H3C--MAC地址表故障处理手册

目 录

1 MAC地址表故障处理·························································································································· 1-1 

1.1 源MAC地址攻击导致端口流量瞬断故障处理 ····················································································· 1-1 

1.1.1 故障描述 ································································································································· 1-1 1.1.2 故障处理步骤 ·························································································································· 1-1 1.2 故障诊断命令 ····································································································································· 1-3 

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1 MAC地址表故障处理

1.1 源MAC地址攻击导致端口流量瞬断故障处理

1.1.1 故障描述

如图1-1所示，Switch下接DSLAM（Digital Subscriber Line Access Multiplexer，数字用户线接入复用器），Switch上接BAS（Broadband Access Server，宽带接入服务器），由BAS终结用户的拨号PPPoE报文。在Switch上配置灵活QinQ功能，对应的用户侧VLAN为824，网络侧VLAN为1003。BAS上作为网关的MAC地址为0090-1aa0-d47a。

故障现象为：DSLAM下挂用户不定时大规模掉线。出现问题时，发现端口Ethernet3/0/4的流量急剧下降。大约5分钟左右，端口Ethernet3/0/4的流量开始慢慢回升，证明此时大规模掉线已经结束，DSLAM下挂用户又在逐步恢复连接，网络在逐步恢复正常状态。 图1-1 源MAC地址攻击导致端口流量瞬断故障处理组网图

1.1.2 故障处理步骤

(1) 检查接口信息

执行display interface命令显示端口Ethernet3/0/4的相关信息：

display interface ethernet 3/0/4 Ethernet3/0/4 current state: UP

IP Packet Frame Type: PKTFMT_ETHNT_2, Hardware Address: 000f-e200-8048 Description: Ethernet3/0/4 Interface Loopback is not set

Media type is twisted pair, port hardware type is 100_BASE_TX 100Mbps-speed mode, full-duplex mode

Link speed type is autonegotiation, link duplex type is autonegotiation Flow-control is not enabled The Maximum Frame Length is 9022

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Broadcast MAX-ratio: 100% Unicast MAX-ratio: 100% Multicast MAX-ratio: 100% Allow jumbo frame to pass PVID: 100 Mdi type: auto

Port link-type: trunk VLAN passing : 824 VLAN permitted: 824

Trunk port encapsulation: IEEE 802.1q Port priority: 0

Last clearing of counters: Never

Peak value of input: 4 bytes/sec, at 2012-04-26 12:07:48 Peak value of output: 81 bytes/sec, at 2012-04-26 12:09:24 Last 300 seconds input: 1 packets/sec 147 bytes/sec Last 300 seconds output: 1 packets/sec 179 bytes/sec Input (total): 271 packets, 12250 bytes

0 unicasts, 150 broadcasts, 121 multicasts, 0 pauses Input (normal): 271 packets, 12250 bytes

0 unicasts, 150 broadcasts, 121 multicasts, 0 pauses Input: 0 input errors, 0 runts, 0 giants, 0 throttles 0 CRC, 0 frame, - overruns, 0 aborts - ignored, - parity errors

Output (total): 1522 packets, 183608303 bytes

0 unicasts, 13 broadcasts, 860 multicasts, 0 pauses Output (normal): 1522 packets, - bytes

0 unicasts, 13 broadcasts, 860 multicasts, 0 pauses Output: 0 output errors, - underruns, 1 buffer failures

0 aborts, 0 deferred, 0 collisions, 0 late collisions 0 lost carrier, - no carrier

可以看到， 端口在故障时只有组播和广播报文的记数。 (2) 原因分析

从现象上看，Switch的端口Ethernet3/0/4的入方向报文计数在故障时只有组播和广播报文，基本上可以确定：由于某种原因，导致从Switch的端口Ethernet3/0/4进来的单播报文被全部丢弃了。DSLAM下挂用户正常通信的流量以单播报文为主，这种丢弃行为导致用户下线，然后重新认证，所以端口流量大大减小。

因此从报文被丢弃的原因着手分析问题。这种报文丢弃特征和“源端口返回报文”丢弃很吻合，也就是说很可能端口收到了目的MAC地址和本端口学习到的目的MAC地址一致的报文。正常工作状态下，BAS的MAC地址（也就是从Switch的端口Ethernet3/0/4上来的PPPoE单播流量的目的MAC地址）只会学习到Switch的端口GigabitEthernet2/0/1上的VLAN 1003，不会学习到用户侧端口，除非网络中有环路或者其他原因。 下面是正常情况下Switch的MAC地址表信息：

display mac-address

MAC ADDR VLAN ID STATE PORT INDEX AGING TIME(s) 0090-1aa0-d47a 1003 Learned GE2/0/1 AGING 0090-1aa0-d47a 1101 Learned GE2/0/1 AGING 0090-1aa0-d47a 1201 Learned GE2/0/1 AGING 0090-1aa0-d47a 4093 Learned GE2/0/1 AGING

再看看出故障时学习到Switch的端口Ethernet3/0/4的MAC地址：

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display mac-address interface ethernet 3/0/4

MAC ADDR VLAN ID STATE PORT INDEX AGING TIME(s) 0090-1aa0-d47a 824 Learned Eth3/0/4 AGING

这个MAC地址是BAS的MAC地址，却学习到了用户侧Switch的端口Ethernet3/0/4的VLAN 824。这时，从Switch的端口Ethernet3/0/4上来的PPPoE正常业务报文的目的MAC地址正是这个MAC地址，结果被作为源端口返回报文全部丢弃。

重新检视故障前后的定位信息，发现了这样的规律：大规模掉线时（端口大量丢包），BAS的MAC地址学习到了Switch的端口Ethernet3/0/4，而在业务流量逐渐恢复到正常的期间，这个MAC地址被老化掉了。

因为DSLAM和Switch是直连，中间没有其他交换机导致环路，网络拓扑一直处于稳定状态，因此可以得出结论：DSLAM下挂用户通过仿冒网关MAC地址的方法对Switch进行攻击，导致该DSLAM下面用户大规模掉线。从用户的攻击心理看，攻击大多数是为了获取更大的带宽，把其他用户踢下线，让网络系统重新来一次初始化，攻击者会明显感觉网速比以前要快了。 (3) 解决故障

在Switch的上行口GigabitEthernet2/0/1配置一个用户侧VLAN 824的静态网关MAC地址，这样网关MAC地址就永远不会学习到Switch的端口Ethernet3/0/4了，攻击者仿冒MAC地址的攻击也就不起作用了，方法如下：

# 把端口GigabitEthernet2/0/1加入到VLAN 824。

system-view

[Switch] interface gigabitethernet 2/0/1

[Switch-GigabitEthernet2/0/1] port link-type trunk [Switch-GigabitEthernet2/0/1] port trunk permit vlan 824 [Switch-GigabitEthernet2/0/1] quit

# 在端口GigabitEthernet2/0/1的VLAN 824设置一个网关静态MAC地址。

[Switch] mac-address static 0090-1aa0-d47a interface gigabitethernet 2/0/1 vlan 824

配置了静态MAC地址后，DSLAM下挂的用户运行很稳定，再未出现掉线的情况。

• 在使能灵活QinQ的情况下，为了防止用户进行源MAC地址攻击，建议在设备的上行口配置网

关的静态MAC地址。

• 在有多个上行口的情况下，在任意一个上行口配置该静态MAC地址即可。

1.2 故障诊断命令

命令

display interface display mac-address

说明

显示以太网接口的相关信息 显示MAC地址表信息

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