19216811.liveChapter 4 WAN Setup
P-660R-F1 Series User’s Guide
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Do not specify a nailed-up connection unless your telephone company offers flat-rate service or you
need a constant connection and the cost is of no concern
4.1.7
NAT
NAT (Network Address Translation - NAT, RFC 1631) is the translation of the IP address of a host in
a packet, for example, the source address of an outgoing packet, used within one network to a
different IP address known within another network.
4.2
Metric
The metric represents the "cost of transmission". A router determines the best route for
transmission by choosing a path with the lowest "cost". RIP routing uses hop count as the
measurement of cost, with a minimum of "1" for directly connected networks. The number must be
between "1" and "15"; a number greater than "15" means the link is down. The smaller the
number, the lower the "cost".
The metric sets the priority for the ZyXEL Device’s routes to the Internet. If any two of the default
routes have the same metric, the ZyXEL Device uses the following pre-defined priorities:
•
Normal route: designated by the ISP (see
Section 4.5 on page 48
)
•
Traffic-redirect route (see
Section 4.7 on page 58
)
•
WAN-backup route, also called dial-backup (see
Section 4.8 on page 60
)
For example, if the normal route has a metric of "1" and the traffic-redirect route has a metric of
"2" and dial-backup route has a metric of "3", then the normal route acts as the primary default
route. If the normal route fails to connect to the Internet, the ZyXEL Device tries the traffic-redirect
route next. In the same manner, the ZyXEL Device uses the dial-backup route if the traffic-redirect
route also fails.
If you want the dial-backup route to take first priority over the traffic-redirect route or even the
normal route, all you need to do is set the dial-backup route’s metric to "1" and the others to "2"
(or greater).
IP Policy Routing overrides the default routing behavior and takes priority over all of the routes
mentioned above.
4.3
Traffic Shaping
Traffic Shaping is an agreement between the carrier and the subscriber to regulate the average rate
and fluctuations of data transmission over an ATM network. This agreement helps eliminate
congestion, which is important for transmission of real time data such as audio and video
connections.
Peak Cell Rate (PCR) is the maximum rate at which the sender can send cells. This parameter may
be lower (but not higher) than the maximum line speed. 1 ATM cell is 53 bytes (424 bits), so a
maximum speed of 832Kbps gives a maximum PCR of 1962 cells/sec. This rate is not guaranteed
because it is dependent on the line speed.