1. Virtual LAN (VLAN)

2. VLAN Trunking
ISL encapsulation
IEEE 802.1Q tagged frame

3. 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 forwarding 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.

4. 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.
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.
(1) id of sender bridge
(2) id of believed root
(3) distance in hops to root

5. Spanning Tree Algorithm
A message is considered better than the recorded info if:
it identifies a root with smaller id or
it identifies 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).

6. 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.

7. Token Ring (IEEE 802.5)
Ethernet and the similar protocols are contention-based protocol
Token ring, token bus are examples of contention-free protocol

8. Properties
The ring is a single, shared medium, not point to point links in a loop.
All nodes see all frames.
A distributed algorithm determines when a node can transmit.
Data always flows in one direction.
Basic idea: A token circulates around the ring. When a host has a frame to transmit, it seizes the token and injects the frame on the medium. The frame is forwarded by intermediate nodes until arriving at the destination. The destination puts the frame back after receiving it, but with a “special mark”. The frame circulates back to the sender.

9. Physical Properties
Question: What happens if a node on the ring fails?
As long as a node provides power to the relay, the relay stays open. When the node fails, the relay closes bypassing the node.
Multi-Station Access Unit: Several relays packaged together. Multiple MSAUs can be plugged together to make a larger network.
Typical token ring data rates go from 4 Mbps to 16 Mbps. Number of stations can be as high as 260.

10. Token Ring Frame Format (IEEE 802.5)

11. Switches
Connected to a set of links: each one runs a data-link (layer 2) protocol.
Primary job: receive incoming frames/packets from one link and output them to the appropriate link.
Each input or output is a port (bidirectional).
Question: How does a switch decide what output port to use?
Alternatives:
Datagrams, or Connection-less approach
Virtual circuit, or Connection-oriented approach
Source routing.

12. Datagrams Destination Port A 3 B C D E 2 F 1 G HC D E 2 F 1 G H
Connectionless: Each datagram is forwarded on its own.
forwarding table for switch 2

13. Connectionless Networks
A host can send a packet anywhere, anytime.
When a packet is sent, it is not known whether the network can deliver it.
Each packet is forwarded independently of other packets that may have gone to the same destination.
A switch or link failure is no big deal: an alternate route can be found and the forwarding table updated.

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

15. Simple internetwork (example)

16. Service model Host-to-host service
Philosophy: Define a model that is so undemanding that almost any network technology is able to provide the required service. (IP)