二层快收敛优化(各种fast)

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1、PostfastUplinkfastBackbonefastportfast特性是思科在PVST中提出的快速收敛特性。针对连接终端的端口，由于这些端口不会形成环路，配置了PortFast特性后，此端口直接从Blocking进入Forwarding状态（跳过Listening和Learning状态），收敛时间为0秒。若未配置portfast特性，则这些连接终端的端口收敛时间为30秒（即2个转发延迟）在RSTP中，借鉴思科的PortFast特性，引入边缘端口（egde-port）的概念。在思科设备上，为了命令的一致性，因此还是沿用portfast的配置

2、命令。【配置】方式1：全局启用portfastSW(config)#spanning-treeportfastdefault针对本交换机的所有Access端口都启用PortFast特性intf0/1spanning-treeportfastdisable关闭此端口的portfast特性方式2：接口下启用portfastSW(config)#intf0/1SW(config-if)#switchportmodeaccessSW(config-if)#spanning-treeportfast接口下启用portfastSW(config)#intf0/1S

3、W(config-if)#switchportmodetrunkSW(config-if)#spanning-treeportfasttrunk若希望一个Trunk端口具备portfast特性，必须在接口下启用并明确带上trunk参数。注：当此trunk端口连接到路由器时，可以启用portfast特性；但是交换机互连的trunk端口，若启用portfast特性可能造成交换环路。【查看】1）使用命令showspanning-treeintf0/1Switch#showspanning-treeintf0/1VlanRoleStsCostPrio.NbrT

4、ype---------------------------------------------------------------------------VLAN0001DesgFWD19128.1P2pEdge注：edge表示此端口启用了PortFast特性（在PVST中）或Edge-Port（在Rapid-PVST和MST中）2）使用命令showspanning-treeintf0/1detaila）如果PortFast特性是在接口下使用spanning-treeportfast命令启用的，则显示如下：Switch#showspanning-tr

5、eeintf0/1detailPort1(FastEthernet0/1)ofVLAN0001isdesignatedforwardingPortpathcost19,Portpriority128,PortIdentifier128.1.Designatedroothaspriority32769,address000a.b71c.6c00Designatedbridgehaspriority32769,address000b.be3d.0980Designatedportidis128.1,designatedpathcost19Timers:me

6、ssageage0,forwarddelay0,hold0Numberoftransitionstoforwardingstate:1TheportisintheportfastmodeLinktypeispoint-to-pointbydefaultBPDU:sent1085,received1b）如果PortFast特性是在全局下使用spanning-treeportfastdefault命令启用的，则显示如下：Switch#showspanning-treeintf0/1detailPort1(FastEthernet0/1)ofVLAN0001

7、isdesignatedforwardingPortpathcost19,Portpriority128,PortIdentifier128.1.Designatedroothaspriority32769,address000a.b71c.6c00Designatedbridgehaspriority32769,address000b.be3d.0980Designatedportidis128.1,designatedpathcost19Timers:messageage0,forwarddelay0,hold0Numberoftransition

8、stoforwardingstate:1Theportisintheportfastmodeb