3Com Switch 7750 Configuration Guide Guide

550 CHAPTER 52: ARP CONFIGURATION
load and network traffic. To solve this problem, the device allows you to configure
the gratuitous ARP update interval.
Introduction to ARP
Proxy
ARP proxy: A host in a network sends an ARP request to an isolated port in the
same network or to a host in another network. Devices enabled with the ARP
proxy function forward the ARP request, so as to realize the Layer 3 connectivity
among the Lay 2 isolated ports.
In order to realize Lay 3 connectivity among ports in one of the following
conditions, you need to enable the ARP proxy function (Proxy ARP).
■ Super VLAN function is enabled on the Switch 7750 Family.
■ The isolate-user-vlan function is enabled on Layer 2 switches connecting with
the Switch 7750 Family.
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■ After ARP proxy is enabled, ports in the same VLAN are interconnected by
default, so that the ARP proxy only processes the ARP requests from different
VLANs and does not deal with the ARP requests from the same VLAN.
■ When isolate-user-vlan function is enabled on the Layer 2 switches connected
with the Switch 7750 Family, ports in the same VLAN cannot communicate
with each other. To realize Layer 3 connectivity among Layer 2 isolated ports in
the same VLAN, you need to enable the VLAN ARP proxy function on the
Switch 7750 Family to make the ARP proxy process the ARP request from the
same VLAN.
Introduction to ARP
Source Suppression
ARP source suppression function is that a switch classifies the received ARP
packets first, and then limits the maximum number of ARP packets with the same
type that can be sent to the CPU at a time to protect CPU from being attacked by
the illegal ARP packets generated when the host scans ARP in the whole network.
The Switch 7750 Family classifies the received ARP packets to the following types:
■ Arbitrary ARP packets, whose source IP address and destination IP address are
not distinguished.
■ ARP pass-by packets with the same source IP address. (their destination IP
addresses are not the IP address of the current switch )
■ ARP packets that with the same source IP address and their destination
addresses are the IP address of the current switch.
According to these types, you can set the maximum number of ARP packets of
each type that can be sent to the CPU at a time on the switch. When the number
of ARP packets received at a time exceeds the corresponding setting, the switch
will regard the exceeding part of ARP packets as illegal ARP packets and discard
them.
ARP Configuration ARP entries in the Switch 7750 Family can either be static entries or dynamic
entries, as described in
Table 418.