LLT Over UDP

VCS 4.1 provides the option of using LLT over the UDP (User Datagram Protocol) layer for clusters using wide-area networks and routers. UDP makes LLT packets routable and thus able to span longer distances more economically.

When to Use LLT Over UDP

Use LLT over UDP when:

• LLT must be used over WANs
• When hardware, such as blade servers, do not support LLT over Ethernet

Performance Considerations

Because LLT over UDP is slower that LLT over Ethernet, LLT over UDP should only be used when the hardware configuration makes it necessary.

Configuring LLT Over UDP

The following is a checklist for configuring LLT over UDP. Examples are provided in the sections that follow.

  Make sure that each NIC has an IP address configured before configuring LLT. Each link must be in a different subnet. See the examples in the following sections.

  Make sure that each link has a unique non-well known UDP port. See Selecting UDP Ports.

  Make sure to configure the netmask and broadcast address when nodes reside on different subnets. See Configuring LLT on Subnets.

  Set the broadcast address correctly for direct-attached (non-routed) links.

  For links that cross an IP router, disable broadcast features and specify the IP address of each link manually in the /etc/llttab file. See Sample Configuration: Links Crossing IP Routers.

The link Command in the /etc/llttab File

The following table describes the fields of the link command shown in the /etc/llttab file examples that follow; see Sample Configuration: Direct-Attached Links, and Sample Configuration: Links Crossing IP Routers. Note that some of these fields differ from the command for standard LLT links.

The set-addr Command in the /etc/llttab File

The set-addr command in the /etc/llttab file is required when the broadcast feature of LLT is disabled, such as when LLT must cross IP routers. The following table describes the fields of the set-addr command; see Sample Configuration: Links Crossing IP Routers.

Selecting UDP Ports

When selecting a UDP port, select an available 16-bit integer from the range described below.

• Use available ports (that is, ports that are not in use)] in the private range 49152 to 65535
• Do not use:
• Ports from the range of well-known ports, 0 to 1023
• Ports from the range of registered ports, 1024 to 49151

To check which ports are defined as defaults for a node, examine the file /etc/services. You should also use the netstat command to list the ports currently in use. For example:

Look in the UDP section of the output; UDP ports listed under Local Address are already in use. If a port is listed in the /etc/services file, its associated name is displayed rather than the port number in the output of netstat -a.

Configuring LLT on Subnets

You need to make sure to properly configure the netmask and broadcast address when nodes reside on different subnets.

Configuring the Netmask

If you have nodes on different subnets, set the netmask so that the nodes can access the subnets in use.

For example:

• For lan2

IP address=192.168.30.1, Broadcast address=192.168.30.255, Netmask=255.255.255.0
• For lan3

IP address=192.168.31.1, Broadcast address=192.168.31.255, Netmask=Mask:255.255.255.0

Configuring the Broadcast Address

If you have nodes on different subnets, set the broadcast address in /etc/llttab depending on the subnet that the links are on.

An example of a typical /etc/llttab file when nodes are on different subnets. Note the explicitly set broadcast address for each link.

Sample Configuration: Direct-Attached Links

The following illustration depicts a typical configuration of direct-attached links employing LLT over UDP.

Click the thumbnail above to view full-sized image.

The configuration represented by the following /etc/llttab file for Node 0 has directly attached crossover links or links connected through a hub or switch. These links do not cross routers.

Because LLT broadcasts requests to peer nodes to discover their addresses, the addresses of peer nodes do not need to be specified in the /etc/llttab file using the set-addr command. For direct attached links, you do need to set the broadcast address of the links in the /etc/llttab file. Verify that the IP addresses and broadcast addresses are set correctly by using the ifconfig -a command.

The file for Node 1 would resemble:

Sample Configuration: Links Crossing IP Routers

The following illustration depicts a typical configuration of links crossing an IP router employing LLT over UDP. The illustration shows just two nodes of a four-node cluster.

Click the thumbnail above to view full-sized image.

The configuration represented by the following /etc/llttab file for Node 1 has links crossing IP routers. Notice that IP addresses are shown for each link on each peer node. The broadcast features are disabled because LLT is unable to broadcast requests for addresses across routers. Since broadcasts are disabled, the broadcast address does not need to be set in the in the link command of the /etc/llttab file.

The /etc/llttab file on Node 0 would resemble: