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information. So, for “one-hop” routing this operation happens once during
transfer of a packet from its source to its destination.
v The IBM implementation can operate in networks where some routers do not
support NHRP. If the next-hop router is not capable of providing NHRP support,
shortcut VCs can be established to the “last” server in the path. See “Disallowed
Router-to-Router Shortcuts” on page 355 and “Exclude Lists” on page 354.
v The customer may configure the 2210 to establish shortcuts only when traffic to a
destination exceeds a given data rate. This can eliminate the creation of VCs for
low volume or one-time traffic (for example, SNMP traps). See “data-rate
parameter” on page 366 and “attempt shortcuts? parameter” on page 365.
v The router provides solutions for the “domino” effect that is described in the
NHRP draft. See “attempt shortcuts? parameter” on page 365.
v All ATM-attached routers on the routed path should support NHRP for the optimal
benefit, although the 2210 can still operate and provide shortcuts in a mixed
network.
Performance Characteristics
NHRP is used during initial contact from a source device to a destination. Once a
shortcut VC has been established, NHRP is not involved in actual data transfer.
Safeguards ensure that NHRP traffic is not retried for every packet. Also, the IBM
implementation provides an option for NHRP shortcuts to be requested only when
traffic to a certain destination exceeds a configurable data rate threshold. This can
prevent, for example, the establishment of virtual circuits that would only be used
for one SNMP trap frame that is generated by an IP host.
NHRP operation does not affect the performance of the router fastpath and will not
significantly affect the slowpath. When shortcuts are available, the performance is
improved by the elimination of extraneous hops over the ATM network. Also, the
performance of intermediate routers that are bypassed by NHRP shortcuts should
be improved, as they handle less traffic.
Note: If a configuration does not include a 1577 interface (that is, the router is
configured only for ELANs), shortcut VCs can be established to the router
only from clients that support the IBM extensions. This limitation can be
avoided simply by defining a 1577 interface on the router.
Examples of NHRP Configurations
The following paragraphs give examples of NHRP configurations.
NHRP in an RFC 1577 Classic IP Environment with All Devices
NHRP-capable
In this picture, the NHRP clients use RFC 1577 connections to communicate with
the router. They use NHRP protocol to learn from the NHRP server about each
other’s ATM addresses. Then they establish a direct virtual circuit between them for
IP traffic.
Using NHRP
Chapter 11. Using NHRP 347