Filter
Top Products
The second identifier for the network is one or more ZoneNames. These ZoneName
strings are not unique throughout the internet. The end station is uniquely identified
by a combined object:type:ZoneName-string.
A router first learns about a network when the new net range appears in the RTMP
routing update from a neighboring router. The router then queries the neighbor for
the ZoneNames of the new network. Note that the net range is repeated in every
new RTMP update but that the ZoneNames are requested only once.
The end stations obtain the network numbers from the broadcasted RTMP (routing
information) packets and then choose a node number. This net/node pair is then
AARP’d for (AARP Probe) to see if any other end station has already claimed its
use. If another station responds, another net/node pair is chosen by the end station
and the process repeated until no responses are received.
Why ZoneName Filters?
When the typical AppleTalk end station wants to use a service (printer, file server)
on the Apple Internet, it first looks at all available Zones and selects one. It then
chooses a service type and requests a list of all names advertising the type in the
chosen Zone. Several problems arise from this mechanism.
v A large internet may have many Zones. Presenting the user with a long list to
choose from obscures the needed ones (thereby inhibiting usability of the list).
v The server may not want to make itself available throughout the internet (for
security reasons). If the Zone that the service is in is not visible to the client,
security is enhanced.
v Restricting the Zones that are visible from a department to the rest of the internet
will allow the internet administration to let the department control (or not) its own
domain while not increasing the overhead for the rest of the internet (reducing
administration).
The filtering of network numbers further enhances the security and administration of
the internet. Network access is only indirectly controlled by Zone filtering. An
unregulated department could add networks with the same Zone names but new
net numbers that conflict with other departments. Network number filtering can be
used to prevent these random additions of zone names and net numbers from
impacting the rest of the network.
How Do You Add Filters?
The router is configured with an exclusive (meaning block the specified zones) or
inclusive (meaning allow only these zones) list of Zones for each direction on each
interface. The specified interface will not readvertise filtered Zone information in the
defined direction. If all Zones in a network’s Zonelist are filtered, network
information will also be filtered across the interface.
v Use configuration commands add and delete, to create the filter list for an
interface.
v Use configuration commands enable and disable to specify how the filter list is
applied.
Use similar commands to create network number filters.
Using AppleTalk Phase 2
Chapter 3. Using AppleTalk Phase 2 215