From: vinny (vinny@foxmail.com)
Date: Sat Feb 07 2009 - 23:29:30 ARST
Hi
To my understanding, RSTP is faster than CST based on four improvments
1. Less states to go through. CST has 5 states while RSTP has only three. This improves the overall forward delay.
2. More port roles. In RSTP, there are four port roles. Other than the classic RP and DP, Alternative Port(AP) and Backup Port(BP) are added. When there's a topology change, without running another SPT calculation, the AP can switch to RP and BP can switch to DP immediately. This improves the delay cased by port role election.
3. It included cisco improvement to CST - Portfast and called it edge ports. This improves the delay at the edge.
4. A new link type is defined. It differentiates P2P link and shared link so that fast negotiation becomes possible on a P2P link. This improves the convergence on a P2P link.
Correct me if I'm wrong.
Cheers
2009-02-08
vinny
7"<~HK#: operator sid
7"KMJ1<d#: 2009-02-06 20:27:59
JU<~HK#: ccielab@groupstudy.com
3-KM#:
VwLb#: Understanding RSTP - Please can someone explain this?
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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