Ethernet Basics- Notes

Here are my notes on the Ethernet Basics chapter of the Cisco Press Exam Certification Guide book.  This is pretty much one of those chapters that is all theory and not very helpful in relation to the lab.  But it’s stuff that everyone should know.

Also, an FYI on how I’ll categorize my future posts.  “Notes” posts will be my own study notes cut and pasted into the post.  So they won’t be real descriptive or polished.  “Overview” posts will be focused on a certain topic and will be written a more reader-friendly manner.

10/100 Ethernet uses pins 1-3 and 6 for communications.

Pins 1-2 are transmit and pins 3,6 are receive for a PC/Router (switches are opposite)

Auto-MDIX on switches can compensate for using wrong cable types (straight-through or cross-over)

Speed/duplex auto-negotiation

• Cisco switches sense speed using the Fast Link Pulses (FLP) of the auto-negotiation process
• If one side has auto-negotiation turned off, the other switch can still detect speed based on incoming electrical signal
• Duplex auto-negotiation requires both sides to participate
• If one side does not participate, 10/100 links default to half-duplex and Gig links default to full
• To turn off auto-negotiation on a switch port, statically set a speed/duplex setting in the interface config mode
• Cannot set duplex on an interface until speed has been set
• CDP can detect duplex mismatches and send notification, but cannot remedy

CSMA/CD process

• Device with a frame to send listens to the wire and waits for a free line (no carrier signal sensed)
• When the line is free, the device starts sending the frame
• Sender listens to make sure no collision occurred
• If there was a collision, all devices that sent a frame send a jamming signal
• After the jamming signal is complete, each device that sent a frame that collided set a random back-off timer
• Once the timer expires, the devices can begin sending again

Ethernet hubs

• Operate at layer 1
• Repeat (regenerate) signals to improve cabling distances
• Forward data received on one port out all others
• Creates a single collision domain

When switches receive multiple frames of different switch ports, they stores frames in memory to prevent collisions

NICs operating in half-duplex mode use loopback circuitry when transmitting a frame. This loops the transmitted frame back to the receive side of the NIC, so that when the NIC receives a frame over the cable, the combined loopback signal and received signal allows the NIC to notice that a collision occurred

In switch interface stats, collisions are detected in first 64 bytes. Late collisions are detected after 64 bytes have been sent. Deferred packets mean that the half-duplex switch port waited until it was not receiving data to send it.

Ethernet type fields

• Ethernet DIX
  • 2 byte Type field

• 802.3
  • 1 byte DSAP

• 802.3 with SNAP header
  • 2 byte type field in SNAP header, DSAP of 0xAA

Multicast packets identified by I/G bit being a 1 (8th bit of the address)

U/L bit (7th bit) identifies if address is vendor assigned or locally assigned

Switch MAC address (CAM table) aging timeout is 300 seconds by default

802.3u defines 100MB Ethernet over fiber and copper

802.3z (optical) and 802.3ab (copper) define Gig Ethernet

Switch internal processing

• Store and forward- switch fully receives frame before forwarding
• Cut-through- switch performs table lookup once destination address field is received. Does not allow switch to drop frames that fail FCS check
• Fragment free- like cut-through, but waits to receive first 64 bytes before forwarding. Should avoid forwarding frames damaged by collisions