Netgear XCM8903SF-10000S Datasheet download pdf (Page 4)

• Per VLAN RSTP is similar to IEEE 802.1w (rapid STP), but

implemented per VLAN. Per VLAN RSTP has in built support for

FastUplink and FastBackbone, hence are not required.

On a given switch, only one mode is conﬁgurable at any instant, RSTP,

STP, MSTP, PVSTP or PVRSTP. Enabling PVSTP or PVRSTP disables

other spanning tree modes on the switch. STP (01:80:c2:00:00:00)

and SSTP (shared spanning-tree protocol – 01:00:0c:cc:cc:cd) frames

are generated for each STP instance when these features are enabled.

24. How FastUplink feature works?

The FastUplink feature is used for quick selection of a port with low-

est cost, when the root port fails. In other words, FastUplink feature

is used to reduce the time taken to converge, when a link fails. This

feature is similar to Cisco® Uplink Fast feature. When the primary link

fails, FastUplink will create an alternate path immediately. Since the

ports that apply FastUplink don’t have to wait for normal convergence

time, this will speed up the transition from the failed primary link to the

backup link (the port in a blocking state).

FastUplink feature is only for PVSTP. It is built-in for PVRSTP (no explicit

conﬁguration).

“Cisco” is a registered trademark of Cisco Technology, Inc.

25. How does the FastBackbone feature work?

The FastBackbone feature allows for faster convergence time when

an indirect link to root fails. When a root port or blocked port receives

an inferior BPDU from the designated switch on that port, the switch

infers that an indirect link to the root has failed. To speed up conver-

gence, the max age timer is immediately expired and the port is put

through the Listening and Learning states.

Normal Timers =

Max Age (20 sec by default) + Listening (15 sec) + Learning (15 sec)

With FastBackbone =

Max Age (0 Expired) + Listening (15 sec) + Learning (15 sec)

FastBackbone feature is only for PVSTP. It is built-in for PVRSTP

(no explicit conﬁguration).

26. Is NETGEAR PV(R)STP interoperable with

Cisco® PVST+?

Yes, NETGEAR PVSTP/PVRSTP and Cisco® PVST+/RPVST+

are interoperable.

PVSTP is similar to the MSTP protocol as dened by IEEE 802.1s, the

main dierence being PVSTP runs one instance per VLAN. In other

words, each congured VLAN runs an independent instance of PVSTP.

The protocol is equivalent to Cisco® PVST+ and can interoperate with

each other.

PVRSTP is similar to the RSTP protocol as dened by IEEE 802.1w, the

main dierence being PVRSTP runs one instance per VLAN. In other

words, each congured VLAN runs an independent instance of PVRSTP.

Each PVRSTP instance elects a root bridge independent of the other.

Hence there would be as many Root Bridges in the region as there are

VLANs congured for PVRSTP. PVRSTP is equivalent to Cisco® RPVST+

protocol and can interoperate with each other.

“Cisco” is a registered trademark of Cisco Technology, Inc.

27. What is the dierence between PV(R)STP and MSTP

(Multiple Spanning Tree)?

The dierence between MSTP and PV(R)STP is primarily in the way the

protocol maps spanning tree instances to VLANs: PV(R)STP creates a

spanning tree instance for every VLAN, whereas MSTP maps one or

more VLANs to each MST instance.

28. What if there is a mismatch between one switch

running PVSTP and another switch running PVRSTP?

PVSTP is based on 802.1s and PVRSTP is based on 802.1w. When

a switch running PVRSTP receives PVSTP BPDU, then the switch

falls back to the lower version (PVSTP) aer its migration-delay

timer expires.

29. How are BPDUs transmitted with PV(R)STP?

The switch running PV(R)STP transmits IEEE standard spanning

tree BPDUs along with SSTP BPDUs. SSTP BPDUs are PV(R)STP

dedicated BPDUs.

On Access Ports, the IEEE standard BPDUs and the SSTP BPDUs are

transmitted as untagged packets. These ports are usually member

of only one Access VLAN, and intend to connect devices that are not

VLAN aware (untagged trac on the device side).

On Trunk Ports, the IEEE standard BPDUs and the SSTP BPDUs are

transmitted as untagged packets on the Native VLAN (PVID). For all

other VLANs that are tagged, only SSTP BPDUs are transmitted, as

tagged packets with respective VLANs.

30. How PV(R)STP interact with other IEEE standards

STP/RSTP/MSTP?

A switch running PV(R)STP can communicate with a switch running

IEEE standard spanning tree protocol.

To communicate each other, a switch running PV(R)STP will use the

access VLAN instance on untagged ports (access ports), or the native

VLAN (PVID) instance on tagged ports to send IEEE standard spanning

tree BPDUs that are not tagged. If this switch receives IEEE spanning

tree BPDU packets, it will include and process them in the access VLAN

or native VLAN (PVID) instance on the port.

A switch running IEEE standard spanning tree protocol will use its CIST

to communicate. If the switch receives the SSTP format BPDUs, it

doesn’t treat them as standard BPDUs. Hence the received SSTP for-

matted BPDUs are ooded on all the ports of the corresponding VLAN.

The SSTP BPDUs are multicasted over the region. Only the untagged

IEEE standard BPDU packets received from the switch running PV(R)

STP are processed as standard BPDUs.

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M6100 Chassis Switch 1

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