The understanding of hubs and bridges

The contents of this article are organized from network. Included concepts are: Hub (hub), bridge.

-----HUB

Hub (hub) belongs toData Communication Systemsin the basic device, it andTwisted pair Wiresuch as transmission media, isA hardware device that does not require any software support or that requires minimal management of software management. It is widely used in various occasions. hubs work in a local area network (LAN) environment , applied to the first layer of the OSI Reference Model and is therefore referred to as the physical layer Equipment . The internal hub uses an electrical interconnect, when the environment that maintains the LAN is a logical bus orring-shaped structure, it is possible to create a physical star or tree network structure with a hub. In this regard, the role of the hub is equivalent to the multi-portRepeater Device. In fact, the hub is actually a kind of repeater, the difference is only that the hub can provide more port services, so the hub is also calledmulti-port relays. The hub can be divided into passive hub, active hub and smart hub according to the processing mode of input signal.
Working process: The working process of the hub is very simple, it can be described as simple: first, the node signals to the line, the hub receives the signal, because the signal in the cable transmission attenuation, the hub receives the signal after the attenuation of the signal shaping amplification, The last hub forwards the amplified signal broadcast to all other ports.

Category:

• Passive Hub It's the lowest level. one that does not do any processing of the signal, Of the media The transmission distance is not extended and has a certain effect on the signal. each computer connected to the hub receives a signal from all other computers on the same hub;
• Active Hub the difference from the passive hub is is that it can amplify or regenerate the signal, thus extending the effective transmission distance;
• Smart Hub In addition to having all the functions of the active hub, there are Network Management and routing capabilities. In the Smart Hub network, not every machine can receive a signal, only the same address as the signal destination address port computer to receive. Some smart hubs can choose the best path for themselves, which is a good way to manage the network . (Hzhsan: personally feel that the smart hub here is a bit beyond the scope of the hub.) )

-----Network Bridge

1. Understanding the meaning of "bridge"

In addition to extending the physical connection range of the network, the Network Bridge can also partition the MAC address, Bridge is a two-tier device Span style= "color: #333333; Background-color:inherit ".

We often hear the saying that" bridge "is a two-layer network device that can connect different network segments (as well as a two-tier switch), one port can connect to a network segment How can bridges connect different network segments? In fact, this "network segment" is more accurately said to be called "Physical network segment", refers to the IP Addresses belong to the same network address segment (that is, the network ID in the IP address), different LAN segments in different geographic locations, and are based on geographic areas of physical significance. What we used to say is that the IP address belongs to a network or subnet of different network address segments, and is a three-layer concept , in fact, this should be called Logical network segment, is based on the logical meaning of the Network address division. (Hzhsan: that is, the network segment here is a physical network segment, not the IP segment we normally call, not the concept on the three layer )

Whether it's a bridge or a two-layer switch, each port can connect to a network segment, but the hosts they connect to are on the same network, or on the same subnet. If the connected host is in a different office or in a different building, two or more small LANs can take the same network address, To form a large LAN that can be managed uniformly. but note that Because the bridge has only two ports So the host of the two physical network segments connected is usually a centralized connection by the hub at that time (bridge ports are usually not directly connected to the host). The bridge that is commonly referred to in software, such as the bridging work mode in VMware, is the function of the bridge, It is also connected to two network segments in the same network or subnet.

Hzhsan question: Are there only two ports on a bridge? Should there be more than one port?

Hzhsan answer: The basic bridge has only two ports, there is a bridge called multi-port bridge, multi-port bridge has multiple ports.

Figure 5-34 Two physical network segments of a bridge connection

2. Network Bridge Working principle analysis

in front of it. Bridge has two main characteristics: one is based on the physical network segment of the MAC address to learn, the second is to isolate the conflict domain. The following is an example of parsing.

Assume that the physical segment 1 and the host in physical network Segment 2 as shown in Figure 5-34 are both The backplane channel of the bridge is not a shared Network bridge data forwarding principle 5-35 is shown. The following is a concrete parsing of the

description mac Address Table This is what is commonly referred to as CAM (content addressable memory, contents addressable storage) Table, The mapping of the corresponding MAC address host to the connected switch port is saved on the switch you can pass some commands (such as the Cisco switch is to use show mac-address-table command) to view. Here is an example of viewing the MAC address and Port mapping table on the switch, which lists the static (static) MAC address assigned to the CPU in the switch and the dynamic MAC address that is learned from the learning function, where the ports column shows the corresponding Mac The port to which the address host is connected, and the VLAN column is the VLAN to which the corresponding host connection port belongs.

Figure 5-35 Network Bridge data Forwarding principle

Hzhsan: For a detailed explanation, see the following text section. Detailed and understandable.

  
 

   
  

  
Switch#show mac-address-table
   
  

  
Mac Address Table
   
  

  
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Vlan Mac Address Type Ports
   
  

  
---- ----------- -------- -----
   
  

  
All 0100.0CCC.CCCC STATIC CPU
   
  

  
All 0100.0CCC.CCCD STATIC CPU
   
  

  
All FFFF.FFFF.FFFF STATIC CPU
   
  

  
1 0000.0C07.ACCB DYNAMIC GI0/1
   
  

  
1 0002.8501.de00 DYNAMIC GI0/1
   
  

  
1 0015.f915.8e80 DYNAMIC GI0/1
   
  

  
1 0016.7694.c009 DYNAMIC GI0/1
   
  

  
1 0020.ed14.399c DYNAMIC GI0/1
   
  

  
1 0030.b637.8e10 DYNAMIC GI0/1
   
  

  
1 0050.ba10.404a DYNAMIC GI0/1
   
  

  
0007.847b.c40a DYNAMIC GI0/1
   
  

  
00D0.D3A4.7CEC DYNAMIC GI0/1
   
  

  
0006.28BB.71C0 DYNAMIC GI0/1
   
  

  
00D0.D3A4.7CEC DYNAMIC GI0/1
   
  

  
0000.b497.8250 DYNAMIC FA0/20
   
  

  
0002.b3d8.68e7 DYNAMIC FA0/20
   
  

  
0002.b3d8.6928 DYNAMIC FA0/20
   
  

  
0003.A03A.03FC DYNAMIC FA0/19
  
 

Now assume that a PC in the network shown in Figure 5-34 wants to send data to another PC. Because a hub is also a physical layer device that does not recognize the MAC address in a frame, no matter which host is sending data, it is broadcast on the hub, and all nodes connected to the hub receive this broadcast frame, including the port to which the bridge is connected to the hub.

1) When the bridge receives a broadcast frame for the hub, the bridge compares the source MAC address and the destination MAC address in the frame with the MAC address table saved in the bridge cache.

2) Initially, there is no MAC address in the Network Bridge cache, so at first it does not know which host is on which physical network segment, all received frames are forwarded directly to the other port in flood mode (also copy the original data frame), and the source Mac in the data frame The physical network segment corresponding to the address is recorded ( in fact, corresponds to the corresponding bridge port ).

3) After the data frame is received by a PC, the corresponding physical network segment corresponding to the destination MAC address is recorded in the Mac table in the cache. In this way, after many such records, you can in the MAC address table in the entire network host MAC address and the corresponding physical network segment all recorded. Because the port of the bridge is usually a connection hub, a bridge port is mapped to multiple host MAC addresses.

4) When a network bridge receives a data frame in which the source MAC address and the destination MAC address are found in the Bridge MAC Address table, the bridge compares whether the two MAC addresses belong to the same physical network segment. If it is the same physical network segment, then the bridge will not forward the frame to the next port, directly discarded, play a role in the conflict domain isolation. Conversely, if two MAC addresses are not on the same physical network segment, the bridge forwards frames from one physical network segment to the other physical network segment , and then broadcasts the replication through the connected hub.

The understanding of hubs and bridges