H3C S6520X-24ST-SI 交换机做服务器，终端DHCP地址获取慢

初始组网：

变更后组网：

终端设备通过初始组网图向DHCP服务器请求地址，获取速度较慢。后客户变更组网，终端直连DHCP服务器，现象依旧。  

第一步：故障时在连接PC的端口上进行抓包以及debug分析。

抓包截图如下，可以看出终端设备向dhcp服务器发送了许多DHCP Discover报文，但交换机未应答。

从debug信息看，设备收到如下discover报文时，有时会不回复offer报文，从报文方面来看，未见明显异常。

*Sep 30 15:10:52:411 2021 core-1F DHCPS/7/PACKET: From 0.0.0.0 port 68, interface Vlan-interface4

    Message type: REQUEST (1)

    Hardware type: 1, Hardware address length: 6

    Hops: 0, Transaction ID: 2001877267

    Seconds: 8, Broadcast flag: 0

    Client IP address: 0.0.0.0   Your IP address: 0.0.0.0

    Server IP address: 0.0.0.0   Relay agent IP address: 0.0.0.0

    Client hardware address: 0c38-3e19-a2b0

    Server host name: not configured

    Boot file name: not configured

DHCP message type: DHCPDISCOVER (1)

第二步：查看设备的logbuffer，从logbuff中看到设备有收到比较频繁的TC消息，收到TC后会删除mac地址表项，报文上送时会检查有没有mac地址，如果没有就被丢掉了，故让客户把连接终端的端口配置成边缘端口进行测试。

%Sep 29 22:41:04:772 2021 core-1F STP/6/STP_NOTIFIED_TC: Instance 0's port Ten-GigabitEthernet1/0/4 was notified a topology change.

%Sep 29 22:41:46:801 2021 core-1F STP/6/STP_NOTIFIED_TC: Instance 0's port Ten-GigabitEthernet1/0/4 was notified a topology change.

从平台的debugging信息中搜收和发，数量是一致的，说明平台收到的报文基本都回复了，大部分都是回复offer和ack，有时会回复icmp echo。

Search "DHCPS/7/EVENT: R" (1490 hits in 1 file)

         Line 38: *Sep 30 15:10:05:496 2021 core-1F DHCPS/7/EVENT: Receive a DHCPINFORM message from Vlan-interface5.

         Line 62: *Sep 30 15:10:05:497 2021 core-1F DHCPS/7/EVENT: Receive a DHCPINFORM message from Vlan-interface5.

         Line 86: *Sep 30 15:10:08:487 2021 core-1F DHCPS/7/EVENT: Receive a DHCPINFORM message from Vlan-interface5.

         Line 99: *Sep 30 15:10:08:488 2021 core-1F DHCPS/7/EVENT: Receive a DHCPINFORM message from Vlan-interface5.

         … …

Search "DHCPS/7/EVENT: S" (1490 hits in 1 file).

         Line 520: *Sep 30 15:10:43:389 2021 core-1F DHCPS/7/EVENT: Sent DHCPACK to 192.168.2.90.

         Line 544: *Sep 30 15:10:43:390 2021 core-1F DHCPS/7/EVENT: Sent DHCPACK to 192.168.2.90.

         Line 568: *Sep 30 15:10:43:554 2021 core-1F DHCPS/7/EVENT: Sent DHCPACK to 192.168.51.79.

         Line 592: *Sep 30 15:10:43:555 2021 core-1F DHCPS/7/EVENT: Sent DHCPACK to 192.168.51.79.

         Line 620: *Sep 30 15:10:44:414 2021 core-1F DHCPS/7/EVENT: Send an ICMP echo request to 192.168.4.60.

         Line 637: *Sep 30 15:10:44:988 2021 core-1F DHCPS/7/EVENT: Send a DHCPOFFER message on Vlan-interface4.

         Line 665: *Sep 30 15:10:48:412 2021 core-1F DHCPS/7/EVENT: Send an ICMP echo request to 192.168.4.60.

         Line 694: *Sep 30 15:10:48:767 2021 core-1F DHCPS/7/EVENT: Sent DHCPACK to 192.168.3.36.

第三步：配置完成边缘端口后，仍有故障现象。经远程排查，发现核心下连的AC会反弹报文给交换机，导致交换机MAC地址漂移，进而导致终端获取地址慢的问题。

注：知了社区有报文回弹导致MAC地址漂移问题案例，

链接为：https://zhiliao.h3c.com/Theme/details/66426

第四步：无线侧进行分析后发现AC 开启了dhcp snooping的功能，R5441P02版本以前不支持AC与交换机上同时开启dhcp snooping功能，原因为交换机1口收到dhcp discovery转发到2口给ac，此时ac会把这个报文复制一份从有线口原路发回去，从而导致交换机的mac地址学习出现错误引起mac漂移。

1. 将AC侧的DHCP Snooping功能关闭

2. 升级AC版本