Cisco Catalyst 4500 Series, Software Configuration Manual

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Catalyst 4500 Series Switch, Cisco IOS Software Configuration Guide - Cisco IOS XE 3.9.xE and IOS 15.2(5)Ex
Chapter 25 Configuring Resilient Ethernet Protocol
About 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. 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 these situations occur:
• 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 and PVLAN
promiscuous 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 milliseconds for the local segment.
VLAN Load Balancing
One edge port in the REP segment acts as the primary edge port; the other as the secondary edge port.
It is the primary edge port that 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:
•
By entering the port ID of the interface.
To identify the port ID of a port in the segment, enter the show interface rep detail interface
configuration command for the port.
•
By entering the neighbor offset number of a port in the segment, which identifies the downstream
neighbor port of an edge port.