乾颐堂安德CCIEv5.0实验指南：STP中通过Port-priority完成Vlan间流量的负载均衡
第39章 通过Port-priority完成Vlan间流量的负载均衡
39.1 理论基础
在交换网络中，通常在汇聚层会做主根和备份根完成冗余。假设两台设备分别为SW1和SW2，而网络中有两个VLAN，vlan10和vlan 100，很多时候会把SW1做为vlan10的主根，同时作为vlan100的备份根（secondary root），相反的SW2做为vlan100的主根，作为vlan10 的备份根。
本文档讲述的是在两条trunk链路上应用port-priority对不同vlan的流量做负载的情况，即端口优先级的用法
让我们讲回生成树，当我们的根已经确定下来的时候，非根交换机通过两条trunk链路连接到根，在非根交换机上有且只有一个根端口，如何选举:选出一条到达根的最小开销的链路（但此时两条trunk链路cost同为19）；接下来通过最小的发送者的Bridge ID（2字节优先级+6字节的MAC地址组成）选举，但是此时发送者是同一台交换机，Bridge ID相同，无法选出；
最后最小的Port ID（由端口优先级+Port ID组成）优选，这将是我们研究的内容。需要注意的是分为默认的发送者的端口ID以及根设备上收到的端口ID（见上一章生成树操作部分）
不得不说的是，这是RS CCIE考试中经常遇到的题目之一
其在MST模式下的命令格式如下
spanning-tree mst instance_id port-priority priority
本实验演示vlan中的配置命令
39.2 拓扑描述
SW1和SW2之间Trunk链路，Sw1为两个VLAN的根，两个Vlan：Vlan 10和Vlan 100
图39-1 修改STP参数
39.3 实验步骤
39.3.1 完成Vlan和Trunk的配置
在Sw1和Sw2上分别做如下配置
SW2(config)#vlan 10
SW2(config-vlan)#vlan
SW2(config-vlan)#vlan 100
！
SW2(config)#int range f0/5 - 6
SW2(config-if-range)# switchport trunk encapsulation dot1q
SW2(config-if-range)# switchport mode trunk
SW2#show interfaces trunk
Port Mode Encapsulation Status Native vlan
Fa0/5 on 802.1q trunking 1
Fa0/6 on 802.1q trunking 1
Port Vlans allowed on trunk
Fa0/5 1-4094
Fa0/6 1-4094
把SW1和SW2的10口划入Vlan 100
SW2(config)#int f0/10
SW2(config-if)#switchport mode access
SW2(config-if)#switchport access vlan 100
验证：
SW2#show vlan brief
VLAN Name Status Ports
---- -------------------------------- --------- -------------------------------
1 default active Fa0/1, Fa0/3, Fa0/4, Fa0/7
Fa0/8, Fa0/9, Fa0/11, Fa0/12
Fa0/13, Fa0/14, Fa0/15, Fa0/16
Fa0/17, Fa0/18, Fa0/19, Fa0/20
Fa0/21, Fa0/22, Fa0/23, Fa0/24
Gi0/1, Gi0/2
10 VLAN0010 active
100 VLAN0100 active Fa0/10
39.3.2 把SW1配置成为Vlan10和vlan100的根
来看默认的情况
SW1(config)#spanning-tree vlan 100 priority 0
SW1(config)#spanning-tree vlan 10 priority 0
两台路由器上在相应接口配置地址
验证：
R1#ping 10.1.1.2
Type escape sequence to abort.
Sending 5, 100-byte ICMP Echos to 10.1.1.2, timeout is 2 seconds:
!!!!!
Success rate is 100 percent (5/5), round-trip min/avg/max = 1/1/4 ms
最主要的是验证生成树的情况：
SW1# sh spanning-tree vlan 100
VLAN0100
Spanning tree enabled protocol ieee
Root ID Priority 100
Address 9caf.ca47.8c00
This bridge is the root
Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec
Bridge ID Priority 100 (priority 0 sys-id-ext 100)
Address 9caf.ca47.8c00
Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec
Aging Time 15 sec
Interface Role Sts Cost Prio.Nbr Type
------------------- ---- --- --------- -------- --------------------------------
Fa0/5 Desg FWD 19 128.7 P2p ---sw1是根交换机，所以其接口都为转发状态
Fa0/6 Desg FWD 19 128.8 P2p
Fa0/10 Desg FWD 19 128.12 P2p
查看SW2的情况：
SW2#sh spanning-tree vlan 100
VLAN0100
Spanning tree enabled protocol ieee
Root ID Priority 100
Address 9caf.ca47.8c00
Cost 19
Port 5 (FastEthernet0/5)
Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec
Bridge ID Priority 32868 (priority 32768 sys-id-ext 100)
Address 0016.9dd7.5180
Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec
Aging Time 15
Interface Role Sts Cost Prio.Nbr Type
------------------- ---- --- --------- -------- --------------------------------
Fa0/5 Root FWD 19 128.5 P2p
Fa0/6 Altn BLK 19 128.6 P2p---就像我们前边分析的一样，根据cost（19）和BID无法选出根端口，此时的情况是由于默认情况下6口的port id大于5口的端口ID ，也就是说现在VLAN100的流量经过的是F0/5
Fa0/10 Desg FWD 19 128.10 P2p Edge
如果此时VLAN10也有流量的话，其实也是经过f0/5的，我们查看spanning的情况：
SW2#sh spanning-tree vlan 10
VLAN0010
Spanning tree enabled protocol ieee
Root ID Priority 10
Address 9caf.ca47.8c00
Cost 19
Port 5 (FastEthernet0/5)
Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec
Bridge ID Priority 32778 (priority 32768 sys-id-ext 10)
Address 0016.9dd7.5180
Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec
Aging Time 300
Interface Role Sts Cost Prio.Nbr Type
------------------- ---- --- --------- -------- --------------------------------
Fa0/5 Root FWD 19 128.5 P2p
Fa0/6 Altn BLK 19 128.6 P2p
39.3.3 通过修改Cost值或者port-priority可以做到Vlan间的负载均衡
不过两者的不同之处是port-priority在根上修改来影响，端口的Cost在非根上修改，这一点我们在上一章讲解STP对比过程时的最后一步提及
SW1(config)#int f0/6
SW1(config-if)#spanning-tree vlan 100 port-priority ?
<0-240> port priority in increments of 16
对端口优先级来说，默认是128，越小越优先
SW1(config-if)#spanning-tree vlan 100 port-priority 0
我们针对vlan100修改了优先级0，那么此时f0/6会由阻塞变为转发
SW2#sh spanning-tree vlan 100
Interface Role Sts Cost Prio.Nbr Type
------------------- ---- --- --------- -------- --------------------------------
Fa0/5 Altn BLK 19 128.5 P2p
Fa0/6 Root FWD 19 128.6 P2p
Fa0/10 Desg FWD 19 128.10 P2p Edge
我们在具体的查看一下SW1：
SW1#sh spanning-tree interface f0/6 detail
Port 8 (FastEthernet0/6) of VLAN0010 is designated forwarding
Port path cost 19, Port priority 128, Port Identifier 128.8.
Designated root has priority 10, address 9caf.ca47.8c00
Designated bridge has priority 10, address 9caf.ca47.8c00
Designated port id is 128.8, designated path cost 0
Timers: message age 0, forward delay 0, hold 0
Number of transitions to forwarding state: 2
Link type is point-to-point by default
BPDU: sent 533, received 153
Port 8 (FastEthernet0/6) of VLAN0100 is designated forwarding
Port path cost 19, Port priority 0, Port Identifier 0.8.
Designated root has priority 100, address 9caf.ca47.8c00
Designated bridge has priority 100, address 9caf.ca47.8c00
Designated port id is 0.8, designated path cost 0
Timers: message age 0, forward delay 0, hold 0
Number of transitions to forwarding state: 1
Link type is point-to-point by default
BPDU: sent 476, received 2
此时如果有Vlan10 的流量，那么是经过5口转发的：
SW2#sh spanning-tree vlan 10
Interface Role Sts Cost Prio.Nbr Type
------------------- ---- --- --------- -------- --------------------------------
Fa0/5 Root FWD 19 128.5 P2p
Fa0/6 Altn BLK 19 128.6 P2p
自然也可以通过修改非根设备的cost来做到这种效果，我们演示一下vlan10的情况：
SW2(config-if)#int f0/6
SW2(config-if)# spanning-tree vlan 10 cost 1---自然我们要把cost改小,那么将优选该接口
SW2#show spanning-tree vlan 10
VLAN0010
Spanning tree enabled protocol ieee
Root ID Priority 10
Address 9caf.ca47.8c00
Cost 1
Port 6 (FastEthernet0/6)
Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec
Bridge ID Priority 32778 (priority 32768 sys-id-ext 10)
Address 0016.9dd7.5180
Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec
Aging Time 300
Interface Role Sts Cost Prio.Nbr Type
------------------- ---- --- --------- -------- --------------------------------
Fa0/5 Altn BLK 19 128.5 P2p
Fa0/6 Root FWD 1 128.6 P2p
实验完成
