Bridged Format RFC 1483 PDU
LLC and SNAP headers use a routed format or a bridged format. A bridged format does not necessarily mean that the encapsulated protocol is not routable. Instead, it is used when one side of the link supports only the bridged-format Protocol Data Units (PDUs), such as in these applications:
- Connection between a router and a Catalyst switch in a corporate campus ATM network.
- Connection between a router and digital subscriber line (DSL) users that connect through a DSL access multiplexer (DSLAM).
In both applications, the ATM router interface usually serves as the default gateway for the remote users. Then, integrated routing and bridging (IRB), routed bridge encapsulation (RBE) or bridged-style PVCs provide the mechanism for routing traffic off-network.
The LLC header consists of three one-octet fields:
The SNAP header, identified with an LLC value of 0xAA-AA-03, uses this format:
The organizational unique identifier (OUI) field identifies the organization administering the meaning of the two-octet Protocol Identifier (PID) field. Together, the OUI and PID fields identify a distinct routed or bridged protocol.
Use the debug atm packet interface atm command to view these LLC or SNAP header values.
7200-2#show debug ATM packets debugging is on Displaying packets on interface ATM5/0.1 only 06:07:06: ATM5/0.1(O): VCD:0x3 VPI:0x1 VCI:0x32 DM:0x0 SAP:AAAA CTL:03 OUI:0080C2 TYPE:0007 Length:0x80 06:07:06: 0000 0030 9475 10A0 0000 0CD5 F07C 0800 4500 0064 000F 0000 FF01 B785 0101 06:07:06: 0101 0101 0102 0800 58EC 05DF 05A3 0000 0000 0150 188C ABCD ABCD ABCD ABCD 06:07:06: ABCD ABCD ABCD ABCD ABCD ABCD ABCD ABCD ABCD ABCD ABCD ABCD ABCD ABCD ABCD 06:07:06: ABCD ABCD ABCD ABCD ABCD ABCD ABCD ABCD ABCD ABCD ABCD ABCD ABCD
This output means:
- ATM5/0.1(O)—The interface transmits an output packet.
- VCD:0x3 VPI:0x1 VCI:0x32—The PVC uses a virtual circuit descriptor (VCD) of 3, a virtual path identifier (VPI) of 1, and a virtual channel identifier (VCI) of 0x32 or decimal 50. The router presents all the header values in hexadecimal format. Convert these values to decimal to ensure that the ATM headers use the correct values.
- SAP:AAAA—A SNAP header follows.
- OUI:0080C2—The OUI is assigned to the IEEE 802.1 committee. It identifies an Ethernet bridged-format PDU.
- TYPE:0007—The type or protocol ID field is used with Ethernet media to indicate whether the sending ATM bridge retained or removed the Ethernet frame's frame check sequence (FCS). An ATM adaptation layer 5 (AAL5) encapsulation trailer includes a four-byte CRC that provides the same protection against changes during transmission as does the Ethernet FCS.
- 0x00-01 - Ethernet FCS is preserved
- 0x00-07 - Ethernet FCS is not preserved.
- ABCD ABCD ABCD—Cisco ping packets use a default payload pattern of ABCD.
In addition to data packets, bridged ATM interfaces send spanning tree packets when configured to run either the IEEE or Digital Equipment Corporation (DEC) version of this protocol. Enable spanning tree with the help of the bridge {group#} protocol {ieee | dec} command unless remote users have no alternate way into your bridged network. In this case, disabling spanning tree reduces the amount of calculation that the router needs to perform to build a loop-free topology of your network.
Spanning tree hello packets use a Type value of 0x000E. A router that acts as a bridge transmits a hello packet every two seconds by default.
04:58:11: ATM5/0.1(O):
VCD:0x3 VPI:0x1 VCI:0x32 DM:0x0 SAP:AAAA CTL:03 OUI:0080C2
TYPE:000E Length:0x2F
04:58:11: 0000 0000 0080 0000 000C 99F7 1800 0000 0080 0000 000C
99F7 1880 1200 0014
04:58:11: 0002 000F 0043
04:58:11:
04:58:13: ATM5/0.1(O):
VCD:0x3 VPI:0x1 VCI:0x32 DM:0x0 SAP:AAAA CTL:03 OUI:0080C2 TYPE:000E
Length:0x2F
04:58:13: 0000 0000 0080 0000 000C 99F7 1800 0000 0080 0000 000C 99F7 1880
1200 0014
04:58:13: 0002 000F 0029
