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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.
The feature is supported on Cisco Catalyst Series Switches with 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.
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.
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
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)
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
You should configure REP only in networks with redundancy. Configuring REP in a network without redundancy causes loss of
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.
REP runs on a physical
link basis and not on a per-VLAN basis. Only one hello message is required for
all VLANs, and it reduces 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 an administrative VLAN for the whole
domain or for a particular segment.
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.
By entering the
keyword to select the port that you previously configured as the
preferred alternate port with the
interface configuration command.
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 delayseconds
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
Spanning Tree Interaction
REP does not interact with STP, but it 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. Therefore, STP cannot run on a segment.
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 has
been configured in both directions up to the location of the edge ports, you then configure the edge ports.
REP segments consists of Failed,
Open, or Alternate ports.
A port configured as a regular segment port starts as a failed port.
After the neighbor adjacencies are determined, the port transitions
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 failed 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 that is reconfigured as a spanning tree port restarts
according 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.
How to Configure
A segment is a
collection of ports connected one to the other in a chain and configured with a
segment ID. To configure REP segments, you configure the REP administrative
VLAN (or use the default VLAN 1) and then add the ports to the segment using
interface configuration mode. You should configure two edge ports in the
segment, with one of them the primary edge port and the other by default the
secondary edge port. A segment has only one primary edge port. If you configure
two ports in a segment as the primary edge port, for example, ports on
different switches, the REP selects one of them to serve as the segment primary
edge port. You can also optionally configure where to send segment topology
change notices (STCNs) and VLAN load balancing.
REP is disabled on all interfaces.
When enabled, the interface is a regular segment port unless it is configured
as an edge port.
When REP is enabled, the sending of segment topology change notices
(STCNs) is disabled, all VLANs are blocked, and the administrative VLAN is VLAN
When VLAN load balancing is enabled, the default is manual preemption
with the delay timer disabled. If VLAN load balancing is not configured, the
default after manual preemption is to block all VLANs at the primary edge port.
REP Configuration Guidelines
Follow these guidelines when configuring REP:
We recommend that you begin by configuring one port and then configure contiguous ports to minimize the number of segments
and the number of blocked ports.
If more than two ports in a segment fail when no external neighbors are configured, one port goes into a forwarding state
for the data path to help maintain connectivity during configuration. In the showrepinterface command output, the Port Role for this port shows as “Fail Logical Open”; the Port Role for the other failed port shows as
“Fail No Ext Neighbor”. When the external neighbors for the failed ports are configured, the ports go through the alternate
port state transitions and eventually go to an open state or remain as the alternate port, based on the alternate port selection
REP ports must be Layer 2 IEEE 802.1Q or Trunk ports.
We recommend that you configure all trunk ports in the segment with the same set of allowed VLANs.
Be careful when configuring REP through a Telnet connection. Because REP blocks all VLANs until another REP interface sends
a message to unblock it. You might lose connectivity to the router if you enable REP in a Telnet session that accesses the
router through the same interface.
You cannot run REP and STP on the same segment or interface.
If you connect an STP network to a REP segment, be sure that the connection is at the segment edge. An STP connection that
is not at the edge could cause a bridging loop because STP does not run on REP segments. All STP BPDUs are dropped at REP
You must configure all trunk ports in the segment with the same set of allowed VLANs, or a misconfiguration occurs.
If REP is enabled on two ports on a switch, both ports must be either regular segment ports or edge ports. REP ports follow
There is no limit to the number of REP ports on a switch; however, only two ports on a switch can belong to the same REP segment.
If only one port on a switch is configured in a segment, the port should be an edge port.
If two ports on a switch belong to the same segment, they must be both edge ports, both regular segment ports, or one regular
port and one edge no-neighbor port. An edge port and regular segment port on a switch cannot belong to the same segment.
If two ports on a switch belong to the same segment and one is configured as an edge port and one as a regular segment port
(a misconfiguration), the edge port is treated as a regular segment port.
REP interfaces come up in a blocked state and remain in a blocked state until they are safe to be unblocked. You need to be
aware of this status to avoid sudden connection losses.
REP sends all LSL PDUs in untagged frames on the native VLAN. The BPA message sent to the Cisco multicast address is sent
on the administration VLAN, which is VLAN 1 by default.
You can configure how long a REP interface remains up without receiving a hello from a neighbor. You can use the rep lsl-age-timer value interface configuration command to set the time from 120 ms to 10000 ms. The LSL hello timer is then set to the age-timer
value divided by 3. In normal operation, three LSL hellos are sent before the age timer on the peer switch expires and checks
for hello messages.
EtherChannel port channel interfaces do not support LSL age-timer values less than 1000 ms. If you try to configure a value
less than 1000 ms on a port channel, you receive an error message and the command is rejected.
REP ports cannot be configured as one of the following port types:
Switched Port Analyzer (SPAN) destination port
REP is supported on EtherChannels, but not on an individual port that belongs to an EtherChannel.
There can be a maximum of 26 REP segments per switch.
Configuring REP Administrative VLAN
To avoid the delay created by link-failure messages, and VLAN-blocking notifications during load balancing, REP floods packets
to a regular multicast address at the hardware flood layer (HFL). These messages are flooded to the whole network, and not
just the REP segment. You can control the flooding of these messages by configuring an administrative VLAN.
guidelines when configuring the REP administrative VLAN:
If you do not
configure an administrative VLAN, the default is VLAN 1.
You can configure one admin VLAN on the switch for all segments.
administrative VLAN cannot be the RSPAN VLAN.
To configure the REP
administrative VLAN, follow these steps, beginning in privileged EXEC mode:
Command or Action
Device# configure terminal
Device(config)# rep admin vlan 2
Specifies the administrative VLAN. The range is from 2 to 4094.
To set the admin VLAN to 1, which is the default, enter the no rep admin vlan global configuration command.
Exits global configuration mode and returns to privileged EXEC mode.
show interface[ interface-id] rep detail
Device# show interface gigabitethernet1/1 rep detail
(Optional) Verifies the configuration on a REP interface.
copy running-config startup config
Device# copy running-config startup config
(Optional) Saves your entries in the switch startup configuration file.
For the REP
operation, you must enable REP on each segment interface and identify the
segment ID. This task is required and must be done before other REP
configurations. You must also configure a primary and secondary edge port on
each segment. All other steps are optional.
Follow these steps
to enable and configure REP on an interface:
Command or Action
Enables privileged EXEC mode.
Enter your password if prompted.
interface, and enter interface configuration mode. The interface can be a
physical Layer 2 interface or a port channel (logical interface). The
port-channel range is 1 to 48.
Enables REP on
the interface and identifies a segment number. The segment ID range is from 1
to 1024. These optional keywords are available:
configure two edge ports, including one primary edge port for each segment.
the port as an edge port. Each segment has only two edge ports. Entering the
configures the port as the secondary edge port.
primary—Configures the port as the primary edge
port, the port on which you can configure VLAN load balancing.
no-neighbor—configures a port with no external REP neighbors
as an edge port. The port inherits all properties of edge ports, and you can
configure them the same as any edge port.
segment can have only one primary edge port, if you configure edge ports on two
different switches and enter the
on both switches, the configuration is valid. However, REP selects only one of
these ports as the segment primary edge port. You can identify the primary edge
port for a segment by entering the
showreptopology privileged EXEC command.
preferred—Indicates that the port is the preferred
alternate port or the preferred port for VLAN load balancing.
port as preferred does not guarantee that it becomes the alternate port; it
merely gives the port a slight edge over equal contenders. The alternate port
is usually a previously failed port.
Configures VLAN load balancing on the primary edge port, identifies the REP
alternate port in one of three ways, and configures the VLANs to be blocked on
the alternate port.
idport-id—identifies the alternate port by port ID.
The port ID is automatically generated for each port in the segment. You can
view interface port IDs by entering the
showinterfacetype numberrep [detail] privileged EXEC command.
neighbor_offset—number to identify the alternate
port as a downstream neighbor from an edge port. The range is from -256 to 256,
with negative numbers indicating the downstream neighbor from the secondary
edge port. A value of
0 is invalid.
identify the secondary edge port as the alternate port. See
for an example of neighbor offset numbering.
enter this command at the primary edge port (offset number 1), you cannot enter
an offset value of 1 to identify an alternate port.
preferred—selects the regular segment port
previously identified as the preferred alternate port for VLAN load balancing.
vlanvlan-list—blocks one VLAN or a range of VLANs.
command only on the REP primary edge port.
Configures a preempt time delay.
command if you want VLAN load balancing to automatically trigger after a link
failure and recovery.
delay range is between15 to 300 seconds. The default is manual preemption with
no time delay.
command only on the REP primary edge port.
Configures a time (in milliseconds) for which the REP interface remains up
without receiving a hello from a neighbor.
The range is
from 120 to 10000 ms in 40-ms increments. The default is 5000 ms (5 seconds).
EtherChannel port channel interfaces do not support LSL
age-timer values less than 1000 ms.
ports on the link should have the same LSL-age configured to avoid link flaps.
privileged EXEC mode.
showinterface[ interface-id] rep [detail]
Displays the REP interface configuration.
Saves your entries in the router startup configuration file.
Preemption for VLAN Load Balancing
If you do not enter
therep preempt delayseconds rep
preempt delay seconds interface configuration command on the primary edge port
to configure a preemption time delay, the default is to manually trigger VLAN
load balancing on the segment. Be sure that all other segment configuration has
been completed before manually preempting VLAN load balancing. When you enter
rep preempt delay
segmentsegment-id command, a confirmation message appears
before the command is executed because preemption can cause network disruption.
Command or Action
rep preempt segmentsegment-id
triggers VLAN load balancing on the segment.
You will need to
confirm the command before it is executed.
show rep topology
Traps for REP
You can configure
the router to send REP-specific traps to notify the Simple Network Management
Protocol (SNMP) server of link operational status changes and any port role
Command or Action
Switch# configure terminal
Switch(config)# snmp mib rep trap-rate 500
switch to send REP traps, and sets the number of traps sent per second.
number of traps sent per second. The range is from 0 to 1000. The default is 0
(no limit imposed; a trap is sent at every occurrence).
privileged EXEC mode.
Switch# show running-config
Displays the running configuration, which can be used to verify the REP trap
Switch# copy running-config startup-config
Saves your entries in the switch startup configuration file.
Command or Action
show interface[ interface-id] rep[ detail]
Displays REP configuration and
status for an interface or for all interfaces.
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