H3C Technologies H3C S3600 Series Switches - Manual (Page 624)

5-3
If a VLAN interface and its IP address are configured incorrectly, or the IP address of the gateway is not
on the same network segment as that of the VLAN interface, the gateway’s MAC address may not be
learned correctly.
How MFF Works
Manual mode
If a host’s IP address is manually assigned, you can enable the MFF manual mode on the access layer
switch connected to it.
If the MFF manual mode is enabled, you need to specify gateways. Upon receiving an ARP request for
a host’s MAC address from a specified gateway, the switch directly sends the host’s MAC address to
the gateway based on the static binding entry or the DHCP snooping entry.
Automatic mode
If all the hosts connected to an access layer switch obtain IP addresses through DHCP, you can enable
MFF automatic mode on the switch, provided that DHCP snooping has been enabled.
With MFF automatic mode enabled, a DHCP snooping device, upon receiving a DHCP-ACK message,
resolves Option 3 in the message (Router IP option) to obtain a gateway for the client’s IP-MAC
snooping entry. If the DHCP-ACK message contains multiple gateway addresses, only the first one is
recorded for the entry. If the message contains no gateway IP address, the first gateway recorded by
the current VLAN is used.
To get a gateway’s MAC address, MFF uses the IP and MAC addresses in the first client snooping entry
corresponding to the gateway to forge an ARP request, and sends the ARP request to the gateway.
z In MFF automatic mode, a VLAN can learn and maintain up to 20 gateways. The gateway IP
addresses will not be updated, that is, the gateway information does not age unless MFF is
disabled.
z If the source MAC address of an incoming ARP packet from a gateway is different from the one
recorded, the MFF device will use the new MAC to replace the old MAC for the gateway.
z After you enable MFF manual mode, the hosts that obtain IP address dynamically can get the
gateway’s IP address based on DHCP snooping entries.
ARP reply mechanism
Like proxy ARP, the ARP reply mechanism is used for Layer 3 communication between hosts, which
helps reduce the number of broadcast messages.
An MFF device processes ARP packets as follows: