Specifications

HP Virtual Connect for Cisco Network Administrators (version 1.2x) page 39
PortFast
The Spanning Tree PortFast feature was designed for Cisco switch ports connected to edge devices,
like server NIC ports. This feature allows a Cisco switch port to bypass the ‘listening’ and ‘learning’
stages of spanning tree and quickly transition to the ‘forwarding’ stage. By enabling this feature,
edge devices are allowed to immediately begin communication on the network instead of having
to wait on Spanning Tree to determine if it needs to block the port to prevent a loop – a process that
can take 30+ seconds with default Spanning Tree timers. Since edge devices do not present a loop
on the network, Spanning Tree is not needed to prevent loops and can be effectively bypassed by
using the PortFast feature. The benefit of this feature is that server NIC ports can immediately
communicate on the network when plugged in rather than timing out for 30 or more seconds. This is
especially useful for time sensitive protocols such as PXE and DHCP.
Since VC uplinks operate on the network as an edge device (like teamed server NICs), Spanning
Tree is not needed on the directly connected Cisco switch ports. This means that PortFast can be
enabled on the Cisco switch ports directly connected to VC uplinks.
Note:
The interface command to enable PortFast on a Cisco access port is “spanning-tree portfast”.
The interface command to enable PortFast on a Cisco trunk port is “spanning-tree portfast trunk”.
BPDU Guard
BPDU Guard is a safety feature for Cisco switch ports that have PortFast enabled. Enabling BPDU
Guard allows the switch to monitor for the reception of BPDU frames (spanning tree configuration
frames) on the port configured for PortFast. When a BPDU is received on a switch port with PortFast
and BPDU Guard enabled, BPDU Guard will cause the switch port to err-disable (shutdown). Since
ports with PortFast enabled should never be connected to another switch (which transmits BPDUs),
BPDU Guard protects against PortFast-enabled ports from being connected to other switches. This
prevents loops caused by bypassing Spanning Tree on that port and this prevents any device
connected to that port from becoming the root bridge.
Since VC behaves as an edge device on the network and since VC does not participate in the
data center spanning tree (i.e. doesn’t transmit BPDUs on VC uplinks), BPDU Guard can be used, if
desired, on Cisco switch ports connected to VC uplinks.
Note:
The interface command to enable BPDU Guard on a Cisco port is “spanning-tree bpduguard
enable”.
Unidirectional Link Detection (UDLD)
UDLD is a Cisco proprietary feature that only operates between two directly connected Cisco
devices. The purpose of UDLD is to detect unidirectional links (UniDirectional Link Detection) in
situations where spanning tree is being used to detect and prevent loops. If a unidirectional link
goes undetected, spanning tree won’t properly detect the loop and won’t block the port to
prevent a loop on the network. Since spanning tree is not used between VC uplinks and external
Cisco switches, a spanning tree loop caused by a unidirectional link is not possible. Therefore, UDLD
is not required nor supported on Virtual Connect uplinks.