C:\Program Files\Ethereal>tethereal -V
Capturing on \Device\NPF_{1A07B08D-CE31-4431-90AF-359A22A251B0}
Frame 1 (64 bytes on wire, 64 bytes captured)
Arrival Time: Apr 19, 2003 16:46:04.759425000
Time delta from previous packet: 0.000000000 seconds
Time relative to first packet: 0.000000000 seconds
Frame Number: 1
Packet Length: 64 bytes
Capture Length: 64 bytes
IEEE 802.3 Ethernet
Destination: 01:80:c2:00:00:00 (Spanning-tree-(for-bridges)_00)
Source: 00:01:42:cb:e2:84 (Cisco_cb:e2:84)
Length: 38
Trailer: 000000000000000078000C00
Logical-Link Control
DSAP: Spanning Tree BPDU (0x42)
IG Bit: Individual
SSAP: Spanning Tree BPDU (0x42)
CR Bit: Command
Control field: U, func = UI (0x03)
000. 00.. = Unnumbered Information
.... ..11 = Unnumbered frame
Spanning Tree Protocol
Protocol Identifier: Spanning Tree Protocol (0x0000)
Protocol Version Identifier: Spanning Tree (0)
BPDU Type: Configuration (0x00)
BPDU flags: 0x00
0... .... = Topology Change Acknowledgment: No
.... ...0 = Topology Change: No
Root Identifier: 32768 / 00:01:42:cb:e2:80
Root Path Cost: 0
Bridge Identifier: 32768 / 00:01:42:cb:e2:80
Port identifier: 0x8010
Message Age: 0
Max Age: 20
Hello Time: 2
Forward Delay: 15

上記はWINDOWS版ですがUNIXでも同様の動きをします。