Poison Reverse; Advertisement Address - 3Com corebuilder 3500 Implementation Manual

298 C 11: I
HAPTER NTERNET

Poison Reverse

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Address
P (IP)
ROTOCOL
Poison Reverse is a RIP feature that you use specifically with a scheme
called Split Horizon. Your system enables Poison Reverse by default.
Split Horizon avoids the problems that reverse-route updates can cause.
Reverse-route updates are sent to a neighboring router and include the
routes that are learned from that router. Split Horizon omits the routes
that are learned from one neighbor in the updates that are sent to that
neighbor (the reverse routes).
Poison Reverse is essentially another layer of protection against
advertising reverse routes.
When you enable (default mode) Poison Reverse, the system
advertises reverse routes in updates, but it sets the metrics to 16
(infinity). Setting the metric to infinity breaks the loop immediately
when two routers have routes that point to each other.
When you disable Poison Reverse, such reverse routes are not
advertised.
You can disable Poison Reverse because it augments what Split Horizon
already does, and it puts additional information that you may not need
into RIP updates.
The system uses the advertisement address specified to advertise routes
to other stations on the same network. The system uses this address for
sending updates. (Note that RIP-2 updates depend on the setting of RIP
compatibility mode.)
Each interface that you define initially uses the default broadcast address
(255.255.255.255) as the advertisement address. If you change the
broadcast address, the address that you specify becomes the new RIP
advertisement address. If you subsequently use RIP-2 (configure the
interface to send RIP-2 advertisements) and have the RIP-1 compatibility
mode disabled, the multicast address is used for updates.
Effects and Consequences
After you add an advertisement address, you cannot subsequently
change the broadcast address.
If you are using RIP-2 for the interface and if you want the system to
use the advertisement list instead of the multicast address for RIP
updates, enable RIP compatibility mode.