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Overview of Resilient Ethernet Protocol
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.
Note
REP is supported on Catalyst switches running IP Base, IP Lite, and IP Services. REP is not supported on the LAN Base license.
REP is supported only on Cisco Catalyst 3560-CX switches.
A REP segment is a chain of ports connected to each other and configured with a segment ID. Each segment consists of standard
(nonedge) segment ports and two user-configured edge ports. A device 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 following figure 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 the ports are operational (as in the segment on the left), a single port is blocked, as
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 the hosts connected to devices
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 the ports
to ensure that connectivity is available through the other gateway.
The segment shown in the following figure is a ring segment, with both the edge ports located on the same device. With this
configuration, you can create a redundant connection between any two devices in the segment.
REP segments have the
following characteristics:
If all the 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 the ports forward traffic on all the
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 (any port in the segment).
In access ring-topologies, the neighboring switch might not support REP as shown in the following figure. In this scenario,
you can configure the non-REP-facing ports (E1 and E2) as edge no-neighbor ports. These ports inherit all the 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 scenario, the STP topology change notice (TCN) that is sent is a multiple spanning-tree
(MST) STP message.
REP has these
limitations:
You must configure each segment port; an incorrect configuration might 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 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 the VLANs are blocked on an interface until the neighbor is detected. 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.
A neighbor does not acknowledge a local port as a peer.
Each port creates an adjacency with its immediate neighbor. After the neighbor adjacencies are created, the ports negotiate
with each other to determine the blocked port for the segment, which will function as the alternate port. All the other ports
become unblocked. By default, REP packets are sent to a bridge protocol data unit-class MAC address. The packets can also
be sent to a 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 the devices not running REP.
Fast
Convergence
REP runs on a physical link basis and not on a per-VLAN basis. Only one hello message is required for all the VLANs, and this
reduces the load on the protocol. We recommend that you create VLANs consistently on all the 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 entire 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 entire domain or
for a particular segment.
The
estimated convergence recovery time on fiber interfaces is between 50 ms and
200 ms for the local segment with 200 VLANs configured. Convergence for VLAN
load balancing is 300 ms or less.
VLAN Load
Balancing
One edge port in the REP segment acts as the primary edge port; and another 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 the 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 preferred keyword to select the port that you previously configured as the preferred alternate port with the rep segmentsegment-idpreferred interface configuration command.
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.
Note
Configure offset numbers on the primary edge port by identifying a port’s downstream position from the primary (or secondary)
edge port. Never enter an offset value of 1 because that is the offset number of the primary edge port.
The following figure shows neighbor offset numbers for a segment, where E1 is the primary edge port and E2 is the secondary
edge port. The numbers inside the ring are numbers offset from the primary edge port; the numbers outside of the ring show
the offset numbers from the secondary edge port. Note that you can identify all the 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.
When the REP segment is complete, all the VLANs are blocked. When you configure VLAN load balancing, you must also configure
triggers in one of two ways:
Manually trigger
VLAN load balancing at any time by entering the
rep preempt
segmentsegment-id
privileged EXEC command on the switch that has the primary edge port.
Configure a
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.
Note
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 the interfaces in the segment
about the preemption. When the secondary port receives the message, the message is sent to 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 the 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.
Reconfigure the primary edge port to reconfigure load balancing. When you change the load-balancing configuration, the primary
edge port waits for the rep preempt 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 configuration.
Spanning Tree
Interaction
REP does not interact with the STP or the Flex Link feature, but can coexist with both. 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 an 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. Since each segment
always contains a blocked port, multiple segments means multiple blocked ports and a potential loss of connectivity. After
the segment is configured in both directions up to the location of the edge ports, configure the edge ports.
REP Ports
REP segments consist 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 the VLANs on the
interface. Blocked-port negotiations occur, and when the segment settles, one blocked port remains in the alternate role and
all the other ports become open ports.
When a failure occurs in a link, all the ports move to the Failed state. When the Alternate port receives the failure notification,
it changes to the Open state, forwarding all the 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 to 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 Resilient Ethernet Protocol
A segment is a collection of ports connected to one another in a chain and configured with a segment ID. To configure REP
segments, 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 a segment, with one of them being the primary edge port and the
other the secondary edge port by default. A segment should have only one primary edge port. If you configure two ports in
a segment as primary edge ports, for example, ports on different switches, the REP selects one of them to serve as the segment's
primary edge port. If required, you can configure the location to which segment topology change notices (STCNs) and VLAN load
balancing are to be sent.
Default REP
Configuration
REP is disabled on all the interfaces. When enabled, the interface is a regular segment port unless it is configured as an
edge port.
When REP is enabled, the task of sending segment topology change notices (STCNs) is disabled, all the VLANs are blocked, and
the administrative VLAN is VLAN 1.
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 the VLANs in 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 is displayed as Fail Logical Open; the Port Role for the other failed port is displayed as Fail No Ext Neighbor. When the external neighbors for the failed ports are configured, the ports go through the alternate port transitions and
eventually go to an open state, or remain as the alternate port, based on the alternate port selection mechanism.
REP ports must be
Layer 2 IEEE 802.1Q or Trunk ports.
We recommend that you configure all the trunk ports in a segment with the same set of allowed VLANs.
Be careful when configuring REP through a Telnet connection because REP blocks all the 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 or REP and Flex Links on the same segment or interface.
If you connect an STP network to an REP segment, be sure that the connection is at the segment edge. An STP connection that
is not at the edge might cause a bridging loop because STP does not run on REP segments. All the STP BPDUs are dropped at
REP interfaces.
You must configure all the trunk ports in a segment with the same set of allowed VLANs. If this is not done, misconfiguration
occurs.
If REP is enabled on two ports on a switch, both the ports must be either regular segment ports or edge ports. REP ports
follow these rules:
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 both be edge ports, 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 must, therefore,
be aware of the status of REP interfaces to avoid sudden connection losses.
REP sends all the LSL PDUs in the untagged frames to the native VLAN. The BPA message sent to a Cisco multicast address is
sent to the administration VLAN, which is VLAN 1 by default.
You can configure the duration for which a REP interface remains up without receiving a hello from a neighbor. 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
Tunnel port
Access port
REP is supported on EtherChannels, but not on an individual port
that belongs to an EtherChannel.
There can be a maximum of 64 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.
Follow these
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.
The
administrative VLAN cannot be the RSPAN VLAN.
To configure the REP
administrative VLAN, follow these steps, beginning in privileged EXEC mode:
SUMMARY STEPS
configureterminal
repadminvlanvlan-id
end
show interface[ interface-id] rep detail
copy running-config startup config
DETAILED STEPS
Command or Action
Purpose
Step 1
configureterminal
Example:
Switch# configure terminal
Enters global
configuration mode.
Step 2
repadminvlanvlan-id
Example:
Switch(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.
Step 3
end
Example:
Switch(config)# end
Exits global configuration mode and returns to privileged EXEC mode.
Step 4
show interface[ interface-id] rep detail
Example:
Switch# show interface gigabitethernet1/1 rep detail
(Optional) Verifies the configuration on a REP interface.
Step 5
copy running-config startup config
Example:
Switch# copy running-config startup config
(Optional) Saves your entries in the switch startup configuration file.
Configuring a REP Interface
To configure REP, enable REP on each segment interface and identify the segment ID. This task is mandatory, and must be done
before other REP configurations. You must also configure a primary and secondary edge port on each segment. All the other
steps are optional.
Follow these steps
to enable and configure REP on an interface:
Specifies the interface, and enters interface configuration mode. The interface can be a physical Layer 2 interface or a port
channel (logical interface).
Enables REP on the interface and identifies a segment number. The segment ID range is from 1 to 1024.
Note
You must configure two edge ports, including one primary edge port, for each segment.
These optional keywords are available:
(Optional) edge—Configures the port as an edge port. Each segment has only two edge ports. Entering the keyword edge without the keyword primary configures the port as the secondary edge port.
(Optional) primary—Configures the port as the primary edge port, the port on which you can configure VLAN load balancing.
(Optional) no-neighbor—Configures a port with no external REP neighbors as an edge port. The port inherits all the properties of an edge port, and
you can configure the properties the same way you would for an edge port.
Note
Although each segment can have only one primary edge port, if you configure edge ports on two different switches and enter
the keyword primary on both the 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.
(Optional) preferred—Indicates that the port is the preferred alternate port or the preferred port for VLAN load balancing.
Note
Configuring a 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.
Switch# rep block port id 0009001818D68700 vlan 1-100
(Optional) Configures VLAN load balancing on the primary edge port, identifies the REP alternate port in one of three ways
(id port-id, neighbor_offset, preferred), and configures the VLANs to be blocked on the alternate port.
id port-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. Enter -1 to identify the secondary edge port as the alternate port.
Note
Because you enter the rep block port 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.
vlanall—Blocks all the VLANs.
Note
Enter this
command only on the REP primary edge port.
Step 8
reppreemptdelayseconds
Example:
Switch# rep preempt delay 100
(Optional)
Configures a preempt time delay.
Use this command if you want VLAN load balancing to be automatically triggered after a link failure and recovery.
The time
delay range is between15 to 300 seconds. The default is manual preemption with
no time delay.
Note
Enter this
command only on the REP primary edge port.
Step 9
rep lsl-age-timervalue
Example:
Switch# rep lsl-age-timer 2000
(Optional)
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).
Note
EtherChannel port channel interfaces do not support LSL age-timer values that are less than 1000 ms.
Both the ports on the link should have the same LSL age configured in order to avoid link flaps.
Step 10
end
Example:
Switch(config)# end
Exits global configuration mode and returns to privileged EXEC mode.
Step 11
showinterface[ interface-id] rep [detail]
Example:
Switch(config)# show interface gigabitethernet1/1 rep detail
(Optional)
Displays the REP interface configuration.
(Optional)
Saves your entries in the router startup configuration file.
Setting Manual
Preemption for VLAN Load Balancing
If you do not enter the 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 the other segment configurations have been completed before manually
preempting VLAN load balancing. When you enter the rep preempt delay segment segment-id command, a confirmation message is displayed before the command is executed because preemption might cause network disruption.
SUMMARY STEPS
enable
configureterminal
rep preempt segmentsegment-id
show rep topology
segmentsegment-id
end
DETAILED STEPS
Command or Action
Purpose
Step 1
enable
Example:
Switch> enable
Enables privileged EXEC mode.
Enter your password if prompted.
Step 2
configureterminal
Example:
Switch# configure terminal
Enters global configuration mode.
Step 3
rep preempt segmentsegment-id
Example:
Switch# rep preempt segment 100
The command will cause a momentary traffic disruption.
Do you still want to continue? [confirm]
Manually
triggers VLAN load balancing on the segment.
You need to confirm the command before it is executed.
Step 4
show rep topology
segmentsegment-id
Example:
Switch# show rep topology segment 100
(Optional) Displays REP topology information.
Step 5
end
Example:
Switch# end
Exits privileged EXEC mode.
Configuring SNMP
Traps for REP
You can configure a router to send REP-specific traps to notify the Simple Network Management Protocol (SNMP) server of link-operational
status changes and port role changes.
SUMMARY STEPS
configureterminal
snmpmibreptrap-ratevalue
end
showrunning-config
copyrunning-configstartup-config
DETAILED STEPS
Command or Action
Purpose
Step 1
configureterminal
Example:
Switch# configure terminal
Enters global configuration mode.
Step 2
snmpmibreptrap-ratevalue
Example:
Switch(config)# snmp mib rep trap-rate 500
Enables the switch to send REP traps, and sets the number of traps sent per second.
Enter the number of traps sent per second. The range is from 0 to 1000. The default is 0 (no limit is imposed; a trap is
sent at every occurrence).
Step 3
end
Example:
Switch(config)# end
Returns to
privileged EXEC mode.
Step 4
showrunning-config
Example:
Switch# show running-config
(Optional)
Displays the running configuration, which can be used to verify the REP trap
configuration.
Step 5
copyrunning-configstartup-config
Example:
Switch# copy running-config startup-config
(Optional)
Saves your entries in the switch startup configuration file.
Device# show rep topology
REP Segment 1
BridgeName PortName Edge Role
---------------- ---------- ---- ----
10.64.106.63 Te5/4 Pri Open
10.64.106.228 Te3/4 Open
10.64.106.228 Te3/3 Open
10.64.106.67 Te4/3 Open
10.64.106.67 Te4/4 Alt
10.64.106.63 Te4/4 Sec Open
REP Segment 3
BridgeName PortName Edge Role
---------------- ---------- ---- ----
10.64.106.63 Gi50/1 Pri Open
SVT_3400_2 Gi0/3 Open
SVT_3400_2 Gi0/4 Open
10.64.106.68 Gi40/2 Open
10.64.106.68 Gi40/1 Open
10.64.106.63 Gi50/2 Sec Alt
Configuration Examples for Resilient Ethernet Protocol
This section provides the following configuration examples:
Example: Configuring the REP Administrative VLAN
This example shows how to configure the administrative VLAN as VLAN 100, and verify the configuration by entering the show interface rep detail command on one of the REP interfaces:
The following example shows how to create an administrative VLAN per segment. Here, VLAN 2 is configured as the administrative
VLAN only for REP segment 2. All the remaining segments that are not configured have VLAN 1 as the administrative VLAN by
default.
Switch# configure terminalSwitch(config)# rep admin vlan 2 segment 2Switch(config)# end
Example: Configuring a REP Interface
This example shows how to configure an interface as the primary edge port for segment 1, to send STCNs to segments 2 through
5, and to configure the alternate port as the port with port ID 0009001818D68700 to block all the VLANs after a preemption
delay of 60 seconds after a segment port failure and recovery. The interface is configured to remain up for 6000 ms without
receiving a hello from a neighbor.
This example shows how to configure the VLAN blocking configuration shown in the Figure 5. The alternate port is the neighbor
with neighbor offset number 4. After manual preemption, VLANs 100 to 200 are blocked at this port, and all the other VLANs
are blocked at the primary edge port E1 (Gigabit Ethernet port 1/1).
Feature Information for Resilient Ethernet Protocol
The following table provides release information about the feature or features described in this module. This table lists
only the software release that introduced support for a given feature in a given software release train. Unless noted otherwise,
subsequent releases of that software release train also support that feature.
Use Cisco Feature Navigator to find information about platform support and Cisco software image support. To access Cisco
Feature Navigator, go to www.cisco.com/go/cfn. An account on Cisco.com is not required.
Table 1. Feature Information for Resilient Ethernet Protocol
Feature Name
Release
Feature Information
Resilient Ethernet Protocol
Cisco IOS Release 15.2(6)E1
This feature was introduced.
In Cisco IOS Release 15.2(6)E1, this feature is supported on Cisco Catalyst 2960-L Series Switches, Cisco Catalyst 2960-X
Series Switches, and Cisco Digital Building.