Cisco Systems A9014CFD manual 12-2, REP Open Segments

Chapter 12 Configuring Resilient Ethernet Protocol

Understanding Resilient Ethernet Protocol (REP)

Figure 12-1shows an example of a segment consisting of six ports spread across four switches. Ports E1 and E2 are configured as edge ports. When all ports are operational (as in the segment on the left), a single port is blocked, shown by the diagonal line. When there is a network failure, as shown on the right of the diagram, the blocked port returns to the forwarding state to minimize network disruption.

Figure 12-1 REP Open Segments

E1 Edge port

Blocked port

Link failure

E1 E2 E1 E2

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The segment shown in Figure 12-1is an open segment; there is no connectivity between the two edge ports. The REP segment cannot cause a bridging loop, and you can safely 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 host cannot access its usual gateway because of a failure, REP unblocks all ports to ensure that connectivity is available through the other gateway.

The segment shown in Figure 12-2, 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 12-2 REP Ring Segment

E1

E2

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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.

	Cisco ASR 901 Series Aggregation Services Router Software Configuration Guide
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