3Com Router User Manual

18
CONFIGURING HDLC
This chapter contains information on the following topics:
■Configure HDLC
■Display and Debug HDLC
Configure HDLCHDLC (High Data Link Control) is a bit-oriented link layer protocol. Its most 
prominent feature is that it can transparently transmit any kind of bit flow without 
the restriction that the data must be character set. Protocols of standard HDLC 
protocol group operate upon the synchronous serial lines, e.g., DDN. The address 
field of HDLC is 8 bits, its control field is 8...

288CHAPTER 18: CONFIGURING HDLC
Enable HDLC packet debuggingdebugging hdlc packet [ 
interface type number ] 

19
CONFIGURING BRIDGE
This chapter contains information on the following topics:
■Bridge Overview
■Configure Bridge’s Routing Function
■Display and Debug Bridge
■Typical Bridge Configuration
Bridge OverviewBridge is a type of network device on the data link layer, which interconnects Local 
Area Networks (LANs) and transfers data between them.
 In some small-sized 
networks
, especially in the networks widely dispersed, using bridges can reduce 
the network maintenance cost, and the network terminal...

290CHAPTER 19: CONFIGURING BRIDGE
■Supports bridging on the Frame Relay links.
■Supports bridging on the sub-interfaces of VLAN.
■Supports bridging on BDR and dialing standby.
■Supports binding of multiple ports and load sharing.
■Support both routing and bridging function for specified protocol.
■Support filtering Ethernet frames according to the MAC address or Ethernet 
frame format.
■Provides command configuration and management functions.
■Provides functions of logging, alarming and debugging.
Main...

Bridge Overview291
Figure 102   Bridge learns that Workstation A is connected with Port 1
Once Workstation B responds to Workstation A, the bridge can detect the 
responding Ethernet frame from Workstation B and learn that Workstation B is 
also connected to Bridge port 1 because the frame is detected on port 1 too. As a 
result, the correlation between the MAC address of Workstation B and Bridge port 
1 is added to the bridging table too, as shown in the following figure:
Figure 103   Bridge learns that...

292CHAPTER 19: CONFIGURING BRIDGE
Figure 104   Final bridging address table
Forward and Filter
The bridge will make the decision to forward frames or not (that is, to filter 
frames) depending on the following three conditions:
■If Workstation A sends an Ethernet frame whose destination is Workstation C, 
the bridge will detect this frame and learn that Workstation C corresponds to 
Bridge port 2 by looking up its bridging table. So, it will forward the frame to 
Bridge port 2, as shown in the following...

Bridge Overview293
Figure 106   Filter (not forward)
■Suppose that Workstation A sends an Ethernet frame to Workstation C, and 
the bridge does not find the correlation between the MAC address of 
Workstation C and the port in the bridging address table, what will the bridge 
do? The bridge will forward this frame destined to an unknown MAC address 
to all ports except the one on which it is received. In this case, the bridge 
actually plays the role of a hub to make sure the continuous information...

294CHAPTER 19: CONFIGURING BRIDGE
Figure 108   Preliminary examination state of bridging loops
As shown in the following figure, the broadcast frame is forwarded over Ethernet 
segment 2 and Ethernet segment 3 that are connected with Bridge Z. Upon 
detecting two copies of this frame on two different ports, Bridge Z forwards them 
to Ethernet segment 3 and Ethernet segment 2 again. Thus, Ethernet segment 2 
and Ethernet segment 3 receive a copy of this frame for the second time. Like this, 
the frame is...

Bridge Overview295
will also specify which bridge to be the “root bridge” and which bridges to be the 
“leaf nodes”.
A BPDU contains the following information:
■Root Identifier: Consists of the Bridge Priority and the MAC address of the root 
bridge.
■Root Path Cost: Path cost from the individual leaf nodes to the root bridge.
■Bridge Identifier: Consists of the Bridge priority and the MAC address of the 
current bridge.
■Port Identifier: Consists of the Port Priority and the Port Number.
■Message Age of...

296CHAPTER 19: CONFIGURING BRIDGE
Figure 110   Spanning tree topology
BPDU Forwarding Mechanism
Upon the initiation of the network, all the bridges assume themselves as the root 
bridge. The designated interface of the bridge regularly sends its BPDU once a 
Hello Time. If it is the root port receives the BPDU, it will increase the Message Age 
carried in the BPDU and enable the timer to time this BPDU. If a path fails, the root 
port on this path will not receive new BPDUs any more and old BPDUs will be...