ZyXEL Communications 1000 User Manual

 Chapter 15 Policy and Static Routes

ZyWALL USG 1000 User’s Guide

• Policy routes are more flexible than static routes. You can select more criteria 

for the traffic to match and can also use schedules, NAT, and bandwidth 

management.

• Policy routes are only used within the ZyWALL itself. Static routes can be 

propagated to other routers using RIP or OSPF. 

• Policy routes take priority over static routes. If you need to use a routing policy 

on the ZyWALL and propagate it to other routers, you could configure a policy 

route and an equivalent static route.

QoS is used to prioritize source-to-destination traffic flows. All packets in the same 
flow are given the same priority. CoS (class of service) is a way of managing traffic 
in a network by grouping similar types of traffic together and treating each type as 
a class. You can use CoS to give different priorities to different packet types. 

DiffServ (Differentiated Services) is a class of service (CoS) model that marks 
packets so that they receive specific per-hop treatment at DiffServ-compliant 
network devices along the route based on the application types and traffic flow. 
Packets are marked with DiffServ Code Points (DSCPs) indicating the level of 
service desired. This allows the intermediary DiffServ-compliant network devices 
to handle the packets differently depending on the code points without the need to 
negotiate paths or remember state information for every flow. In addition, 
applications do not have to request a particular service or give advanced notice of 
where the traffic is going.

DiffServ defines a new DS (Differentiated Services) field to replace the Type of 
Service (TOS) field in the IP header. The DS field contains a 2-bit unused field and 
a 6-bit DSCP field which can define up to 64 service levels. The following figure 
illustrates the DS field. 

DSCP is backward compatible with the three precedence bits in the ToS octet so 
that non-DiffServ compliant, ToS-enabled network device will not conflict with the 
DSCP mapping.

The DSCP value determines the forwarding behavior, the PHB (Per-Hop Behavior), 
that each packet gets across the DiffServ network. Based on the marking rule, 
different kinds of traffic can be marked for different kinds of forwarding. Resources 
can then be allocated according to the DSCP values and the configured policies.

DSCP (6 bits)

Unused (2 bits)