Configuring Flex Links

This chapter describes how to configure Flex Links, a pair of Layer 2 interfaces, where one interface is configured to act as a backup to the other.

Restrictions for Configuring Flex Links

  • You can configure only one Flex Link backup link for any active link, and it must be a different interface from the active interface.

  • An interface can belong to only one Flex Link pair. An interface can be a backup link for only one active link. An active link cannot belong to another Flex Link pair.

  • Neither of the links can be a port that belongs to an EtherChannel nor port channel

  • A backup link does not have to be the same type (Fast Ethernet, Gigabit Ethernet) as the active link.

  • STP is disabled on Flex Link ports. If STP is configured on the switch, Flex Links do not participate in STP in all VLANs in which STP is configured. With STP not running, be sure that there are no loops in the configured topology.

  • Flex link is only supported on trunk EFP.

  • In bi-directional traffic, FlexLink Convergence will be high in one-direction due to mac address black holing.

Information About Flex Links

The feature provides an alternative solution to the Spanning Tree Protocol (STP), allowing you to turn off STP and still provide basic link redundancy. Flex Links are typically configured in service provider or enterprise networks, where, you do not want to run STP on the router. If the router is running STP, it is not necessary to configure Flex Links, because STP already provides link-level redundancy or backup. Flex Links are supported only on Trunk EFP and are not supported on other EVCs.

Following are the two flex link modes supported:

  • Active-Alone Forwarding Method

  • Active-Backup-Both Forwarding Method

Active-Alone forwarding Method

From the schematic representation, ports 1 and 2 on switch A are connected to uplink switches B and C. Because they are configured as Flex Link in active-backup both forwarding mode, both the interfaces will be forwarding traffic. If port 1 is the active link, all mutually inclusive VLANs (common VLANs configured in both active / backup interface) would be forwarded on active interface and mutually exclusive VLANs would be forwarded from the respective active / backup interfaces. If port 1 goes down, then port 2 will start forwarding only the traffic for the common VLANs along with its specific exclusive vlans. All traffic belonging to the exclusive VLANS as part of active interface configuration would be dropped until port 1 comes back to operational state.

Figure 1. Active-Alone Forwarding Method


Configuring Active Alone Forwarding Method

SUMMARY STEPS

  1. enable
  2. configure terminal
  3. interface interface-id
  4. no shutdown
  5. ethernet backup interface interface-id
  6. end

DETAILED STEPS

  Command or Action Purpose
Step 1

enable

Example:
Router> enable

Enables privileged EXEC mode.

  • Enter your password if prompted.

Step 2

configure terminal

Example:
Router# configure terminal

Enters global configuration mode.

Step 3

interface interface-id

Example:
Router(config)# interface gigabitEthernet 0/0/5

Specify the 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.

Step 4

no shutdown

Example:
Router(config-if)# no shutdown

Enable the port, if necessary. By default, UNIs are disabled, and NNIs are enabled.

Step 5

ethernet backup interface interface-id

Example:
Router(config)# ethernet backup interface gigabitEthernet 0/0/5

Configure a physical Layer 2 interface (or port channel) as part of a Flex Link pair with the interface. When one link is forwarding traffic, the other interface is in standby mode.

Step 6

end

Example:
Router(config-if)# end

Return to privileged EXEC mode.

Configuration Example 
On Active interface(Port 5)

Router> enable
Router# configure terminal
Router# service instance trunk 1000 ethernet
Router# encapsulation dot1q 1-1000
Router# rewrite ingress tag pop 1 symmetric
Router# bridge-domain from-encapsulation
 

  
Backup interface (Port 6)

Router> enable
Router# configure terminal
Router# service instance trunk 1000 ethernet
Router# encapsulation dot1q 1-1000
Router# rewrite ingress tag pop 1 symmetric
Router# bridge-domain from-encapsulation



Flexlink Configuration

Router> enable
Router# configure terminal
Router(config)# interface gigabitEthernet 0/0/5
Router(config-if)# no shutdown
Router(config-if)# ethernet backup interface gigabitEthernet 0/0/6
Router(config-if)# end

Verifying Active Alone Forwarding Method Configuration

SUMMARY STEPS

  1. enable
  2. configure terminal
  3. show ethernet backup detail

DETAILED STEPS

  Command or Action Purpose
Step 1

enable

Example:
Router> enable

Enables privileged EXEC mode.

  • Enter your password if prompted.

Step 2

configure terminal

Example:
Router# configure terminal

Enters global configuration mode.

Step 3

show ethernet backup detail

Example:
Router# show ethernet backup detail

This displays the flex link configuration.

Configuration Output 

Switch Backup Interface Pairs:
Active Interface        Backup Interface        State
------------------------------------------------------------------------
GigabitEthernet0/0/5    Te0/0/12                Active Up/Backup Standby
        Preemption Mode  : off
        Multicast Fast Convergence  : Off
        Bandwidth : 1000000 Kbit (Gi0/0/3), 1000000 Kbit (Te0/0/12)
        Mac Address Move Update Vlan : auto
        Forwarding  : Active-Only

Active-Backup-Both forwarding Method

From the schematic representation, ports 1 and 2 on switch A are connected to uplink switches B and C. Because they are configured as Flex Link in active-backup both forwarding mode, both the interfaces will be forwarding traffic. If port 1 is the active link, all mutually inclusive vlans (common vlans configured in both active / backup interface) would be forwarded on active interface and mutually exclusive vlans would be forwarded from the respective active / backup interfaces. If port 1 goes down, then port 2 will start forwarding only the traffic for the common vlans along with its specific exclusive vlans. All traffic belonging to the exclusive vlans as part of active interface configuration would be dropped until port 1 comes back to operational state.

Figure 2. Active-Backup-Both Forwarding Method


Configuring Active Backup Both Forwarding Method

SUMMARY STEPS

  1. enable
  2. configure terminal
  3. interface interface-id
  4. no shutdown
  5. ethernet backup interface interface-id prefer forwarding
  6. end

DETAILED STEPS

  Command or Action Purpose
Step 1

enable

Example:
Router> enable

Enables privileged EXEC mode.

  • Enter your password if prompted.

Step 2

configure terminal

Example:
Router# configure terminal

Enters global configuration mode.

Step 3

interface interface-id

Example:
Router(config)# interface gigabitEthernet 0/0/8

Specify the 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.

Step 4

no shutdown

Example:
Router(config-if)# no shutdown

Enable the port, if necessary. By default, UNIs are disabled, and NNIs are enabled.

Step 5

ethernet backup interface interface-id prefer forwarding

Example:
Router(config)# ethernet backup interface gigabitEthernet 0/0/8 prefer forwarding

Configure a physical Layer 2 interface (or port channel) as part of a Flex Link pair with the interface. When one link is forwarding traffic, the other interface is in standby mode.

Step 6

end

Example:
Router(config-if)# end

Return to privileged EXEC mode.

Configuration Example 
On Active interface(Port 7)

Router> enable
Router# configure terminal
Router# service instance trunk 1000 ethernet
Router# encapsulation dot1q 1-512
Router# rewrite ingress tag pop 1 symmetric
Router# bridge-domain from-encapsulation
 

  
Backup interface (Port 8)

Router> enable
Router# configure terminal
Router# service instance trunk 1000 ethernet
Router# encapsulation dot1q 512-1000
Router# rewrite ingress tag pop 1 symmetric
Router# bridge-domain from-encapsulation



Flexlink Configuration

Router> enable
Router# configure terminal
Router(config)# interface gigabitEthernet 0/0/8
Router(config-if)# no shutdown
Router(config-if)# ethernet backup interface gigabitEthernet 0/0/8 prefer forwarding

Router(config-if)# end

Verifying Active-Backup-Both Forwarding Method Configuration

SUMMARY STEPS

  1. enable
  2. configure terminal
  3. show ethernet backup detail

DETAILED STEPS

  Command or Action Purpose
Step 1

enable

Example:
Router> enable

Enables privileged EXEC mode.

  • Enter your password if prompted.

Step 2

configure terminal

Example:
Router# configure terminal

Enters global configuration mode.

Step 3

show ethernet backup detail

Example:
Router# show ethernet backup detail

This displays the flex link configuration.

Configuration Output 

Switch Backup Interface Pairs:
Active Interface        Backup Interface        State
------------------------------------------------------------------------
GigabitEthernet0/0/3    Te0/0/12                Active Up/Backup Standby
        Preemption Mode  : off
        Multicast Fast Convergence  : Off
        Bandwidth : 1000000 Kbit (Gi0/0/3), 1000000 Kbit (Te0/0/12)
        Mac Address Move Update Vlan : auto
        Forwarding  : Active-Backup-Both

Unsupported Functions

Following functions are not supported:

  • MMU Notification

  • IGMP Fast convergence

  • Preemption Support

  • Flex links support on a Port channel interface.

  • Flex links support on EVC

  • Flex links with VLB

  • Flex links on IP configured Physical interface.

  • Flexlink cannot be configured on a REP / G8032 configured interface and vice-versa.

  • STP can be enabled globally but will not be applied on flex link configured interfaces alone.

Additional References

Related Documents

Related Topic Document Title

Cisco IOS commands

https://www.cisco.com/c/en/us/td/docs/ios-xml/ios/mcl/allreleasemcl/all-book.html

Standards and RFCs

Standard/RFC Title

No specific Standards and RFCs are supported by the features in this document.

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