Next Hop Resolution Protocol (NHRP), defined in RFC 2332, is a Layer 2 address resolution protocol and cache, like Address Resolution Protocol (ARP) and Frame Relay Inverse-ARP. NHRP is used by a branch router connected to a non-broadcast, multi-access (NBMA) sub-network to determine the IP address of the “NBMA next hop”; in this case, the headend router or the destination IP address of another branch router.
When a branch router is first established onto a DMVPN network, it registers its IP address with the headend router whose IP address is already pre-configured on the branch router. This registration enables the mGRE interface on the headend router to build a dynamic tunnel back to the registering branch router without having to know the branch tunnel destination through a CLI configuration. NHRP maps a tunnel IP address to an NBMA IP address. NHRP tells the mGRE interface where to tunnel a packet to reach a certain address. When the packet is encapsulated in the mGRE packet, the IP destination address is the NBMA address.
If the destination address is connected to the NBMA sub-network, the headend router is the destination itself. Otherwise, the headend route is the egress router closest to the branch requesting a destination IP address.
Headend and branch routers should be configured with an NHRP holdtime, which sets the length of time that routers instruct other routers to keep their NHRP information. This information is kept in the NHRP cache until the NHRP holdtime expires and the information must be relearned. The default NHRP holdtime is two hours; however, the recommended value is ten minutes. The NHRP cache can be populated with either static or dynamic entries. On the headend router, all entries are added dynamically via registration or resolution requests. The branch router is configured with a static NHRP map pointing to the headend router. To participate in one NHRP registration process, all routers must belong to the same NHRP network by a network ID. The NHRP network ID defines an NHRP domain.
Branch routers must be configured with the NBMA address of the headend router as their next hop server (NHS) to register with the headend router. The branch routers send a registration to the headend router that contains the tunnel IP address and the NBMA address. The headend router creates an entry in its NHRP cache and returns a registration reply. The branch router now views the headend router as a valid NHS and uses it as a source to locate any other branches and networks in the NHRP domain.
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