Ghost compatibility with IGMP

Products

• Ghost Solution Suite

Issue/Introduction

You want to use Internet Group Management Protocol (IGMP)-capable switches and IGMP-capable routers to reduce network traffic. Do Ghost Multicast and Ghost Enterprise Console work with IGMP?

Environment

GSS 3.x

Resolution

Ghost works with IGMP version 2 and requires IGMP-enabled routers or IGMP-enabled switches for multicasting.

When a network does not include IGMP-enabled routers or switches, the network does not support multicasting. In this situation, Ghost versions prior to Symantec Ghost 7.5 will either send the image data to all computers on the network, which floods the network with packets, or send no data through the routers and switches.

In Symantec Ghost 7.5 and later, Ghost provides alternatives to multicasting. Ghost includes the option Data Transfer Mode for selecting the method for sending packets over a network: multicast, directed broadcast, or unicast.

More information
IGMP is a type of control packet designed to prevent multicast traffic from flooding the network: Packets are sent only to the computers that have requested it.

For instance, when Ghost joins a multicast session it sends out an IGMP control packet to let the network know that the client computer wants to receive all multicast traffic that is sent to a particular address. This constitutes joining a Ghost Multicast Session. On an IGMP-enabled network that uses both IGMP-capable routers and IGMP-capable switches, the router detects the packet and notifies the IGMP-capable switch to add that client computer (defined by the MAC address) to the multicast session. In the Cisco documentation referred to later, this is called "Joining a Multicast Group."

Similarly, when the session has ended Ghost will send out another IGMP packet to leave the Ghost Multicast Session. In the Cisco documentation referred to later, this is called "Leaving the Multicast Group."

This process decreases network traffic in two ways:

1. Switches send multicast traffic only to those ports that contain client computers that have joined the group (in Ghost's case, the "group" is the Ghost Multicast Session). The port can have one or many client computers (by means of a hub).
2. Routers pass multicast traffic only to subnets that include clients that have joined the group.

Each router or switch filters out multicast traffic that is not needed by any of the computers on that branch, resulting in less network traffic on that branch. Although the router or switch might be flooded with packets, the router or switch prevents those packets from flooding the entire network.

Also see Cisco documentation on their Catalyst Switches. See the sections "How CGMP Works," "Joining a Multicast Group," and "Leaving a Multicast Group" on the Cisco web site.