From: Ravi Singh (way2ccie@googlemail.com)
Date: Fri Feb 06 2009 - 14:37:11 ARST
As I said . I stand to be corrected .. Thanks for correcting me operator sid
. I haven't labbed up much on switching yet, I think. Between RSTP & STP ,
PVST simply slipped out of my mind. Here are a few clarifications
You can enable RSTP in a Cisco switch by using the *spanning-tree mode
rapid-pvst *global command. Alternatively, you can simply enable 802.1s MST,
which by definition uses 802.1w RSTP
For the command to change the link type I just read a command *spanning-tree
link-type point-to-point* . I have never tested this, so I am not sure if
this really is the command.
Do you have the book Certification guide 3rd edition by Wendell Odom. there
is a bit of more (different)stuff on this .
I have a few rack sessions scheduled this weekend and I will definitely try
these things out. Thanks for the eye opener , yet again ;-)
Ravi
On Fri, Feb 6, 2009 at 3:31 PM, operator sid <ccie1@live.co.uk> wrote:
>
> Thank Ravi
>
> But few things dont seem correct.
>
> You said
>
> "RSTP does not require any additional configuration by itself , though the
> features which make it RSTP , obviously need to be configured . For example
> you won't explicitly enable RSTP but you would enable portfast, uplinkfast
> or backbonefast with their relevant commands."
>
>
> However Cisco doc says, you have to enable RSTP by using rapid-pvst mode
> instead of the old pvst+ mode as shown below: (For MST, RSTP is built
> within, so you just need to enable MST mode)
>
> Services1(config)#spanning-tree mode rapid-pvst
>
> Further Cisco doc says:
>
> "Spanning Tree UplinkFast and BackboneFast features are PVST+ features.
> These are disabled when you enable rapid-PVST+ because those features are
> built within rapid-PVST+. Therefore, during the migration you can remove
> those commands. "
>
> "However, The configuration of the features such as PortFast, BPDUguard,
> BPDUfilter, root guard, and loopguard are applicable in rapid-PVST+ mode."
>
> Regarding the questions i asked
>
> 1) you seem to agree with Cisco doc on this, however i agree with what Fred
> Quoted in his email from his ARCH book. I Quote again:
>
> "Even if the recommended design does not depend on STP to resolve link
> or node failure events, STP is required to protect against user-side
> loops. There are many ways that a loop can be introducted on the
> user-facing access-layer ports. Wiring mistakes, misconfigured end
> stations, or malicious users can create a loop. STP is required to
> ensure a loop-free topology and to protect the rest of the network
> from problems created in the access layer."
>
> This is very contradictory, as this is also from cisco source.
>
> 2) yOU mentioned change the Duplex setting, in many implementations i have
> seen Auto rather then Full-duplex configured on links, and link may
> negotiate as half 100 (although this may not be ideal) does that mean RSTP
> will not work and will revert back to legacy STP. How many people actually
> notice this? Seems like something you can easily get wrong inadvertently.
>
> Regards...
>
>
> ----------------------------------------
> > Date: Fri, 6 Feb 2009 14:54:22 +0000
> > Subject: Re: Understanding RSTP - Please can someone explain this?
> > From: way2ccie@googlemail.com
> > To: ccie1@live.co.uk
> > CC: ccielab@groupstudy.com
> >
> > Well .. The document really has everything you need to know. Anyhow , I
> will
> > try to clear your doubts in simpler terms .
> >
> > RSTP does not require any additional configuration by itself , though the
> > features which make it RSTP , obviously need to be configured . For
> example
> > you won't explicitly enable RSTP but you would enable portfast,
> uplinkfast
> > or backbonefast with their relevant commands.
> > RSTP is called Rapid only because of these features hence these are a
> part
> > of RSTP.
> >
> > coming to your questions
> > 1) should STP be disabled on edge ports all together ? As a good practice
> ,
> > Yes. Your explanation has the answer "If RSTP is not disabled on edge
> ports,
> > convergence times will be excessive for packets traversing those ports "
> .
> > There is no use for STP to be enabled to a port connected to a PC. so why
> > wait for 30 seconds for the port to realise not to run STP on it.
> >
> > 2)What is the configuration to explicitly configure RSTP link types?
> Again
> > your explanation has the answer "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". So change the duplex settings explicitly and you
> > can change the link type .
> >
> > HTH, and I definitely stand to be corrected so anyone with any comments
> is
> > most welcome.
> >
> > Regards,
> > Ravi
> > On Fri, Feb 6, 2009 at 2:37 PM, operator sid wrote:
> >
> >>
> >> I have read it,and unfortunately its not a good document, i was hoping
> >> someone could explain better...
> >>
> >>
> >> ________________________________
> >>> Date: Fri, 6 Feb 2009 14:09:48 +0000
> >>> Subject: Re: Understanding RSTP - Please can someone explain this?
> >>> From: way2ccie@googlemail.com
> >>> To: ccie1@live.co.uk
> >>> CC: ccielab@groupstudy.com
> >>>
> >>> This might help if you have not read it before
> >>>
> >>>
> >>>
> >>>
> >>
> http://www.cisco.com/en/US/tech/tk389/tk621/technologies_white_paper09186a0080094cfa.shtml
> >>>
> >>>
> >>>
> >>> Ravi
> >>>
> >>>
> >>> On Fri, Feb 6, 2009 at 12:25 PM, operator sid> wrote:
> >>>
> >>> Hi Group
> >>>
> >>>
> >>>
> >>> 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
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