Switches, Routers, Bridges and LANs/Bridges


“A device used to connect two separate Ethernet networks into one extended Ethernet. Bridges only forward packets between networks that are destined for the other network. Term used by Novell to denote a computer that accepts packets at the network layer and forward them to another network.”

Why Use Bridges?Edit

Bridges are important in some network because the networks are geographically divided into many parts. Something is required to join these networks so that we can connect the whole network. Take for example LAN, if there is no medium to join these LAN an enterprise may be limited in its growth potential. The bridge is one of the tools to join these LANS.

Secondly LAN (for example Ethernet) can be limited in its distance. We can eliminate this problem using bridges so that we can connect the network within the building or campus using bridges. The geographically challenged networks can be connected using Bridges.

Third, the network administrator can control the amount of traffic going through bridges sent across the expensive network media.

Fourth, the bridge is plug and play device so there is no need to configure bridge. And suppose any machine was taken out from the network then there is no need for network administrator to update the information as bridges are self configured.

The MAC BridgeEdit

Bridges are used to connect LANs. Therefore in determining how to transmit traffic between LANs they use a destination MAC address. Bridges pushes the function of network layer such as route discovery and forwarding to the data link layer. There is no conventional network layer for bridge.

The bridges can not maintain the data integrity. For example suppose there is a error in one frame and that frame is not transmitted properly the bridge will not give any acknowledgement to retransmit that frame. If the bridge becomes congested the frames can be discarded to make the traffic smooth. On the other hand the bridges are easy to implement and no need to configure them.

Types of BridgesEdit

  1. Transparent basic bridge
  2. Source routing bridge
  3. Transparent learning bridge
  4. Transparent spanning bridge

The Transparent Basic BridgeEdit

The simplest type of bridge is the transparent basic bridge. It stores the traffic until it can transmit it to the next network. The amount of time the data is stored is very brief. Traffic is sent to all ports except the port from which the bridge received the data. No conversion of traffic is performed by a bridge. In this regard, the bridge is similar to a repeater.

Source Routing BridgeEdit

The route through the LAN internet is determined by the source (originator) of the traffic hence this bridge is called as source routing bridge. The routing information field (RIF) in the LAN frame header, contains the information of route followed by the LAN network.

The intermediate nodes that are required to receive and send the frame must be identified by the routing information. For this reason source routing requires that the user traffic should follow the path determined by the routing information field.

The frames of the source routing protocol are different from the other bridge frames because the source routing information must be contained within the frame. The architecture of the other bridges and the source routing bridges are similar. Both uses MAC relay entity at the LAN node. Interfaces are provided through MAC relay entity and LLC.

The figure shown above shows the functional architecture of the source routing bridges. There are two primitives who are invoked between MAC entities and the LLC. The first is M_UNITDATA.request, and the second is M_UNITDATA.indication

The parameter in these primitives must provide the information about to create the frame (frame control), MAC address, the routing information which is used to forward the traffic through the LAN. The frame check sequence value should be included if frame check sequence operations are to be performed. The primitive also contain a user priority parameter, a data parameter, and a service class parameter. A user parameter and a service class parameter are used only with token rings and not used for other LANs, for example Ethernet or Token bus.

The Transparent Learning BridgeEdit

The transparent bridge finds the location of user using the source and destination address. When the frame is received at the bridge it checks its source address and the destination address. The destination address is stored if it was not found in a routing table. Then the frame sent to all LAN excluding the LAN from which it came. The source address is also stored in the routing table. If another frame is arrived in which the previous source address is now its destination address then it is forwarded to that port.

The physical topology of transparent bridges cannot allow the loops in the network. This is the restriction over transparent learning bridge. The whole operation of this bridge is operated by the bridge processor which is responsible for routing traffic across its ports. The processor decides the destination ports of associated MAC addresses by accessing a routing database. When a frame arrives the processor will check the output port in the database on which the frame will be relayed. If the destination address is not in the database then the processor will broadcast that frame onto all ports except the port from which the frame was arrived. The bridge processor also stores the source address in the frame because this source address may be the destination address for another incoming frame.


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The bridge processes the incoming frame as shown above in figure . The Bridge will check the source and destination address of the incoming frame. In this case the source address is ‘B’ and the destination address is ‘A’. After accessing its routing table, it finds that the destination address is not present in the database and it broadcast the frame to all outgoing ports except the port from which it was arrived. After forwarding the frame it will check the SA (source address) and confirms that it knows it. If SA is present in the database, it will refresh the database and update this entry by refreshing the timer which means that this address is still ‘timely and valid’. In the shown example it does not know about the SA of ‘B’. It stores ‘B’ into database that ‘B’ is an active station on the LAN and from the view point of the bridge ‘B’ can be found on the port 1.


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As shown in figure , a frame arrives at the bridge on port 3 with DA (destination address) as ‘B’ and SA (source address) as C. The bridge will access its database and check the destination address is present or not. The bridge finds the destination port and determines that ’B’ can reach through its port 1. This decision was made from the previous operation in which a frame arrived from port 1 with SA= ’B’ and DA= ’A’. This source address becomes the destination address for this frame. So bridge know the exact destination port no. This time the bridge will not broadcast the frame because it knows the DA. It will forward the frame to port 1 and not on port 2. The bridge also stores in its routing table that the SA ‘C’ can be reached through port 3. This information will be useful in coming next frames. The learning bridge assumes that the frame received on an incoming port has been properly delivered by the other bridges and LANs so it does not forward the frame to the port from where it arrives. The learning bridge is totally based on trust.

In some situation bridges will not forward the frame to any of the port. The total filtering is possible as shown in the figure 5. a frame arrives to the bridge with SA= ’D’ and DA = ’B’ from port 1. The bridge will access its database and check the SA and DA and reveals that DA=’B’ can be found on port 1. The frame is arrived from the same port so it will not forward that frame to port 2 or port 3 and also not backward to port 1. It also checks the SA is present in the database. The SA =’D’ is not present in the database so it will add this entry to the database and time is attached to the entry.

The multicasting and broadcasting is permitted in learning bridge. AS shown in figure a frame arrives from port 1 with DA = ‘ALL’ (All 1s in the destination address field) and SA= ‘D’. The bridge will not update its routing table as ‘D’ is already present in the database. In this example traffic is sent to ports two and three.

The figure 6 shows examples of how a bridge forward and filter the incoming frame. A frame transmitted from station ‘A’ to station ‘B’ is not forwarded by bridge one because it assumes that the traffic was successfully transformed from ‘A’ to ‘B’ using broadcasting. The traffic sent by station ‘A’ to station ‘C’ must be forwarded by bridge 1 and discarded by bridge 2. And the traffic sent by station ‘A’ to station ’D’ must be forwarded by both of the bridges.

The following flow chart shows the overall working logic of the learning bridges. When the bridge will receive incoming frame from port Z, it will look for its destination MAC address in the database. If it is not present the bridge will broadcast that frame to all outgoing ports except from the arrival frame port. If the DA is present in the database, it will forward the frame else if the DA is the port from where it arrives, it will discard that frame. However if the DA is different then it will forward the frame to appropriate port. Then the bridge will check the SA is present in the database. If it is present then it will refresh the timer else it will add that SA to the database.

The Transparent Spanning Tree BridgeEdit

The last type of bridge is transparent spanning bridge. These bridges use a subnet of the full topology to create a loop free operation. The following table show the logic of transparent spanning tree bridge. The received frame is checked by the bridge in following manner. The destination address of arrived frame is checked with routing table in the database. Here more information is required for bridge so the bridge port is also stored in the database. This information is known as port state information and it helps in deciding that, a port can be used for this destination address or not. The port can be in a block state to fulfill the requirements of spanning tree operations or in a forwarding state. If the port is in forwarding state the frame is routed across the port. The port can have different status such as; it may be in “disabled” state for the maintenance reason or may also be unavailable temporarily if databases are being changed in the bridge because of result of the change in the routed network.

Spanning tree ProtocolEdit

Some site uses two or more bridges in parallel between the pair of LANs to increase the reliability of the network as shown in figure 7. This arrangement introduces some looping problem in the network.


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In above figure frame F, with unknown destination is handled.Bridges don’t know the destination address so each of the two bridges broadcast that frame. For this example this means copying it to LAN 2. After that bridge 1 sees the frame F2 which is a frame with unknown destination address, which it copies to LAN1, generating another frame F3 which is not shown in figure. Same way bridge 2 copies frame F1 to LAN1 generating F4 which is also not shown in figure. Bridge 1 now forward frame F4 and bridge 2 copies frame F3. This cycle goes on and on. This is where looping problem came in picture.

The solution to this looping problem is bridges should communicate with each other and change their actual topology with spanning tree that reaches to each LAN in the network. In the spanning tree some bridges in the network are discarded as we want to construct the loop free topology. For example in figure shown below there are 10 routers connecting 9 different LANs. This configuration can be changed to loop free topology shown by the graph in figure 9. Here LANs are shown as node and the LANs are connected by the bridges shown by an arc. This graph can be reduced to the spanning tree by dropping the arc shown by dotted lines. Now there is only single path from one LAN to another. In this way looping problem was solved by the spanning tree and there is only single path from each source to each destination. Loops are totally removed using spanning 3.

To construct the spanning tree follow following Spanning tree Algorithm

  1. First of all select the root bridge. The root bridge is the bridge with the lowest serial number (this number is provided by the router manufacturer). All ports which are coming to the bridge or going out from the bridge are designated port. In our given example in figure the root bridge is ‘A’ and the ports coming from LAN 1 and LAN 2 are the designated ports.
  2. Then select a root port for the non-root bridge. Root port for the non-root bridge is the port with the lowest path cost to the root bridge. In our example the incoming port to bridge ‘B’ is lowest cost path. Same logic applies for the other bridges.
  3. Select a designated port on each segment. The designated port has the lowest cost to the root bridge. In our example the outgoing port from bridge ‘B’ is designated port which has the lowest cost to the root bridge. Same logic applies for the other bridges.
  4. After spanning tree algorithm determine the lowest cost spanning tree, it enables all root ports and the designated ports, and disables all other ports.
  5. The spanning tree algorithm continues to run during normal operation.

The advantages of bridging over routing are as follows:

  1. Transparent bridges are plug and play as they are self learning and do not require any configuration. For the assignment of network address routers require definition for each interface. These addresses should be unique.
  2. Bridging has less overhead for handling packets as compared to routing.
  3. Bridging is protocol independent while routing is protocol dependent.
  4. Bridging will forward all LAN protocols while router can route packets only.

Packet formatsEdit

The Configuration Message:

The following fig. shows the format for the configuration message. It is also called as a bridge protocol data unit (BPDU). The following parameters are set to zero:

  1. The protocol identifier,
  2. Version identifier,
  3. Message type.
Octets Parameters
2 Protocol Identifier
1 Protocol Version
1 Type of BPDU
1 Flags
8 Root identifier
4 Cost to root
8 Bridge identifier
2 Port identifier
2 Message age
2 Max age
2 Hello time
2 Forward Delay

The root identifier field contains the identity of the root bridge, and a 2 octet field which is used for deciding the priority of the root bridge and the designated bridge. The root path cost field indicates the total cost from the transmitting bridge to the bridge which is listed in the root id. field.

The flag field in the bridge message contains a topology change notification flag which is used to inform non root bridges that they should update station entries in cache. This field can also be used to indicate topology change notification bit. The bridges do not have to inform a parent bridge that a topology changes has occurred using the previous field. The parent bridge will perform this task.

The priority and ID of the bridge that is sending the configuration message can be indicated by the bridge and port identifier. The message age field indicates the time in 1/256th of a second. The hello time field also indicates 1/256th of a second, defines the time between the sending of message by the root bridge. The forward delay field, also in 1/256th of a second, is the time during which a port should stay in an intermediate state such as learning or listening before moving from blocking state to forwarding state.

Port statesEdit

Each port on a bridge has a state that controls how it handles the packets it receives. The port can forward frames as normal, or can discard the frames. In addition, a port may choose to learn MAC addresses from frames it discards (in addition to learning from frames it forwards). It's theoretically possible that a port could forward a packet without learning the MAC address from it, but this isn't used in practice. The possible combinations are given the following names:

No forwarding or learning takes place
The port learns MAC addresses, but discards the frames
The port both learns addresses and forwards frames

Remote bridgesEdit

Bridges are used to connect two (or more than 2) different distant LANs. For example a company may have different department at different locations each with its own LAN. The whole network should be connected so that it will act like one large LAN. This can be achieved by the placing a bridge on each LAN and connecting the bridges with the lines (the line given by the telephone company). In the figure10 the three LANs are connected with the remote bridges. As given in the figure we can join number of LANs using remote bridges.


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Rapid Spanning Tree Protocol (RSTP)Edit

In 2001 the IEEE introduced Rapid Spanning Tree Protocol, which is an extension of the original STP that supports faster convergence after a topology change. The new protocol requires a new BPDU format, but supports falling back to the old format in the event of a plain STP bridge being identified. This means that the STP protocol as such is no longer needed, and was later obsoleted in the 2004 release of the 802.1D standard.

Last modified on 30 March 2014, at 21:59