Ethernet 101

Circuit Switched

• Dedicated connections end to end

• Remains established

Packet Switched

• No end to end dedicated circuit required

• Network delivers packet based on address

Ethernet Switching (Bridges)

• Perform 4 functions - Think of how switches work ARP etc

  • Learn

  • Forward

  • Flood

  • Discard

VLANS (802.1q)

Basic Ethernet Frame Format (802.3)

Ethernet Header (14 Bytes)

• 6 Byte Destination MAC Address

• 6 Byte Source MAC Address

• 2 Byte (16 bit) Ether Type (ET) Field

  • 0x0800 = IPv4

  • 0x0806 = ARP

  • 0x086DD = IPv6

Payload (Minimum 46 Bytes)

• Payload must be AT LEAST 46 Bytes long (or padded to 46 Bytes)

• This is because the minimum frame size is 64 Byte Frame

  • Ethernet Header = 14 Bytes

  • Payload = 46 Bytes

  • FCS = 4 Bytes

    • TOTAL = 64 Bytes

Ethernet Frame Sizes

• Untagged = 1518 Bytes

  • Ethernet Header (14 Bytes)

  • IP Header (20 Bytes)

  • TCP/UDP/ICMP Header (20 Bytes)

  • Payload (1460 Bytes)

  • TOTAL = 1518 Bytes

• Tagged (VLAN) - Extra 4 Bytes

  • TOTAL = 1522 Bytes

• Provider Bridged (2 VLANS) - Extra 4 Bytes

  • TOTAL = 1526 Bytes

VLAN Tagged Ethernet Frame

• Customer Tag (C-Tag) = 4 Bytes (32 Bits)

  • TPID (Tag Protocol ID) = 8100 for C Tag -> 16 bits

    • For S-Tag TPID would be 88a8

  • PCP (Priority Code Point) indicates Class of Service (CoS) -> 3 bits

  • DEI (Discard Eligibility Indicator) -> 1 bit

  • C-VID (Customer VLAN ID) - between 0-4095

    • VLAN ID 0 = Priority Tag

ENNI Frames - an Ethernet frame sent from one CEN (Carrier Ethernet Network) provider to another must have a provider bridge formatted S-TAG

Remember, Cisco had proprietary 802.1ad (Q-in-Q) based on the 802.1q (VLAN specification) Here Cisco used 2 C-Tags TPID 8100 where by Provider Bridging uses C and S Tags with TPID's of 8100 and 88a8 respectively

Service Frames

• Service Frame - Ethernet Frame transmitted across a UNI towards either the Service Provider or Subscriber

• The Egress/Ingress is determined from the Service Providers perspective

ENNI Frames

• An Ethernet Frame sent from one Operator CEN (Carrier Ethernet Network) across the ENNI to another Operator CEN. Must have a Provider Bridge formatted S-TAG (see above "Double Tagged Ethernet Frames")