ZyXEL Communications ZyWall USG20-VPN, Руководство пользователя

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Chapter 9 Interfaces
USG20(W)-VPN Series User’s Guide
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The gateway is an optional setting for each interface. If there is more than one gateway, the USG
uses the gateway with the lowest metric, or cost. If two or more gateways have the same metric,
the USG 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 USG restricts the amount of traffic into and out of the USG through each interface.
• Egress bandwidth sets the amount of traffic the USG sends out through the interface to the
network.
• Ingress bandwidth sets the amount of traffic the USG allows in through the interface from the
network.At the time of writing, the USG does not support ingress bandwidth management.
If you set the bandwidth restrictions very high, you effectively remove the restrictions.
The USG 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 USG 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.
DHCP Settings
Dynamic Host Configuration Protocol (DHCP, RFC 2131, RFC 2132) provides a way to automatically
set up and maintain IP addresses, subnet masks, gateways, and some network information (such
as the IP addresses of DNS servers) on computers in the network. This reduces the amount of
manual configuration you have to do and usually uses available IP addresses more efficiently.
In DHCP, every network has at least one DHCP server. When a computer (a DHCP client) joins the
network, it submits a DHCP request. The DHCP servers get the request; assign an IP address; and
provide the IP address, subnet mask, gateway, and available network information to the DHCP
client. When the DHCP client leaves the network, the DHCP servers can assign its IP address to
another DHCP client.
In the USG, some interfaces can provide DHCP services to the network. In this case, the interface
can be a DHCP relay or a DHCP server.
As a DHCP relay, the interface routes DHCP requests to DHCP servers on different networks. You
can specify more than one DHCP server. If you do, the interface routes DHCP requests to all of
them. It is possible for an interface to be a DHCP relay and a DHCP client simultaneously.
As a DHCP server, the interface provides the following information to DHCP clients.