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S/N/K-Series use of the 'ip multicast-arp-learning' command


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Article ID: 14129

Products
S-Series, firmware 7.31.03.0010 and higher
Matrix N-Series DFE, firmware 7.31.03.0010 and higher
K-Series, all firmware

Goals
Use of the 'ip multicast-arp-learning' command.

Solution
The Configuration/CLI Guides provide this background regarding the Multicast ARP Learning command:As specified in RFC 1812, by default the router must not believe any ARP packet that claims the packet MAC address is broadcast or multicast. Use this command to remove the multicast restriction. The broadcast restriction remains unchanged.

K Chassis(rw-config)->interface vlan 1
K Chassis(rw-config-intf-vlan.0.1)->ip multicast-arp-learning

The "no" form of this command reinstates the multicast restriction for ARP packets.[/code]As further background, the command allows the ARP cache to dynamically populate unicast IP addresses each bound to a MAC address with the group bit set to 1, representing a multicast MAC address.
For example, "1.1.1.1 01-01-01-ae-ae-ae" would not normally be a valid entry in the router ARP cache, but this command allows such an entry to exist.

The command is expected to be used in virtualization environments where MicroSoft Network Load Balancing (MS NLB) is running. The unicast IP address <-> multicast MAC address load balance behavior allows all devices in a VLAN to receive traffic since the multicast MAC is never used as a source address or learned in the Forwarding DataBase (FDB).

Firewall loadsharing schemes may also use this methodology for a similar purpose.

In order to be able to route such traffic, the router needs an entry in the arp cache. As discussed in 5708 this is typically accomplished via static ARP entry, but this command permits a specialized form of dynamic ARP entry instead.

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