lecture11

Slide1: 

Bridges and LAN Switches

Last time: Datagrams: 

Last time: Datagrams forwarding table Connectionless: Each datagram is forwarded on its own.

Virtual Circuit Switching: 

Virtual Circuit Switching Establish a connection between source and destination between any data is communicated. Two part process: (1) Connection setup (2) Data transfer Virtual Circuit Table: virtual circuit identifier incoming interface outgoing interface [a different VCI for outgoing packets]

Source Routing: 

Source Routing The sender of a packet knows the precise route that will take data across the network, all the way to the destination host. Each packet carries with it the entire route from source to destination.

Bridges and LAN Switches: 

Bridges and LAN Switches

Bridges: 

Bridges You want to interconnect two Ethernets. You could use repeaters as we discussed previously: ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ Question: Is this solution always going to work? Why not? You could also use “bridges”, i. e. special multiport nodes or switches that can forward packets from one LAN to the other. LAN 1 LAN 2

Leaning Bridges: 

Leaning Bridges LAN 1 LAN 2 Question: Does a bridge need to forward all the packets it receives? Question: Do you need human intervention to configure the bridge? How could it be automated? A bridge can inspect all the packets it receives; looking at the source addresses it receives, it can map hosts to ports building a table like the one above. Questions: What should the bridge do if it receives a packet to a destination it doesn’t know? Questions: How many entries should the bridge have? Should it keep entries forever after created?

Spanning Tree: 

Spanning Tree Extended LANs can have cycles because either they were built without global knowledge or for the sake of having redundant paths between LANs. In order to populate routing tables so that packets don’t end up looping forever, first we have to remove cycles from the network graph and find a spanning tree.

Spanning Tree Algorithm: 

Spanning Tree Algorithm Question: How can you find a spanning tree of a graph if you don’t know the full graph? Bridges have ids, just as nodes do. When a bridge is started up, it exchanges configuration messages with others and elects one bridge to be the root of the spanning tree. (1) id of sender bridge (2) id of believed root (3) distance in hops to root configuration message Initially every bridge thinks it’s the root and sends out configuration messages on all its ports. When it starts receiving messages from other bridges, it checks to see if the new message improves the configuration recorded for that port.

Spanning Tree Algorithm: 

Spanning Tree Algorithm A message is considered better than the recorded info if: it ids a root with smaller id or it ids a root with equal id but shorter distance or the root id and distance are equal, but the sending bridge has smaller id. Before updating the info with the received message, the bridge adds 1 to the hop count. When a bridge discovers it is not the root, it stops sending out messages of its own (only forwards those from other bridges after adding 1 to the hop count).

Spanning Tree Algorithm: 

Spanning Tree Algorithm When a bridge discovers it’s not the designated bridge for that port, it stops sending configuration messages over that port. Eventually, the system stabilizes: only the root sends out configuration messages and the other bridges only forward them around.

Broadcast and Multicast: 

Broadcast and Multicast Broadcast messages should still reach all the nodes in the extended LAN. Multicast messages should reach all the nodes in a multicast group independently of the LAN to which the nodes belong. Question: How can one implement these kinds of messages on the extended LAN?

Limitations of bridges?: 

Limitations of bridges?