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Chapter 14 Policy and Static Routes
ISG50 User’s Guide
299
14.4 Policy Routing Technical Reference
Here is more detailed information about some of the features you can configure in policy routing.
NAT and SNAT
NAT (Network Address Translation - NAT, RFC 1631) is the translation of the IP address in a packet
in one network to a different IP address in another network. Use SNAT (Source NAT) to change the
source IP address in one network to a different IP address in another network.
Assured Forwarding (AF) PHB for DiffServ
Assured Forwarding (AF) behavior is defined in RFC 2597. The AF behavior group defines four AF
classes. Inside each class, packets are given a high, medium or low drop precedence. The drop
precedence determines the probability that routers in the network will drop packets when
congestion occurs. If congestion occurs between classes, the traffic in the higher class (smaller
numbered class) is generally given priority. Combining the classes and drop precedence produces
the following twelve DSCP encodings from AF11 through AF43. The decimal equivalent is listed in
brackets.
Port Triggering
Some services use a dedicated range of ports on the client side and a dedicated range of ports on
the server side. With regular port forwarding, you set the port(s) and IP address to forward a
service (coming in from the remote server) to a client computer. The problem is that port
forwarding only forwards a service to a single IP address. In order to use the same service on a
different computer, you have to manually replace the client computer's IP address with another
client computer's IP address.
Port triggering allows the client computer to take turns using a service dynamically. Whenever a
client computer’s packets match the routing policy, it can use the pre-defined port triggering setting
Metric Metric represents the “cost” of transmission for routing purposes. IP routing uses
hop count as the measurement of cost, with a minimum of 1 for directly connected
networks. Enter a number that approximates the cost for this link. The number
need not be precise, but it must be 0~127. In practice, 2 or 3 is usually a good
number.
OK Click OK to save your changes back to the ISG50.
Cancel Click Cancel to exit this screen without saving.
Table 89 Configuration > Network > Routing > Static Route > Add (continued)
LABEL DESCRIPTION
Table 90 Assured Forwarding (AF) Behavior Group
CLASS 1 CLASS 2 CLASS 3 CLASS 4
Low Drop Precedence AF11 (10) AF21 (18) AF31 (26) AF41 (34)
Medium Drop Precedence AF12 (12) AF22 (20) AF32 (28) AF42 (36)
High Drop Precedence AF13 (14) AF23 (22) AF33 (30) AF43 (38)