Software
Manual SCA-340 / SCA-340-L
sysmik.de
35
4.3.2
Ethernet Ring Monitoring
Ethernet ports LAN3 and LAN4 allow to build an Ethernet ring, provided both ports
belong to
eth0
. The Ethernet ring connects Ethernet field devices with integrated
unmanaged switch (with at least two Ethernet ports, like SysMik's SCC devices) and
ensures Ethernet communication even when one connection in the ring is broken
(see Fig. 4.3.1.5).
If the ring is completely closed and the involved switches are unmanaged ones, a so
called broadcast storm is likely to occur: because broadcast and multicast
messages are always forwarded, they circulate in the ring and create high traffic
load. The Ethernet ring monitoring observes always the state of the ring and
deactivates LAN4 when the ring is faultless. If the ring connection is broken at one
point, LAN4 is activated.
Note:
The time for complete recovery of the communication in the ring is defined by the
aging-time of the switches in the ring.
4.3.3
Rapid-Spanning-Tree-Protocol (RSTP)
The Rapid-Spanning-Tree-Protocol allows to build arbitrary redundant Ethernet
structures, also beyond simple ring structures, by detecting redundant paths in the
system and disabling them as long as they are not needed. As soon as a connection
is broken it will be replaced by a redundant path (provided there is one). RSTP
needs just a few seconds to re-establish a broken connection.
Note:
Inside an RSTP structure, all involved Ethernet switches must support RSTP and must
be adequately configured.
Tables
4.3.3.1
to Table
4.3.3.4
show the possible combinations regarding RSTP. A
LAN port can be configured for RSTP if there is at least one further LAN port
assigned to the same IP interface.
eth
LAN
RSTP configurations
0
1
A)
2
A)
3
B)
4
B)
A), B)
RSTP topologies independent of each other
Table 4.3.3.1:
LAN1-4 at
eth0
,
eth1
deactivated
eth
LAN
RSTP configurations
1
1
0
2
3
4
Table 4.3.3.2:
LAN2/3/4 at
eth0
, LAN1 at
eth1