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Cisco ME 3400 Ethernet Access Switch Software Configuration Guide
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Chapter 17 Configuring Resilient Ethernet Protocol
Understanding REP
Figure 17-1 REP Open Segments
The segment shown in Figure 17-1 is an open segment; there is no connectivity between the two edge
ports. The REP segment cannot cause a bridging loop and it is safe to connect the segment edges to any
network. All hosts connected to switches inside the segment have two possible connections to the rest
of the network through the edge ports, but only one connection is accessible at any time. If a failure
causes a host to be unable to access its usual gateway, REP unblocks all ports to ensure that connectivity
is available through the other gateway.
The segment shown in Figure 17-2, with both edge ports located on the same switch, is a ring segment.
In this configuration, there is connectivity between the edge ports through the segment. With this
configuration, you can create a redundant connection between any two switches in the segment.
Figure 17-2 REP Ring Segment
REP segments have these characteristics:
• If all ports in the segment are operational, one port (referred to as the alternate port) is in the blocked
state for each VLAN. If VLAN load balancing is configured, two ports in the segment control the
blocked state of VLANs.
• If one or more ports in a segment is not operational, causing a link failure, all ports forward traffic
on all VLANs to ensure connectivity.
• In case of a link failure, the alternate ports are unblocked as quickly as possible. When the failed
link comes back up, a logically blocked port per VLAN is selected with minimal disruption to the
network.
E2E1 E2E1
E1 Edge port
Blocked port
Link failure
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E2E1
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