1. Making bigger LANs out of small ones What technology is available to us for connecting small LANs together into larger ones?

2. Creating a MAN (medium area network) Most places are not big enough to have a full WAN, but they’re too big for a single LAN. For example, let’s say you are at a small college in Wisconsin. You might have a LAN in each dorm, one in the library, and one for each classroom building. How can you integrate them all into one network?

3. Connecting devices

4. Invisible devices Passive hub –One of the most common devices. –Many ethernet cables enter. Only one leaves. –Doesn’t do anything except provide a way for many devices to share one cable. Repeater –Takes an attenuated signal in –Regenerates the original signal and sends it out. Active hub –Operates as both a hub and a repeater.

5. Bridges Operates in the physical layer as a repeater. Operates in the data link layer as a way of extending the capabilities of a network to include filtering. Newer ones learn their routing tables. Loop problem: To ensure reliability, we frequently have redundant bridges on a network. If they haven’t learned complete routing yet, that can lead to a loop. A frame is received by two bridges. They don’t know where the receiver is, so they both broadcast the frame. Each receives the other’s broadcast and rebroadcasts it. Etc. –Solved using a spanning tree. A spanning tree is a graph without any loops; in network terms, a topology where there is only one path from one LAN to another.

6. Spanning trees in networks 1.Every bridge on the MAN broadcasts its unique ID. 2.The bridge with the lowest ID becomes the “root” of the spanning tree. 3.Costs are assigned to each hop in the MAN. 4.Now we find the lowest cost path from the root to every other bridge and LAN. The combination of paths becomes the spanning tree. 5.Ports on the spanning tree are marked as forwarding ports. Ports not on the spanning tree are marked as blocking ports. 6.Bridges send out special packets at regular intervals to update the tree.

7. Two-layer switches Essentially a bridge with a separate port for each device connected to it. Two-layer switches operate in the physical and data link layers. Routing decisions are made by MAC address.