lag - Link aggregation (also called trunking) introductory
information
Link aggregation, or trunking, enables administrators to
combine two or more physical Ethernet Network Interface
Cards (NICs) and create a single virtual link. (Upperlayer
software sees this link aggregation group as a single
virtual interface.) The single virtual link can carry
traffic at higher data rates than a single interface
because the traffic is distributed across all of the physical
ports that make up the link aggregation group.
Using link aggregation provides the following capabilities:
Increased network bandwidth - The increase is incremental
based on the number and type of ports, or Network
Interface Cards (NICs), added to the link aggregation
group. See the "Load Sharing" section for more information.
Fault tolerance - If a port in a link aggregation
group fails, the software detects the failure and reroutes
traffic to the other available ports. See the "Fault Tolerance"
section for more information. Load sharing -
Traffic is distributed across all ports of a link aggregation
group. See the "Load Sharing" section for more information.
You can use a link aggregation group virtual interface for
the following point-to-point connections: server-to-server
and server-to-switch. For server-to-switch connections,
the switch must support link aggregation. See your switch
documentation for information on configuring your switch.
Link aggregation requires an optional kernel subsystem
(lag.mod). You can verify the presence of the link aggregation
subsystem by issuing the sysconfig -s lag command.
If the lag subsystem is not loaded, you can load it using
either of the following methods: Dynamically load it using
the sysconfig -c lag command. This method does not persist
across system reboots. Edit the system configuration
file, add an options LAG entry to it, and build a new kernel
by issuing the doconfig command. Then, reboot the system.
This method loads the subsystem each time the system
reboots.
After the subsystem is loaded, you can configure a link
aggregation group.
Link Aggregation Configuration [Toc] [Back]
You can configure link aggregation groups either in multiuser
mode or at boot time with the lagconfig command.
When you configure the group, you can specify a virtual
interface number, a key, a distribution algorithm, and a
Media Access Control (MAC) address.
After you create a link aggregation group, you can then
enable ports (interfaces) for link aggregation. The
enabled ports attach to the link aggregation group with
the corresponding key. If the port fails in some way, the
port detaches from the group and traffic is rerouted to
the remaining port or ports.
Any link aggregation configuration done in multiuser mode
does not persist across system reboots. If you want link
aggregation groups configured at boot time, you must
include the appropriate lagconfig and ifconfig commands in
the /etc/inet.local file. See the Network Administration:
Connections manual for an example.
On platforms where I/O bandwidth may be a limiting factor,
you might increase link aggregation performance by distributing
the NICs across different portions of the I/O
infrastructure (for example, different PCI buses).
Fault Tolerance [Toc] [Back]
The link aggregation subsystem monitors the link state of
ports that are enabled for link aggregation. When the link
aggregation subsystem detects that a port's link state is
down, the subsystem detaches the port from its link aggregation
group and redistributes traffic among the remaining
ports.
When the link aggregation subsystem detects that the
port's link state is up, the subsystem reattaches the port
to its link aggregation group. The port then starts handling
part of the traffic load again. The amount of time
it takes to detect a link state change and fail over
depends on the device and driver in use. For DE60x devices
using the ee driver, average failover times are typically
1 to 2 seconds. To achieve faster failover, reduce the
value of the ee subsystem link_check_interval attribute. A
value of 20 (0.2 seconds) typically provides average
failover times of 0.1 to 0.2 seconds.
For DEGPA devices using the alt driver, average failover
times are less than 1 second.
Load Sharing [Toc] [Back]
A link aggregation group performs load sharing of both
inbound and outbound traffic. Distribution of inbound
packets is determined by the server or switch to which the
link aggregation group is connected. When transmitting
packets, the system uses a load distribution algorithm to
determine on which attached port to transmit the packets.
The following load distribution algorithms are supported:
For IP packets, the port is selected based on a hash of
the destination IP address. For non-IP packets, the port
is selected based on a hash of the destination MAC
address. All traffic addressed to a specific destination
IP address uses the same port in the link aggregation
group.
This algorithm can utilize the combined bandwidth
of a link aggregation group in environments where
traffic is destined to a large number of different
IP addresses (for example, a web server).
However, this algorithm might not produce the
expected bandwidth utilization in environments
where the majority of traffic is destined to a single
IP address (for example, a private server-toserver
interconnect). The port is selected based
on a hash of the destination MAC address. All
traffic addressed to a specific destination MAC
address uses the same port in the link aggregation
group.
This algorithm can utilize the combined bandwidth
of a link aggregation group in environments where
traffic is destined to a large number of different
MAC addresses (for example, a server that sends
most of its traffic to clients on the same LAN).
However, this algorithm might not produce the
expected bandwidth utilization in environments
where the majority of traffic is destined to a
small number of MAC addresses (for example, a
server-to-server interconnect, or a server that
sends most of its traffic through a router). For
TCP or UDP packets originating on the system, the
port is selected based on a hash of the source and
destination TCP or UDP port numbers. For all other
packets, including TCP and UDP packets being forwarded
by the system, the Destination IP address
(dstip) algorithm is used. All traffic addressed
to a specific source+destination port pair uses the
same port in the link aggregation group.
This algorithm can utilize the combined bandwidth
of a link aggregation group in environments where
traffic is destined to a single IP or MAC address,
but is exchanged between a number of different TCP
or UDP port number pairs (for example, a server-toserver
interconnect). The port is selected on a
rotating basis.
This algorithm can utilize the combined bandwidth
of a link aggregation group in most environments.
However, this algorithm may result in reordering of
packets belonging to the same flow (for example, a
TCP connection), which in turn may adversely affect
performance.
The following restrictions apply: Supports only DEGPA
(alt), DEGXA (bcm), and DE60x (ee) network interface cards
(NICs). Supports only Ethernet (802.3 CSMA/CD) links.
NetRAIN virtual interfaces cannot be included in link
aggregation groups. Ports must be operating in full
duplex mode. Ports in the same link aggregation group
must operate at the same data rate. Ports in a link
aggregation group must be attached to the same system,
either server-to-server or server-to-switch.
Commands: lagconfig(8)
System Attributes: sys_attrs_ee(5), sys_attrs_lag(5)
Files: inet.local(4)
Technical Overview
Network Administration: Connections
lag(7)
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