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都知道交换机的stp协议是通过桥id(bid)来选出根桥（RB）的，其他的交换机都是非根桥也可以说是网桥。 桥ID组成： 优先级+MAC地址 优先级默认是：32768 MAC地址：为本交换机MAC用来选举， 一般一个二层交换机只有一个MAC地址，可以查看那些比较低端的交换机所有端口mac地址都是一样的。 （一个二层交换机不一定有一个mac地址，也可以有上千个，多个mac地址只会选取第一个作为桥id使用，应该是最小的算第一个吧） 不过有些二层交换机也有存在多个mac地址比如一些cisco2916xl 里面就是有多个mac地址具体多少我不知道，没看过官方文档，但是交换机会分配一个范围的mac地址，stp生成树的桥id就是取用这个范围里面的第一个mac作为桥id的组成，下面国外文献参考虽然有点老。 下面是一个华为的端口信息，发现所以端口都是这个mac地址，就展示了一个，意思是这个mac是用来管理交换机的。
关于stp 桥id mac地址问题介绍： For L2 switches, there are mac addresses for different functions (to be able to communicate (when we want to telnet, ping or do snmp to the switch) and spanning tree (to form the bridge ID). On some manufactures switches, they use 1 mac address for all functions, Cisco does not. On the 4/5/6x00 devices, there are 1024 mac addresses assigned to the upervisor (1 or more for the switch, 1000 for spanning tree (PVST, each instance has its' own mac address)). To see the addresses on those switches, do a show module. On the XL series, there is a range, but it seems much smaller (at least on my 2916XL), if you do a show mac self, you would see the range of addresses assigned to the switch. It will use the first one in the range for it self and as part of the bridge id for spanning tree. For Cisco, they are burned in an eprom...but it varies on platform as to were they are located (mother board, supervisor or in the case of the 5500, on the backplane). Catalyst 6000 family switches have a pool of 1024 MAC addresses that can be used as bridge identifiers for VLANs running under PVST+ or for MISTP instances. You can use the show module command to view the MAC address range. stp信息对比：
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