Resilient Ethernet Protocol (REP) is a Cisco proprietary protocol that provides an alternative to Spanning Tree Protocol (STP) to control network loops, handle link failures, and improve convergence time. REP controls a group of ports connected in a segment, ensures that the segment does not create any bridging loops, and responds to link failures within the segment. REP provides a basis for constructing more complex networks and supports VLAN load balancing.
REP is supported on Catalyst switches running the Network Essentials license.
A REP segment is a chain of ports connected to each other and configured with a segment ID. Each segment consists of standard (non-edge) segment ports and two user-configured edge ports. A router can have no more than two ports that belong to the same segment, and each segment port can have only one external neighbor. A segment can go through a shared medium, but on any link only two ports can belong to the same segment. REP is supported only on Trunk Ethernet Flow Point (EFP) interfaces.
The figure below shows 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 failure in the network, the blocked port returns to the forwarding state to minimize network disruption.
Figure 1. REP Open Segment
The segment shown in the figure above is 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 routers 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 failure occurs on any segment or on any port on a REP segment, REP unblocks all ports to ensure that connectivity is available through the other gateway.
The segment shown in the figure below is a ring segment with both edge ports located on the same router. With this configuration, you can create a redundant connection between any two routers in the segment.
Figure 2. REP Ring Segment
REP segments have the following characteristics:
If all ports in a 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.
If one or more ports in a segment is not operational, and cause a link failure, all ports forward traffic on all VLANs to ensure connectivity.
In case of a link failure, alternate ports are unblocked as quickly as possible. When the failed link is up, a logically blocked port per VLAN is selected with minimal disruption to the network.
You can construct almost any type of network based on REP segments. REP also supports VLAN load balancing, which is controlled by the primary edge port occurring at any port in the segment.
In access ring topologies, the neighboring switch might not support REP as shown in the figure below. In this case, you can configure the non-REP facing ports (E1 and E2) as edge no-neighbor ports. These ports inherit all properties of edge ports, and you can configure them the same as any edge port, including configuring them to send STP or REP topology change notices to the aggregation switch. In this case, the STP topology change notice (TCN) that is sent is a multiple spanning-tree (MST) STP message.
Figure 3. Edge No-Neighbor Ports
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.
REP does not use an end-to-end
polling function 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 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 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.
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,
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. The neighbor offset number range is
–256 to +256; a value of 0 is invalid. The primary edge port has an offset
number of 1; positive numbers above 1 identify downstream neighbors of the
primary edge port. Negative numbers indicate the secondary edge port (offset
number -1) and its downstream neighbors.
You configure offset
numbers on the primary edge port by identifying a port’s downstream position
from the primary (or secondary) edge port. You would never enter an offset
value of 1 because that is the offset number of the primary edge port itself.
The figure below shows
neighbor offset numbers for a segment where E1 is the primary edge port and E2
is the secondary edge port. The red numbers inside the ring are numbers offset
from the primary edge port; the black numbers outside of the ring show the
offset numbers from the secondary edge port. Note that you can identify all
ports (except the primary edge port) by either a positive offset number
(downstream position from the primary edge port) or a negative offset number
(downstream position from the secondary edge port). If E2 became the primary
edge port, its offset number would then be 1 and E1 would be -1.
Figure 4. Neighbor Offset
Numbers in a Segment
When the REP segment
is complete, all VLANs are blocked. When you configure VLAN load balancing,you
must also configure triggers in one of two ways:
VLAN load balancing at any time by entering the
privileged EXEC command on the switch that has the primary edge port.
preempt delay time by entering the rep preempt delay
interface configuration command. After a link failure and recovery, VLAN load
balancing begins after the configured preemption time period elapses. Note that
the delay timer restarts if another port fails before the time has elapsed.
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 out 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.
primary edge port to reconfigure load balancing. When you change the load
balancing configuration, the primary edge port again waits for the
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