MTU 1500 bytes

When we enter "show interface f0" and get "MTU 1500 bytes", The book I'm reading says it refers to the maximum PACKET size that can traverse this interface is 1500 bytes. Since "packet" is a layer 3 unit term, if the packet is actually 1500 bytes, does it mean when the packet gets to layer 2 (Data layer) and is encapsulated into a frame, the frame size would be larger than 1500 bytes. Correct?

Does it get even larger when a frame goes from layer 2 (data layer) to the layer one (physical layer)?

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Comments

NetworkVeteran

In IOS, the 'mtu' command refers to the size of the maximum Ethernet payload.

I.e., 1500 does not include DstMac, SrcMac, Type/Length, or FCS.

The book I'm reading says it refers to the maximum PACKET size that can traverse this interface is 1500 bytes.

This would be an accurate characterization that jives with what I said.

if the packet is actually 1500 bytes, does it mean when the packet gets to layer 2 (Data layer) and is encapsulated into a frame, the frame size would be larger than 1500 bytes. Correct?

Yup, 1518 bytes to be exact.

Does it get even larger when a frame goes from layer 2 (data layer) to the layer one (physical layer)?

L1 doesn't deal with frames, so the 'frame' doesn't get any larger going from L2 to L1. However, there is additional overhead when you look beyond the frame, such as the 'preamble' and 'carrier extensions'.

blueberries

Layer one doesn't care about encapsulation. MTU 1500 bytes is for ethernet frames. If a packet is carrying more in its payload, then it will segment into separate frames.

networker050184

In Cisco IOS when you specify the MTU it is without the L2 overhead. For example an ethernet port with MTU set to 1500 actually is 1514 to allow for the ethernet header.

You have to be careful though because not all vendors (or even all operating systems for the same vendor - XR) implement this the same way.

iamme4eva

networker050184 wrote: »

You have to be careful though because not all vendors (or even all operating systems for the same vendor - XR) implement this the same way.

I can vouch for that.

We have had problems at work recently with a specific device working on the MTU being including the L2 overhead. We also had problems because people can't do basic addition, but that's another story...

blueberries

iamme4eva wrote: »

I can vouch for that.

We have had problems at work recently with a specific device working on the MTU being including the L2 overhead. We also had problems because people can't do basic addition, but that's another story...

Math lessons during tea time, what do ya say?

vanquish23

I hate implementing jumbo frames.