Computer Networks · free preview
Bridges and LAN Switches
Segments are joined into an extended local area network by bridges, now called layer 2 switches, which learn which hosts live on which port by watching source addresses and flood only what they have not yet learned, entries age out after a couple of minutes so mobility never strands them, loops would make flooded frames circulate forever so the switches run a distributed spanning tree algorithm that elects a root by smallest identifier and disables just enough ports to break every cycle, and the design's scaling limits, a flat spanning tree and broadcast radiation, are partly answered by virtual LANs that color ports into separate broadcast domains.
1. Learning Bridges
Learning Bridges
2. Loops Are Fatal
Loops Are Fatal
3. Growing the Tree Distributedly
Growing the Tree Distributedly
4. The Limits of Layer 2
The Limits of Layer 2
5. Learning from sources
A frame addressed to host X arrives at a bridge's port 2, and X is not in the forwarding table. What does the bridge do with the frame, and what does it record?
6. The aging timer
Why does each entry in a bridge's forwarding table carry a time to live that refreshes on every frame from that host?
7. Why loops kill
Three switches are wired in a triangle and a host broadcasts one frame. What happens without the spanning tree algorithm?
8. Root election
Switches with identifiers 4, 9, and 2 join a bridged network. According to the configuration message rules, which becomes the root, and how do the others come to agree?
9. Designated switches
Two switches are equally close to the root and both attach to the same shared link. Which one forwards data frames onto that link, and why exactly one?
10. Why layer 2 stops growing
What two properties of switched Ethernet limit its growth to tens of switches?
11. Learning signal
A learning bridge fills its table by recording which field of each arriving frame, against the port it arrived on?
12. The unknown default
When a frame's destination is not found in the bridge's table, the frame is sent out all ports except the arrival port, an action called what?
13. The tree property
A spanning tree keeps every vertex of the network graph while eliminating every what, which is the property that stops frames from circulating?
14. VLAN identifier
In a virtual LAN, each switch port is configured with an identifier, sometimes called a color, and frames may travel only between segments sharing the same what?
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