ZyXEL Communications USG-300 - V2.20 ED 2, Manual

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Chapter 15 Policy and Static Routes
ZyWALL USG 300 User’s Guide
381
Policy Routes Versus Static Routes
• 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.
DiffServ
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.
DSCP Marking and Per-Hop Behavior
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)