DHCP Tunnels Support
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DHCP Tunnels SupportLast Updated: September 30, 2012
The DHCP--Tunnels Support feature provides the capability to configure the node (or spoke) of the generic routing encapsulation (GRE) tunnel interfaces dynamically using DHCP. In a Dynamic Multipoint VPN (DMVPN) network each participating spoke must have a unique IP address belonging to the same IP subnet. It is difficult for a network administrator to configure the spoke addresses manually on a large DMVPN network. Hence, DHCP is used to configure the spoke address dynamically on a DMVPN network. Finding Feature InformationYour software release may not support all the features documented in this module. For the latest caveats and feature information, see Bug Search Tool and the release notes for your platform and software release. To find information about the features documented in this module, and to see a list of the releases in which each feature is supported, see the feature information table at the end of this module. Use Cisco Feature Navigator to find information about platform support and Cisco software image support. To access Cisco Feature Navigator, go to www.cisco.com/go/cfn. An account on Cisco.com is not required. Information About DHCP Tunnels SupportDHCP OverviewDHCP is based on the Bootstrap Protocol (BOOTP), which provides the framework for passing configuration information to hosts on a TCP/IP network. DHCP adds the capability to automatically allocate reusable network addresses and configuration options to Internet hosts. DHCP consists of two components: a protocol for delivering host-specific configuration parameters from a DHCP server to a host and a mechanism for allocating network addresses to hosts. DHCP is built on a client/server model, where designated DHCP server hosts allocate network addresses and deliver configuration parameters to dynamically configured hosts. See the "DHCP" section of the Cisco IOS IP Addressing Configuration Guide for more information. DHCP Behavior on a Tunnel NetworkDMVPN spoke nodes establish a tunnel with a preconfigured DMVPN next hop server (NHS) (hub node) and exchange IP packets with the NHS before an IP address is configured on the tunnel interface. This allows the DHCP client on the spoke and the DHCP relay agent or the DHCP server on the NHS to send and receive the DHCP messages. A DHCP relay agent is any host that forwards DHCP packets between clients and servers. When the tunnel on a spoke is in the UP state or becomes active, the spoke establishes a tunnel with the preconfigured hub node. The tunnel formation may include setting up IP Security (IPsec) encryption for the tunnel between the spoke and the hub. DHCP receives the GRE tunnel interface UP notification only after the spoke establishes a tunnel with the hub. The DHCP client configured on the spoke must exchange the DHCP IP packets with the hub (DHCP relay agent or server) to obtain an IP address for the GRE tunnel interface. Therefore, the spoke-to-hub tunnel must be in active state before the GRE tunnel interface UP notification is sent to the DHCP server or the relay agent. IP packets that are broadcast on the DMVPN spoke reache the DMVPN hub. The spoke broadcasts a DHCPDISCOVER message to the DHCP relay agent on the DMVPN hub, prior to the spoke having an IP address on the GRE tunnel interface. By using the DHCPDISCOVER message, DHCP unicasts the offer back to the client. The hub cannot send IP packets to the spoke before the hub receives a Next Hop Resolution Protocol (NHRP) registration from the spoke. The DHCP relay agent configured on the DMVPN hub adds mapping information to the DHCP client packets (DHCPDISCOVER and DHCPREQUEST). The mapping information is added to the DHCP client so that it is available for the DMVPN hub to relay the DHCP server response.
DMVPN Hub as a DHCP Relay AgentRelay agents are not required for DHCP to work. Relay agents are used only when the DHCP client and server are in different subnets. The relay agent acts as a communication channel between the DHCP client and server. The DHCP--Tunnels Support feature requires the DMVPN hub to act as a relay agent to relay the DHCP messages to the DHCP server. The DHCP server is located outside the DMVPN network and is accessible from the DMVPN hub nodes through a physical path. The spoke nodes reach the DHCP servers through the hub-to-spoke tunnel (GRE tunnel). The DHCP server is not directly reachable from the DMVPN spoke. The DHCP relay agent on the DMVPN hub helps the DHCP protocol message exchange between the DHCP client on the spoke and the DHCP server. DMVPN TopologiesDual-Hub Single-DMVPN TopologyIn a dual-hub single-DMVPN topology, both the hubs must be connected to the same DHCP server that has the high availability (HA) support to maintain DMVPN redundancy. If the hubs are connected to different DHCP servers, they must be configured with mutually exclusive IP address pools for address allocation. Dual-Hub Dual-DMVPN TopologyIn the dual-hub dual-DMVPN topology, each hub is connected to a separate DHCP server. The DMVPN hubs (DHCP relay agents) include a client-facing tunnel IP address in the relayed DHCP requests. DHCP requests are used by the DHCP server to allocate an IP address from the correct pool. Hierarchical DMVPN TopologyIn a DMVPN hierarchical topology, there are multiple levels of DMVPN hubs. However, all the tunnel interface IP addresses are allocated from the same IP subnet address. The DHCP client broadcast packets are broadcast to the directly connected hubs. Hence, the DMVPN hubs at all levels must either be DHCP servers or DHCP relay agents. If DHCP servers are used then the servers must synchronize their databases. The DMVPN hubs must be configured as DHCP relay agents to forward the DHCP client packets to the central DHCP servers. If the DHCP server is located at the central hub, all DHCP broadcasts are relayed through the relay agents until they reach the DHCP server. How to Configure DHCP Tunnels Support
Configuring a DMVPN Spoke to Acquire an IP Address from the DHCP Server
SUMMARY STEPS
DETAILED STEPS Configuring the DHCP Relay Agent to Unicast DHCP RepliesPerform this task to configure the DHCP relay agent to unicast DHCP replies. By default, the DHCP replies are broadcast from the DMVPN hub to the spoke. Therefore a bandwidth burst occurs. The DHCP--Tunnels Support feature does not function if the DHCP messages are broadcast. Hence, you must configure the DHCP relay agent to unicast the DHCP messages for the DHCP to be functional in a DMVPN environment. DETAILED STEPS Configuring a DMVPN Spoke to Clear the Broadcast FlagPerform this task to configure a DMVPN spoke to clear the broadcast flag. By default, DMVPN spokes set the broadcast flag in the DHCP DISCOVER and REQUEST messages. Therefore the DHCP relay agent is forced to broadcast the DHCP replies back to the spokes, even though the relay agent has sufficient information to unicast DHCP replies. Hence, you must clear the broadcast flag from the DMVPN spoke. DETAILED STEPS Configuration Examples for DHCP Tunnels Support
Additional ReferencesRelated DocumentsMIBsTechnical Assistance
Feature Information for DHCP Tunnels SupportThe 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. Use Cisco Feature Navigator to find information about platform support and Cisco software image support. To access Cisco Feature Navigator, go to www.cisco.com/go/cfn. An account on Cisco.com is not required.
Cisco and the Cisco logo are trademarks or registered trademarks of Cisco and/or its affiliates in the U.S. and other countries. To view a list of Cisco trademarks, go to this URL: www.cisco.com/go/trademarks. Third-party trademarks mentioned are the property of their respective owners. The use of the word partner does not imply a partnership relationship between Cisco and any other company. (1110R) Any Internet Protocol (IP) addresses and phone numbers used in this document are not intended to be actual addresses and phone numbers. Any examples, command display output, network topology diagrams, and other figures included in the document are shown for illustrative purposes only. Any use of actual IP addresses or phone numbers in illustrative content is unintentional and coincidental. © 2012 Cisco Systems, Inc. All rights reserved.
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