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Cisco ASR 901 Series Aggregation Services Router Software Configuration Guide
OL-23826-09
Chapter 12 Configuring Resilient Ethernet Protocol
Understanding Resilient Ethernet Protocol (REP)
Link Integrity
REP does not use an end-to-end polling mechanism between edge ports to verify link integrity. It
implements local link failure detection. The REP Link Status Layer (LSL) detects its REP-aware
neighbor and establishes connectivity within the segment. All VLANs are blocked on an interface until
it detects the neighbor. After the neighbor is identified, REP determines which neighbor port should
become the alternate port and which ports should forward traffic.
Each port in a segment has a unique port ID. The port ID format is similar to that used by the spanning
tree algorithm: a port number (unique on the bridge), associated to a MAC address (unique in the
network). When a segment port is coming up, its LSL starts sending packets that include the segment ID
and the port ID. The port is declared operational after it performs a three-way handshake with a neighbor
in the same segment.
A segment port does not become operational if:
• No neighbor has the same segment ID.
• More than one neighbor has the same segment ID.
• The neighbor does not acknowledge the local port as a peer.
Each port creates an adjacency with its immediate neighbor. After the neighbor adjacencies are created,
the ports negotiate to determine one blocked port for the segment, the alternate port. All other ports
become unblocked. By default, REP packets are sent to a BPDU class MAC address. The packets are
dropped by devices not running REP.
Fast Convergence
Because REP runs on a physical link basis and not a per-VLAN basis, only one hello message is required
for all VLANs, reducing the load on the protocol. We recommend that you create VLANs consistently
on all switches in a given segment and configure the same allowed VLANs on the REP trunk ports. To
avoid the delay introduced by relaying messages in software, REP also allows some packets to be
flooded to a regular multicast address. These messages operate at the hardware flood layer (HFL) and
are flooded to the whole network, not just the REP segment. Switches that do not belong to the segment
treat them as data traffic. You can control flooding of these messages by configuring a dedicated
administrative VLAN for the whole domain.
The estimated convergence recovery time on fiber interfaces is less than 200 ms for the local segment
with 200 VLANs configured. Convergence for VLAN load balancing is 300 ms or less.
VLAN Load Balancing (VLB)
One edge port in the REP segment acts as the primary edge port; the other as the secondary edge port.
The primary edge port always participates in VLAN load balancing in the segment. REP VLAN
balancing is achieved by blocking some VLANs at a configured alternate port and all other VLANs at
the primary edge port. When you configure VLAN load balancing, you can specify the alternate port in
one of three ways:
• Enter the port ID of the interface. To identify the port ID of a port in the segment, use the show
interface rep detail interface configuration command for the port.