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　　时间过得真是快啊，11月份参加了Cisco CCNA 网络管理培训，到现在已经整整3个月了，原来学过的东西又忘得差不多了。所以从这周开始打算重新看一次ICND (Interconnecting Cisco Network Devices)课程的培训资料，为CCNA考试作准备，时间真的不多了，希望自己能坚持自己定下来的计划。

1.Configuring Catalyst Switch Operations

Basic Layer 2 Switching and Bridging Functions
2层交换机和网桥工作在OSI第二层－data-link layer，HUB集线器工作在OSI第一层
交换机功能：Address learning(MAC); Forward/filter decision; Loop avoidance(STP)
三种传输方式：store and forward; cut-through; fragment-free
Cisco Catalyst 1900 series switch default switching method: fragment-free
Cisco Catalyst 2950 series switch default switching method: store and forward
adaptive cut-through:if the error rate is too high, the switch or bridge can be set, either manually or automatically, to use the store and forward mode instead.

MAC Address Table: learning the source addresses
Catalyst 1900 series 1024 entries, Catalyst 2950 series 8192 entries
flooding: forwarding a frame to all connected ports except the incoming port
age time: Catalyst 1900 series switch - 5 minutes by default

Redundant Topology Overview
redundant links and devices can introduce 3 problems: broadcast storms, multiple frame transmission, and MAC database instability.
broadcast storm: without some loop avoidance process in operation, each switch will flood broadcasts endlessly
some layer 3 protocols implement a Time to Live(TTL) mechanism that limits the number of times a packet can be retransmitted by a layer 3 networking device

Spanning Tree Protocol Overview
The STP is a Layer 2 link management protocol that is used to maintain a loop-free network(by placing certain ports in the blocking state).
Catalyst 1900 series and 2950 series use the IEEE 802.1d STP, STP is enabled by default in Catalyst switches.
Two key concepts: bridge ID(BID) and path cost.
Three steps:
1.Elects one root bridge: the bridge with the lowest BID(Bridge Priority:default 32768 +MAC Address)
2.Selects the root port on the nonroot bridges;
3.Selects the designated port on each segment.

Configuring a Catalyst Switch

2.Extending Switched Networks with VLANs

VLAN Operation Overview

Configuring VLANs