KB.15.15

When reconfiguring the MAC address, you may specify a keepalive timeout to transmit heartbeat

packets that advertise the new MAC address

By configuring the MAC address of the previously installed router as the MAC address of each

VLAN interface on an HP Switch, you can swap the physical port of a router to the HP Switch after

the switch has been properly configured in the network.

Handling incoming and outgoing VLAN Traffic

Incoming VLAN data packets and ARP requests

These are received and processed on the routing switch according to the MAC address of the

previously installed router that is configured for each VLAN interface.

Outgoing VLAN traffic

This uses the MAC address of the HP Sswitch as the source MAC address in packet headers. The

MAC address configured on VLAN interfaces is not used on outbound VLAN traffic.

When the routing switch receives an ARP request for the IP address configured on a VLAN interface,

the ARP reply uses the reconfigured MAC address in both the:

• ARP Sender MAC address field

• Source MAC address field in the Ethernet frame header

When proxy ARP is enabled on a VLAN interface, the "gracious" ARP reply sent for an ARP request

received from VLAN devices located outside the directly connected IP subnets also contains the

reconfigured MAC address in both the:

• ARP Sender MAC address field

• Source MAC address field in the Ethernet frame header

NOTE: The Virtual Router Redundancy Protocol (VRRP) is not supported on VLAN interfaces on

which the MAC address for incoming traffic has been reconfigured.

To hosts in the network, VLAN traffic continues to be routed (using the reconfigured MAC address

as destination address), but outbound VLAN traffic appears to be sent from another router attached

to the same subnet (using the HP Switch MAC address as source address) attached to the same

subnet . Although it appears as an asymmetric path to network hosts, the MAC address configuration

feature enables Layer 3 VLAN migration. (A successful VLAN migration is achieved because the

hosts do not verify that the source MAC address and the destination MAC address are the same

when communicating with the routing switch.)

Sending heartbeat packets with a configured MAC Address

On the VLAN interfaces of a routing switch, the user-defined MAC address only applies to inbound

traffic. As a result, any connected switches need to learn the new address that is included in the

Ethernet frames of outbound VLAN traffic transmitted from the routing switch.

If a connected switch does not have the newly configured MAC address of the routing switch as

a destination in its MAC address table, it floods packets to all of its ports until a return stream

allows the switch to learn the correct destination address. As a result, the performance of the switch

is degraded as it tries to send Ethernet packets to an unknown destination address.

To allow connected switches to learn the user-configured MAC address of a VLAN interface, the

HP routing switch can send periodic heartbeat-like Ethernet packets. The Ethernet packets contain

the configured MAC address as the source address in the packet header. IP multicast packets or

Ethernet service frames are preferred because they do not interrupt the normal operation of client

devices connected on the segment.

Because the aging time of destination addresses in MAC address tables varies on network devices,

you must also configure a time interval to use for sending heartbeat packets.

Introducing tagged VLAN technology into networks running untagged VLANs 63