Microsoft Knowledge Base Article 892422 contains the following summary and introduction:

SUMMARY

This article discusses communications behavior that occurs between nodes that are in a cluster. Additionally, this article discusses why event IDs 1123 and 1122 may be logged in the System log of Microsoft Windows Server 2003-based and Microsoft Windows 2000-based server clusters.

INTRODUCTION Cluster nodes periodically communicate with each other on a network that is designated for cluster use. Each node has access to the following communication types:

Internal cluster communications only
This type is also known as a private network.
All communications
This type is also known as a mixed network. A mixed network is a combination of a private network and a public network.

Note A public network is designated for client network access. A public network is not used by the Cluster service for intra-node communications. However, the public network will be monitored by the cluster network driver to make sure it is available for all cluster nodes.

For intra-node communications, cluster nodes communicate over User Datagram Protocol (UDP) port 3343. Each node in the cluster periodically exchanges sequenced, unicast UDP datagrams with every other node in the cluster. The purpose of this exchange is to determine whether all nodes are running correctly and to monitor the health of network links.

The cluster network driver (Clusnet.sys) manages cluster communications. The cluster network driver performs the following functions:

Provides a uniform interface for cluster node communications that are independent of the network infrastructure.
Monitors the status of all communication paths in the cluster.
Routes intra-cluster messages over the optimal paths.
Detects node failures by using periodic messages that are known as "heartbeats".
Detects failures in network and TCP/IP communications.

Event ID 1123 and event ID 1122 may be logged consecutively to the System log in your cluster. Frequently, these events indicate temporary interruptions in intra-cluster communication and can be ignored.

Event messages that are similar to the following are logged:

Event ID: 1123
Source: ClusSvc
Description:
The node lost communication with cluster node ComputerName on network 'Public Network'.

Event ID: 1123
Source: ClusSvc
Description:
The node lost communication with cluster node ComputerName on network 'Private Network'.

Event ID: 1122
Source: ClusSvc
Description:
The node (re)established communication with cluster node ComputerName on network 'Public Network'.

Event ID: 1122
Source: ClusSvc
Description:
The node (re)established communication with cluster node ComputerName on network 'Private Network'.