Hubs, Bridges, and Switches in Java

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departments are interconnected by a bridge, as in Figure 56-2, we again refer to the entire interconnected network as a LAN, and we again refer to each of the departmental portions of the network as LAN segments But in contrast to the multi-tier hub design in Figure 56-1, each LAN segment is now an isolated collision domain
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Figure 56-2: Three departmental LANs interconnected with a bridge Bridges can overcome many of the problems that plague hubs First, bridges permit inter-departmental communication while preserving isolated collision domains for each of the departments Second, bridges can interconnect different LAN technologies, including 10 Mbps and 100 Mbps Ethernets Third, there is no limit to how big a LAN can be when bridges are used to interconnect LAN segments: in theory, using bridges, it is possible to build a LAN that spans the entire globe
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Bridge Forwarding and Filtering
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Filtering is the ability to determine whether a frame should be forwarded to an interface or should just be dropped When the frame should be forwarded, forwarding is the ability to determine which of the interfaces the frame should be directed to Bridge filtering and forwarding are done with a bridge table For each node on the LAN, the bridge table contains (1) the LAN address of the node, (2) the bridge interface that leads towards the node, (3) and the time at which the entry for the node was placed in the table An example Table for the LAN in Figure 562 is shown in Figure 56-3 This description of frame forwarding may sound similar to our discussion of datagram forwarding in 4 We note here that the addressees used by bridges are physical addresses (not network addresses) We will also see shortly that a bridge table is constructed in a very different manner than routing tables Address 62-FE-F7-11-89-A3 7C-BA-B2-B4-91-10 Interface Time 1 3 9:32 9:36
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Figure 56-3: Portion of a bridge table for the LAN in Figure 562
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file:///D|/Downloads/Livros/computa o/Computer%20Netwoch%20Featuring%20the%20Internet/transparent_bridgeshtm (3 of 12)20/11/2004 15:52:39
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Hubs, Bridges, and Switches
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To understand how bridge filtering and forwarding works, suppose a frame with destination address DD-DD-DD-DDDD-DD arrives to the bridge on interface x The bridge indexes its table with the LAN address DD-DD-DD-DD-DDDD and finds the corresponding interface y
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If x equals y, then the frame is coming from a LAN segment that contains adapter DD-DD-DD-DD-DD-DD There being no need to forward the frame to any of the other interfaces, the bridge performs the filtering function by discarding the frame If x does not equal y, then the frame needs to be routed to the LAN segment attached to interface y The bridge performs its forwarding function by putting the frame in an output buffer that precedes interface y
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These simple rules allow a bridge to preserve separate collision domains for each of the different LAN segments connected to its interfaces The rules also allow the nodes on different LAN segments to communicate Let's walk through these rules for the network in Figures 56-2 and its bridge table in Figure 56-3 Suppose that a frame with destination address 62-FE-F7-11-89-A3 arrives to the bridge from interface 1 The bridge examines its table and sees that the destination is on the LAN segment connected to interface 1 (ie, the Electrical Engineering LAN) This means that the frame has already been broadcast on the LAN segment that contains the destination The bridge therefore filters (ie, discards) the frame Now suppose a frame with the same destination address arrives from interface 2 The bridge again examines its table and sees that the destination is the direction of interface 1; it therefore forwards the frame to the output buffer preceding interface 1 It should be clear from this example that as long as the bridge table is complete and accurate, the bridge isolates the departmental collision domains while permitting the departments to communicate Recall that when a hub (or a repeater) forwards a frame onto a link, it just sends the bits onto the link without bothering to sense whether another transmission is currently taking place on the link In contrast, when a bridge wants to forward a frame onto a link, it runs the CSMA/CD algorithm discussed in Section 53 In particular, the bridge refrains from transmitting if it senses that some other node on the LAN segment is transmitting; furthermore, the bridge uses exponential backoff when one of its transmissions results in a collision Thus bridge interfaces behave very much like node adapters But technically speaking, they are not node adapters because neither a bridge nor its interfaces have LAN addresses Recall that a node adapter always inserts its LAN address into the source address of every frame it transmits This statement is true for router adapters as well as host adapters A bridge, on the other hand, does not change the source address of the frame One significant feature of bridges is that they can be used to combine Ethernet segments using different Ethernet technologies For example, if in Figure 56-2, Electrical Engineering has a 10Base2 Ethernet, Computer Science has a 100BaseT Ethernet, and Electrical Engineering has a 10BaseT Ethernet, then a bridge can be purchased that can interconnect the three LANs With Gigabit Ethernet bridges, it is possible to have an additional 1 Gbps connection to a router, which in turn connects to a larger university network As we mentioned earlier, this feature of being able to interconnect different link rates is not available with hubs Also, when bridges are used as interconnection devices, there is no theoretical limit to the geographical reach of a LAN In theory, we can build a LAN that spans the globe by interconnecting hubs in a long, linear topology, with each pair of neighboring hubs interconnected by a bridge Because in this design each of the hubs has its own collision domain, there is no limit on how long the LAN can be We shall see shortly, however, that it is undesirable to build very large networks exclusively using bridges as interconnection devices -- large networks need routers as well
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