Extensions
The NHRP protocol includes Extensions. Extensions are appended to NHRP
packets. Extensions are used to request additional functions from the NHRP
participants. The use of the extensions parameter lets you determine if the router
sends certain extensions:
vpath information extensions
vIBM vendor-private extensions

Path Information Extensions:

Three extensions are defined in NHRP to provide
path information. These extensions can be used to help monitor the request itself,
to determine the path taken by the request, to determine who generated the reply,
and the path taken by the reply.The path information extensions are:
vForward Transmit - Each Next-Hop Server (NHS) that forwards the request along
the way should append information about itself.
vResponder Address - The Next-Hop Server (NHS) that generates the reply
should append information about itself.
vReverse Transmit - Each Next-Hop Server (NHS) that forwards the reply along
the way should append information about itself.
The router can be configured to send any or all of these extensions in Next Hop
Resolution Request packets that it generates. The information received in the reply
packets is displayed in the router’s NHRP ELS messages.

IBM Vendor-Private Extensions:

Tosupport NHRP in an Emulated LAN
environment, the server adds vendor-unique extensions to NHRP packets. These
extensions act as “queries”; the NHRP client places them in the Next Hop
Resolution Request. If the server supports this function, it responds with three
corresponding extensions containing ELAN address information (MAC address,
ATM address and Routing information); these extensions are included in the Next
Hop Resolution Reply.
The router can be configured so that it does not support the IBM-specific
extensions. If the IBM specific extensions are not used, shortcuts directly to ELAN
devices are not possible. Use the “Exclude List” option to disallow shortcuts
selectively to certain ELAN devices.
Disallowed Router-to-Router Shortcuts
Operation of NHRP may result in establishing transit paths across NBMA network
between routers. However, establishing an NHRP shortcut across a boundary
where information used in route selection is lost may result in a routing loop. Such
situations include the loss of BGP path vector information, and the interworking of
multiple routing protocols with dissimilar metrics. Under such circumstances, NHRP
shortcuts between routers should be disallowed. This situation can be avoided if
there are no “back door” paths between the entry and egress router outside the
NBMA network.
The server allows router-to-router (R2R) shortcuts by default. However, by
configuring disallowed R2R shortcuts, you can create a list of destination or router
addresses for which the router does not allow shortcuts.
Tocreate a disallowed R2R shortcut, you must specify both a protocol address and
a mask. The protocol address is either the destination or a router, and the mask
allows for a range of addresses.
Using NHRP
Chapter11. Using NHRP 355