Comments on: MTU and ping size confusion

By: mmahmoud

mmahmoud — Fri, 16 Apr 2010 15:22:55 +0000

Hi,

Initially I though it was related to the minimum Ethernet frame size of 64 bytes, but when doing the math (and some Wireshark) I found out that it didn’t, if you used 36 bytes datagram size, this means an Ethernet frame of a total of 54 bytes (14 bytes Ethernet Header + 20 bytes IP Header + 8 bytes ICMP Header + 8 bytes ICMP data + 4 bytes CRC), and you’ll find that the Cisco router pads the Ethernet frame with extra 10 bytes (all zeros) to comply with the minimum Ethernet frame size (JUNOS and other OSs don’t do this anymore, it is useless with Full Duplex Ethernet anyway, remember that the minimum Ethernet frame size was mainly forced for insuring the detection of collisions). Anyway I’ll try to dig for an answer for your question.

By: YT.Zhang

YT.Zhang — Wed, 14 Apr 2010 02:24:31 +0000

Hi.
Friend, I saw your post, it’s great, but still I have some confusion. Let me show you:
When you get ping in cisco IOS routers, the min size of
datagram size is 36 bytes, the max size of datagram size is 18024 bytes, right? and I wonder why it’s 36 bytes? it’s contain 20 bytes of ip header and 8 byte of icmp header and 8 byte of data?

By: shivlu jain

shivlu jain — Wed, 03 Mar 2010 12:52:52 +0000

Today we struck with the same issue but our post help us lot. Thanks

By: mmahmoud

mmahmoud — Wed, 24 Feb 2010 15:21:19 +0000

You are very welcomed, I am glade that you’ve found our post useful, and I believe that the upcoming period will have interesting C and J posts, pure vendor-less technical posts and other posts contrasting how both vendors implement the technologies.

Have a nice day.

BR,
Mohammed Mahmoud.

By: Amir Mohamed

Amir Mohamed — Thu, 18 Feb 2010 09:00:03 +0000

Many thanks for helpful information that clear my confusion

By: AlvinJ

AlvinJ — Mon, 08 Feb 2010 06:58:47 +0000

Great! I never concerned about the actually size of the ping packet
now I get it !