632 | Layer 2
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Configuring Redundant Pairs
Configuring Redundant Pairs is supported on platforms: e c s Z
Networks that employ switches that do not support Spanning Tree (STP) — for example, networks with
Digital Subscriber Line Access Mutiplexers (DSLAM) — cannot have redundant links between switches
because they create switching loops (Figure 29-6). The Redundant Pairs feature enables 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.
Assign a backup interface to an interface using the command
switchport backup. The backup interface
remains in 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. FTOS supports
Gigabit, 10-Gigabit, and 40-Gigabit interfaces as backup interfaces.
You must 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 29-6. Configuring Redundant Layer 2 Pairs without Spanning Tree
Note: For details on STP, see Chapter 49, “Spanning Tree Protocol (STP),” on page 1003.
Redundant links create a switching loop.
Without STP broadcast storms occurs.
FTOS(conf-if-gi-4/32)#switchport
FTOS(conf-if-gi-4/32)#no shutdown
fnC0067mp
FTOS(conf-if-gi-3/42)#switchport
FTOS(conf-if-gi-3/42)#no shutdown
Backup Link
Use backup interfaces to create redundant
links in networks without STP
FTOS(conf-if-gi-4/31)#switchport
FTOS(conf-if-gi-4/31)#no shutdown
FTOS(conf-if-gi-3/41)#switchport
FTOS(conf-if-gi-3/41)#switchport backup gi 3/42
FTOS(conf-if-gi-3/41)#no shutdown
R3
R4
3/41
3/42
4/31
4/32
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