Allied Telesis RAPIER I - Manual (Page 211)

Ethernet Protection Switching Ring (EPSR) 13
EPSR and Spanning Tree Operation
EPSR and the Spanning Tree protocols (STP) each address the issue of data
loop prevention, although their method of doing so is quite different. For
information on STP, see the Spanning Tree Chapter of your switch’s Software
Refernce. EPSR is manually configured to explicitly identify which link(s) will
be broken in the defined ring, whereas STP/RSTP calculates where to break
links based upon user provisioned values (metrics) that are compared to
determine the "best" (or lowest cost) paths for data traffic.
At the practical level these two techniques can be employed to create
complementary hybrid EPSR / STP configurations. Such a configuration might
have a high speed fibre loop topology backbone—controlled and managed
using EPSR. Lobes could extend out from each loop node into a user mesh
network. Any loops existing within this mesh network would be controlled
and managed using STP/RSTP.
Figure 1-9 on page 1-13 shows a basic
combined EPSR / STP network.
Figure 1-9: EPSR and Spanning Tree Operation
Note that EPSR and STP cannot share the same ports.
Data
VLA
N
Data V
LAN
Data VLAN
Master
Node
Other
Ports
Other
Ports
Other
Ports
Other
Ports
Other
Ports
Control VLAN (control_ring)
Transit
Node
Transit
Node
Transit
Node
EPSR 5.eps
Data VLAN
Port 1
Primary Port 2
Secondary
Port 1
Port 1 Port 1
Port 2 Port 2
Port 3 Port 4
P S
Node 1
Node 2
Node 3
Node 4
Port 2
Co
nt
ro
l V
LA
N
D
at
a
V
LA
N
Con
tro
l V
LA
N
D
at
a V
LA
N
EPSR Ring
STP/RSTP
Network