Routing and WAN connections
BAT54-Rail/F..
Release
7.54
06/08
11.17
The rapid spanning tree protocol
457
11.17.1Classic and rapid spanning tree
The early version of the spanning-tree protocol compliant with IEEE 802.1D,
here referred to as classic spanning tree, had the problem that changes to
topology after a connection failure were implemented very slowly: Depending
on the complexity of the network, the classic spanning tree takes between 20
seconds and a minute to establish new routes. For many network services a
failure of this length of time is unacceptable.
The spanning tree protocol was improved and published as the "Rapid Span-
ning Tree Protocol" (RSTP), initially as the IEE 802.1t/w standard and later
as a part of the newly published IEEE 802.1D. Even though the classic span-
ning tree protocol was thus withdrawn, it continues to be supported by LCOS.
11.17.2Improvements from rapid spanning tree
As mentioned above, the primary aim of RSTP is to accelerate the activation
of network paths once an active connection has failed. RSTP achieves this
by dispensing with the states "blocking" and "listening" to reduce the time re-
quired to update the network paths to just a few seconds. In case of a net-
work path failure, not all of the links are blocked until the new topology has
been calculated; instead, only the failed connections are unavailable for use.
RSTP also enables the administrator to configure information on network to-
pology.
D A bridge port can be defined as an edge port. An edge port is the only
bridge port leading to the connected LAN segment, i.e. no other bridges
are connected to the LAN segment, but workstations or servers only, for
example. As these ports cannot lead to loops, they change immediately
into the forwarding state without waiting for the network topology to be de-
termined. However, RSTP continues to monitor these ports. Should BP-
DUs be unexpectedly received at an edge port due to another bridge
being connected to the LAN, the ports automatically return to their normal
state.
D A bridge port can also operate as a point-to-point link. In this case the port
is directly connected with an additional bridge. Since no additional sta-
tions can occur between the two bridges, the switch into the forwarding
state can take place faster.
In the ideal case, RSTP immediately resorts to familiar alternative network
paths in case of connection failure.
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