includes the destination protocol address. The routers (that are also NHRP servers)
along the routed path first check to see if the destination protocol address is an
address that it can serve.
If the router can satisfy the request, the router returns a Next Hop Resolution Reply
with the NBMA address of the destination station. The originator can then establish
a direct virtual circuit with the destination. If it cannot satisfy the request, the router
forwards the request to the next-hop router. This forwarding continues until the
request can be satisfied, or it is determined that the destination cannot be reached.
Touse client/server terminology, a device may be both a client and a server. The
client is the device that originates Next Hop Resolution Requests, and the server is
the one that provides Next Hop Resolution Replies with NBMA address information.
The 2210 is such a device; the client conceptually “registers” with the server
function in the same machine, although no Registration Requests actually flow. The
server also supports NHRP Registrations from remote NHRP clients.
The information provided by clients to their server, and by servers to requestors,
must be refreshed periodically and may be purged if conditions dictate. Clients and
Servers maintain caches of resolution information that they have sent and received;
holding times are used to age out the entries or force refreshes.
Benefits of NHRP and the IBM implementation
In general, use of NHRP shortcuts can:
vImprove end-to-end performance, by eliminating hops between routers when the
source and destination are on the same NBMA network and can communicate
directly
vReduce the load on network routers, since they are bypassed for traffic that,
without NHRP,would be handled by the router. This can reduce overall costs as
fewer routers or less bandwidth may be needed.
The IBM implementation of NHRP provides these additional benefits:
vThe NHRP draft does not address using the protocol in an Emulated LAN
environment. However, the IBM implementation of NHRP includes considerations
for such environments; NHRP packets can flow between routers over ELAN
connections, and shortcut VCs can be established.
vOne-hop Routing: ATMdevices that do not support NHRP can be the destination
of shortcut paths, eliminating another router hop for traffic, by expanding the
definition of the devices that are “served” to include devices that share a protocol
subnetwork with the server. For example, all IP addresses on a classical IP
subnet that a server is part of, are “served” by that server. The NHRPfunction
interfaces with classical IP 1577 and LAN Emulation components to use their
existing ATMaddress resolution capabilities and apply them to NHRP requests.
This enhancement can even be used for traffic to legacy LAN-attached devices
that connect to ATMthrough LAN switches; the NHRP server in the router replies
to the client with ATMaddressing information for the LAN switch, allowing the
client to shortcut directly to that switch. For examples of these “one-hop routing”
cases, see Figure 27 on page 345 and Figure 28 on page 348
Note: A hop is an operation performed by a traditional router when forwarding
packets from one subnet to another. In particular those operations are (1)
doing a lookup on a Layer 3 subnet identifier (2) determining the outbound
“next hop” for the packet (3) stripping and replacing the Layer 2 packet
header, removing ingress link information and adding egress link
Using NHRP
346 MRS V3.2 Protocol Config Ref Vol2