17–QLogic Teaming Services
Executive Summary
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When the clients and the system are on different subnets, and incoming traffic has
to traverse a router, the received traffic destined for the system is not load
balanced. The physical adapter that the intermediate driver has selected to carry
the IP flow carries all of the traffic. When the router sends a frame to the team IP
address, it broadcasts an ARP request (if not in the ARP cache). The server
software stack generates an ARP reply with the team MAC address, but the
intermediate driver modifies the ARP reply and sends it over a particular physical
adapter, establishing the flow for that session.
The reason is that ARP is not a routable protocol. It does not have an IP header
and therefore, is not sent to the router or default gateway. ARP is only a local
subnet protocol. In addition, since the G-ARP is not a broadcast packet, the router
will not process it and will not update its own ARP cache.
The only way that the router would process an ARP that is intended for another
network device is if it has Proxy ARP enabled and the host has no default
gateway. This is very rare and not recommended for most applications.
Transmit traffic through a router will be load balanced as transmit load balancing is
based on the source and destination IP address and TCP/UDP port number.
Since routers do not alter the source and destination IP address, the load
balancing algorithm works as intended.
Configuring routers for Hot Standby Routing Protocol (HSRP) does not allow for
receive load balancing to occur in the adapter team. In general, HSRP allows for
two routers to act as one router, advertising a virtual IP and virtual MAC address.
One physical router is the active interface while the other is standby. Although
HSRP can also load share nodes (using different default gateways on the host
nodes) across multiple routers in HSRP groups, it always points to the primary
MAC address of the team.
Generic Trunking
Generic Trunking is a switch-assisted teaming mode and requires configuring
ports at both ends of the link: server interfaces and switch ports. This is often
referred to as Cisco Fast EtherChannel or Gigabit EtherChannel. In addition,
generic trunking supports similar implementations by other switch OEMs such as
Extreme Networks Load Sharing and Bay Networks or IEEE 802.3ad Link
Aggregation static mode. In this mode, the team advertises one MAC Address
and one IP Address when the protocol stack responds to ARP Requests. In
addition, each physical adapter in the team uses the same team MAC address
when transmitting frames. This is possible since the switch at the other end of the
link is aware of the teaming mode and will handle the use of a single MAC
address by every port in the team. The forwarding table in the switch will reflect
the trunk as a single virtual port.
