MTU 1500 bytes
workfrom925
Member Posts: 196
in CCNA & CCENT
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)?
Does it get even larger when a frame goes from layer 2 (data layer) to the layer one (physical layer)?
Comments
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NetworkVeteran
Member Posts: 2,338 ■■■■■■■■□□
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.
This would be an accurate characterization that jives with what I said.The book I'm reading says it refers to the maximum PACKET size that can traverse this interface is 1500 bytes.
Yup, 1518 bytes to be exact.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?
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'.Does it get even larger when a frame goes from layer 2 (data layer) to the layer one (physical layer)? -
blueberries
Banned Posts: 138
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
Mod Posts: 11,962 Mod
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.An expert is a man who has made all the mistakes which can be made. -
iamme4eva
Member Posts: 272
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...Current objective: CCNA Security
My blog: mybraindump.co.uk -
blueberries
Banned Posts: 138
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
Member Posts: 224
I hate implementing jumbo frames.He who SYNs is of the devil, for the devil has SYN'ed and ACK'ed from the beginning. For this purpose, that the ACK might destroy the works of the devil.
