DES-3010F/DES-3010FL/DES-3010G/DES-3016/DES-3018/DES-3026 Fast Ethernet Switch Manual
101
NOTE:
To use IGMP Access Control, the user must first configure the multicast filtering
mode so that it can filter unregistered groups.
Spanning Tree
This Switch supports two versions of the Spanning Tree Protocol; 802.1D STP and 802.1w Rapid STP.
802.1D STP will be familiar to most networking professionals. However, since 802.1w RSTP has been
recently introduced to D-Link managed Ethernet switches, a brief introduction to the technology is provided
below followed by a description of how to set up 802.1D STP and 802.1w RSTP.
802.1w Rapid Spanning Tree
The Switch implements two versions of the Spanning Tree Protocol, the Rapid Spanning Tree Protocol
(RSTP) as defined by the IEEE 802.1w specification and a version compatible with the IEEE 802.1D STP.
RSTP can operate with legacy equipment implementing IEEE 802.1D, however the advantages of using
RSTP will be lost.
The IEEE 802.1w Rapid Spanning Tree Protocol (RSTP) evolved from the 802.1D STP standard. RSTP was
developed in order to overcome some limitations of STP that impede the function of some recent switching
innovations, in particular, certain Layer 3 functions that are increasingly handled by Ethernet switches. The
basic function and much of the terminology is the same as STP. Most of the settings configured for STP are
also used for RSTP. This section introduces some new Spanning Tree concepts and illustrates the main
differences between the two protocols.
Port Transition States
An essential difference between the two protocols is in the way ports transition to a forwarding state and in
the way this transition relates to the role of the port (forwarding or not forwarding) in the topology. RSTP
combines the transition states disabled, blocking and listening used in 802.1D and creates a single state
Discarding. In either case, ports do not forward packets. In the STP port transition states disabled, blocking
or listening or in the RSTP port state discarding, there is no functional difference, the port is not active in the
network topology. Table 6-2 below compares how the two protocols differ regarding the port state
transition.
All two protocols calculate a stable topology in the same way. Every segment will have a single path to the
root bridge. All bridges listen for BPDU packets. However, BPDU packets are sent more frequently - with
every Hello packet. BPDU packets are sent even if a BPDU packet was not received. Therefore, each link
between bridges is sensitive to the status of the link. Ultimately this difference results in faster detection of
failed links, and thus faster topology adjustment. A drawback of 802.1D is this absence of immediate
feedback from adjacent bridges.
802.1w RSTP
802.1D STP
Forwarding
Learning
Discarding Disabled
No
No
Discarding Blocking No
No
Discarding Listening No
No
Learning Learning No
Yes
Forwarding Forwarding Yes
Yes
Table 6- 1. Comparing Port States
RSTP is capable of a more rapid transition to a forwarding state - it no longer relies on timer configurations -
RSTP compliant bridges are sensitive to feedback from other RSTP compliant bridge links. Ports do not
need to wait for the topology to stabilize before transitioning to a forwarding state. In order to allow this
rapid transition, the protocol introduces two new variables: the edge port and the point-to-point (P2P) port.
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