next hop resolution protocol (NHRP)

Next Hop Resolution Protocol (NHRP) is a request-response protocol. In order to participate in NHRP communication, all systems must be able to generate NHRP requests.

The NHRP protocol makes IP routing information visible to routers connected to ATM beyond a simple LIS(Logical IP Subnet) structure. Since ATM is connection-oriented, routing occurs only once when a connection is established at the ATM layer, and the connection is communicated to all ATM switches involved in the route. In doing so, the NHRP protocol dynamically determines the MAC addresses of other routers and hosts connected to the network. It enables efficient routing between IP subnets through direct communication without an intermediate hop. In addition, NHRP is also used in Multiprotocol over ATM( MPoA) to efficiently map Layer 3 addresses to ATM addresses.

The NHRP frame format consists of a Fixed header, which has a length of 20 bytes, a Mandatory Part, whose length is in the Extension Offset field of the Fixed Part, and an optional Extension Part. The Fixed Part header contains all quantities that are independent of the packet type and have a fixed length. These include the address, the protocol type, the hop count value, the total size of the data packet, the version field, and the checksum field.