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17-3
Cisco ME 3400 Ethernet Access Switch Software Configuration Guide
OL-9639-06
Chapter 17 Configuring Resilient Ethernet Protocol
Understanding REP
You can construct almost any type of network based on REP segments. REP also supports VLAN
load-balancing, controlled by the primary edge port but occurring at any port in the segment.
REP has these limitations:
• You must configure each segment port; an incorrect configuration can cause forwarding loops in the
networks.
• REP can manage only a single failed port within the segment; multiple port failures within the REP
segment cause loss of network connectivity.
• You should configure REP only in networks with redundancy. Configuring REP in a network
without redundancy causes loss of connectivity.
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. When enabled on an interface, 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 as 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. Once 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 can
also be sent to the Cisco multicast address, which at present is used only to send blocked port
advertisement (BPA) messages when there is a failure in the segment. 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 is less than 200 ms for the local segment.