2.1.a Implement and Troubleshoot Switch Administration

This section post will discuss mostly ethernet and how it relates to 2.1.a implement troubleshoot switch administration. All of the information here is a good intro to subsections of this section and those concepts build off of this.

I am going to use this parent section to discuss ethernet, and some of its properties. I believe that these details are ones that a CCIE should know.

Ethernet was originally invented to allow two or more hosts to use the same medium/cable. It go into something much more.

Ethernet operates at the datalink layer of the OSI model.

Ethernet addresses are 48 bits long and written as hexadecimal.

The first six digits of a MAC address are the Organizational Unnique Identifier (OUI) which is registered to the manufacturer.
The remaining digits are for the devices they create.

Ethernet frames actually have more bytes than what you may be accused to. With cisco devices we are used to talking about MTU as not including ethernet headers, so the IP MTU is usually 1500.

However when we add the source, destination mac, data length, and FCS we now get up to 1518 Bytes. If we add an 802.1Q header this now becoems 1522 Bytes!

img 5a16246ba6a5e

Now when we see this, we realize that there is EVEN MORE than 1522 Bytes in an IP packet that’s 1500 Bytes.

There’s a preamble, a SOF, and an interpacket gap! That’s another 20 bytes totalling to 1542 bytes!

Let’s recap, if we have an IP packet that totals 1500 bytes (meaning the IP payload is 1580, and the headers are 20 bytes) we can actually have between 38-42 extra bytes of overhead we are unaware of.

You should know that there are 8 ethernet headers (excluding payload)

  • preamble (7 bytes)
  • start of frame delimiter (1 byte)
  • DST MAC (6 bytes)
  • SRC MAC (6 bytes)
  • .1q header (4 bytes)
  • ethertype (2 Bytes)
  • Frame check sequence (4 Bytes)
  • interpacket gap (12 Bytes)


Leave a comment