Dell Emulex Family of Adapters - Manual (Page 748)

OneCommand NIC Teaming and VLAN Manager User Manual P009415-01A Rev. A
1. Overview
Receive-Side Scaling
748
Transmit load balancing is achieved by creating a hashing table using the hashing
algorithm based on load distribution type. When the virtual teaming adapter selects a
physical adapter (or port) to carry all the frames to the destination, the unique MAC
address of the physical adapter is included in the frame, and not the team MAC
address. This is required to comply with the IEEE 802.3 standard. If two adapters
transmit using the same MAC address, a duplicate MAC address situation would occur
that the switch could not handle.
Receive load balancing attempts to load balance incoming traffic for client machines
across physical ports in the team. It uses a modified gratuitous ARP to advertise a
different MAC address for the team IP address in the sender physical and protocol
address. The gratuitous ARP is unicast with the MAC and IP address of a client
machine in the target physical and protocol address respectively. This causes the target
client to update its ARP cache with a new MAC address map to the team IP address.
This has the potential to direct the received traffic to a different adapter than learned
from the transmission. Gratuitous ARPs are not broadcast because this would cause all
clients to send their traffic to the same port. As a result, the benefits achieved through
client load balancing would be eliminated, and could cause out of order frame delivery.
This receive load balancing scheme works as long as all clients and the teamed server
are on the same subnet or broadcast domain.
When the clients and the server are on different subnets, and incoming traffic has to
traverse a router, the received traffic destined for the server 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 needs to send 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 over the particular physical adapter. 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
gratuitous ARP is not a broadcast packet, the router does not process it and does not
update its own ARP cache.
The only way that the router would process an ARP intended for another network
device is if the router has proxy ARP enabled and the host has no default gateway. This
is very rare and not recommended for most applications.
Transmitted traffic through a router is load balanced and 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.
Receive-Side Scaling
Receive-side scaling (RSS) improves the received traffic performance by distributing
multiple incoming traffic sessions to multiple processors.
On the OCe10102 UCNA, receive-side scaling (RSS) works only on incoming packets
that match the MAC address programmed into the NIC port. This limitation applies to
secondary members of the Failover, LACP and 802.3ad team types.