2012年思科认证(CCNA)考试知识复习指导2
09-10
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VLAN&TRUNK
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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