Rapid Spanning Tree Question

All,
I have a question about Rapid Spanning Tree reconfiguration. I have to following situation:
As you can see 3 switches with RSTP, and 2 switches without RSTP (or any other spanning tree, just unmanaged).
The 2 switch will form a loop in my network. Switch 1 will block one of the ports and the other port will forward the traffic.
If I break the link "Just Forwarding", my second switch won't be able to cumminucate for around 40 seconds. It will take some time before the backup link will be up again.
Cisco has the Fastforwarding mechanism. Will this help in this situation? I would like to shorten the 40 seconds time.
Thans in advance.

I'd guess the unmanaged devices run legacy spanning tree, and rapid
pvst switches will run rapid according the "heard" protocol. So if it hears
the legacy bpdu, it will run regular spanning tree, hence the 40 second delay.
chris

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Rapid spanning tree / portfast

hello together,
i have a question about rapid spanning tree.
If I enable per vlan rapid spanning tree do i have to configure portfast on the access ports or is this nativly done in rstp?
best regards
lars

Hi Lars,
In RSTP, the access ports are known as "edge" ports. To configure a port as an "edge port" you use the same command to enable portfast to do this.
"Edge portsIf you configure a port as an edge port on an RSTP switch by using the spanning-tree portfast interface configuration command, the edge port immediately transitions to the forwarding state. An edge port is the same as a Port Fast-enabled port, and you should enable it only on ports that connect to a single end station."
http://www.cisco.com/univercd/cc/td/doc/product/lan/c3550/12113ea1/3550scg/swmstp.htm
HTH,
Bobby
*Please rate helpful posts.

Rapid Spanning Tree, 802.1w

Do any SBTG switches support Rapid Spanning Tree? It appears the 3560x does, but looking for more "cost effective" solution.

Hi Art,
The new and improved 300 series (SRWxxx-K9-NA) which is a refresh for the older SRW series, shows that it supports STP, RSTP and MSTP.
http://www.cisco.com/en/US/products/ps10898/prod_models_comparison.html
regards and seasons greetings
Dave Hornstein

Rapid spanning tree combnation

Dear All,
I am new to Spanning tree technology...and it sounds pretty good to run 802.w on LAN,
Is it posible to run 802.w on switches that support Rapid spanning tree and some old ones that do not ?
Is there any way to prevent BPDU to be send to switch that do not support 802.w ?
Looking forward to hearing from you??
Best regards,
Sholeh

The roles were in fact introduced by RSTP. Because it was also very convenient with regular STP, we added them to our implementation of STP. However, older software are just showing the information defined in STP at that time.
A forwarding port is indeed designated or root. In order to make a difference between the two, you need to check what is the designated bridge ID. If this is the local bridge, the port is designated. If it's a different bridge, it's a root port.
Another simple way: you also get the root port for the vlan in the show spantree. There is only a maximum of one root port on a bridge, so if your forwarding port is not the root port, it is then designated.
Note that STP does not make any difference between backup and alternate port either. For this, you need again to look for the designated bridge ID on this port. If it is the bridge itself, this is a backup port, else, an alternate port (this is useful for uplinkfast, only alternate port can do fast transition).
Regards,
Francois

Spanning tree question

I want to change the spanning tree root on several vlans on my network. My question is, will this cause STP to recalculate for the entire network, which can cause the network to slow down or will it only affect the vlans that I am changing? I want to make sure I am not going to impact anything on the network.

When executed properly, this will only affect the vlans that you wish to change. Use the folllowing command to change spanning tree prio on a vlan:
spanning-tree vlan xx priority 4096 (or a multiple of 4096 for less priority)
Be aware that there is always a risk of unexpected disruptions when you do this. The vlans that you change may still carry user traffic altough there are no users on it. If your topology and traffic flow are not exactly as you assume they are, more vlans may be affected. It is therefore not advisable to alter this setting during peak-hours.
Regards,
Leo

Rapid Spanning Tree Problem

Hi all,
I am experiencing an RSTP problem. I have two swtitches connected via wireless link, the port is in trunk mode, the native vlan is vlan 1 the problem is that bpdu's are exchanged for other vlan's but not for vlan 1, when i connect a second backup wireless link it causes the loop, it seems that there are no bpdu exchanges between switches for vlan 1, also in trunk ports i see that BPDU's for vlan 1 are sent by both switches but they do not receive any BPDU's from each other. Any explanation about thiss issue ?
Thanks in advance

I would need to know some things to troubleshoot this:
1. Is VLAN 1 the native VLAN of the trunk, on both sides?
2. I presume VLAN 1 is in the allowed VLANs list on both sides of the link?
3. If the native VLAN is not 1, is the native VLAN allowed on the trunk, on both sides?
4. What model of switch is it, and what version of the software?
5. Can you do a show run int for each end of each trunk link?
6. Can you do a show int xxx trunk for each end of each trunk link?
7. Can you do a show spanning-tree vlan 1 on each side of each trunk?
Kevin Dorrell
Luxembourg

Spanning Tree questions ...

Hi,
There are four 6500 switches configured as below:
PortChannel
& Trunk & STP
SW1 --------- SW2 ----PC1
NO PortChannel | |No
& No TRunk | |PortChannel
& NO STP | |& No Trunk
| |& No STP &
PC2--------SW3 ---------- SW4
PortChannel &
Trunk & NO STP
Between SW1 & SW2, there is a STP enabled on each PVST. For other connections, there is NO STP enabled.
THe configuration as below:
SW1-SW2 : PortChannel, Trunk & STP enabled
SW1-SW3 : No PortChannel, Trunk & STP disabled
SW2-SW4 : No PortChannel, Trunk & STP disabled
SW3-SW4 : PortChannel & Trunk enanabled, STP disabled
In this configuraiton, the SW2 will block the connection between SW1 & SW2 to prevent loop.
Also, there is only ~20 seconds network interruption on particular connection if any one of connections broken.
I would like to know whether above configuration is supported. Is there any impact or unpredicatble issue?
In addition, I would like to know whether we can enable BackBone Fast feature if SW1 & SW2 are third party switches.
Thanks in advance.
Rgds,
Iavn Cheng

Hi Iavn,
I guess that in your diagram SW1 and SW2 are running STP but SW3 and SW4 are not.
First, especially if you use third party devices, I want to warn you that there is no standard defining what "disabling STP" means, so interpretation may vary. In Cisco's PVST, we flood BPDUs, which allow you indeed to break the loop between SW1 & SW2 (SW1 and SW2 just see redundant point to point links between them).
Your configuration will however -1- be less efficient that running STP everywhere, plus -2- it breaks one of the basic asumption of STP.
-1- When a link fails between two hosts that don't run STP, the reconvergence can only be based on timer. In your example, if the link between SW3 and SW4 breaks, you will need max_age + 2xforward_delay to recover with STP. Even with RSTP your convergence time will depend on timer, which is not efficient.
-2- But the real problem, which is related to the same scenario is that when the network has converged after the failure of the link SW3-SW4, bringing back up this link will result in a temporary bridging loop. This is because SW3 and SW4 are not running STP and will put their ports directly to forwarding. Even temporary, loops are bad in a L2 network:-(
At last, backbonefast is absolutely helpless if you don't run STP on each switch. Backbonefast is supposed to save you waiting max_age when a bridge that does not have any alternate port loses its root port. In the case where you only have two bridges running STP (SW1 and SW2), you will never get into this situation anyway. If you run STP on all 4 switches, then it makes sense. As already mentioned by Georg, Backbonefast is a proprietary feature.
I highly recommend running RSTP (whether Rapid-PVST or MST) on all switches if possible.
Regards,
Francois

About Spanning tree problem

I am a newbie for cisco switch.
I need a failover solution for both switch and AP Bridge link on both side.
I have 2 of location (Location A and Location B)
Location A
There has 3 set of cisco 2960 switch.
switch C is active switch
switch A is redundancy switch , it will be active when primary Wi-FI Link and switch C is failure.
Location B
There has 3 set of cisco 2960 switch
switch D is active switch
switch B is redundancy switch ,it will be active when primary Wi-Fi Link and switch D is failure.
I would like to use spanning tree protocol for this case.
As show my diagram, Can it achive failover for both switch and AP bridge link if I use this network design
Please help to comment
Thanks
John

Hi John,
This is achievable. The best way to do this is, If you can control the client switches,
make the Client switch at location A, the root primary for the STP domain.
On the Client switch at location B, make the STP cost high on the port towards the Switch B.
Assuming all other STP settings are on default values, this should block the link between LocationB client switch and Switch B. So all your traffic will take the path through switchC-SwitchD.
If the Wifi Bridge fails (AP3-AP4), the blocked link will start forwarding (make sure you are using rapid spanning tree for fast transition)
Now the most important thing in this design is to make sure that the Wifi bridges pass STP BPDU traffic, if they don't, this will not work.
Even if one of the switches fails on the active path, the backup path would still kick in..
Let me know how you go with this..
please rate helpful posts.. :)

Integrated switch - supports STP/RSTP? (Spanning Tree Protocol)

Greetings all..
As this device includes a 3-port switch, which can be integrated with the rest of the network, does it support either Spanning Tree Protocol (802.1d) or Rapid Spanning Tree Protocol (802.1w)?
Neither protocol is terribly difficult to implement, and since these things are so heavily marketed in the education space, it seems like it's a feature that should be there.
That said, when I look at the switch port that the AEBS is connected to (in bridge mode, obviously), there is no interaction with STP or RSTP. Is there some sort of "friendly" name for the option in the AirPort Utility that's eluding me?
While I'm not going to create loops in the network by cabling this up, I can't count on someone else coming behind me doing the wrong thing...

I have had sniffers on LAN ports off various AirPort AP before, never a sign of a BPDU. I am fairly certain that it does not support Spanning Tree.

Mono spanning-tree and PVST

Refering to these two links
http://www.cisco.com/en/US/tech/tk389/tk390/technologies_tech_note09186a0080094665.shtml
http://www.experts-exchange.com/Hardware/Routers/Q_21349385.html
IEEE 802.1Q defines a single instance of spanning tree running on the native VLAN for all the VLANs in the network which is called Mono Spanning Tree (MST). This lacks the flexibility and load balancing capability of PVST available with ISL. However, PVST+ offers the capability to retain multiple Spanning Tree topologies with 802.1Q trunking.
IEEE 802.1Q defines a single instance of spanning tree running on the native VLAN for all the VLANs in the network which is called Mono Spanning Tree (MST). This lacks the flexibility and load balancing capability of PVST available with ISL. However, PVST+ offers the capability to retain multiple Spanning Tree topologies with 802.1Q trunking.
http://networking.ringofsaturn.com/Certifications/BCMSN.php
Per-VLAN Spanning Tree (PVST) ? A Cisco proprietary method of connecting through 802.1Q VLAN trunks, the switches maintain one instance of the spanning tree for each VLAN allowed on the trunk, versus non-Cisco 802.1Q switches which maintain one instance for ALL VLANs. This is the default STP used on ISL trunks.
http://www.informit.com/content/images/1587051427/samplechapter/1587051427content.pdf
The 802.1Q standard defines one unique Spanning Tree instance to be used by all VLANs in the network. STP runs on the Native VLAN so that it can communicate with both 802.1Q and non-802.1Q compatible switches. This single instance of STP is often referred to as 802.1Q Mono Spanning Tree or Common Spanning Tree (CST). A single spanning tree
lacks flexibility in how the links are used in the network topology. Cisco implements a protocol known as Per-VLAN Spanning Tree Plus (PVST+) that is compatible with 802.1Q CST but allows a separate spanning tree to be constructed for each VLAN. There is only one active path for each spanning tree; however, in a Cisco network, the active path can be
I could not get exactly what these Terminology (PVST, instance, PVST+, MST, etc) trying to achieve ?
Any URL and online resource help me to do some extar reading to clarify these terminology

Hi,
The URLs that you have provided all explains the same technical details in different fashion.
I will summarise them here for better clarity.
There are two separate technologies that needs clarity.
1) Method of Trunking many vlans across a link
2) Spanning tree
Now for point 1, we have the IEEE standard 802.1q, which mentions how multiple vlans can be carried across a link. As per this standard a 4 byte tag will be inserted in the ethernet packet, ( inserted between the Destination mac address field and the ethertype field)
This tag will contain the vlan identifier info and some other details ( available in the urls that you have highlighted)
Cisco has a proprietary technology called ISL which effectively does the same job in a different fashion but can only be used in cisco devices.
Now for point 2, again we have IEEE standards like 802.1d ( common/mono spanning tree), 802.1w/RSTP ( Rapid spanning tree) and 802.1s/MSTP.
In 802.1d, there will be only one spanning tree process/instance running for the whole network, irrespective of how many vlans are involved in the network. Hence the whole network is treated as one common domain by the STP protocol.
So, there can be only one root bridge in the network and other bridges will intelligently block the redundant links, we wont have much control to effectively utilise the redundant links.
IEEE 802.1w/RSTP also works in the same fashion, but the convergence time is very fast in this protocol.
Here also there is only one spanning tree instance involved.
In both the above STP protocols, there is only one instance/process of the protocol running in the network, which is common for all vlans. Hence these protocols consume only very less CPU utilisation.
In 802.1s/ MSTP ( multiple spanning tree), extends the 802.1w Rapid Spanning Tree Protocol (RSTP) to have multiple STP instances. In this protocol, we can group the desired vlans in to one Instance of the protocol.
Say for example, lets assume a typical campus network with multiple access switches, 2 distribution/core switches.
Access switches having dual connectivity to the distribution/core switches.
In this topology if we deploy 802.1d or 802.1w, the redundant links from the access switches to the distribution/core switches will be blocked. Only one uplink from the access switch to the distribution layer will be working at any point.
In this network, only one distribution/core switch will be root bridge for the entire network.
But if we deploy 802.1s for this network, we can design it as follows.
We can split the vlans in to two groups,
Group1 => vlan 1 to 50
Group2 => vlan 51 to 100
We can create two instance of MSTP protocol with the following mappings
Instance 1 => for Group 1, with one distribution/core switch as the root bridge
Instance 2 => for Group 2, with another distribution switch as the root bridge
--Continued

Which spanning tree protocol is preferred PVST or rapid-PVST and why?

I have WS-C2960G-24TC-L and Cisco 3750G switches, I have option to configure PVST spanning tree or rapid-pvst. Please let me know which is better and why? also send me some document explaining both protocols in detail.

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As Alex has noted, normally rapid-PVST should be preferred.
Depending on needs (and device support), MST might be better yet.

Spanning Tree Config Question

What is the difference between using the following 2 command to gaurentee a certain switch as the root switch? Can I use either one? Is one way more beneficial than the other?
1) spanning-tree vlan 2 priority 8192
2) spanning-tree vlan 2 root primary

There is more to the "root primary" command that just setting the priority. The root command is in fact a macro that configures the priority but also the spanning-tree timers (by macro I mean that this command is in fact expanded into several different configuration lines, one of them being the stp priority. The macro is not showing up in the configuration). This is particularly interesting if you want to adjust your timers based on the diameter of your network in PVST mode.
If it's just a matter of configuring the root bridge, I don't really get the point of using this macro. Configuring a bridge as root can be done in a trivial way with the priority command.
Personally, I don't like the switch to pick up a priority for me and I prefer choosing the value myself with the priority command. At least, I know what is happening... but that's a matter of taste at that stage.
Just be aware that if you use the "root primary" command, you will have your timers also set to their default values.
Regards,
Francois

Switching Best Practice - Spanning Tree andEtherchannel

Dear All,
Regarding best practice related to Spanning Tree and Etherchannel, we have decided to configure following.
1. Manually configure STP Root Bridge.
2. On end ports, enable portfast and bpduguard.
3. On ports connecting to other switches enable root guard.
In etherchannel config, we have kept mode on on both side, need to change to Active and desirable as I have read that mode on may create loops? Please let me know if this is OK and suggest if something missing.
Thank You,
Abhisar.

Hi Abhisar,
Regarding your individual decisions: Manually configuring the Root Bridge is a natural thing to do. You should never leave your network just pick up a root switch based on default switch settings.
On end ports, using PortFast and BPDU Guard is a must especially if you are running Rapid PVST+ or MSTP.
Regarding the Root Guard on ports to other switches - this is something I do not recommend. The Root Guard is a protective mechanism in situations when your network and the network of your customer need to form a single STP domain, yet you want to have the STP Root Bridge in your network part and you do not want your customer to take over this root switch selection. In these cases, you would put the Root Guard on ports toward the customer. However, inside your own network, using Root Guard is a questionable practice. Your network can be considered trustworthy and there is no rogue root switch to protect against. Using Root Guard in your own network could cause your network to be unable to converge on a new workable spanning tree if any of the primary links failed, and it would also prevent your network from converging to a secondary root switch if the primary root switch failed entirely. Therefore, I personally see no reason to use Root Guard inside your own network - on the contrary, I am concerned that it would basically remove the possibility of your network to actually utilize the redundant links and switches.
Regarding EtherChannels - yes, you are right, using the on mode can, under circumstances, lead to permanent switching loops. EtherChannel is one of few technologies in which I wholeheartedly recommend on relying on a signalling protocol to set it up, as opposed to configuring it manually. The active mode is my preferred mode, as it utilizes the open LACP to signal the creation of an EtherChannel, and setting both ends of a link to active helps to bring up the EtherChannel somewhat faster.
If you are using fiber links between switches, I recommend running UDLD on them to be protected against issues caused by uni-directional links. UDLD is not helpful on copper ports and is not recommended to be run on them. However, I strongly recommend running Loop Guard configured globally with the spanning-tree loopguard default. Loop Guard can, and should, be run regardless of UDLD, and they can be used both as they nicely complement each other.
My $0.02...
Best regards,
Peter

Spanning tree - balanced without use vlan ?

Hi, i´m sorry if this is a classic question.
i have implemented rapid pvst like show in the image. The dotted lines are the alternative links. (image 1)
SwitchA# spanning-tree vlan 1 root primary
SwitchB# spanning-tree vlan 1 root secondary
I want to make a kind of balancing like image 2. But the problem is that i have vlan 100 (and other vlans) in side A and Side B.
So, if i make
SwitchB: spanning-tree vlan 100 root primary
SwitchA: spanning-tree vlan 100 root secondary
The SwB it change to primary for vlan 100.
But i want to the switchB be the primary for side A and secondary for side A. No matter the vlan. Is possible?
Thanks a lot!
IMAGE 1
IMAGE 2
PS: Later i will implement HSRP.

Hi, i know that is possible, but doing this the result is unbalanced for mi network. For example vlan 20 reside in all switches and vlan 21 reside in only one switch.
i want to the switchA be the primary for side A and secondary for side B. No matter the vlan. Like image 2.
I hope to be clear.
Thanks.

Changing spanning tree modes / potential outages?

Hi All,
Our core / distribution / access layers are all currently configured to use Cisco's PVST+. We are now a fully populated Cisco network with no standards based STP so we can now migrate to Rapid PVST.
By simply changing the spanning tree mode on a an access switch to Rapid PVST will the vlans with spanning tree enabled suffer an outage ? If so will this be time be based on how the rest of the network is configured ? That is, if the rest of the network is still PVST+ and I change a switch to Rapid PVST will the outage deault to around 45 seconds based on PVST+ timers ?
Furthermore, as I understand it, even though this access switch would now be configured for Rapid PVST, the switch defaults back to PVST until the rest of the network (or VLAN) is configured for Rapid PVST.
My second question is this :
Assuming that all the access layer switches have been migrated to Rapid PVST, what would be the effect of then migrating the distributing and potentially core layer devices to Rapid PVST ? Will they also cause an outage on the VLAN on which STP is enabled ? Again, if so, what would the outage be ? Would this be based on PVST timers or Rapid PVST ?
Thanks in advance.
Chris.

Mike
No problem and please do come back if needed.
One thing I should have answered from your questions but didn't directly was the question of the mac address of the root switch.
The mac address that is important in the root switch election is the one contained in the BPDU not the source mac address of the BPDU. The source mac address is simply that of the port that transmitted the BDPU.
If a switch flushes it's mac address table it would remove that mac address but that would make no difference as to whether the switch believed it had lost it's path to root or not.
In terms of switch to switch communication BPDUs are sent with a multicast destination mac address so removing that mac address has no effect on BPDUs being exchanged.
So the fact that you are seeing the switch reporting it has lost it's path to root is not a direct consequence of the mac address being flushed because it doesn't need that to send and receive BPDUs.
However with all the flooding of end to end devices because of the flushing an indirect consequence may be that BPDUs are getting lost.
Apologies for not making that clearer.
Jon

Rapid Spanning Tree Question

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