思科认证CCNA考试听课笔记3
交换机上配置trunk端口时建议先将端口shutdown，配置完毕后再no shutdown。ISL：思科私有，VLAN范围小，对QoS支持不好，没有native VLAN的概念，新版的CISCO IOS已经不支持ISL，而使用IEEE802.1q。
802.1Q在以太网帧中插入4字节的TAG字段并重新计算CRC值，而ISL不改变以太网帧结构，而采用封装方式，在以太网帧前封装26字节的ISL头部，后面加上4字节的CRC校验。
TRUNK链路双方端口的Native vlan一定要一致，否则以太网帧经过TRUNK链路时VLAN会“跳变”。
PVLAN端口类型：
• Isolated: Communicate with only promiscuous ports
• Promiscuous: Communicate with all other ports
• Community: Communicate with other members of community and all promiscuous ports
A PVLAN uses VLANs in three ways:
•As a primary VLAN: Carries traffic from promiscuous ports to isolated, community, and other promiscuous ports in the same primary VLAN.
•As an isolated VLAN: Carries traffic from isolated ports to a promiscuous port.
•As a community VLAN: Carries traffic between community ports and to promiscuous ports. You can configure multiple community VLANs in a PVLAN.
Isolated and community VLANs are called secondary VLANs.
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