Users Guide

12–Marvell Teaming Services
Executive Summary
Doc No. BC0054508-00 Rev. R
January 21, 2021 Page 149 Copyright © 2021 Marvell
SLB 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 G-ARP is unicast with the MAC and IP Address of a
client machine in the target physical and protocol address, respectively. This
action causes the target client to update its ARP cache with a new MAC address
map to the team IP address. G-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 system are on the same subnet or broadcast domain.
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 specific 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, because 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 situation is very rare and not recommended for most applications.
Transmit traffic through a router is load balanced because transmit load balancing
is based on the source and destination IP address and TCP/UDP port number.
Because 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.