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Configuring IP Addressing
Configuring Address Resolution Methods
IPC-20
Cisco IOS IP Configuration Guide
Each Next Hop Server serves a set of destination hosts, which might be directly connected to the NBMA
network. Next Hop Servers cooperatively resolve the NBMA next hop addresses within their NBMA
network. Next Hop Servers typically also participate in protocols used to disseminate routing
information across (and beyond the boundaries of) the NBMA network, and might support ARP service.
A Next Hop Server maintains a “next hop resolution” cache, which is a table of network layer address
to NBMA address mappings. The table is created from information gleaned from NHRP register packets
extracted from NHRP request or reply packets that traverse the Next Hop Server as they are forwarded,
or through other means such as ARP and preconfigured tables.
Protocol Operation
NHRP requests traverse one or more hops within an NBMA subnetwork before reaching the station that
is expected to generate a response. Each station (including the source station) chooses a neighboring
Next Hop Server to forward the request to. The Next Hop Server selection procedure typically involves
performing a routing decision based upon the network layer destination address of the NHRP request.
Ignoring error situations, the NHRP request eventually arrives at a station that generates an NHRP reply.
This responding station either serves the destination, is the destination itself, or is a client that specified
it should receive NHRP requests when it registered with its server. The responding station generates a
reply using the source address from within the NHRP packet to determine where the reply should be sent.
NHRP Configuration Task List
To configure NHRP, perform the tasks described in the following sections. The tasks in the first section
are required; the tasks in the remaining sections are optional.
• Enabling NHRP on an Interface (Required)
• Configuring a Static IP-to-NBMA Address Mapping for a Station (Optional)
• Statically Configuring a Next Hop Server (Optional)
• Configuring NHRP Authentication (Optional)
• Controlling the Triggering of NHRP (Optional)
• Triggering NHRP Based on Traffic Thresholds (Optional)
• Controlling the NHRP Packet Rate (Optional)
• Suppressing Forward and Reverse Record Options (Optional)
• Specifying the NHRP Responder Address (Optional)
• Changing the Time Period NBMA Addresses Are Advertised as Valid (Optional)
• Configuring a GRE Tunnel for Multipoint Operation (Optional)
• Configuring NHRP Server-Only Mode (Optional)