ZyXEL Communications ZyWall USG 50-H Series, User Manual

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Chapter 8 Interface
ZyWALL USG 50-H User’s Guide
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In many interfaces, you can also let the IP address and subnet mask be assigned by an external
DHCP server on the network. In this case, the interface is a DHCP client. Virtual interfaces,
however, cannot be DHCP clients. You have to assign the IP address and subnet mask
manually.
In general, the IP address and subnet mask of each interface should not overlap, though it is
possible for this to happen with DHCP clients.
In the example above, if the ZyWALL gets a packet with a destination address of 5.5.5.5, it
might not find any entries in the routing table. In this case, the packet is dropped. However, if
there is a default router to which the ZyWALL should send this packet, you can specify it as a
gateway in one of the interfaces. For example, if there is a default router at 200.200.200.100,
you can create a gateway at 200.200.200.100 on wan1. In this case, the ZyWALL creates the
following entry in the routing table.
The gateway is an optional setting for each interface. If there is more than one gateway, the
ZyWALL uses the gateway with the lowest metric, or cost. If two or more gateways have the
same metric, the ZyWALL uses the one that was set up first (the first entry in the routing
table). In PPPoE/PPTP interfaces, the other computer is the gateway for the interface by
default. In this case, you should specify the metric.
If the interface gets its IP address and subnet mask from a DHCP server, the DHCP server also
specifies the gateway, if any.
Interface Parameters
The ZyWALL restricts the amount of traffic into and out of the ZyWALL through each
interface.
• Egress bandwidth sets the amount of traffic the ZyWALL sends out through the interface
to the network.
• Ingress bandwidth sets the amount of traffic the ZyWALL allows in through the interface
from the network.
2
If you set the bandwidth restrictions very high, you effectively remove the restrictions.
The ZyWALL also restricts the size of each data packet. The maximum number of bytes in
each packet is called the maximum transmission unit (MTU). If a packet is larger than the
MTU, the ZyWALL divides it into smaller fragments. Each fragment is sent separately, and
the original packet is re-assembled later. The smaller the MTU, the more fragments sent, and
the more work required to re-assemble packets correctly. On the other hand, some
communication channels, such as Ethernet over ATM, might not be able to handle large data
packets.
Table 76 Example: Routing Table Entry for a Gateway
IP ADDRESS(ES) DESTINATION
0.0.0.0/0 200.200.200.100
2. At the time of writing, the ZyWALL does not support ingress bandwidth management.