NHRP Implementation
NHRP interacts with the router function in the router. When the router function in
the router is forwarding packets along the routed path and NHRP successfully
obtains a shortcut VC, NHRP will update the router function to send the packet
directly over the shortcut VC.
NHRP updates the routing function’s forwarding table after the VC is up. This allows
the switch from routed path to the shortcut path to occur without any packet loss.
When an NHRP shortcut is used, the router transmits frames to a next hop address
on a subnetwork that the router itself is not a part of. So the NET,or interface, that
provides the outbound path for the traffic is called a “virtual” network interface.

Virtual Network Interface (VNI)

Normally, outbound packet flow from a router is constrained by the following:
vInability to send packets directly to network addresses that are not defined on a
network interface.
vInability to send packets to network types (for example, token-ring ELAN) unless
that network type is defined on a network interface.
The Virtual Network Interface (VNI) net-handler removes all of these constraints,
which allows the router to forward packets directly to next hops obtained via NHRP
(shortcut routes). It enables one-hop routing, where NHRP shortcut routes can be
made directly to devices that do not support NHRP.
The VNI supports token-ring, Ethernet V2 and Ethernet DIX ELAN network
interfaces and classic IP network interfaces. When the outbound path is to use a
classic IP (1577) interface, the implementation actually uses the existing 1577
net-handler interface for the VNI. However, when the outbound path is to use a
LANE shortcut, a unique interface is accessed. This is called the LANE Shortcut
Interface (LSI). The LSI is different from a traditional LEC interface because it can
provide more than one LAN encapsulation type; that is, one VC may be established
using token-ring encapsulation while another uses Ethernet V2. Also the LSI
provides connections to more than one Emulated LAN; a traditional LEC interface
connects to only one ELAN.
When you enable NHRP,an LSI is created for each ATM adapter. The LSI is
assigned the next available interface number, and will be listed when you invoke
console functions that display information about the router interfaces.

LANE Shortcut Interface (LSI)

The LANE shortcuts provided by the IBM extensions to NHRP are not compatible
with some LAN Emulation Client (LEC) and end-station protocol stack
implementations. This section describes how these incompatibilities can arise and,
in some cases, how they can be overcome using configuration options.
Paranoid LECs are devices that use the LAN Emulation Flush Protocol to verify that
clients setting up Data Direct VCCs to it are actually members of its ELAN. These
devices will not work with NHRP shortcuts generated by LSIs since the LSI is not
part of the target ELAN.
Note: The “Exclude List” configuration option can be used to prevent shortcuts to
Paranoid LECs as described in “Exclude Lists” on page354.
Using NHRP
352 MRS V3.2 Protocol Config Ref Vol2