The Ethernet switching mode determines how the fabric interconnect behaves as a switching device between the servers and the network. The fabric interconnect operates in either of the following Ethernet switching modes:

End-host mode

Switching mode

In end-host mode, the Cisco UCS presents an end host to an external Ethernet network. The external LAN sees the Cisco UCS Fabric Interconnect as an end host with multiple adapters (see Figure 12-29).

Figure 12-29 UCS FI End-Host Mode Ethernet

End-host mode allows the fabric interconnect to act as an end host to the network, representing all servers (hosts) connected to it through vNICs. This behavior is achieved by pinning (either dynamically pinning or hard pinning) vNICs to uplink ports, which provides redundancy to the network, and makes the uplink ports appear as server ports to the rest of the fabric.

In end-host mode, the fabric interconnect does not run the Spanning Tree Protocol (STP), but it avoids loops by denying uplink ports from forwarding traffic to each other and by denying egress server traffic on more than one uplink port at a time. End-host mode is the default Ethernet switching mode and should be used if either of the following is used upstream:

Layer 2 switching for Layer 2 aggregation

vPC or VSS aggregation layer

Note

When you enable end-host mode, if a vNIC is hard pinned to an uplink port and this uplink port goes down, the system cannot repin the vNIC, and the vNIC remains down.

Server links (vNICs on the blades) are associated with a single uplink port, which may also be a port channel. This association process is called pinning, and the selected external interface is called a pinned uplink port. The pinning process can be statically configured when the vNIC is defined or dynamically configured by the system. In end-host mode, pinning is required for traffic flow to a server.

Static pinning is performed by defining a pin group and associating the pin group with a vNIC. Static pinning should be used in scenarios in which a deterministic path is required. When the target (as shown on Figure 12-30) on Fabric Interconnect A goes down, the corresponding failover mechanism of the vNIC goes into effect, and traffic is redirected to the target port on Fabric Interconnect B.

Figure 12-30 UCS LAN Pinning Group Configuration

If the pinning is not static, the vNIC is pinned to an operational uplink port on the same fabric interconnect, and the vNIC failover mechanisms are not invoked until all uplink ports on that fabric interconnect fail. In the absence of Spanning Tree Protocol, the fabric interconnect uses various mechanisms for loop prevention while preserving an active-active topology.

In the Cisco UCS, two types of Ethernet traffic paths will have different characteristics—Unicast and Multicast/Broadcast.

Unicast traffic paths in the Cisco UCS are shown in Figure 12-31. Characteristics of unicast traffic in the Cisco UCS include the following:

Each server link is pinned to exactly one uplink port (or port channel).

Server-to-server Layer 2 traffic is locally switched. Server-to-network traffic goes out on its pinned uplink port.