1. Bridges Neil Tang 10/10/2008
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2. Bridge Used to connect two or more LANs.
A simple multiple-input and multiple-output switch.
Datagram (connectionless) switching. Whenever receiving a packet from an input port, forward it to all the other output ports.
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3. Routing Table
Host
Port A 1 B C X 2 Y Z
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4. Bridge Compute/maintain the routing table
Human (administrator) maintained routing table.
Automatically maintained table: inspect the source address of each received packet and update the routing table accordingly.
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5. Loop in the Extended LANY
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6. Loop in the Extended LAN
Packets (corrupted, multicast, or normal packets but incomplete routing table) may potentially loop through the extended LAN forever.
Loops are caused by two reasons: 1) An extended LAN may span multiple organization. In this setting, no person knows about the entire configuration. 2) Loops are built intentionally to support fault tolerance.
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7. Spanning Tree Algorithm
The spanning tree algorithm helps each bridge to decide the
ports over which it is or is not willing to forward frames such
that all LANs form a tree without any loop. A C E D B K F H J G I B5 B2 B3 B7 B4 B1 B6
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8. Spanning Tree Algorithm
After running the algorithm
The bridge with smallest ID is elected as the root.
Each bridge is on the shortest path (minimum hop) to the root and knows which of its ports are on the path.
Each LAN has a designated bridge which will be responsible for forwarding packets. Each designated bridge must be the one closest to the root. Bridge ID is used to break a tie.
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9. Spanning Tree Algorithm
Configuration message
The ID for what the sending bridge believes to be the root
The distance to the root in terms of hop count
The ID of the sending bridge
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10. Spanning Tree Algorithm
Procedure
Initially, each bridge thinks it is the root and send out corresponding configuration message.
When receiving a configuration message over a port, it checks if this message is a “better” message (root with smaller id, shorter distance or smaller ID of sending bridge) for that port.
If receiving a message indicating it is not the root, then the bridge stops generate its own configuration message and instead only modifies and forwards messages from other bridges.
If receiving a message indicating it is not the designated bridge for that port, then the bridge stops sending any messages over that port.
Eventually, the system will stabilize and a spanning tree will be constructed.
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11. Spanning Tree Algorithm
Example
B3 receives (B2, 0, B2).
Since 2<3, B3 accepts B2 as root
B3 adds one to the distance advertised by B2 (0) and sends (B2, 1, B3) to B5.
Meanwhile, B2 accepts B1 as the root and it sends (B1,1, B2) to B3.
B5 accepts B1 as the root and sends (B1,1,B5) to B3.
B3 accepts B1 as the root and it realizes that both B2 and B5 are closer to the root than it is. So it stops sending any messages on both ports. A C E D B K F H J G I B5 B2 B3 B7 B4 B1 B6
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12. Fault Tolerance
After the system stabilizes, the root will continue to send the configuration message periodically and the other bridges will continues to forward it.
If a bridge fails, the downstream bridges will not receive the configuration message. After a certain period, they will claim to be the root and start a new procedure again.
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13. Broadcast and Multicast
If the destination address of a received packet is a broadcast or multicast address, a bridge will forward it to all the other ports. Hosts will decide if it will accept this packet or not.
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14. Limitations
Scalability: Spanning tree algorithm and broadcast do not scale well.
Heterogeneity: bridges can only be used to connect networks with same frame format.
Traffic control: congestion, long latency, packet loss.
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