This chapter discusses basic information about bridges and bridge operation. It includes the following sections:
About Bridges
Bridges and Routers
Bridging Methods
How Bridges Work
About Bridges
A bridge is a device that links two or more Local Area Networks (LANs). The bridge accepts data frames from each connected network and then decides whether to forward each frame based on the Medium Access Control (MAC) frame.
You can use bridges to link homogeneous or heterogeneous networks. Homogeneous means that the connected networks use the same bridging method and media types. Heterogeneous means that the connected networks mix different bridging methods and media types, and offer more configuration options. Figure 1-1 illustrates homogeneous and heterogeneous bridging configurations.
Figure 1-1 Homogeneous and Heterogeneous Bridging Configurations
Bridges and Routers
Bridges and routers connect network segments. However, each device uses a different method to establish and maintain the LAN to LAN connections. Routers connect LANs at layer 3 (network layer) of the OSI model while bridges connect LANs at layer 2 (data link layer).
Router Connections
Routers connect distant and diverse LANs more intelligently using network layer protocols. Because of the in-depth network topology related information available at network layer, using routers to connect large networks is recommended.
You must route when a protocol is routable. For example, you must route when mixing Ethernet and Token Ring with protocols that use MAC information in the upper layers. Examples of such protocols are IPX, VINES, and AppleTalk.
Bridge Connections
Bridges connect LANs across a physical link. This connection is essentially transparent to the host connected on the network.
A bridge acts as a relay for frames between networks at the data link layer. The data link layer maintains physical addressing schemes, line discipline, topology reporting, error notification, flow control, and ordered delivery of data frames. The principal service provided by the data link layer to the higher layers is error detection and control. With a fully functional data link layer protocol, the next higher layer may assume virtually error-free transmission over the link.
You must bridge when the protocol is non-routable, that is, it carries no network layer. Examples of such protocols are NetBIOS, SNA, NetBEUI, LAT, LAST, LAD, and LAVC.
Advantages of Bridging
Isolation from upper-layer protocols is one of the advantages of bridging. Since bridges function at the data link layer, they are not concerned with looking at the protocol information that occurs at the upper layers. This provides lower processing overhead and fast communication of network layer protocol traffic.
Bridges can also filter frames based on layer 2 fields. This means that you can configure the bridge to accept and forward only certain type of frames or only frames that originate from a particular network. Filters are very useful for maintaining effective traffic flow.
Bridges are advantageous when dividing large networks into manageable segments. The advantages of bridging in large networks can be summed up as follows:
The decision to choose one method of bridging over another depends on the network's topology and the applications used on the end stations.
The bridge captures MAC frames whose destination addresses are not on the local LAN and forwards them to the appropriate destination LAN. Throughout this process, there is a dialogue between the peer LLC entities in the two end stations. Architecturally, the bridge need not contain an LLC layer since the function of the LLC layer is merely to relay MAC frames.
Figure 1-2 Two-port Bridge Connecting Two LANs
Example 2: Remote Bridging over a Serial Link
Figure 1-3 shows a pair of bridges connected over a serial link. These remote bridges connect LANs with identical LLC and MAC layers (i.e., two Token Ring LANs).
Bridge A captures a MAC frame whose destination address is not on the local LAN and then sends it to bridge B across a serial line using the appropriate serial line encapsulation to identify the bridge frame type. Remote bridge B decapsulates the serial line header and forwards the frame to the local LANs. Throughout this process, there is a dialogue between the peer LLC entities in the two end stations.
Figure 1-3 Bridging Over a Point-to-Point Link
Figure 1-4 Data Encapsulation Over a Point-to-Point Link
Figure 1-5 shows the CSMA/CD and Token Ring MAC frame formats supported by the bridges. The specific frames are detailed in the following section.
Figure 1-5 MAC Frame Format Samples
CSMA/CD (Ethernet) MAC Frames
The following information describes each of the fields found in CSMA/CD (Ethernet) MAC frames: