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lag(7)
NAME
lag - Link aggregation (also called trunking) introductory information
DESCRIPTION
Link aggregation, or trunking, enables administrators to combine two or
more physical Ethernet Network Interface Cards (NICs) and create a single
virtual link. (Upper-layer 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
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
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
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:
Destination IP Address (dstip)
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-to-server
interconnect).
Destination MAC address (dstmac)
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).
Transport Port number (port)
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-to-server interconnect).
Round Robin (roundrobin)
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.
RESTRICTIONS
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.
SEE ALSO
Commands: lagconfig(8)
System Attributes: sys_attrs_ee(5), sys_attrs_lag(5)
Files: inet.local(4)
Technical Overview
Network Administration: Connections
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Index for
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