Amer Networks E5Web GUI - Manual (Page 186)

The Size of the ARP Cache
By default, the ARP Cache is able to hold 4096 ARP entries at the same time. This is adequate for
most scenarios but on rare occasions, such as when there are several very large LANs directly
connected to the security gateway, it may be necessary to adjust this value upwards. This can be
done by modifying the ARP advanced setting ARP Cache Size.
Hash tables are used to rapidly look up entries in the ARP Cache. For maximum efficiency, a hash
table should be twice as large as the entries it is indexing, so if the largest directly connected LAN
contains 500 IP addresses, the size of the ARP entry hash table should be at least 1000. The
administrator can modify the ARP advanced setting ARP Hash Size to reflect specific network
requirements. The default value of this setting is 512.
The setting ARP Hash Size VLAN setting is similar to the ARP Hash Size setting, but affects the
hash size for VLAN interfaces only. The default value is 64.
3.5.3. ARP Publish
Overview
cOS Core supports the publishing of IP addresses on interfaces other than the one the IP address
is actually connected to. This can optionally be done along with a specific MAC address instead
of the publishing interface's MAC address. cOS Core will then send out ARP replies for ARP
requests received on the interface for the published IP addresses. This feature is referred to in
cOS Core as ARP Publish.
Usage
Publishing may be done for a number of reasons:
• To give the impression that an interface in cOS Core has more than one IP address.
This is useful if there are several separate IP spans on a single LAN. The hosts on each IP span
may then use a gateway in their own span when these gateway addresses are published on
the corresponding cOS Core interface.
• Another use is publishing multiple addresses on an external interface, enabling cOS Core to
statically address translate traffic to these addresses and send it onwards to internal servers
with private IPv4 addresses.
• A less common purpose is to aid nearby network equipment responding to ARP in an
incorrect manner.
Methods of Enabling ARP Publishing
ARP publishing is enabled in one of two ways:
• An ARP object can be created for any interface by defining a single address which is to be ARP
published on that interface.
• When a static route is created for an IPv4 address or network, the route option called Proxy
ARP can be used to publish the address or network on all or selected interfaces.
Using ARP objects, single IP addresses only can be published one at a time. However, the
Proxy ARP feature can publish entire networks. It also provides a single step to publish on all
interfaces.
Chapter 3: Fundamentals
186