Srini Vidhun Nice work, this gives a good explanation. Akhil Very useful article on DHCP relay agent. I can imagine your efforts and time behind this. Cheers from Seattle, USA. Massimiliano Sbaraglia Thank you too much max massimilianosbaraglia.
Rajneesh Panwar Very well explained and easiest way with artifacts. Vice nice explanation. Thank you for visiting Netmanias! Please leave your comment if you have a question or suggestion. Related Contents. What is DHCP? Continue to download. Password confirmation. Technical Documents. One-Shot Gallery. K orea ICT News. Korea Communication Market Data. List of Contributors. Become a Contributor. Gigabit Internet. SK Telecom. Network Protocol. Korean Vendors. Private 5G Edge.
Netmanias Private 5G Analysis. China Mobile. M etaswitch. A thonet. The show ip route dhcp command is useful to help you understand any problems with the DHCP relay agent adding routes to clients from unnumbered interfaces. All routes added to the routing table by the DHCP server and relay agent are displayed. To enable route addition by DHCPv6 relay and server for the delegated prefix, use the ipv6 dhcp iapd-route-add command in global configuration mode.
To add routes for individually assigned IPv6 addresses on the relay or server, use the ipv6 dhcp iana-route-add command in global configuration mode. The following example shows how to enable the DHCP server, the relay agent, and the insertion and removal of the DHCP relay information option option In this example, the DHCP server is disabled:.
The following example shows that for subscribers who are being serviced by the same aggregation router, the relay agent information option for ATM subscribers must be processed differently from that for Ethernet digital subscribers.
For ATM subscribers, the relay agent information option is configured to be removed from the packet by the relay agent before forwarding the packet to the client. For Ethernet subscribers, the connected device provides the relay agent information option, and the option is configured to remain in the packet and be forwarded to the client.
The following example shows how to add a unique identifier to the subscriber-identifier suboption of the relay agent information option:. The relay agent will match and identify the relay class from the relay pool and forward the DHCP message to the appropriate DHCP server identified by the relay target command. DHCP relay agent 2 is configured to add its own relay agent information option, including the VPN information, and to encapsulate the relay agent information option received from DHCP relay agent 1.
The DHCP server receives the relay agent information options from both the relay agents, uses this information to assign IP addresses and other configuration parameters, and forwards them back to the client. If the DHCP server has a scope or pool configured for the Because the ip dhcp smart-relay global configuration command is configured, if the router sends three requests using Without the smart relay functionality, the router uses only The Cisco Support and Documentation website provides online resources to download documentation, software, and tools.
Use these resources to install and configure the software and to troubleshoot and resolve technical issues with Cisco products and technologies.
Access to most tools on the Cisco Support and Documentation website requires a Cisco. The Cisco Support website provides extensive online resources, including documentation and tools for troubleshooting and resolving technical issues with Cisco products and technologies.
The following table provides release information about the feature or features described in this module. This table lists only the software release that introduced support for a given feature in a given software release train. Unless noted otherwise, subsequent releases of that software release train also support that feature.
This feature allows a second DHCP relay agent to encapsulate the relay agent information option option 82 from a prior relay agent, add its own option 82, and forward the packet to the DHCP server. The DHCP server can use the VPN information from the second relay agent along with the option 82 information from the first relay agent to send correct address assignments and other configuration parameters for the client devices based on the VRF, option 60, and encapsulated option The following commands were modified by this feature: ip dhcp relay information policy , ip dhcp relay information policy-action.
This feature enhances the DHCP class mechanism to support options 60, 77, , and These options identify the type of client sending the DHCP message. The following command was introduced by this feature: option hex. This feature allows subscribers with different relay information option VPN ID requirements on different interfaces to be reached from one Cisco router. The following command was introduced by this feature: ip dhcp relay information option vpn-id.
This feature enables support for the DHCP relay agent information option option 82 on a per interface basis. The following commands were introduced by this feature: ip dhcp relay information check-reply , ip dhcp relay information option-insert , ip dhcp relay information policy-action.
This feature enables an ISP to add a unique identifier to the subscriber-identifier suboption of the relay agent information option. The following command was introduced by this feature: ip dhcp relay information option subscriber-id.
The following commands were modified by this feature: ip dhcp relay information option , ip helper address. Emerging industry standard upon which tag switching is based. A VRF consists of an IP routing table, a derived forwarding table, a set of interfaces that use the forwarding table, and a set of rules and routing protocols that determine what goes into the forwarding table.
Skip to content Skip to search Skip to footer. Book Contents Book Contents. Find Matches in This Book. PDF - Complete Book 3. You can verify whether they have been disabled by checking your configuration file. If they have been disabled, the no service dhcp command will appear in the configuration file. Use the service dhcp command to reenable the functionality if necessary.
Enter your password if prompted. Step 2 configure terminal Example: Router configure terminal Enters global configuration mode. Step 4 ip helper-address address Example: Router config-if ip helper-address The address argument can be a specific DHCP server address, or it can be the network address if other DHCP servers are on the destination network segment.
Using the network address enables other servers to respond to DHCP requests. If you have multiple servers, you can configure one helper address for each server. The relay agent information option contains related suboptions.
The DHCP server receives the packet, uses the suboptions to assign IP addresses and other configuration parameters to the packet, and forwards the packet back to the client. The suboption fields are stripped off of the packet by the relay agent while forwarding the packet to the client. Figure 1. Note If the ip dhcp relay information command is configured in global configuration mode but not configured in interface configuration mode, the global configuration is applied to all interfaces.
If the ip dhcp relay information command is configured in both global configuration mode and interface configuration mode, the interface configuration command takes precedence over the global configuration command. However, the global configuration is applied to interfaces without the interface configuration. If the ip dhcp relay information command is not configured in global configuration mode but is configured in interface configuration mode, only the interface with the configuration option applied is affected.
All other interfaces are not impacted by the configuration. Step 2 configure terminal Example: Device configure terminal Enters global configuration mode.
This function is disabled by default. If an invalid message is received, the relay agent drops the packet. If a valid message is received, the relay agent removes the option field and forwards the packet.
Use the ip dhcp relay information check command to reenable this functionality if it has been disabled. If the interface is not configured as the DHCP relay, it will discard the message immediately. Since a router's interface forwards only unicast messages and a DHCP message is a broadcast message, the router's interface wraps the broadcast message in a new unicast message and forwards it to the DHCP server. The DHCP server uses the source address of the incoming unicast message to determine the subnet of the client.
For example, if the source address of an incoming message is Once the default gateway IP is determined, the DHCP server checks its pools and finds the pool that uses the same default gateway.
In this case, it finds the pool that is configured with the default gateway IP When the DHCP relay receives a unicast message from the DHCP server, it converts that unicast message in local broadcast message and sends it in local subnet.
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