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Configuring LLT and GAB

VCS uses LLT and GAB to replace the functions of TCP/IP for VCS private network communications. LLT and GAB provide the performance and reliability required by VCS for these and other functions.

LLT and GAB must be configured as described in the following sections.

Configuring low latency transport (LLT)

To configure LLT, set up two files: /etc/llthosts and /etc/llttab on each node in the cluster.

Setting up /etc/llthosts

The file llthosts(4) is a database, containing one entry per system that links the LLT system ID (in the first column) with the LLT host name. You must create an identical file on each node in the cluster.

Use vi, or another editor, to create the file /etc/llthosts that contains entries that resemble:

0 north

1 south

Setting Up /etc/llttab

The /etc/llttab file must specify the system's ID number (or, its node name), and the network links that correspond to the system. In addition, the file can contain other directives. Refer also to the sample llttab file in /opt/VRTSllt.

See LLT directives.

Using vi or another editor, create the file /etc/llttab that contains entries that resemble:

set-node north

set-cluster 2

link lan1 /dev/lan1 - ether - -

link lan2 /dev/lan2 - ether - -

The first line must identify the system on which the file exists. In the example above, the value for set-node could be north, 0, or the file name /etc/nodename, provided the file contains the name of the system (north in this example). The next two lines, beginning with the link command, identify the two private network cards that the LLT protocol uses. The order of directives must be the same as in the sample file /opt/VRTSllt/llttab.

LLT directives

For more information about LLT directives, refer to the llttab(4) manual page.

LLT directives

Directive	Description
set-node	Assigns the system ID or symbolic name. The system ID number must be unique for each system in the cluster, and must be in the range 0-31. The symbolic name corresponds to the system ID listed in /etc/llthosts file.  Note that LLT fails to operate if any systems share the same ID.
link	Attaches LLT to a network interface. At least one link is required, and up to eight are supported. The first argument to link is a user-defined tag shown in the lltstat(1M) output to identify the link. It may also be used in llttab to set optional static MAC addresses.  The second argument to link is the device name of the network interface. Its format is device_name:device_instance_number. The remaining four arguments to link are defaults; these arguments should be modified only in advanced configurations. There should be one link directive for each network interface. LLT uses an unregistered Ethernet SAP of 0xCAFE. If the SAP is unacceptable, refer to the llttab(4) manual page for information on how to customize SAP. Note that IP addresses do not need to be assigned to the network device; LLT does not use IP addresses.
set-cluster	Assigns a unique cluster number. Use this directive when more than one cluster is configured on the same physical network connection. LLT uses a default cluster number of zero.
link-lowpri	Use this directive in place of link for public network interfaces. This directive prevents VCS communication on the public network until the network is the last link, and reduces the rate of heartbeat broadcasts. Note that LLT distributes network traffic evenly across all available network connections and, in addition to enabling VCS communication, broadcasts heartbeats to monitor each network connection.

For more information about LLT directives, refer to the llttab(4) manual page.

Additional considerations for LLT

You must attach each network interface that is configured for LLT to a separate and distinct physical network.

Configuring group membership and atomic broadcast (GAB)

To configure GAB, use vi or another editor to set up an /etc/gabtab configuration file on each node in the cluster. The following example shows a simple /etc/gabtab file:

/sbin/gabconfig -c -nN

Where the -c option configures the driver for use and -nN specifies that the cluster is not formed until at least N systems are ready to form the cluster. By default, N is the number of systems in the cluster.

  Note   Symantec does not recommend the use of the -c -x option for /sbin/gabconfig. Using -c -x dramatically increases configuration time for the Gigabit Ethernet controller and can lead to a split-brain condition.