Chapter 8 Routing
USG FLEX H Series User’s Guide
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How You Can Use Policy Routing
• Source-Based Routing – Network administrators can use policy-based routing to direct traffic from
different users through different connections.
• Cost Savings – IPPR allows organizations to distribute interactive traffic on high-bandwidth, high-cost
paths while using low-cost paths for batch traffic.
• Load Sharing – Network administrators can use IPPR to distribute traffic among multiple paths.
• NAT - The Zyxel Device performs NAT by default for traffic going to or from the WAN interfaces. A
routing policy’s SNAT allows network administrators to have traffic received on a specified interface
use a specified IP address as the source IP address.
Note: The Zyxel Device automatically uses SNAT for traffic it routes from internal interfaces to
external interfaces. For example LAN to WAN traffic.
Static Routes
The Zyxel Device usually uses the default gateway to route outbound traffic from computers on the LAN
to the Internet. To have the Zyxel Device send data to devices not reachable through the default
gateway, use static routes.
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 take priority over static routes. If you need to use a routing policy on the Zyxel Device
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.
DSCP (6 bits) Unused (2 bits)
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