Deploying the Dell PowerConnect 8100 with the Cisco Catalyst
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RSTP
(Based on IEEE 802.1w) - Rapid STP. A standards -based protocol that is considerably
faster at network convergence (after a topology change) than its STP predecessor in
most network implementations. It is the default for the Dell PowerConnect 8100,
and is compatible with STP.
MST
(Based on IEEE 802.1s) – Multiple Spanning Tree protocol or MSTP. Allows for
multiple instances of spanning tree across a network. Each inst ance contains one or
more VLANs. Enables load balancing of traffic through the network while minimizing
CPU cycles.
PVST+
Per VLAN spanning Tree. A Cisco proprietary protocol similar to STP that allows
each VLAN to run its own instance of Spanni ng Tree. It is available on the Cisco
Catalyst 6500.
RPVST+
Rapid PVST+. A Cisco proprietary protocol that is considerably faster at network
convergence (after a topology change) than its PVST+ predecessor in most network
implementations. It is the default on the Cisco Catalyst 6500.
CST
Common Spanning Tree. A single spanning tree instance for an entire network
regardless of the number of VLANs on that network.
BPDU
Bridge Protocol Data Unit. Packets that are exchanged between network devices to
help detect loops in a network.
802.1d
IEEE standard that allows spanning tree ports to be in one of the following states:
listen, learn, forward, block, or disabled.
802.1w
IEEE standard that allows spanning tree ports to be in one of the following states:
learn, forward, or discard. Discard replaces listen, block, and disable states. A
great improvement in convergence speed over 802.1d.
801.1s
IEEE standard that allows for multiple instances of spanning tree across a physical
bridged network.
Attempting to recover from a failed network device is a common difficulty that arises on a Cisco
Catalyst network using RPVST+, when standards-based edge devices are attached in a redundant loop
topology. RPVST+ ports from the Cisco Catalyst switch that are connected to the Dell PowerConnect
8100 no longer receive BPDUs from VLANs not in the CST. This causes a delay in network activity until
the forward-delay timer expires.
To minimize downtime when using Cisco proprietary protocols (PVST+, RPVST+) together with standard
protocols (STP, RSTP) on the Dell PowerConnect switches, additional instruction on the network is
required. Below are two common methods to get the job done.
Method 1: Portfast
Ports on the Cisco Catalyst switch that are directly connected to the Dell PowerConnect 8100 can be
forced into forwarding state after a network change by using the portfast option. The rapid spanning
tree protocols quickly blocks any loops detected in the network during convergence. The block occurs
at a location away from the Cisco core network. In t his scenario, the link between the Dell
PowerConnect switches is blocked. This is the most recommended method.