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网桥与局域网交换机
网段借助网桥——现称为二层交换机——互连成一个扩展局域网。这些设备通过监听源地址来学习主机所在的端口，只对尚未学习到的帧进行泛洪；表项在几分钟后自动老化，因此主机移动不会使旧条目滞留。若存在环路，泛洪帧将无限循环，故交换机运行分布式生成树算法：以最小标识符选举根网桥，并禁用恰好足够的端口以消除所有环路。该设计的扩展性瓶颈——平面生成树与广播辐射——部分地由虚拟局域网（VLAN）来缓解，VLAN 将端口划分为独立的广播域。
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?