Learning Bridges Ch15z

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15.1

Learning Bridges

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15.2

Bridges

Operate in both physical and data link layers

As physical it regenerate signals as it receive

Used to divide a network into smaller segments

May also relay frames between separate LANs

Keeps traffic from each segment separate;useful for controlling congestion and provides

isolation, as well as security Checks address of frame and only forwards to

segment to which address belongs



15.3

Function of a Bridge



15.4

Transparent Bridges & Learning Bridges

Stations are completely unaware of Bridge’spresence

Builds table by examining destination and source

address of each packet it receives Learning bridges

If address not recognized, packet is relayed to allstations; called F l ood i ng

Stations respond and bridge updates routing table withsegment and station ID info

Changes on the network are updated as they occur



15.5

Figure 15.6 A learning bridge and the process of learning



15.6

Loop Problemin a Learning Bridge

Redundant bridges may be installed toprovide reliability, which may create loop

To prevent infinite looping of packetsbetween bridges, a spanning tree algorithm is used to identify

any redundant paths Source Routing, source dictates the path upto

destination


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Figure 15.7 Loop problem in a learning bridge



Spanning Tree

To prevent infinite looping of packetsbetween bridges, a spanning treealgorithm is used to identify any

redundant paths Path with lowest cost will be identified and

used as the primary route that

communications will be routed through;



Spanning Tree

Spanning Tree is a graph in which there isno loop In Bridged LAN, it creates a topology in

which each LAN can be reached from any

other LAN through one path only (no loop Process involves Three steps:

Selection of r oo t b r i dge

Mark one port of each bridge as r oo t po r t Choose a des i gna t ed b r i dge for each LAN



Spanning Tree

Process involves Three steps: Selection of r oo t b r i dge , one with the smallest ID

selected as root, as every bridge has a unique ID Mark one port of each bridge (except for the root

bridge) as r oo t po r t . A root port is the port with theleast cost path from the bridge to the root bridge.

Least cost criteria can be minimum number of hops ormay be minimum delay and maximum bandwidth.

Choose a des i gna t ed b r i dge for each LAN. A designated bridge has the least cost path between theLAN and root bridge, called as designated port

Root port and designated port as forwarding ports and other as blocking ports. A forwarding portforwards the frame it receives, blocking does not.



Spanning Tree Algorithm



Spanning Tree Algorithm -- cont



Figure 15.8 A system of connected LANs and its graph representation



Figure 15.9 Finding the shortest paths and the spanning

tree in a system of bridges



Figure 15.10 Forwarding and blocking ports after using spanning

tree algorithm


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Source Routing

Sender of packet defines bridges androutes that packet should take

Complete path of bridge IDs and

destination address is defined within theframe

Bridge routing table is not used

Designed to be used with Token RingLANs Not as common today

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Learning Bridges Ch15z