Cisco WS-X6416-GE-MT - Interface Module - Expansion Manual de usuario descargar pdf (Pagina 25)

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expire. This brings the convergence time from 50 seconds to three to five seconds or even

subseconds.

In the case of an indirect failure in a Layer 2 network, Spanning Tree BackboneFast reduces the

convergence time by the “maximum age” timer value (which defaults to 20 seconds).

Finally, Spanning Tree PortFast causes an access port to enter the forwarding state immediately,

bypassing the listening and learning states. The feature is used on switch ports connected to a

single workstation, IP phone, server, and so on and allows these devices to connect to the network

immediately, rather than waiting for spanning tree to converge. Because access ports do not

typically transmit or receive bridge protocol data units (BPDUs) from attached devices, PortFast

mode is supported on both nontrunking access ports and trunk ports in both the Cisco Catalyst OS

and Cisco IOS Software.

Below are the configuration tasks associated with the aforementioned enhancements to PVST+:

Cisco Catalyst OS Cisco IOS Software

set spantree uplinkfast enable

set spantree backbonefast enable

set spantree portfast 3/1 enable

IOS(config)# spanning-tree uplinkfast

IOS(config)# spanning-tree backbonefast

IOS(config)# int range fa3/1

IOS(config-if)# switchport

IOS(config-if)# spanning-tree portfast

Rapid PVST

Rapid PVST+ is based on the IEEE 802.1w standard and uses the existing configuration for

PVST+ to provide for faster Spanning Tree Protocol convergence times. With Rapid PVST+,

entries are flushed immediately on a per-port basis on topology changes. UplinkFast and

BackboneFast configurations are ignored in this mode, as both features are included in the Rapid

Spanning Tree Protocol (IEEE 802.1w).

Cisco Catalyst OS Cisco IOS Software

Set spantree mode rapid-pvst+ IOS(config)#spanning-tree mode rapid-pvst

IEEE 802.1S (MST)

Multiple Spanning Tree (MST) is based on the IEEE 802.1s standard and extends the IEEE

802.1w rapid spanning tree (RST) algorithm to multiple spanning trees. This provides both rapid

convergence and load balancing in a VLAN environment while converging even faster than

PVST+.

MST allows the formation of spanning trees over trunks, to provide multiple forwarding paths for

data traffic. This improves fault tolerance, as a single failure does not directly affect other

instances of spanning tree. Additionally, by grouping multiple VLANs into single instances of

spanning trees, the overall CPU of the system decreases significantly.

One major difference between the configuration of MST on the operating systems is the MST

configuration submode in Cisco IOS Software. This mode is used to both enter and to display the

MST configuration:

Cisco Catalyst OS Cisco IOS Software

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