Windows Server 2008 Failover Clusters: Networking (Part 1)

The Windows Server 2008 Failover Clustering feature provides high availability for services and applications. To ensure applications and services remain highly available, it is imperative the cluster service running on each node in the cluster function at the highest level possible. Providing redundant and reliable communications connectivity among all the nodes in a cluster plays a large role in ensuring for the smooth functioning of the cluster. Configuring proper communications connectivity within a failover cluster not only provides access to highly available services required by clients but also guarantees the connectivity the cluster requires for its own internal communications needs. The sections that follow discuss Windows Server 2008 Failover Clustering networking features, functionality and recommended processes for the proper configuration and implementation of network connectivity within a cluster.

The following sections provide the information needed to understand failover cluster networking and to properly implement it.

Windows Server 2008 Failover Cluster networking features

Windows Server 2008 Failover Clustering introduces new networking capabilities that are a major shift away from the way things have been done in legacy clusters (Windows 2000\2003 and NT 4.0). Some of these take advantage of the new networking features that are included as part of the operating system and others are a result of feedback that has been received from customers. The new features include:

  • A new cluster network driver architecture
  • The ability to locate cluster nodes on different, routed networks in support of multi-site clusters
  • Support for DHCP assigned IP addresses
  • Improvements to the cluster health monitoring (heartbeat) mechanism
  • Support for IPv6

New cluster network driver architecture

The legacy cluster network driver (clusnet.sys) has been replaced with a new NDIS level driver called the Microsoft Failover Cluster Virtual Adapter (netft.sys). Whereas the legacy cluster network driver was listed as a Non-Plug and Play Driver, the new fault tolerant adapter actually appears as a network adapter when hidden devices are displayed in the Device Manager snap-in (Figure 1).


Figure 1: Device Manger Snap-in

The driver information is shown in Figure 2.


Figure 2: Microsoft Failover Cluster Virtual Adapter driver

The cluster adapter is also listed in the output of an ipconfig /all command on each node (Figure 3).


Figure 3: Microsoft Failover Cluster Virtual Adapter configuration information

The Failover Cluster Virtual Adapter is assigned a Media Access Control (MAC) address that is based on the MAC address of the first enumerated (by NDIS) physical NIC in the cluster node (Figure 4) and uses an APIPA (Automatic Private Internet Protocol Addressing) address.


Figure 4: Microsoft Failover Cluster Virtual Adapter MAC address

The goal of the new driver model is to sustain TCP/IP connectivity between two or more systems despite the failure of any component in the network path. This goal can be achieved provided at least one alternate physical path is available. In other words, a network component failure (NIC, router, switch, hub, etc…) should not cause inter-node cluster communications to break down, and communication should continue making progress in a timely manner (i.e. it may have a slower response but it will still exist) as long as an alternate physical route (link) is still available. If cluster communications cannot proceed on one network, the switchover to another cluster-enabled network is automatic. This is one of the primary reasons that each cluster node must have multiple network adapters available to support cluster communications and each one should be connected to different switches.

The failover cluster virtual adapter is implemented as an NDIS miniport adapter that pairs an internally constructed virtual route with each network found in a cluster node. The physical network adapters are exposed at the IP layer on each node. The NETFT driver transfers packets (cluster communications) on the virtual adapter by tunneling through the best available route in its internal routing table (Figure 5).


Figure 5: NetFT traffic flow diagram

Here is an example to illustrate this concept. A 2-Node cluster is connected to three networks that each node has in common (Public, Cluster and iSCSI). The output of an ipconfig /all command from one of the nodes is shown in Figure 6.


Figure 6: Example Cluster Node IP configuration

Note: Do not be concerned with the name ‘Microsoft Virtual Machine Bus Network Adapter’ as these examples were derived from cluster nodes running as Guests in Hyper-V.

The Microsoft Failover Cluster Virtual Adapter configuration information for each node is shown in Figure 7. Keep in mind; the default port for cluster communication is still TCP\UDP: 3343.


Figure 7: Node Failover Cluster Virtual Adapter configuration information

When the cluster service starts, and a node either Forms or Joins a cluster, NETFT, along with other components, is responsible for determining the node’s network configuration and connectivity with other nodes in the cluster. One of the first actions is establishing connectivity with the Microsoft Failover Cluster Virtual Adapter on all nodes in the cluster. Figure 8 shows an example of this in the cluster log.


Figure 8: Microsoft Failover Cluster Virtual Adapter information exchange

Note: You can see in Figure 8 that the endpoint pairs consist of both IPv4 and IPv6 addresses. The NETFT adapter prefers to use IPv6 and therefore will choose the IPv6 addresses for each end point to use.

As the cluster service startup continues, and the node either Forms or Joins a cluster, routing information is added to NETFT. Using the three networks mentioned previously, Figure 9 shows each route being added to a cluster.


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Figure 9: Routes discovered and added to NETFT

Each ‘real’ route is added to the ‘virtual’ routes associated with the virtual adapter (NETFT). Again, note the preference for NETFT to use IPv6 as the protocol of choice.

The capability to place cluster nodes on different, routed networks in support of Multi-Site Clusters

Beginning with Windows Server 2008 failover clustering, individual cluster nodes can be located on separate, routed networks. This requires that resources that depend on IP Address resources (i.e. Network Name resources), implement an OR logic since it is unlikely that every cluster node will have a direct local connection to every network the cluster is aware of. This facilitates IP Address and hence Network Name resources coming online when services\applications failover to remote nodes. Here is an example (Figure 10) of the dependencies for the cluster name on a machine connected to two different networks.


Figure 10: Cluster Network Name resource with an OR dependency

All IP addresses associated with a Network Name resource, which come online, will be dynamically registered in DNS (if configured for dynamic updates). This is the default behavior. If the preferred behavior is to register all IP addresses that a Network Name depends on, then a private property of the Network Name resource must be modified. This private property is called RegisterAllProvidersIP (Figure 11). If this property is set equal to 1, all IP addresses will be registered in DNS and the DNS server will return the list of IP addresses associated with the A-Record to the client.


Figure 11: Parameters for a Network Name resource

Since cluster nodes can be located on different, routed networks, and the communication mechanisms have been changed to use reliable session protocols implemented over UDP (unicast), the networking requirements for Geographically Dispersed (Multi-Site) Clusters have changed. In previous versions of Microsoft clustering, all cluster nodes had to be located on the same network. This required ‘stretched’ VLANs be implemented when configuring multi-site clusters. Beginning with Windows Server 2008, this requirement is no longer necessary in all scenarios.

Support for DHCP assigned IP addresses

Beginning with Windows Server 2008 Failover Clustering, cluster IP address resources can obtain their addressing from DHCP servers as well as via static entries. If the cluster nodes themselves have at least one NIC that is configured to obtain an IP addresses from a DHCP server, then the default behavior will be to obtain an IP address automatically for all cluster IP address resources. The new ‘wizard-based’ processes in Failover Clustering understand the network configuration and will only ask for static addressing information when required. If the cluster node has statically assigned IP addresses, the cluster IP address resources will have to be configured with static IP addresses as well. Cluster IP address resource IP assignment follows the configuration of the physical node and each specific interface on the node. Even if the nodes are configured to obtain their IP addresses from a DHCP server, individual IP address resources can be changed to static addresses (Figure 12).


Figure 12: Changing DHCP assigned to Static IP address

Improvements to the cluster ‘heartbeat’ mechanism

The cluster ‘heartbeat’, or health checking mechanism, has changed in Windows Server 2008. While still using port 3343, it is no longer a broadcast communication. It is now unicast in nature and uses a Request-Reply type process. This provides for higher security and more reliable packet accountability. Using the Microsoft Network Monitor protocol analyzer to capture communications between nodes in a cluster, the ‘heartbeat’ mechanism can be seen (Figure 13).


Figure 13: Network Monitor capture

A typical frame is shown in Figure 14.


Figure 14: Heartbeat frame from a Network Monitor capture

There are properties of the cluster that address the heartbeat mechanism; these include SameSubnetDelay, CrossSubnetDelay, SameSubnetThreshold, and CrossSubnetThreshold (Figure 16).


Figure 16: Properties affecting the cluster heartbeat mechanism

The default configuration (shown here) means the cluster service will wait 5.0 seconds before considering a cluster node to be unreachable and have to regroup to update the view of the cluster (One heartbeat sent every second for five seconds). The limits on these settings are shown in Figure 17. Make changes to the appropriate settings depending on the scenario. The CrossSubnetDelay and CrossSubnetThreshold settings are typically used in multi-site scenarios where WAN links may exhibit higher than normal latency.


Figure 17: Heartbeat Configuration Settings

These settings allow for the heartbeat mechanism to be more ‘tolerant’ of networking delays. Modifying these settings, while a worthwhile test as part of a troubleshooting procedure (discussed later), should not be used as a substitute for identifying and correcting network connection delays.

Support for IPv6

Since the Windows Server 2008 OS will be supporting IPv6, the cluster service needs to support this functionality as well. This includes being able to support IPv6 IP Address resources and IPv4 IP Address resources either alone or in combination in a cluster. Clustering also supports IPv6 Tunnel Addresses. As previously noted, intra-node cluster communications by default use IPv6. For more information on IPv6, please review the following:

Microsoft Internet Protocol Version 6

In the next segment, I will discuss Implementing networks in support of Failover Clusters (Part 2) . See ya then.

Chuck Timon
Senior Support Escalation Engineer
Microsoft Enterprise Platforms Support