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Dell EMC Configuration Guide for the S3100 Series 9.14.2.4

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Configure Redundant Pairs

Important Points about Configuring Redundant Pairs

Networks that employ switches that do not support the spanning tree protocol (STP) — for example, networks with digital subscriber line access multiplexers (DSLAM) — cannot have redundant links between switches because they create switching loops (as shown in the following illustration). The redundant pairs feature allows you to create redundant links in networks that do not use STP by configuring backup interfaces for the interfaces on either side of the primary link.
NOTE For more information about STP, refer to Spanning Tree Protocol (STP).

Assign a backup interface to an interface using the switchport backup command. The backup interface remains in a Down state until the primary fails, at which point it transitions to Up state. If the primary interface fails, and later comes up, it becomes the backup interface for the redundant pair. Dell EMC Networking OS supports Gigabit, 10 Gigabit, and 40-Gigabit interfaces as backup interfaces.

Apply all other configurations to each interface in the redundant pair such that their configurations are identical, so that transition to the backup interface in the event of a failure is transparent to rest of the network.

Figure 1. Configuring Redundant Layer 2 Pairs without Spanning Tree
Illustration of configuring redundant Layer 2 pairs without spanning tree.

You configure a redundant pair by assigning a backup interface to a primary interface with the switchport backup interface command. Initially, the primary interface is active and transmits traffic and the backup interface remains down. If the primary fails for any reason, the backup transitions to an active Up state. If the primary interface fails and later comes back up, it remains as the backup interface for the redundant pair.

Following primary/backup interfaces are also supported:
  • primary interface is a physical interface, the backup interface can be a physical interface
  • primary interface is a physical interface, the backup interface can be a static or dynamic LAG
  • primary interface is a static or dynamic LAG, the backup interface can be a physical interface
  • primary interface is a static or dynamic LAG, the backup interface can be a static or dynamic LAG

In a redundant pair, any combination of physical and port-channel interfaces is supported as the two interfaces in a redundant pair. For example, you can configure a static (without LACP) or dynamic (with LACP) port-channel interface as either the primary or backup link in a redundant pair with a physical interface.

To ensure that existing network applications see no difference when a primary interface in a redundant pair transitions to the backup interface, be sure to apply identical configurations of other traffic parameters to each interface.

If you remove an interface in a redundant link (remove the line card of a physical interface or delete a port channel with the no interface port-channel command), the redundant pair configuration is also removed.

  • You may not configure any interface to be a backup for more than one interface, no interface can have more than one backup, and a backup interface may not have a backup interface.
  • The active or backup interface can be a LAG, but it cannot be a member port of a LAG.
  • The active and standby do not have to be of the same type (1G, 10G, and so on).
  • You may not enable any Layer 2 protocol on any interface of a redundant pair or to ports connected to them.
As shown in the above illustration, interface 3/41 is a backup interface for 3/42, and 3/42 is in the Down state. If 3/41 fails, 3/42 transitions to the Up state, which makes the backup link active. A message similar to the following message appears whenever you configure a backup port.
02:28:04: %RPM0-P:CP %IFMGR-5-L2BKUP_WARN: Do not run any Layer2 protocols on Gi 3/41 and Te 3/42
02:28:04: %RPM0-P:CP %IFMGR-5-OSTATE_DN: Changed interface state to down: Gi 3/42
02:28:04: %RPM0-P:CP %IFMGR-5-STATE_ACT_STBY: Changed interface state to standby: Gi 3/42

Example of Configuring Redundant Layer 2 Pairs

DellEMC(conf-if-range-Gi-3/41-42)#switchport backup interface GigabitEthernet 3/42
DellEMC(conf-if-range-Gi-3/41-42)#show config
!
interface GigabitEthernet 3/41
  no ip address
  switchport
  switchport backup interface GigabitEthernet 3/42
  no shutdown
!
interface GigabitEthernet 3/42
  no ip address
  switchport
  no shutdown
DellEMC(conf-if-range-Gi-3/41-42)#
DellEMC(conf-if-range-Gi-3/41-42)#do show ip int brief | find 3/41
GigabitEthernet 3/41   unassigned         YES Manual up       up
GigabitEthernet 3/42   unassigned         NO Manual up        down
[output omitted]
DellEMC(conf-if-range-Gi-3/41-42)#interface gig 3/41
DellEMC(conf-if-Gi-3/41)#shutdown
00:24:53: %RPM0-P:CP %IFMGR-5-ASTATE_DN: Changed interface Admin state to down: Gi 3/41
DellEMC(conf-if-Gi-3/41)#00:24:55: %RPM0-P:CP %IFMGR-5-OSTATE_DN: Changed interface state to
down: Gi 3/41
00:24:55: %RPM0-P:CP %IFMGR-5-INACTIVE: Changed Vlan interface state to inactive: Vl 1
00:24:55: %RPM0-P:CP %IFMGR-5-OSTATE_UP: Changed interface state to up: Gi 3/42
00:24:55: %RPM0-P:CP %IFMGR-5-ACTIVE: Changed Vlan interface state to active: Vl 1
00:24:55: %RPM0-P:CP %IFMGR-5-STATE_STBY_ACT: Changed interface state from standby to active:
Gi 3/42

DellEMC(conf-if-Gi-3/41)#do show ip int brief | find 3/41
GigabitEthernet 3/41   unassigned      NO Manual administratively down down
GigabitEthernet 3/42   unassigned      YES Manual up                   up
[output omitted]

Example of Configuring Redundant Pairs on a Port-Channel 

DellEMC#show interfaces port-channel brief
Codes: L - LACP Port-channel

  LAG  Mode  Status  Uptime     Ports
  1    L2    up      00:08:33   Gi 1/1 (Up)
  2    L2    up      00:00:02   Gi 2/1 (Up)
DellEMC#configure
DellEMC(conf)#interface port-channel 1
DellEMC(conf-if-po-1)#switchport backup interface port-channel 2
Apr 9 00:15:13: %STKUNIT0-M:CP %IFMGR-5-L2BKUP_WARN: Do not run any Layer2 protocols on Po 1 and Po 2
Apr 9 00:15:13: %STKUNIT0-M:CP %IFMGR-5-OSTATE_DN: Changed interface state to down: Po 2
Apr 9 00:15:13: %STKUNIT0-M:CP %IFMGR-5-STATE_ACT_STBY: Changed interface state to standby: Po 2
DellEMC(conf-if-po-1)#
DellEMC#
DellEMC#show interfaces switchport backup
Interface        Status   Paired Interface     Status
Port-channel 1   Active   Port-chato mannel 2  Standby
Port-channel 2   Standby  Port-channel 1       Active
DellEMC#

DellEMC(conf-if-po-1)#switchport backup interface gigabitethernet 1/2
Apr 9 00:16:29: %STKUNIT0-M:CP %IFMGR-5-L2BKUP_WARN: Do not run any Layer2 protocols on Po 1 and Gi 1/2
DellEMC(conf-if-po-1)#

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