For simplicity, I will stick to EtherType the IP protocol and not include other encapsulations (or encapsulation combinations).
Media MTU = Encapsulation Overhead + Protocol MTU
Transmission Unit (up to the MTU):
Obviously, this doesn’t take into account to host OS’s varying calculations of MTU… but typically it is the Maximum IP Payload Size
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