A1- 6
Another problem with STP is that it requires that all links must pass
through a lengthy period of address learning, even if the link is a
point-to-point link to a device such as an ordinary PC [36].
As an alternative, Rapid Spanning Tree Protocol (RSTP
IEEE802.3W) was suggested to solve STP's problem with failover time by
a number of means [31]. Whereas STP switches store only the best path to
the root switch, RSTP switches store all potential paths. When links fail,
RSTP has pre-calculated routes to fall back upon. Additionally, unlike STP
switches, an RSTP switch will respond to another switch that advertises an
inferior or incorrect route to the root switch. This information allows the
switch with incorrect information to be rapidly trained [ 34 ].
RSTP solves STP's problem with lengthy recovery time by
introducing a procedure called proposing-agreeing. Proposing and agreeing
works after a better path to the root is restored by "shuffling" the restored
part of the network one hop at a time towards the network edge. This
method also enables the network to come up quickly at inception [ 36 ].
RSTP also introduces a method for quickly bringing up ports at the edge of
the network, while still protecting them against loops [ 37 ].
A 1 .4 Full-Duplex Ethernet
One of the limitations of old Ethernet standard like 1 0Base5 and
l 0 Base2 is that communication is half-duplex ( 10 Base-T is always full-
duplex); a station can either send or receive, but may not do both at the
same time [30]. The next step in the evolution was to move from switched
Ethernet to full-duplex switched Ethernet where there is no need for the
CSMA/CD (Carrier Sense Multiple Access / Collision Detection) method.
This means that each station is connected to the switch via two separate
links and can send and receive independently without worrying about
collision. The full-duplex mode increases the capacity of each domain from
10 to 20 Mbps [29]. Figure A 1 .4 shows a switched Ethernet in full-duplex