Cisco Catalyst 6880-X Specifications download pdf (Page 55)

Distribution Layer April 2014

A critical aspect of the Cisco Catalyst VSS is the control plane and data plane operating models. From a control

plane standpoint the VSS uses an active-standby operating model. This means that supervisor hardware on

one chassis becomes the active control plane for the entire VSS while the other supervisor hardware on the

paired chassis becomes the standby. The control plane handles protocol operations like IP routing, peering, route

table updates, and spanning tree BPDUs. The dataplane handles the hardware forwarding of packets, and both

switches are actively forwarding traffic in an active-active operating model.

The VSL allows the switches to communicate and stay in synchronization. The VSS uses the Stateful Switchover

(SSO) redundancy facility to keep the control plane synchronized between the two switches. As a result, the VSS

appears to devices in adjoining layers as a single switch with a single MAC address.

Step 5: Configure dual-active detection mechanism.

In the event that the VSL is severed (that is, all links are down), or for any reason communication is lost over

the VSL (such as excessive high CPU utilization), both switches would assume the active control plane role,

thus creating a dual-active condition, which can result in network instability. To prevent a dual-active scenario

from causing an outage in the network, VSS supports multiple unique dual-active detection and recovery

mechanisms.

The dual-active detection mechanisms are used to trigger a VSS recovery mode. In the VSS recovery mode

only one switch chassis is allowed to remain active, the other switch (the previous VSS active switch) enters

recovery mode, and shuts down all of its interfaces except the VSL interfaces, thereby preventing instability in

the network. Once the VSL is repaired, and communication over the VSL is reestablished, then the VSS reloads

the switch in recovery mode and returns the VSS to a normal operating state.

You can use the following methods to detect this dual-active condition:

• Ethernet Fast-Hello (VSLP) link

• Enhanced Port Aggregation Protocol (PAgP) hellos with an adjacent switch

This design uses the Fast-Hello (VSLP) link for dual-active detection. To configure the link, use a Gigabit Ethernet

interface on each VSS switch chassis and connect them together (similar to a VSL connection) in a back-to-back

fashion. This link does not require high bandwidth because it is only a detection link with control plane hellos on

it.

Figure 22 - VSLP

VSL 10GbE

EtherChannel

VSLP Dual-Active

Detect Link

Active Hot

Standby

2108

D6500-VSS(config)# switch virtual domain 100

D6500-VSS(config-vs-domain)#dual-active detection fast-hello

D6500-VSS(config)#interface range gigabit1/1/24, gigabit2/1/24

D6500-VSS(config-if-range)#dual-active fast-hello

D6500-VSS(config-if-range)#no shutdown

%VSDA-SW2_SPSTBY-5-LINK_UP: Interface Gi2/1/24 is now dual-active detection

capable

%VSDA-SW1_SP-5-LINK_UP: Interface Gi1/1/24 is now dual-active detection capable

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Cisco Catalyst 6880-X Specifications Page 55

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