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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

1 ACCEPTED SOLUTION

Accepted Solutions

Re: mono spanning-tree and PVST

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

3 REPLIES

Re: mono spanning-tree and PVST

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

Re: mono spanning-tree and PVST

--Continuation--

In this fashion for the same sample network( which we discussed above), we can have two root bridges in the network.

One root bridge serving for vlan 1 to 50 and other one serving for vlan 51 to 100.

Hence from the access layer switches, both the uplinks will be active, the first link carrying traffic pertaining to group1 and the second link carrying traffic pertaining to the group2.

In case of a link failure or Distribution switch failure, the other switch/link will serve for all the groups.

802.1s gives us the power to control the redundant links/switches, effectively in our network.

All the above protocols are IEEE based.

PVST and PVST+ are cisco proprietary protocols.

PVST => one Spanning tree instance will be running for one vlan in the network.

If we have 100 vlans in our network, then 100 PVST process/instance will be running in all the switches, 100 root bridges will be elected in the network for corresponding vlans. We have full control over managing redundant links, but the demand for CPU utilisation will go high.

Additionally when vlan trunking involved, PVST protocol will run only when ISL trunking is used.

PVST cannot recognise BPDUs travelling via a 802.1q tagged link. Refer to ISL standards in the URL provided by you.

PVST+ will works well with IEEE 802.1q tagged links also.

In nut shell we can summarise the following points.

802.1d

******

One STP instance for all vlans, less CPU utilisation, inefficient control over utilising redundant links

802.1w

******

One STP instance for all vlans, less CPU utilisation, inefficient control over utilising redundant links. But convergence time is faster when compared to 802.1d

PVST

****

One STP instance for one vlan in the network. More cPu utilisation, when the number of vlans increased. Effective management of redundant link.

Will run only over ISL tagged links. Will not recognise 802.1q tagged links.

PVST+

*****

One STP instance for one vlan in the network. More cPu utilisation, when the number of vlans increased. Effective management of redundant link. Will run over ISL tagged as well as 802.1q tagged link.

802.1s

******

Multiple instances of protocol mapping one or a group of vlans in the network respectively.

Can reduce the number of instances required for the vlans. Less cpu utilisation when compared with PVST/PVST+, Effective utilisation of the links.

For more details on the mechanism of the protocols, you can follow the links provided by you to have more understanding.

HTH

-VJ

Re: mono spanning-tree and PVST

To add to VJ's exhaustive post, the documents you quoted also use old Cisco's terminology for MST.

In old Cisco docs:

MST = Mono Spanning Tree

SSTP = Shared Spanning Tree = PVST

In IEEE terms:

MST = Multiple Spanning Trees

SSTP = Single Spanning Tree

So IEEE and Cisco's terminology use same acronyms for exactly opposite concepts;-) Recent Cisco documentation is now using IEEE terms when describing MST and Rapid-PVST (I hope at least;-)).

Regards,

Francois

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