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1 纯BPF过滤表达式分析思科PVST/PVST+/Rapid-PVST+常见网络故障

1.1 一、思科PVST/PVST+/Rapid-PVST+帧结构参考

1.1.1 关键特征:

  • 目的MAC地址01-00-0C-CC-CC-CD(Cisco PVST组播地址)
  • 协议标识符:Cisco私有协议,通常使用SNAP封装
  • VLAN标签:PVST+为每个VLAN发送单独的BPDU,带802.1Q标签

1.1.2 PVST+帧结构(BPF偏移计算):

0-5:    目的MAC (01:00:0C:CC:CC:CD)
6-11:   源MAC
12-13:  Ethertype (0x8100 - 802.1Q标签)
14-15:  VLAN标签 (TCI: 优先级+CFI+VLAN ID)
16-17:  内部协议类型 (0x010B - Cisco PVST)
18-...: PVST+ BPDU数据

1.1.3 Rapid-PVST+ BPDU结构:

PVST+头部(14字节以太网 + 4字节802.1Q + 2字节协议类型)
从字节20开始:标准RSTP BPDU结构

1.2 二、基础PVST/PVST+/Rapid-PVST+捕获表达式

# 1. 捕获所有思科生成树协议流量
ether dst 01:00:0C:CC:CC:CD

# 2. 捕获所有带Cisco PVST标记的流量
ether[12:2] == 0x8100 and ether[16:2] == 0x010B

# 3. 组合捕获:所有PVST/PVST+/Rapid-PVST+
ether dst 01:00:0C:CC:CC:CD and ether[12:2] == 0x8100 and ether[16:2] == 0x010B

# 4. 捕获特定VLAN的PVST流量
ether dst 01:00:0C:CC:CC:CD and ether[12:2] == 0x8100 and (ether[14:2] & 0x0fff) == VLAN_ID

# 5. 捕获非802.1Q封装的PVST(罕见,但可能存在)
ether dst 01:00:0C:CC:CC:CD and ether[12:2] == 0x010B

1.3 三、PVST(原始PVST)分析

1.3.1 PVST BPDU结构(偏移从以太网头部开始):

14:     协议版本 (STP=0x00, PVST可能有变种)
15:     BPDU类型 (配置=0x00, TCN=0x80)
16:     标志位
17-24:  根桥ID
25-28:  根路径开销
29-36:  桥ID
37-38:  端口ID
39-...: 标准STP计时器
# 1. 捕获PVST配置BPDU
ether dst 01:00:0C:CC:CC:CD and ether[12:2] == 0x8100 and ether[16:2] == 0x010B and ether[20] == 0x00

# 2. 捕获PVST TCN BPDU
ether dst 01:00:0C:CC:CC:CD and ether[12:2] == 0x8100 and ether[16:2] == 0x010B and ether[20] == 0x80

# 3. 检查PVST标志位(字节20)
ether dst 01:00:0C:CC:CC:CD and ether[12:2] == 0x8100 and ether[16:2] == 0x010B and (ether[21] & 0x01) == 0x01  # TC标志

# 4. 检查PVST根桥ID(字节17-24)
ether dst 01:00:0C:CC:CC:CD and ether[12:2] == 0x8100 and ether[16:2] == 0x010B and ether[17:8] == 预期根桥ID

1.4 四、PVST+(增强PVST)分析

1.4.1 PVST+与PVST的区别:

  • 使用802.1Q标签
  • 支持VLAN 1-4094
  • 每个VLAN独立BPDU
# 1. 捕获特定VLAN的PVST+ BPDU
ether dst 01:00:0C:CC:CC:CD and ether[12:2] == 0x8100 and ether[16:2] == 0x010B and (ether[14:2] & 0x0fff) == 0x0001  # VLAN 1

# 2. 捕获Native VLAN(VLAN 1)的PVST+
ether dst 01:00:0C:CC:CC:CD and ether[12:2] == 0x8100 and ether[16:2] == 0x010B and (ether[14:2] & 0x0fff) == 0x0001

# 3. 捕获用户VLAN的PVST+
ether dst 01:00:0C:CC:CC:CD and ether[12:2] == 0x8100 and ether[16:2] == 0x010B and (ether[14:2] & 0x0fff) > 0x0001

# 4. 检查VLAN标签中的优先级(前3位)
ether dst 01:00:0C:CC:CC:CD and ether[12:2] == 0x8100 and ether[16:2] == 0x010B and (ether[14:2] & 0xe000) != 0x0000

1.5 五、Rapid-PVST+(快速PVST+)分析

1.5.1 Rapid-PVST+关键特征:

  • 使用RSTP协议(802.1w)
  • 版本号:0x02
  • BPDU类型:0x02
# 1. 捕获Rapid-PVST+ BPDU
ether dst 01:00:0C:CC:CC:CD and ether[12:2] == 0x8100 and ether[16:2] == 0x010B and ether[20] == 0x02

# 2. 捕获Rapid-PVST+特定VLAN
ether dst 01:00:0C:CC:CC:CD and ether[12:2] == 0x8100 and ether[16:2] == 0x010B and ether[20] == 0x02 and (ether[14:2] & 0x0fff) == VLAN_ID

# 3. 检查Rapid-PVST+标志位(字节21)
# 端口角色:位2-3
ether dst 01:00:0C:CC:CC:CD and ether[12:2] == 0x8100 and ether[16:2] == 0x010B and ether[20] == 0x02 and (ether[21] & 0x0C) == 0x08  # 根端口
ether dst 01:00:0C:CC:CC:CD and ether[12:2] == 0x8100 and ether[16:2] == 0x010B and ether[20] == 0x02 and (ether[21] & 0x0C) == 0x0C  # 指定端口

# 4. 检查Rapid-PVST+端口状态(位4-5)
ether dst 01:00:0C:CC:CC:CD and ether[12:2] == 0x8100 and ether[16:2] == 0x010B and ether[20] == 0x02 and (ether[21] & 0x30) == 0x00  # 阻塞
ether dst 01:00:0C:CC:CC:CD and ether[12:2] == 0x8100 and ether[16:2] == 0x010B and ether[20] == 0x02 and (ether[21] & 0x30) == 0x30  # 转发

1.6 六、协议版本区分

# 1. 区分PVST(STP版本0x00)
ether dst 01:00:0C:CC:CC:CD and ether[12:2] == 0x8100 and ether[16:2] == 0x010B and ether[20] == 0x00

# 2. 区分Rapid-PVST+(RSTP版本0x02)
ether dst 01:00:0C:CC:CC:CD and ether[12:2] == 0x8100 and ether[16:2] == 0x010B and ether[20] == 0x02

# 3. 捕获TCN BPDU(类型0x80)
ether dst 01:00:0C:CC:CC:CD and ether[12:2] == 0x8100 and ether[16:2] == 0x010B and ether[20] == 0x80

# 4. 捕获配置BPDU(类型0x00)
ether dst 01:00:0C:CC:CC:CD and ether[12:2] == 0x8100 and ether[16:2] == 0x010B and ether[20] == 0x00

1.7 七、VLAN相关故障分析

1.7.1 故障1: VLAN不一致

# 捕获VLAN ID超出有效范围(1-4094)
ether dst 01:00:0C:CC:CC:CD and ether[12:2] == 0x8100 and ether[16:2] == 0x010B and (ether[14:2] & 0x0fff) == 0x0000
ether dst 01:00:0C:CC:CC:CD and ether[12:2] == 0x8100 and ether[16:2] == 0x010B and (ether[14:2] & 0x0fff) > 0x0fff

# 捕获Native VLAN不匹配(两端不同)
# 需要比较两个方向的流量,BPF难以直接实现
ether dst 01:00:0C:CC:CC:CD and ether[12:2] == 0x8100 and ether[16:2] == 0x010B and (ether[14:2] & 0x0fff) == 0x0001

1.7.2 故障2: VLAN修剪问题

# 捕获未配置VLAN的PVST+ BPDU
ether dst 01:00:0C:CC:CC:CD and ether[12:2] == 0x8100 and ether[16:2] == 0x010B and (ether[14:2] & 0x0fff) == 未配置VLAN_ID

# 捕获VLAN列表中的缺失VLAN(需要持续监控)
# 以下捕获特定VLAN范围,检查是否存在
ether dst 01:00:0C:CC:CC:CD and ether[12:2] == 0x8100 and ether[16:2] == 0x010B and (ether[14:2] & 0x0fff) >= 0x0002 and (ether[14:2] & 0x0fff) <= 0x000a

1.7.3 故障3: VLAN跳数问题

# 检查VLAN内的根桥一致性(通过根桥ID)
# 需要比较同一VLAN内不同BPDU的根桥ID
ether dst 01:00:0C:CC:CC:CD and ether[12:2] == 0x8100 and ether[16:2] == 0x010B and (ether[14:2] & 0x0fff) == VLAN_ID | \
比较根桥ID字段

1.8 八、根桥选举故障

1.8.1 PVST+根桥ID结构:

字节17-18: 优先级(2字节)
字节19-24: MAC地址(6字节)
# 1. 捕获特定根桥优先级
ether dst 01:00:0C:CC:CC:CD and ether[12:2] == 0x8100 and ether[16:2] == 0x010B and ether[17:2] == 0x8000  # 默认32768

# 2. 捕获根桥优先级为0(可能配置错误)
ether dst 01:00:0C:CC:CC:CD and ether[12:2] == 0x8100 and ether[16:2] == 0x010B and ether[17:2] == 0x0000

# 3. 捕获根桥MAC地址
ether dst 01:00:0C:CC:CC:CD and ether[12:2] == 0x8100 and ether[16:2] == 0x010B and ether[19:6] == 预期MAC

# 4. 检测多个VLAN有不同根桥(PVST+特性,正常)
# 但可检查特定VLAN的根桥ID是否一致
ether dst 01:00:0C:CC:CC:CD and ether[12:2] == 0x8100 and ether[16:2] == 0x010B and (ether[14:2] & 0x0fff) == VLAN_ID

1.9 九、计时器问题

1.9.1 PVST+计时器字段(偏移从字节39开始):

39-40: 消息年龄
41-42: 最大年龄
43-44: Hello时间
45-46: 转发延迟
# 1. 检查Hello时间(默认2秒)
ether dst 01:00:0C:CC:CC:CD and ether[12:2] == 0x8100 and ether[16:2] == 0x010B and ether[43:2] != 0x0002

# 2. 检查Hello时间为0
ether dst 01:00:0C:CC:CC:CD and ether[12:2] == 0x8100 and ether[16:2] == 0x010B and ether[43:2] == 0x0000

# 3. 检查消息年龄超过最大年龄
ether dst 01:00:0C:CC:CC:CD and ether[12:2] == 0x8100 and ether[16:2] == 0x010B and ether[39:2] > ether[41:2]

# 4. 检查转发延迟(默认15秒)
ether dst 01:00:0C:CC:CC:CD and ether[12:2] == 0x8100 and ether[16:2] == 0x010B and ether[45:2] != 0x000f

# 5. 检查最大年龄(默认20秒)
ether dst 01:00:0C:CC:CC:CD and ether[12:2] == 0x8100 and ether[16:2] == 0x010B and ether[41:2] != 0x0014

1.10 十、端口状态和角色故障

1.10.1 PVST+端口ID字段(字节37-38):

高4位:端口优先级
低12位:端口号
# 1. 检查端口ID有效性
ether dst 01:00:0C:CC:CC:CD and ether[12:2] == 0x8100 and ether[16:2] == 0x010B and (ether[37:2] & 0x0fff) == 0x0000

# 2. 检查端口优先级(默认128)
ether dst 01:00:0C:CC:CC:CD and ether[12:2] == 0x8100 and ether[16:2] == 0x010B and (ether[37:2] & 0xf000) == 0x8000

# 3. Rapid-PVST+端口角色检查(字节21的位2-3)
ether dst 01:00:0C:CC:CC:CD and ether[12:2] == 0x8100 and ether[16:2] == 0x010B and ether[20] == 0x02 and (ether[21] & 0x0C) == 0x00  # 未知角色

# 4. Rapid-PVST+端口状态检查(字节21的位4-5)
ether dst 01:00:0C:CC:CC:CD and ether[12:2] == 0x8100 and ether[16:2] == 0x010B and ether[20] == 0x02 and (ether[21] & 0x30) == 0x20  # 异常:转发但未学习

1.11 十一、拓扑变更相关故障

# 1. 捕获TCN BPDU
ether dst 01:00:0C:CC:CC:CD and ether[12:2] == 0x8100 and ether[16:2] == 0x010B and ether[20] == 0x80

# 2. 捕获TC标志设置(位0)
ether dst 01:00:0C:CC:CC:CD and ether[12:2] == 0x8100 and ether[16:2] == 0x010B and (ether[21] & 0x01) == 0x01

# 3. 捕获TCA标志(拓扑变更确认,位7)
ether dst 01:00:0C:CC:CC:CD and ether[12:2] == 0x8100 and ether[16:2] == 0x010B and (ether[21] & 0x80) == 0x80

# 4. 捕获频繁的拓扑变更(需要时间分析)
ether dst 01:00:0C:CC:CC:CD and ether[12:2] == 0x8100 and ether[16:2] == 0x010B and (ether[21] & 0x01) == 0x01 | \
按时间频率分析

1.12 十二、与其他协议的互操作问题

1.12.1 PVST+与标准STP/RSTP互操作:

# 1. 检测混合环境(PVST+和标准STP同时存在)
ether dst 01:00:0C:CC:CC:CD or ether dst 01:80:c2:00:00:00

# 2. 检测PVST+与802.1D STP互操作问题
ether dst 01:80:c2:00:00:00 and ether[19] == 0x00 and \
ether dst 01:00:0C:CC:CC:CD and ether[12:2] == 0x8100 and ether[16:2] == 0x010B

# 3. 检测PVST+与802.1w RSTP互操作问题
ether dst 01:80:c2:00:00:00 and ether[19] == 0x02 and \
ether dst 01:00:0C:CC:CC:CD and ether[12:2] == 0x8100 and ether[16:2] == 0x010B and ether[20] == 0x02

1.12.2 PVST+与MSTP互操作:

# 检测PVST+与MSTP混合环境
ether dst 01:00:0C:CC:CC:CD and ether[12:2] == 0x8100 and ether[16:2] == 0x010B or \
ether dst 01:80:c2:00:00:00 and ether[19] == 0x03

1.13 十三、思科特有扩展检查

1.13.1 BPDU过滤和防护:

# 1. 检测BPDU Guard应阻止的BPDU(来自主机端口)
ether dst 01:00:0C:CC:CC:CD and ether[12:2] == 0x8100 and ether[16:2] == 0x010B and \
not ether[6:3] = 思科交换机OUI(如00:00:0C, 00:01:42等)

# 2. 检测Root Guard应阻止的更优BPDU
# 需要比较根桥优先级,BPF难以直接实现
ether dst 01:00:0C:CC:CC:CD and ether[12:2] == 0x8100 and ether[16:2] == 0x010B and ether[17:2] < 当前根桥优先级

1.13.2 PortFast和BPDU Filter:

# 检测PortFast端口是否发送BPDU(不应发送)
ether src 主机端口MAC and ether dst 01:00:0C:CC:CC:CD and ether[12:2] == 0x8100 and ether[16:2] == 0x010B

# 检测BPDU Filter是否生效(端口不应接收/发送BPDU)
ether dst 01:00:0C:CC:CC:CD and ether[12:2] == 0x8100 and ether[16:2] == 0x010B and \
ether[6:6] = 启用了BPDU Filter的端口MAC

1.14 十四、组合故障诊断表达式

1.14.1 综合PVST+健康检查:

ether dst 01:00:0C:CC:CC:CD and ether[12:2] == 0x8100 and ether[16:2] == 0x010B and (
    # 协议版本错误
    ether[20] != 0x00 and ether[20] != 0x02 and ether[20] != 0x80 or
    
    # VLAN ID无效
    (ether[14:2] & 0x0fff) == 0x0000 or
    (ether[14:2] & 0x0fff) > 0x0fff or
    
    # 计时器异常
    ether[43:2] == 0x0000 or  # Hello时间为0
    ether[39:2] > ether[41:2] or  # 消息年龄超时
    ether[45:2] == 0x0000 or  # 转发延迟为0
    
    # 根桥ID异常
    ether[17:8] == 0x0000000000000000 or
    ether[17:2] == 0x0000 or  # 优先级为0
    
    # 端口ID异常
    (ether[37:2] & 0x0fff) == 0x0000 or
    
    # Rapid-PVST+特定检查
    (ether[20] == 0x02 and (
        (ether[21] & 0x0C) == 0x00 or  # 未知端口角色
        (ether[21] & 0x30) == 0x20     # 异常状态
    ))
)

1.14.2 严重故障过滤器:

# 可能导致环路的严重故障
ether dst 01:00:0C:CC:CC:CD and ether[12:2] == 0x8100 and ether[16:2] == 0x010B and (
    # BPDU过期但端口仍在转发
    ether[39:2] >= ether[41:2] and ether[20] == 0x02 and (ether[21] & 0x20) == 0x20 or
    
    # 多个VLAN有不同的根桥但配置要求相同(需要具体配置信息)
    # 这里检查根桥ID与桥ID相同但非根桥
    ether[17:8] == ether[29:8] and ether[25:4] != 0x00000000 or
    
    # Native VLAN不匹配(VLAN 1但另一端不是)
    (ether[14:2] & 0x0fff) == 0x0001 and 需要检查对端
)

1.14.3 VLAN相关故障:

# VLAN配置问题
ether dst 01:00:0C:CC:CC:CD and ether[12:2] == 0x8100 and ether[16:2] == 0x010B and (
    # VLAN ID在允许范围外
    (ether[14:2] & 0x0fff) < 0x0001 or
    (ether[14:2] & 0x0fff) > 0x0ffe or
    
    # 特定VLAN缺失(假设VLAN 10应存在)
    not (ether[14:2] & 0x0fff) == 0x000a and 时间段内应有BPDU or
    
    # VLAN优先级异常(非0)
    (ether[14:2] & 0xe000) != 0x0000
)

1.14.4 性能问题过滤器:

# 可能影响网络性能的问题
ether dst 01:00:0C:CC:CC:CD and ether[12:2] == 0x8100 and ether[16:2] == 0x010B and (
    # 过多的VLAN导致BPDU泛滥
    # 通过统计不同VLAN ID的数量来检测,BPF无法直接实现
    
    # 频繁的拓扑变更
    (ether[21] & 0x01) == 0x01 and ether[39:2] < 0x0002 or
    
    # Hello时间过长
    ether[43:2] > 0x0004 or
    
    # Rapid-PVST+端口状态频繁变化
    ether[20] == 0x02 and ether[39:2] < 0x0003  # 新BPDU
)

1.15 十五、特定VLAN根桥一致性检查

# 检查特定VLAN(如VLAN 10)的所有BPDU根桥ID是否一致
ether dst 01:00:0C:CC:CC:CD and ether[12:2] == 0x8100 and ether[16:2] == 0x010B and (ether[14:2] & 0x0fff) == 0x000a | \
比较ether[17:8]字段的一致性

# 检查不同VLAN是否有相同根桥(正常PVST+特性,但可验证)
ether dst 01:00:0C:CC:CC:CD and ether[12:2] == 0x8100 and ether[16:2] == 0x010B | \
按VLAN分组比较根桥ID

1.16 十六、BPF表达式优化

# 1. 预编译PVST+过滤器
# 基本PVST+捕获
ether[0:6] = 01:00:0C:CC:CC:CD and ether[12:2] == 0x8100 and ether[16:2] == 0x010B

# 2. 特定VLAN的Rapid-PVST+
ether[0:6] = 01:00:0C:CC:CC:CD and ether[12:2] == 0x8100 and ether[16:2] == 0x010B and ether[20] == 0x02 and (ether[14:2] & 0x0fff) == VLAN_ID

# 3. 使用掩码同时检查多个条件
ether dst 01:00:0C:CC:CC:CD and ether[12:2] == 0x8100 and ether[16:2] == 0x010B and (ether[21] & 0x81) != 0x00  # TC或TCA标志设置

# 4. 排除非PVST+流量
ether dst 01:00:0C:CC:CC:CD and not (ether[12:2] == 0x8100 and ether[16:2] == 0x010B)

# 5. 组合检查计时器
ether dst 01:00:0C:CC:CC:CD and ether[12:2] == 0x8100 and ether[16:2] == 0x010B and (ether[43:2] == 0x0000 or ether[45:2] == 0x0000)

1.17 十七、常见故障场景与BPF表达式

故障现象 BPF表达式 可能原因
VLAN间环路 检查不同VLAN根桥ID一致性 PVST+配置错误
收敛缓慢 ether[43:2]>0x0004 Hello时间过长
端口无法转发 ether[20]==0x02 and (ether[21]&0x30)==0x00 Rapid-PVST+端口阻塞
BPDU丢失 监控特定VLAN BPDU间隔 VLAN修剪或过滤
Native VLAN不匹配 比较两端VLAN 1的BPDU 配置不一致
根桥震荡 监控根桥ID变化频率 链路不稳定或配置

1.18 十八、注意事项

  1. 思科设备特定:PVST/PVST+/Rapid-PVST+是Cisco私有协议
  2. VLAN标签:PVST+ BPDU通常带802.1Q标签,VLAN 1可能例外
  3. 目的MAC:固定为01:00:0C:CC:CC:CD
  4. 协议类型0x010B(Cisco PVST)
  5. 兼容性:与标准STP/RSTP/MSTP不直接兼容
  6. 性能影响:每个VLAN独立BPDU可能产生大量流量

1.19 总结

纯BPF表达式分析思科PVST/PVST+/Rapid-PVST+故障的关键点:

  1. 识别协议:目的MAC=01:00:0C:CC:CC:CD,协议类型=0x010B,802.1Q标签
  2. 区分版本:PVST(0x00)、Rapid-PVST+(0x02)、TCN(0x80)
  3. VLAN分析:每个VLAN独立BPDU,检查VLAN标签和一致性
  4. 根桥检查:每个VLAN可能有不同根桥,验证配置意图
  5. 计时器验证:Hello、MaxAge、Forward Delay

思科PVST+系列协议的主要优势是每个VLAN独立的生成树,但这也增加了故障排查的复杂性。这些BPF表达式可以帮助快速识别协议问题、VLAN配置错误和收敛问题。对于复杂故障,建议结合Cisco IOS命令如show spanning-tree vlandebug spanning-tree等进行综合分析。

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