From: Abdul (rslab007@gmail.com)
Date: Fri Feb 06 2009 - 14:15:44 ARST
Alright...let me see if I can help. Operator Sid, ..basically let me see if
this helps you. Backbone fast, Uplink fast are features which are inherrent
in Rapid spanning tree. Cisco was a head of curve by adding features into
Spanning tree that helped in better convergence and better loop dection than
what the default spanning tree standard 802.1d stardard called for. Then the
industry caught up, and incorporated alot of the features Cisco was
deploying (Uplinkfast, Backbone fast) into Rapid Spanning tree 802.1w.
Port Fast is a feature that simply allows for the port to go into forwarding
while spanning tree is calculated for the port. Now if that spanning tree is
Rapid or the old 802.1d it doesn't matter. Once the algorith is calculated,
if the port should be blocked, then it state would get block, else it keeps
forwarding.
The key to all of this is to understand the basics (801.2d, 802.1w)
differences, similarities then the features that help in faster convergence,
yet protect in loop detection. The protocols themselves can do this without
the added help. But Cisco has given us these features to help us in creating
faster convergence, yet protect us from creating additional loops.
So for example, Portfast..is used for faster convergence (allowing edge
ports non-root or alternate root path ports to go into forwarding mode and
not wait for spanning tree to calculate). BPDUguard, BPDUfilter, root guard
are all tools to help us in adding better protection against loops. They
also add to faster convergence as well.
Other tools MST (multiple spanning tree protocol - 802.1s), PVST, per vlan
spanning tree are a bit more complicated to explain. But in the simplest
explaination, both of them are about how many instances of spanning tree you
are going to run in your enviroment. But fundamentally understanding the
basicas above will go along way to understanding how many instances of the
protocol you wish to run.
I know..it seems confusing..expecially when your first learning it..but
you'll get it. Good questions though.
On Fri, Feb 6, 2009 at 10:31 AM, 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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> >>>
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