Filter
Top Products
12-6
Cisco ASR 901 Series Aggregation Services Router Software Configuration Guide
OL-23826-09
Chapter 12 Configuring Resilient Ethernet Protocol
Understanding Resilient Ethernet Protocol (REP)
Note When VLAN load balancing is configured, it does not start working until triggered by either manual
intervention or a link failure and recovery.
When VLAN load balancing is triggered, the primary edge port sends a message to alert all interfaces in
the segment about the preemption. When the secondary port receives the message, it is reflected into the
network to notify the alternate port to block the set of VLANs specified in the message and to notify the
primary edge port to block the remaining VLANs.
You can also configure a particular port in the segment to block all VLANs. Only the primary edge port
initiates VLAN load balancing, which is not possible if the segment is not terminated by an edge port on
each end. The primary edge port determines the local VLAN load balancing configuration.
Reconfigure the primary edge port to reconfigure load balancing. When you change the load balancing
configuration, the primary edge port again waits for the rep preempt segment command or for the
configured preempt delay period after a port failure and recovery before executing the new
configuration. If you change an edge port to a regular segment port, the existing VLAN load balancing
status does not change. Configuring a new edge port might cause a new topology configuration.
Spanning Tree Interaction
REP does not interact with MSTP, but the two can coexist. A port that belongs to a segment is removed
from spanning tree control, and STP BPDUs are not accepted or sent from segment ports.
To migrate from an STP ring configuration to REP segment configuration, begin by configuring a single
port in the ring as part of the segment, and continue by configuring contiguous ports to minimize the
number of segments. Each segment always contains a blocked port, so multiple segments means multiple
blocked ports and a potential loss of connectivity. When the segment is configured in both directions to
the edge ports, you then configure the edge ports.
REP Ports
Ports in REP segments are in the Failed, Open, or Alternate states. The various states REP ports go
through are as follows:
• A port configured as a regular segment port starts as a failed port.
• After the neighbor adjacencies are determined, the port changes to alternate port state, blocking all
VLANs on the interface. Blocked port negotiations occur and when the segment settles, one blocked
port remains in the alternate role, and all other ports become open ports.
• When a failure occurs in a link, all ports move to the open state. When the alternate port receives
the failure notification, it changes to the open state, forwarding all VLANs.
A regular segment port converted to an edge port, or an edge port converted to a regular segment port,
does not always result in a topology change. If you convert an edge port into a regular segment port,
VLAN load balancing is not implemented unless it has been configured. For VLAN load balancing, you
must configure two edge ports in the segment.
A segment port reconfigured as a spanning tree port restarts according to the spanning tree configuration.
By default, this is a designated blocking port. If PortFast is configured or if STP is disabled, the port
goes into the forwarding state.
For instructions on how to configure REP, see Configuring Resilient Ethernet Protocol (REP), page 12-7.