Spanning Tree Basics
15-14 Configuring Spanning Tree
RSTP Operation
RSTP optimizes convergence by significantly reducing the time to reconfigure the network’s active
topology when physical topology or configuration parameter changes occur. RSTP provides rapid
connectivity following the failure of a switching device, switch port, or the addition of a switch
into the network.
A new root port may forward as soon as any recent root ports are put into blocking.
A designated port may forward with the exchange of two BPDUs in rapid succession. The
designated port presents new BPDU information with a proposal request. The attached port
processes the BPDU and may respond immediately with an agreement. Upon reception of that
agreement BPDU, the designated port may move to forwarding. Another feature of RSTP is that
designated ports transmit periodic BPDUs regardless of reception of BPDUs at the root port. This
insulates the network from jitter in receiving BPDUs, particularly at the edge.
Important STP timers are max age, hello time, and forward delay. The default values for the timers
are:
• Hello time – 2 seconds
• Forward delay – 15 seconds
• Max age – 20 seconds
The operational values from these timers are derived from the root bridge. The current IEEE
standard for Spanning Tree fixes hello time at 2 seconds. The Enterasys switches covered in this
document do not enforce this restriction to allow existing configurations to remain compatible. It
is not recommended that a value other than 2 seconds be used. Other values may not interact well
with other non-variable protocol times such as edgeDelayWhile or mDelayWhile. The max age
timer may be adjusted to change the network diameter. Take care to consider that failure in the
network may cause the topology to “unravel” causing the diameter to become larger than
anticipated. An insufficient value could cause devices near or at the edge of the network to
become unreachable. For example, in a ring topology of 10 bridges, no bridge is more than 5 hops
from the root. A max age that accounts for 6 hops would be sufficient. A failure of ports
immediately interconnecting a bridge with the root would break the ring topology and change the
furthest hop from the root from 5 to 9. Any bridges beyond the configured network diameter of 6
would cause the Spanning Tree topology not to converge.
MSTP Operation
MSTP makes it possible for VLAN switching devices to use multiple Spanning Trees, allowing
traffic belonging to different VLANs to flow over potentially different paths within the LAN. It
builds upon the advancements of RSTP with its decreased time for network re-spans. MSTP’s
principle objective is to increase bandwidth utilization by allowing:
• Frames assigned to different VLANs to follow different data routes
• Ports to block for some Spanning Trees and forward for others
• Every inter-switch link in the topology to be forwarding for at least one Spanning Tree
MSTP is the default Spanning Tree mode on all Enterasys switch devices.
Common and Internal Spanning Tree (CIST)
MSTP uses all Spanning Tree region information to create a single Common and Internal
Spanning Tree (CIST) that represents the connectivity of the entire network. This is equivalent to
the single Spanning Tree used for STP and RSTP.
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