MTU calculation on CISCO routers

[from88]

                   Hello, just wanted to clarify my knowledge on MTU.

interface commands -

IP MTU: is calculating    LAYER 3 HEADER + ITS PAYLOAD, which makes max MTU of 1500.

MTU: ETHERNET HEADER + LAYER 3 HEADER + ITS PAYLOAD, which makes 1514 mtu if no mpls, no tags are used. I AM right ? Looking at some vendors other than CISCO (for example) alcatel -lucent this seems to be correct.

But i saw on some cisco routers MTU and IP MTU - 1500 bytes. Why MTU is only 1500, how CISCO calculates MTU, it doesn't count ethernet header or what ? So my calculation above is bad considering CISCO ?

Thanks!

[Giuseppe Larosa]

Hello Kajatonas88,

your understanding is correct.

What actually happens is that in IOS MTU is referred to layer3  PDU only, and applies to all possible Layer3 protocols not only to IPv4, this behaviour has changed for example in IOS XR where MTU considers the OSI Layer2  without the 4 bytes of FCS as you have seen in other vendor routers.

I agree that the way MTU is treated in IOS is somewhat misleading for this reason.

Another example is the command mpls mtu that says how big an MPLS frame can be with no reference to the MPLS label stack depth ( how many labels are in the MPLS frame? this is left unspecified).

Hope to help

Giuseppe

[from88]

thanks for reply, Giuseppe.

Can you clarify HOW cisco calculates Interface MTU ?

[Mohamed Sobair · Accepted solution]

Hello,

Sorry Giuseppe, but I have to disagree with your statement that ((IOS MTU is referred to layer 3 PDU only)) and with the ((MPLS MTU)) command.

Infact, how Cisco treat MTU is what should be understood, at least from my point of view and experience.

The IOS indeed calculate the (IP MTU, Layer-2 MTU and MPLS MTU), below how its calculated:

1- The 1500 of (IP MTU) considers the layer-3 Header and the Payload.

2- The Layer-2 MTU of (1500), considers the Ethernet Header and CRC Trailer of additional 22 bytes as well, So the Default of  (MTU 1500) doesnt mean the header and FCS is not considered, the fact, the total layer-2 MTU is equal to 1522, but when you see it 1500 on the IOS, it doesnt mean its not considerable, its calculated as a sum and its shown as 1500, So the 1500 considers the Ethernet header and CRC in its Calculations.

I have seen many implementation where MTU were a big issue resulting in performance degradation and frame/packet drops if not increased, Especially when having MPLS and Dot1q Tunneling.

The Layer-2 MTU from my point MUST be increased when using Dot1q Tunneling to consider the Inner VLAN tag of 4 Bytes, Like Wise, the MPLS MTU on IOS Platforms must be increased to accomodate the additional labels.

I hope this gives an idea of how Cisco Calculates its MTU ,

Regards,

Mohamed

[Giuseppe Larosa]

Hello Mohamed,

I realize I have been unclear in my answer.

I agree with you and your explanation.

You have answered to the original poster.  team work!

Thanks for your correction

Rated as it deserves

Best Regards

Giuseppe

[Mohamed Sobair]

You are very Welcome Jiuseppe,  I am always impressed by your posts & Knowledge & that what makes this forum valuable.

Regards,

Mohamed

[kunapuri]

Hello Mohamed, Giuseppe

I am trying to calculate MTU overhead in below IP DSL Scenario

CE-(ATM)-LAC--(Etherent)--LNS/PE

Ethernet interface has of default MTU of 1500 bytes.

When i am sending ping packet from CE to PE, i am able to send 1500 bytes of traffic with DF bit set.

As mentioned above IP header is already considered in L3 MTu(1500) Default and L2 header already consider in L2 MTu(1500) default.

However why UDP header and L2TP header(20 bytes) is not included, I was expecting the packet to be dropped because of 20 bytes of UDP and L2tp header.

Waiting for your kind response.

Thanks
Kunal