To understand the switching mode behavior, you need to understand the fabric-based failover feature for Ethernet in the Cisco UCS. Each adapter in the Cisco UCS is a dual-port adapter that connects to both fabrics (A and B). The two fabrics in the Cisco UCS provide failover protection in the event of planned or unplanned component downtime in one of the fabrics. Typically, host software—such as NIC teaming for Ethernet and PowerPath or multipath I/O (MPIO) for Fibre Channel—provides failover across the two fabrics (see Figure 12-28).

Figure 12-28 UCS Fabric Traffic Failover Example

A vNIC in the Cisco UCS is a host-presented PCI device that is centrally managed by the Cisco UCS Manager. The fabric-based failover feature, which you enable by selecting the high-availability vNIC option in the service profile definition, allows network interface virtualization (NIV)-capable adapters (Cisco virtual interface card, or VIC) and the fabric interconnects to provide active-standby failover for Ethernet vNICs without any NIC-teaming software on the host.

For unicast traffic failover, the fabric interconnect in the new path sends gratuitous Address Resolution Protocols (gARPs). This process refreshes the forwarding tables on the upstream switches.

For multicast traffic, the new active fabric interconnect sends an Internet Group Management Protocol (IGMP) Global Leave message to the upstream multicast router. The upstream multicast router responds by sending an IGMP query that is flooded to all vNICs. The host OS responds to these IGMP queries by rejoining all relevant multicast groups. This process forces the hosts to refresh the multicast state in the network in a timely manner.

Cisco UCS fabric failover is an important feature because it reduces the complexity of defining NIC teaming software for failover on the host. It does this transparently in the fabric based on the network property that is defined in the service profile.