Trying to understand RSTP - Please can someone explain this?

Unanswered Question
Feb 6th, 2009


I am still confused about how RSTP is implemented. From what i understand the major difference is that STP

used Timers for Loop prevention whereas RSTP coordinates between neighbors via messages (proposal/aggreement) to turn on links more quickly after topology changes and is "timer free".

However, I have not noticed any difference in the configuration from the legacy STP configurations and the RSTP configuration on cisco devices. Or are there any differences??

I have read in documentation that RSTP natively includes features like UplinkFast, BackboneFast and PortFast. So are these features now obsolete

and not needed to be configured if you are running RSTP. (Although i have seen Portfast still configured along with RSTP on many switches)

Also can someone explain the below Points from Cisco Documentation

1) should STP be disabled on edge ports all together as suggested below?

"STP edge ports are bridge ports that do not need STP enabled, where loop protection is not needed out

of that port or an STP neighbor does not exist out of that port. For RSTP, it is important to disable STP

on edge ports, which are typically front-side Ethernet ports, using the command bridge

bridge-group-number spanning-disabled on the appropriate interface. If RSTP is not disabled on edge

ports, convergence times will be excessive for packets traversing those ports."

2) It seems RSTP relies on duplex setting to determine inter-switch links. What is the configuration to explicitly

configure RSTP link types? (I couldnt find this in the documentation)

"RSTP can only achieve rapid transition to the forwarding state on edge ports and on point-to-point links.

The link type is automatically derived from the duplex mode of a port. A port that operates in fullduplex

is assumed to be point-to-point, while a half-duplex port is considered as a shared port by default. This automatic link type setting can be overridden by explicit configuration. In switched

networks today, most links operate in full-duplex mode and are treated as point-to-point links by RSTP.

This makes them candidates for rapid transition to the forwarding state."

Also i am a bit rough on my RSTP knowledge even after skimming a few Cisco documents. Can someone please explain this in simple way.

Thanks in advance

I have this problem too.
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siddiqirf Fri, 02/06/2009 - 06:51

Does it even matter what the media is, Whether it is copper or Optical? Are you saying spanning tree behaves differently for each ??

dominic.caron Fri, 02/06/2009 - 12:18

It does not matter on what media type you are on your LAN/WAN. What matters is the technology. RSTP is not configure the same on ONS platform and your reference looks like something out of an ONS configuration guide and not a Catalyst.

Giuseppe Larosa Fri, 02/06/2009 - 08:18

Hello Irfan,

RSTP includes and implements several proprietary features that are present in Cisco PVST+.

1) edge ports is the RSTP name for end user port: where we don't need to go through the STP states in 802.1D.

It is important to define what ports are edge ports for example using

spanning-tree portfast

because this helps RSTP: the handshake mechanism with proposal / agreeemnt don't need to be used on these ports.

When a change needs to be handled it doesn't involve leaf ports (edge ports).

The wave of propagation of changes is called synchronization and happens on a single inter-switch link at a time so that it can be loop-free.

The process is not applied to edge ports.

So it is very important in deployment to classify ports.

2) the handshake mechanism requires a true point-to-point link the communication can involve only two parties

Because hub support only half-duplex operation RSTP when finds a normal port with duplex half revert to usage of timers because it supposes this can be a connection one to many via an hub.

to tell RSTP to treat an half duplex link like a normal inter-switch link and to use the handshake you need

int fx/y

spanning-tree link-type ?

point-to-point Consider the interface as point-to-point

shared Consider the interface as shared

spanning-tree link-type point-to-point

As you see there is also an option to declare the link shared

Hope to help


Maximiliano Gus... Fri, 02/06/2009 - 12:03

Hi all,

Just to clarify a bit more about port-fast, edge-port, and RSTP..

If you don't enable portfast on ports connected to end stations (or routers), when there is a TC (Topology Change), the agreement-proposal handshake does not work fine on those ports. The switch running RSTP sends a BPDU with the proposal bit set but there will be no answer from the router/end station, so the switch thinks there is a legacy switch running STP connected to that port and the transition from blocking to forwarding takes 30 seconds. That's why you need to configure these ports as edge-ports (with the "portfast" command).




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