了解通过第3层跳的数据包的DHCP监听

简介

本文档介绍当数据包通过任何L3设备以及数据包是否封装在L3报头中时，DHCP监听的交互和行为。

先决条件

要求

Cisco 建议您了解以下主题：

• Cisco IOS® XE在配置和操作命令方面的熟练程度。
• 熟悉Cisco Catalyst 9000系列交换机硬件和架构。
• 深入了解DHCP协议操作和DHCP监听机制。
• 对DHCP选项82和中继代理的作用的概念性理解。
• 基本的路由协议知识。

使用的组件

本文档中的信息基于以下软件和硬件版本：

• 核心/分布层交换机：Cisco Catalyst 9600X系列
• 接入层交换机:Cisco Catalyst 9300 系列
• DHCP 客户端:终端主机设备
• DHCP 服务器:集中式网络服务提供商

本文档中的信息都是基于特定实验室环境中的设备编写的。本文档中使用的所有设备最初均采用原始（默认）配置。如果您的网络处于活动状态，请确保您了解所有命令的潜在影响。

背景信息

本文检查DHCP监听如何在DHCP数据包经过第3层跳时检查并拦截它们。通过实际配置示例和相关数据包捕获的分析，它演示了DHCP监听在Cisco Catalyst 9000系列网络环境中第3层跳上的交互。

网络图

拓扑

非工作场景

首先，概述在第3层跳上丢弃DHCP发现数据包的场景，特别是在中央交换机上。现在，从DHCP客户端逐跳分析数据包传输过程。

远程站点交换机数据包处理

MAC地址为7010.5cdf.31e9的主机传输DHCP发现数据包，该数据包到达远程站点交换机上的接口GigabitEthernet1/0/1。

远程站点配置

!
!
Remote-Site#show run int vlan 113
Building configuration...

Current configuration : 170 bytes
!
interface Vlan113
ip address 10.93.150.254 255.255.255.0
ip helper-address 10.88.2.63
no ip redirects
no ip proxy-arp
end
!
!
! Host Access Port
Remote-Site#show run int gig1/0/1
Building configuration...

Current configuration : 90 bytes
!
interface GigabitEthernet1/0/1
switchport access vlan 113
switchport mode access
end
!
!
! Uplink Port
Remote-Site#show run int gig1/0/3
Building configuration...

Current configuration : 121 bytes
!
interface GigabitEthernet1/0/3
no switchport
ip address 10.92.250.6 255.255.255.252
end
!
!
Remote-Site#show run | sec dhcp
ip dhcp snooping vlan 1-999
no ip dhcp snooping information option
ip dhcp snooping
!
!
Remote-Site#show cdp neighbors 
Capability Codes: R - Router, T - Trans Bridge, B - Source Route Bridge
S - Switch, H - Host, I - IGMP, r - Repeater, P - Phone, 
D - Remote, C - CVTA, M - Two-port Mac Relay

Device ID     Local Intrfce    Holdtme      Capability    Platform       Port ID

Edge-Switch   Gig 1/0/3        153          R S I         C9300-48H      Gig 1/0/3
!
!
Remote-Site#show ip eigrp neighbors
EIGRP-IPv4 Neighbors for AS(100)
H Address Interface Hold Uptime SRTT RTO Q Seq
(sec) (ms) Cnt Num
0 10.92.250.5 Gi1/0/3 14 00:04:40 1 100 0 20
!
!
Remote-Site#show ip route 10.88.2.63
Routing entry for 10.88.2.0/24
 Known via "eigrp 100", distance 90, metric 3072, precedence routine (0), type internal
 Redistributing via eigrp 100
 Last update from 10.92.250.5 on GigabitEthernet1/0/3, 00:05:15 ago
 Routing Descriptor Blocks:
 * 10.92.250.5, from 10.92.250.5, 00:05:15 ago, via GigabitEthernet1/0/3
     Route metric is 3072, traffic share count is 1
     Total delay is 20 microseconds, minimum bandwidth is 1000000 Kbit
     Reliability 255/255, minimum MTU 1500 bytes
     Loading 1/255, Hops 1
!
!

远程站点交换机入口

进入远程站点交换机的广播DHCP发现数据包。

!
!
Remote-Site#show monitor capture tac parameter 
     monitor capture tac control-plane BOTH
     monitor capture tac interface GigabitEthernet1/0/1 BOTH
     monitor capture tac match any
     monitor capture tac file location flash:pcport.pcap
!
!
Remote-Site#show monitor capture file flash:pcport.pcap packet-number 667 detailed
Starting the packet display ........ Press Ctrl + Shift + 6 to exit

Frame 667: 345 bytes on wire (2760 bits), 345 bytes captured (2760 bits) on interface 0
~
~
[Frame is marked: False]
[Frame is ignored: False]
[Protocols in frame: eth:ethertype:ip:udp:bootp]
Ethernet II, Src: 70:10:5c:df:31:e9 (70:10:5c:df:31:e9), Dst: ff:ff:ff:ff:ff:ff (ff:ff:ff:ff:ff:ff)
Destination: ff:ff:ff:ff:ff:ff (ff:ff:ff:ff:ff:ff)
Address: ff:ff:ff:ff:ff:ff (ff:ff:ff:ff:ff:ff)
.... ..1. .... .... .... .... = LG bit: Locally administered address (this is NOT the factory default)
.... ...1 .... .... .... .... = IG bit: Group address (multicast/broadcast)
Source: 70:10:5c:df:31:e9 (70:10:5c:df:31:e9)
Address: 70:10:5c:df:31:e9 (70:10:5c:df:31:e9)
.... ..0. .... .... .... .... = LG bit: Globally unique address (factory default)
.... ...0 .... .... .... .... = IG bit: Individual address (unicast)
Type: IPv4 (0x0800)
Internet Protocol Version 4, Src: 0.0.0.0, Dst: 255.255.255.255
0100 .... = Version: 4
.... 0101 = Header Length: 20 bytes (5)
Differentiated Services Field: 0x00 (DSCP: CS0, ECN: Not-ECT)
0000 00.. = Differentiated Services Codepoint: Default (0)
.... ..00 = Explicit Congestion Notification: Not ECN-Capable Transport (0)
Total Length: 331
Identification: 0x0405 (1029)
Flags: 0x0000
0... .... .... .... = Reserved bit: Not set
.0.. .... .... .... = Don't fragment: Not set
..0. .... .... .... = More fragments: Not set
...0 0000 0000 0000 = Fragment offset: 0
Time to live: 255
Protocol: UDP (17)
Header checksum: 0xb69d [validation disabled]
[Header checksum status: Unverified]
Source: 0.0.0.0
Destination: 255.255.255.255
User Datagram Protocol, Src Port: 68, Dst Port: 67
Source Port: 68
Destination Port: 67
Length: 311
Checksum: 0x33de [unverified]
[Checksum Status: Unverified]
[Stream index: 3]
Bootstrap Protocol (Discover)
Message type: Boot Request (1)
Hardware type: Ethernet (0x01)
Hardware address length: 6
Hops: 0
Transaction ID: 0x000026ee
Seconds elapsed: 0
Bootp flags: 0x8000, Broadcast flag (Broadcast)
1... .... .... .... = Broadcast flag: Broadcast
.000 0000 0000 0000 = Reserved flags: 0x0000
Client IP address: 0.0.0.0
Your (client) IP address: 0.0.0.0
Next server IP address: 0.0.0.0
Relay agent IP address: 0.0.0.0
Client MAC address: 70:10:5c:df:31:e9 (70:10:5c:df:31:e9)
Client hardware address padding: 00000000000000000000
Server host name not given
Boot file name not given
Magic cookie: DHCP
Option: (53) DHCP Message Type (Discover)
Length: 1
DHCP: Discover (1)
Option: (57) Maximum DHCP Message Size
Length: 2
Maximum DHCP Message Size: 1200
Option: (61) Client identifier
Length: 29
Type: 0
Client Identifier: cisco-7010.5cdf.31e9-Gi2/0/7
Option: (12) Host Name
Length: 2
Host Name: PC
Option: (55) Parameter Request List
Length: 8
Parameter Request List Item: (1) Subnet Mask
Parameter Request List Item: (6) Domain Name Server
Parameter Request List Item: (15) Domain Name
Parameter Request List Item: (44) NetBIOS over TCP/IP Name Server
Parameter Request List Item: (3) Router
Parameter Request List Item: (33) Static Route
Parameter Request List Item: (150) TFTP Server Address
Parameter Request List Item: (43) Vendor-Specific Information
Option: (60) Vendor class identifier
Length: 8
Vendor class identifier: ciscopnp
Option: (255) End
Option End: 255
!
!

远程站点交换机出口

从远程站点交换机到边缘交换机的中继单播DHCP发现数据包。

!
!
Remote-Site#show monitor capture tac1 parameter 
 monitor capture tac1 control-plane BOTH
 monitor capture tac1 interface GigabitEthernet1/0/3 BOTH
 monitor capture tac1 match any
 monitor capture tac1 file location flash:remote_edge.pcap
!
!
Remote-Site#show monitor capture file flash:pcport.pcap packet-number 669 detailed
Starting the packet display ........ Press Ctrl + Shift + 6 to exit

Frame 669: 345 bytes on wire (2760 bits), 345 bytes captured (2760 bits) on interface 0
~
~
[Frame is marked: False]
[Frame is ignored: False]
[Protocols in frame: eth:ethertype:ip:udp:bootp]
Ethernet II, Src: 5c:5a:c7:1f:11:58 (5c:5a:c7:1f:11:58), Dst: 08:f3:fb:de:46:05 (08:f3:fb:de:46:05)
Destination: 08:f3:fb:de:46:05 (08:f3:fb:de:46:05)
Address: 08:f3:fb:de:46:05 (08:f3:fb:de:46:05)
.... ..0. .... .... .... .... = LG bit: Globally unique address (factory default)
.... ...0 .... .... .... .... = IG bit: Individual address (unicast)
Source: 5c:5a:c7:1f:11:58 (5c:5a:c7:1f:11:58)
Address: 5c:5a:c7:1f:11:58 (5c:5a:c7:1f:11:58)
.... ..0. .... .... .... .... = LG bit: Globally unique address (factory default)
.... ...0 .... .... .... .... = IG bit: Individual address (unicast)
Type: IPv4 (0x0800)
Internet Protocol Version 4, Src: 10.93.150.254, Dst: 10.88.2.63
0100 .... = Version: 4
.... 0101 = Header Length: 20 bytes (5)
Differentiated Services Field: 0x00 (DSCP: CS0, ECN: Not-ECT)
0000 00.. = Differentiated Services Codepoint: Default (0)
.... ..00 = Explicit Congestion Notification: Not ECN-Capable Transport (0)
Total Length: 331
Identification: 0x001c (28)
Flags: 0x0000
0... .... .... .... = Reserved bit: Not set
.0.. .... .... .... = Don't fragment: Not set
..0. .... .... .... = More fragments: Not set
...0 0000 0000 0000 = Fragment offset: 0
Time to live: 255
Protocol: UDP (17)
Header checksum: 0x0c94 [validation disabled]
[Header checksum status: Unverified]
Source: 10.93.150.254
Destination: 10.88.2.63
User Datagram Protocol, Src Port: 67, Dst Port: 67
Source Port: 67
Destination Port: 67
Length: 311
Checksum: 0xe48f [unverified]
[Checksum Status: Unverified]
[Stream index: 4]
Bootstrap Protocol (Discover)
Message type: Boot Request (1)
Hardware type: Ethernet (0x01)
Hardware address length: 6
Hops: 1
Transaction ID: 0x000026ee
Seconds elapsed: 0
Bootp flags: 0x8000, Broadcast flag (Broadcast)
1... .... .... .... = Broadcast flag: Broadcast
.000 0000 0000 0000 = Reserved flags: 0x0000
Client IP address: 0.0.0.0
Your (client) IP address: 0.0.0.0
Next server IP address: 0.0.0.0
Relay agent IP address: 10.93.150.254
Client MAC address: 70:10:5c:df:31:e9 (70:10:5c:df:31:e9)
Client hardware address padding: 00000000000000000000
Server host name not given
Boot file name not given
Magic cookie: DHCP
Option: (53) DHCP Message Type (Discover)
Length: 1
DHCP: Discover (1)
Option: (57) Maximum DHCP Message Size
Length: 2
Maximum DHCP Message Size: 1200
Option: (61) Client identifier
Length: 29
Type: 0
Client Identifier: cisco-7010.5cdf.31e9-Gi2/0/7
Option: (12) Host Name
Length: 2
Host Name: PC
Option: (55) Parameter Request List
Length: 8
Parameter Request List Item: (1) Subnet Mask
Parameter Request List Item: (6) Domain Name Server
Parameter Request List Item: (15) Domain Name
Parameter Request List Item: (44) NetBIOS over TCP/IP Name Server
Parameter Request List Item: (3) Router
Parameter Request List Item: (33) Static Route
Parameter Request List Item: (150) TFTP Server Address
Parameter Request List Item: (43) Vendor-Specific Information
Option: (60) Vendor class identifier
Length: 8
Vendor class identifier: ciscopnp
Option: (255) End
Option End: 255
!
!

边缘交换机数据包处理

对于DHCP流量问题，边缘交换机是一种L2交换机，仅用于将数据包分发到中央/核心交换机和远程站点交换机之间或它们之间的交换机。信任所有DHCP数据包。

边缘交换机配置

!
!
Edge-Switch#show run int gig1/0/3
Building configuration...

Current configuration : 152 bytes
!
interface GigabitEthernet1/0/3
switchport access vlan 565
ip flow monitor IPv4_NETFLOW input
ip dhcp snooping trust
end
!
!
Edge-Switch#show run int gig1/0/1
Building configuration...

Current configuration : 119 bytes
!
interface GigabitEthernet1/0/1
switchport trunk native vlan 999
switchport mode trunk
ip dhcp snooping trust
end
!
!
Edge-Switch#show run | sec dhcp
ip dhcp snooping vlan 1-4094
no ip dhcp snooping information option
ip dhcp snooping
!
!
Edge-Switch#show cdp neighbors 
Capability Codes: R - Router, T - Trans Bridge, B - Source Route Bridge
S - Switch, H - Host, I - IGMP, r - Repeater, P - Phone, 
D - Remote, C - CVTA, M - Two-port Mac Relay

Device ID        Local Intrfce   Holdtme   Capability   Platform     Port ID
Remote-Site      Gig 1/0/3       153       R S I        C9300L-48    Gig 1/0/3
Central-Switch   Gig 1/0/1       170       R S I        C9606R       Fif 2/1/0/1
!
!

边缘交换机入口

从远程站点交换机传出的数据包按原样到达边缘交换机。

边缘交换机出口

由于这是数据包的第2层跃点，因此它会在进入下一跳时退出交换机，而交换机保持不变。边缘交换机将数据包转发到中央交换机，而不在数据包中发现任何修改。

边缘交换机出口数据包是中央交换机入口数据包。

!
!
Edge-Switch#show monitor capture tac parameters
 monitor capture tac control-plane BOTH
 monitor capture tac interface GigabitEthernet1/0/1 BOTH
 monitor capture tac match any
 monitor capture tac file location flash:edge_central.pcap
!
!
Edge-Switch#show monitor capture file flash:edge_central.pcap packet-number 25597 detail
Starting the packet display ........ Press Ctrl + Shift + 6 to exit

Frame 25597: 345 bytes on wire (2760 bits), 345 bytes captured (2760 bits) on interface /tmp/epc_ws/wif_to_ts_pipe, id 0
~
~
[Frame is marked: False]
[Frame is ignored: False]
[Protocols in frame: eth:ethertype:ip:udp:dhcp]
Ethernet II, Src: 5c:5a:c7:1f:11:58 (5c:5a:c7:1f:11:58), Dst: 08:f3:fb:de:46:05 (08:f3:fb:de:46:05)
Destination: 08:f3:fb:de:46:05 (08:f3:fb:de:46:05)
Address: 08:f3:fb:de:46:05 (08:f3:fb:de:46:05)
.... ..0. .... .... .... .... = LG bit: Globally unique address (factory default)
.... ...0 .... .... .... .... = IG bit: Individual address (unicast)
Source: 5c:5a:c7:1f:11:58 (5c:5a:c7:1f:11:58)
Address: 5c:5a:c7:1f:11:58 (5c:5a:c7:1f:11:58)
.... ..0. .... .... .... .... = LG bit: Globally unique address (factory default)
.... ...0 .... .... .... .... = IG bit: Individual address (unicast)
Type: IPv4 (0x0800)
Internet Protocol Version 4, Src: 10.93.150.254, Dst: 10.88.2.63
0100 .... = Version: 4
.... 0101 = Header Length: 20 bytes (5)
Differentiated Services Field: 0x00 (DSCP: CS0, ECN: Not-ECT)
0000 00.. = Differentiated Services Codepoint: Default (0)
.... ..00 = Explicit Congestion Notification: Not ECN-Capable Transport (0)
Total Length: 331
Identification: 0x001c (28)
Flags: 0x0000
0... .... .... .... = Reserved bit: Not set
.0.. .... .... .... = Don't fragment: Not set
..0. .... .... .... = More fragments: Not set
Fragment offset: 0
Time to live: 255
Protocol: UDP (17)
Header checksum: 0x0c94 [validation disabled]
[Header checksum status: Unverified]
Source: 10.93.150.254
Destination: 10.88.2.63
User Datagram Protocol, Src Port: 67, Dst Port: 67
Source Port: 67
Destination Port: 67
Length: 311
Checksum: 0xe48f [unverified]
[Checksum Status: Unverified]
[Stream index: 5]
[Timestamps]
[Time since first frame: 0.000000000 seconds]
[Time since previous frame: 0.000000000 seconds]
Dynamic Host Configuration Protocol (Discover)
Message type: Boot Request (1)
Hardware type: Ethernet (0x01)
Hardware address length: 6
Hops: 1
Transaction ID: 0x000026ee
Seconds elapsed: 0
Bootp flags: 0x8000, Broadcast flag (Broadcast)
1... .... .... .... = Broadcast flag: Broadcast
.000 0000 0000 0000 = Reserved flags: 0x0000
Client IP address: 0.0.0.0
Your (client) IP address: 0.0.0.0
Next server IP address: 0.0.0.0
Relay agent IP address: 10.93.150.254
Client MAC address: 70:10:5c:df:31:e9 (70:10:5c:df:31:e9)
Client hardware address padding: 00000000000000000000
Server host name not given
Boot file name not given
Magic cookie: DHCP
Option: (53) DHCP Message Type (Discover)
Length: 1
DHCP: Discover (1)
Option: (57) Maximum DHCP Message Size
Length: 2
Maximum DHCP Message Size: 1200
Option: (61) Client identifier
Length: 29
Type: 0
Client Identifier: cisco-7010.5cdf.31e9-Gi2/0/7
Option: (12) Host Name
Length: 2
Host Name: PC
Option: (55) Parameter Request List
Length: 8
Parameter Request List Item: (1) Subnet Mask
Parameter Request List Item: (6) Domain Name Server
Parameter Request List Item: (15) Domain Name
Parameter Request List Item: (44) NetBIOS over TCP/IP Name Server
Parameter Request List Item: (3) Router
Parameter Request List Item: (33) Static Route
Parameter Request List Item: (150) TFTP Server Address
Parameter Request List Item: (43) Vendor-Specific Information
Option: (60) Vendor class identifier
Length: 8
Vendor class identifier: ciscopnp
Option: (255) End
Option End: 255
!
!

中央交换机数据包处理

中央交换机配置

!
!
Central-Switch#show cdp neighbors 
Capability Codes: R - Router, T - Trans Bridge, B - Source Route Bridge
S - Switch, H - Host, I - IGMP, r - Repeater, P - Phone, 
D - Remote, C - CVTA, M - Two-port Mac Relay

Device ID      Local Intrfce   Holdtme   Capability   Platform    Port ID

Edge-Switch    Fif 2/1/0/1     177       R S I        C9300-48H   Gig 1/0/1
!
!
Central-Switch#show run int fif2/1/0/1
Building configuration...

Current configuration : 115 bytes
!
interface FiftyGigE2/1/0/1
switchport trunk native vlan 999
switchport mode trunk
end
!
!
Central-Switch#show run int fif2/1/0/3
Building configuration...

Current configuration : 87 bytes
!
interface FiftyGigE2/1/0/3
no switchport
ip address 10.88.2.254 255.255.255.0
end
!
!
interface Vlan565
ip address 10.92.250.5 255.255.255.252
!
!
!
Central-Switch#show run | sec dhcp
ip dhcp snooping vlan 1-4094
no ip dhcp snooping information option
ip dhcp snooping
!
!
Central-Switch#show ip eigrp neighbors 
EIGRP-IPv4 Neighbors for AS(100)
H Address Interface Hold Uptime SRTT RTO Q Seq
(sec) (ms) Cnt Num
0 10.92.250.6 Vl565 11 00:10:32 3 100 0 19
~
~
!
!
router eigrp 100
  network 10.0.0.0
  network 10.88.0.0 0.0.255.255
  passive-interface default
  no passive-interface Vlan565
  eigrp router-id 10.255.255.254
!
!

中央交换机入口

如前所述，边缘交换机出口数据包是中央交换机入口数据包。

当数据包到达接口fif 2/1/0/1时，将观察此日志。

%DHCP_SNOOPING-5-DHCP_SNOOPING_NONZERO_GIADDR: DHCP_SNOOPING drop message with non-zero giaddr or option82 value on untrusted port, message type: DHCPDISCOVER, MAC sa: 5c5a.c71f.1158

接口Fif 2/1/0/1被配置为DHCP监听不可信端口。

第3层中继的DHCP发现数据包通过此端口进入。虽然数据包在第3层中继和封装，但DHCP监听仍会在第2层端口上检查数据包。由于此端口不受信任，并且数据包包含网关IP（中继代理）地址，而不使用选项82，因此DHCP监听将丢弃此不受信任端口上的数据包。此行为是预期行为，且是设计行为。

中央交换机出口

在这种情况下，中央交换机将丢弃DHCP发现数据包，并且不会出现任何MAC地址为70:10:5c:df:31:e9的主机发起的DHCP发现数据包从接口Fif 2/1/0/3进入。

如何解决问题？

1.在DHCP客户端的第一跳上启用选项82。 
在当前示例中，请在远程交换机上配置ip dhcp snooping information option命令。

2.必须在从DHCP客户端到DHCP服务器的DHCP路径中启用DHCP监听信任。

在当前示例中，在中央交换机的接口fif2/1/0/1上配置ip dhcp snooping trust命令。

Central-Switch#show run int fif2/1/0/1
Building configuration...

Current configuration : 115 bytes
!
interface FiftyGigE2/1/0/1
switchport trunk native vlan 999
switchport mode trunk
ip dhcp snooping trust
end

在这些更改后，Discover数据包将开始从中央交换机发送到DHCP服务器。

Frame 9: 345 bytes on wire (2760 bits), 345 bytes captured (2760 bits) on interface 0
~
~
[Frame is marked: False]
[Frame is ignored: False]
[Protocols in frame: eth:ethertype:ip:udp:dhcp]
Ethernet II, Src: 08:f3:fb:de:46:05 (08:f3:fb:de:46:05), Dst: 00:aa:6e:f3:6c:e4 (00:aa:6e:f3:6c:e4)
Destination: 00:aa:6e:f3:6c:e4 (00:aa:6e:f3:6c:e4)
Address: 00:aa:6e:f3:6c:e4 (00:aa:6e:f3:6c:e4)
.... ..0. .... .... .... .... = LG bit: Globally unique address (factory default)
.... ...0 .... .... .... .... = IG bit: Individual address (unicast)
Source: 08:f3:fb:de:46:05 (08:f3:fb:de:46:05)
Address: 08:f3:fb:de:46:05 (08:f3:fb:de:46:05)
.... ..0. .... .... .... .... = LG bit: Globally unique address (factory default)
.... ...0 .... .... .... .... = IG bit: Individual address (unicast)
Type: IPv4 (0x0800)
Internet Protocol Version 4, Src: 10.93.150.254, Dst: 10.88.2.63
0100 .... = Version: 4
.... 0101 = Header Length: 20 bytes (5)
Differentiated Services Field: 0x00 (DSCP: CS0, ECN: Not-ECT)
0000 00.. = Differentiated Services Codepoint: Default (0)
.... ..00 = Explicit Congestion Notification: Not ECN-Capable Transport (0)
Total Length: 331
Identification: 0x02ff (767)
Flags: 0x0000
0... .... .... .... = Reserved bit: Not set
.0.. .... .... .... = Don't fragment: Not set
..0. .... .... .... = More fragments: Not set
...0 0000 0000 0000 = Fragment offset: 0
Time to live: 254
Protocol: UDP (17)
Header checksum: 0x0ab1 [validation disabled]
[Header checksum status: Unverified]
Source: 10.93.150.254
Destination: 10.88.2.63
User Datagram Protocol, Src Port: 67, Dst Port: 67
Source Port: 67
Destination Port: 67
Length: 311
Checksum: 0x061b [unverified]
[Checksum Status: Unverified]
[Stream index: 0]
[Timestamps]
[Time since first frame: 0.000000000 seconds]
[Time since previous frame: 0.000000000 seconds]
Dynamic Host Configuration Protocol (Discover)
Message type: Boot Request (1)
Hardware type: Ethernet (0x01)
Hardware address length: 6
Hops: 1
Transaction ID: 0x00000563
Seconds elapsed: 0
Bootp flags: 0x8000, Broadcast flag (Broadcast)
1... .... .... .... = Broadcast flag: Broadcast
.000 0000 0000 0000 = Reserved flags: 0x0000
Client IP address: 0.0.0.0
Your (client) IP address: 0.0.0.0
Next server IP address: 0.0.0.0
Relay agent IP address: 10.93.150.254
Client MAC address: 70:10:5c:df:31:e9 (70:10:5c:df:31:e9)
Client hardware address padding: 00000000000000000000
Server host name not given
Boot file name not given
Magic cookie: DHCP
Option: (53) DHCP Message Type (Discover)
Length: 1
DHCP: Discover (1)
Option: (57) Maximum DHCP Message Size
Length: 2
Maximum DHCP Message Size: 1200
Option: (61) Client identifier
Length: 29
Type: 0
Client Identifier: cisco-7010.5cdf.31e9-Gi2/0/7
Option: (12) Host Name
Length: 2
Host Name: PC
Option: (55) Parameter Request List
Length: 8
Parameter Request List Item: (1) Subnet Mask
Parameter Request List Item: (6) Domain Name Server
Parameter Request List Item: (15) Domain Name
Parameter Request List Item: (44) NetBIOS over TCP/IP Name Server
Parameter Request List Item: (3) Router
Parameter Request List Item: (33) Static Route
Parameter Request List Item: (150) TFTP Server Address
Parameter Request List Item: (43) Vendor-Specific Information
Option: (60) Vendor class identifier
Length: 8
Vendor class identifier: ciscopnp
Option: (255) End
Option End: 255

Discover将到达DHCP服务器，DHCP服务器将回复DHCP Offer。

DHCP提供数据包：

Frame 128: 353 bytes on wire (2824 bits), 353 bytes captured (2824 bits) on interface 0
~
~
[Frame is marked: False]
[Frame is ignored: False]
[Protocols in frame: eth:ethertype:ip:udp:bootp]
Ethernet II, Src: 00:aa:6e:f3:6c:e4 (00:aa:6e:f3:6c:e4), Dst: 08:f3:fb:de:46:05 (08:f3:fb:de:46:05)
Destination: 08:f3:fb:de:46:05 (08:f3:fb:de:46:05)
Address: 08:f3:fb:de:46:05 (5c:5a:c7:1f:11:58)
.... ..0. .... .... .... .... = LG bit: Globally unique address (factory default)
.... ...0 .... .... .... .... = IG bit: Individual address (unicast)
Source: 00:aa:6e:f3:6c:e4 (00:aa:6e:f3:6c:e4)
Address: 00:aa:6e:f3:6c:e4 (00:aa:6e:f3:6c:e4)
.... ..0. .... .... .... .... = LG bit: Globally unique address (factory default)
.... ...0 .... .... .... .... = IG bit: Individual address (unicast)
Type: IPv4 (0x0800)
Internet Protocol Version 4, Src: 10.88.2.63, Dst: 10.93.150.254
0100 .... = Version: 4
.... 0101 = Header Length: 20 bytes (5)
Differentiated Services Field: 0x00 (DSCP: CS0, ECN: Not-ECT)
0000 00.. = Differentiated Services Codepoint: Default (0)
.... ..00 = Explicit Congestion Notification: Not ECN-Capable Transport (0)
Total Length: 339
Identification: 0x0006 (6)
Flags: 0x0000
0... .... .... .... = Reserved bit: Not set
.0.. .... .... .... = Don't fragment: Not set
..0. .... .... .... = More fragments: Not set
...0 0000 0000 0000 = Fragment offset: 0
Time to live: 252
Protocol: UDP (17)
Header checksum: 0x0fa2 [validation disabled]
[Header checksum status: Unverified]
Source: 10.88.2.63
Destination: 10.93.150.254
User Datagram Protocol, Src Port: 67, Dst Port: 67
Source Port: 67
Destination Port: 67
Length: 319
Checksum: 0x9c4c [unverified]
[Checksum Status: Unverified]
[Stream index: 2]
Bootstrap Protocol (Offer)
Message type: Boot Reply (2)
Hardware type: Ethernet (0x01)
Hardware address length: 6
Hops: 0
Transaction ID: 0x00000563
Seconds elapsed: 0
Bootp flags: 0x8000, Broadcast flag (Broadcast)
1... .... .... .... = Broadcast flag: Broadcast
.000 0000 0000 0000 = Reserved flags: 0x0000
Client IP address: 0.0.0.0
Your (client) IP address: 10.93.150.11
Next server IP address: 0.0.0.0
Relay agent IP address: 10.93.150.254
Client MAC address: 70:10:5c:df:31:e9 (70:10:5c:df:31:e9)
Client hardware address padding: 00000000000000000000
Server host name not given
Boot file name not given
Magic cookie: DHCP
Option: (53) DHCP Message Type (Offer)
Length: 1
DHCP: Offer (2)
Option: (61) Client identifier
Length: 29
Type: 0
Client Identifier: cisco-7010.5cdf.31e9-Gi2/0/7
Option: (54) DHCP Server Identifier
Length: 4
DHCP Server Identifier: 10.88.2.63
Option: (51) IP Address Lease Time
Length: 4
IP Address Lease Time: (73119s) 20 hours, 18 minutes, 39 seconds
Option: (58) Renewal Time Value
Length: 4
Renewal Time Value: (36559s) 10 hours, 9 minutes, 19 seconds
Option: (59) Rebinding Time Value
Length: 4
Rebinding Time Value: (63973s) 17 hours, 46 minutes, 13 seconds
Option: (1) Subnet Mask
Length: 4
Subnet Mask: 255.255.255.0
Option: (6) Domain Name Server
Length: 4
Domain Name Server: 8.8.8.8
Option: (255) End
Option End: 255

DHCP服务器回复的提供数据包将命中CORE/Central交换机的接口fif2/1/0/3。

Central-Switch#show run int fif2/1/0/3
Building configuration...

Current configuration : 87 bytes
!
interface FiftyGigE2/1/0/3
no switchport
ip address 10.88.2.254 255.255.255.0
end

此时没有DHCP监听端的信任，并且OFFER正在进入，那么Offer为什么没有在不信任的端口上丢弃？

如您所知，在启用DHCP监听的网络中，端口被分类为可信或不可信。

可信端口连接到合法DHCP服务器或其他可信交换机。所有DHCP消息都允许在其上。
不可信端口通常连接到最终用户设备（客户端）。 它们受到限制以防止“DHCP欺骗”或“非法DHCP服务器”攻击。
DHCP Offer和DHCP ACK数据包被视为“服务器端”消息。根据安全规则，这些消息只能来自合法服务器。
当DHCP中继代理或任何L3路由器/交换机在配置为不可信的L2端口上收到DHCP提供时，DHCP监听机制会将此行为标识为安全违规。它假设非法DHCP服务器尝试向网络中的客户端提供未经授权的IP地址。
即使数据包是单播（具体针对中继代理），物理或逻辑端口的信任状态优先。无论数据包是广播还是单播，都将在端口级别执行安全检查。
由于端口不受信任，系统将阻止DHCP提供，以保护网络免受来自未授权源的潜在恶意或错误配置数据的侵害。

第3层端口（路由接口）在第3层运行。由于它是路由接口，因此它不会遵守应用于交换机端口的第2层DHCP监听信任/不信任规则。
客户端发送广播请求。中继代理（L3设备）会拦截此请求并将单播请求发送到DHCP服务器。
DHCP服务器将单播DHCP提供发送回中继代理的IP地址（特别是转发器或网关IP地址）。
当数据包到达中继代理的L3端口或到达数据包路径中的任何L3跳时，设备会接受数据包作为合法路由流量。
与早期场景（L2不受信任端口）不同，交换机正在查找来自面向客户端的端口的“服务器端”消息，L3端口充当服务器响应的预期目标或合法L3跃点。
如果L3端口丢弃了单播DHCP提供，DHCP中继机制将永远无法工作，因为服务器将永远无法与中继代理通信。

最后，Offer数据包将经过直到到达客户端。

主机/客户端将使用DHCP请求进行回复。

Frame 20: 363 bytes on wire (2904 bits), 363 bytes captured (2904 bits) on interface 0
~
~
[Frame is marked: False]
[Frame is ignored: False]
[Protocols in frame: eth:ethertype:ip:udp:bootp]
Ethernet II, Src: 70:10:5c:df:31:e9 (70:10:5c:df:31:e9), Dst: ff:ff:ff:ff:ff:ff (ff:ff:ff:ff:ff:ff)
Destination: ff:ff:ff:ff:ff:ff (ff:ff:ff:ff:ff:ff)
Address: ff:ff:ff:ff:ff:ff (ff:ff:ff:ff:ff:ff)
.... ..1. .... .... .... .... = LG bit: Locally administered address (this is NOT the factory default)
.... ...1 .... .... .... .... = IG bit: Group address (multicast/broadcast)
Source: 70:10:5c:df:31:e9 (70:10:5c:df:31:e9)
Address: 70:10:5c:df:31:e9 (70:10:5c:df:31:e9)
.... ..0. .... .... .... .... = LG bit: Globally unique address (factory default)
.... ...0 .... .... .... .... = IG bit: Individual address (unicast)
Type: IPv4 (0x0800)
Internet Protocol Version 4, Src: 0.0.0.0, Dst: 255.255.255.255
0100 .... = Version: 4
.... 0101 = Header Length: 20 bytes (5)
Differentiated Services Field: 0x00 (DSCP: CS0, ECN: Not-ECT)
0000 00.. = Differentiated Services Codepoint: Default (0)
.... ..00 = Explicit Congestion Notification: Not ECN-Capable Transport (0)
Total Length: 349
Identification: 0x3be5 (15333)
Flags: 0x0000
0... .... .... .... = Reserved bit: Not set
.0.. .... .... .... = Don't fragment: Not set
..0. .... .... .... = More fragments: Not set
...0 0000 0000 0000 = Fragment offset: 0
Time to live: 255
Protocol: UDP (17)
Header checksum: 0x7eab [validation disabled]
[Header checksum status: Unverified]
Source: 0.0.0.0
Destination: 255.255.255.255
User Datagram Protocol, Src Port: 68, Dst Port: 67
Source Port: 68
Destination Port: 67
Length: 329
Checksum: 0xed98 [unverified]
[Checksum Status: Unverified]
[Stream index: 1]
Bootstrap Protocol (Request)
Message type: Boot Request (1)
Hardware type: Ethernet (0x01)
Hardware address length: 6
Hops: 0
Transaction ID: 0x00000563
Seconds elapsed: 0
Bootp flags: 0x8000, Broadcast flag (Broadcast)
1... .... .... .... = Broadcast flag: Broadcast
.000 0000 0000 0000 = Reserved flags: 0x0000
Client IP address: 0.0.0.0
Your (client) IP address: 0.0.0.0
Next server IP address: 0.0.0.0
Relay agent IP address: 0.0.0.0
Client MAC address: 70:10:5c:df:31:e9 (70:10:5c:df:31:e9)
Client hardware address padding: 00000000000000000000
Server host name not given
Boot file name not given
Magic cookie: DHCP
Option: (53) DHCP Message Type (Request)
Length: 1
DHCP: Request (3)
Option: (57) Maximum DHCP Message Size
Length: 2
Maximum DHCP Message Size: 1200
Option: (61) Client identifier
Length: 29
Type: 0
Client Identifier: cisco-7010.5cdf.31e9-Gi2/0/7
Option: (54) DHCP Server Identifier
Length: 4
DHCP Server Identifier: 10.88.2.63
Option: (50) Requested IP Address
Length: 4
Requested IP Address: 10.93.150.11
Option: (51) IP Address Lease Time
Length: 4
IP Address Lease Time: (73119s) 20 hours, 18 minutes, 39 seconds
Option: (12) Host Name
Length: 2
Host Name: PC
Option: (55) Parameter Request List
Length: 8
Parameter Request List Item: (1) Subnet Mask
Parameter Request List Item: (6) Domain Name Server
Parameter Request List Item: (15) Domain Name
Parameter Request List Item: (44) NetBIOS over TCP/IP Name Server
Parameter Request List Item: (3) Router
Parameter Request List Item: (33) Static Route
Parameter Request List Item: (150) TFTP Server Address
Parameter Request List Item: (43) Vendor-Specific Information
Option: (60) Vendor class identifier
Length: 8
Vendor class identifier: ciscopnp
Option: (255) End
Option End: 255

DHCP服务器将最终确认IP地址分配。

Frame 25: 353 bytes on wire (2824 bits), 353 bytes captured (2824 bits) on interface 0
~
~
[Frame is marked: False]
[Frame is ignored: False]
[Protocols in frame: eth:ethertype:ip:udp:bootp]
Ethernet II, Src: 08:f3:fb:de:46:05 (08:f3:fb:de:46:05), Dst: 5c:5a:c7:1f:11:58 (5c:5a:c7:1f:11:58)
Destination: 5c:5a:c7:1f:11:58 (5c:5a:c7:1f:11:58)
Address: 5c:5a:c7:1f:11:58 (5c:5a:c7:1f:11:58)
.... ..0. .... .... .... .... = LG bit: Globally unique address (factory default)
.... ...0 .... .... .... .... = IG bit: Individual address (unicast)
Source: 08:f3:fb:de:46:05 (08:f3:fb:de:46:05)
Address: 08:f3:fb:de:46:05 (08:f3:fb:de:46:05)
.... ..0. .... .... .... .... = LG bit: Globally unique address (factory default)
.... ...0 .... .... .... .... = IG bit: Individual address (unicast)
Type: IPv4 (0x0800)
Internet Protocol Version 4, Src: 10.88.2.63, Dst: 10.93.150.254
0100 .... = Version: 4
.... 0101 = Header Length: 20 bytes (5)
Differentiated Services Field: 0x00 (DSCP: CS0, ECN: Not-ECT)
0000 00.. = Differentiated Services Codepoint: Default (0)
.... ..00 = Explicit Congestion Notification: Not ECN-Capable Transport (0)
Total Length: 339
Identification: 0x0007 (7)
Flags: 0x0000
0... .... .... .... = Reserved bit: Not set
.0.. .... .... .... = Don't fragment: Not set
..0. .... .... .... = More fragments: Not set
...0 0000 0000 0000 = Fragment offset: 0
Time to live: 252
Protocol: UDP (17)
Header checksum: 0x0fa1 [validation disabled]
[Header checksum status: Unverified]
Source: 10.88.2.63
Destination: 10.93.150.254
User Datagram Protocol, Src Port: 67, Dst Port: 67
Source Port: 67
Destination Port: 67
Length: 319
Checksum: 0x1e0f [unverified]
[Checksum Status: Unverified]
[Stream index: 2]
Bootstrap Protocol (ACK)
Message type: Boot Reply (2)
Hardware type: Ethernet (0x01)
Hardware address length: 6
Hops: 0
Transaction ID: 0x00000563
Seconds elapsed: 0
Bootp flags: 0x8000, Broadcast flag (Broadcast)
1... .... .... .... = Broadcast flag: Broadcast
.000 0000 0000 0000 = Reserved flags: 0x0000
Client IP address: 0.0.0.0
Your (client) IP address: 10.93.150.11
Next server IP address: 0.0.0.0
Relay agent IP address: 10.93.150.254
Client MAC address: 70:10:5c:df:31:e9 (70:10:5c:df:31:e9)
Client hardware address padding: 00000000000000000000
Server host name not given
Boot file name not given
Magic cookie: DHCP
Option: (53) DHCP Message Type (ACK)
Length: 1
DHCP: ACK (5)
Option: (61) Client identifier
Length: 29
Type: 0
Client Identifier: cisco-7010.5cdf.31e9-Gi2/0/7
Option: (54) DHCP Server Identifier
Length: 4
DHCP Server Identifier: 10.88.2.63
Option: (51) IP Address Lease Time
Length: 4
IP Address Lease Time: (86400s) 1 day
Option: (58) Renewal Time Value
Length: 4
Renewal Time Value: (43200s) 12 hours
Option: (59) Rebinding Time Value
Length: 4
Rebinding Time Value: (75600s) 21 hours
Option: (1) Subnet Mask
Length: 4
Subnet Mask: 255.255.255.0
Option: (6) Domain Name Server
Length: 4
Domain Name Server: 8.8.8.8
Option: (255) End
Option End: 255

DORA完成，主机将成功接收IP地址。

摘要

请参阅下表以了解数据包转发场景。

端口类型	信任状态	中继数据包类型	结果	原因
L2端口	不受信任	单播DHCP提供/确认	已丢弃	DHCP监听可防止服务器消息出现在不可信的第2层端口上。
L3端口	不适用（路由）	单播DHCP提供/确认	已接受	路由端口是中继代理通信的预定节点。
L2端口	可信	单播DHCP发现与选项82	已接受	可信端口允许所有DHCP信号。
L2端口	不受信任	单播DHCP发现与选项82	已接受	具有中继代理IP地址和选项82的数据包是合法的。
L2端口	可信	不带选项的单播DHCP发现82	已接受	可信端口允许所有DHCP信号。
L2端口	不受信任	不带选项的单播DHCP发现82	已丢弃	具有中继代理IP地址且没有选项82的数据包是欺诈数据包。

修订历史记录

版本	发布日期	备注
1.0	22-Jul-2026	初始版本