Protocol Types and the 802.3 Length Field
Each of the three types of Ethernet header shown in Figure 1-4 has a field identifying the format
of the Data field in the frame. Generically called a Type field, these fields allow the receiver of an
Ethernet frame to know how to interpret the data in the received frame. For instance, a router might
want to know whether the frame contains an IP packet, an IPX packet, and so on.
DIX and the revised IEEE framing use the Type field, also called the Protocol Type field. The
originally-defined IEEE framing uses those same 2 bytes as a Length field. To distinguish the style
of Ethernet header, the Ethernet Type field values begin at 1536, and the length of the Data field
in an IEEE frame is limited to decimal 1500 or less. That way, an Ethernet NIC can easily
determine whether the frame follows the DIX or original IEEE format.
The original IEEE frame used a 1-byte Protocol Type field (DSAP) for the 802.2 LLC standard type
field. It also reserved the high-order 2 bits for other uses, similar to the I/G and U/L bits in MAC
addresses. As a result, there were not enough possible combinations in the DSAP field for the needs
of the market—so the IEEE had to define yet another type field, this one inside an additional IEEE
SNAP header. Table 1-6 summarizes the meaning of the three main Type field options with Ethernet.
Ethernet Type Fields
Type Field Description
Protocol Type DIX V2 Type field; 2 bytes; registered values now administered by the IEEE
DSAP 802.2 LLC; 1 byte, with 2 high-order bits reserved for other purposes; registered
values now administered by the IEEE
SNAP SNAP header; 2 bytes; uses same values as Ethernet Protocol Type; signified by
an 802.2 DSAP of 0xAA
20 Chapter 1: Ethernet Basics
Example 1-2 Command Output Showing MAC Address Table Learning
Switch1# show mac-address-table dynamic
Mac Address Table
------------------------------------------
Vlan Mac Address Type Ports
---- ----------- ---- -----
1 000f.2343.87cd DYNAMIC Fa0/13
1 0200.3333.3333 DYNAMIC Fa0/3
1 0200.4444.4444 DYNAMIC Fa0/13
Total Mac Addresses for this criterion: 3
! Above, Switch1’s MAC address table lists three dynamically learned addresses,
! including Switch4’s FA 0/13 MAC.
! Below, Switch1 pings Switch4’s management IP address.
Switch1# ping 10.1.1.4
Type escape sequence to abort.
Sending 5, 100-byte ICMP Echos to 10.1.1.4, timeout is 2 seconds:
!!!!!
Success rate is 100 percent (5/5), round-trip min/avg/max = 1/2/4 ms
! Below Switch1 now knows the MAC address associated with Switch4’s management IP
! address. Each switch has a range of reserved MAC addresses, with the first MAC
! being used by the switch IP address, and the rest being assigned in sequence to
! the switch interfaces – note 0xcd (last byte of 2nd address in the table above)
! is for Switch4’s FA 0/13 interface, and is 13 (decimal) larger than Switch4’s
! base MAC address.
Switch1# show mac-address-table dynamic
Mac Address Table
------------------------------------------
Vlan Mac Address Type Ports
---- ----------- ---- -----
1 000f.2343.87c0 DYNAMIC Fa0/13
1 000f.2343.87cd DYNAMIC Fa0/13
1 0200.3333.3333 DYNAMIC Fa0/3
1 0200.4444.4444 DYNAMIC Fa0/13
Total Mac Addresses for this criterion: 4
! Not shown: PC1 ping 10.1.1.23 (R3) PC1's MAC in its MAC address table
------------------------------------------
Vlan Mac Address Type Ports
---- ----------- ---- -----
1 000f.2343.87c0 DYNAMIC Fa0/13
1 000f.2343.87cd DYNAMIC Fa0/13
1 0010.a49b.6111 DYNAMIC Fa0/13
1 0200.3333.3333 DYNAMIC Fa0/3
1 0200.4444.4444 DYNAMIC Fa0/13
Total Mac Addresses for this criterion: 5
! Above, Switch1 learned the PC’s MAC address, associated with FA 0/13,
! because the frames sent by the PC came into Switch1 over its FA 0/13.
! Below, Switch4’s MAC address table shows PC1’s MAC off its FA 0/6
switch4# show mac-address-table dynamic
Mac Address Table
-------------------------------------------
Vlan Mac Address Type Ports
---- ----------- -------- -----
1 000a.b7dc.b780 DYNAMIC Fa0/13
1 000a.b7dc.b78d DYNAMIC Fa0/13
1 0010.a49b.6111 DYNAMIC Fa0/6
1 0200.3333.3333 DYNAMIC Fa0/13
1 0200.4444.4444 DYNAMIC Fa0/4
Total Mac Addresses for this criterion: 5
! Below, for example, the aging timeout (default 300 seconds) is shown, followed
! by a command just listing the mac address table entry for a single address.
switch4# show mac-address-table aging-time
Vlan Aging Time
---- ----------
1 300
switch4# show mac-address-table address 0200.3333.3333
Mac Address Table
-------------------------------------------
Vlan Mac Address Type Ports
---- ----------- -------- -----
1 0200.3333.3333 DYNAMIC Fa0/13
Total Mac Addresses for this criterion: 1