link on cisco MISTP

Answered Question
Sep 1st, 2008

hi everybody!

I need some god link on cisco Mistp.

Could anybody help me with this please?

thanks a lot

I have this problem too.
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Correct Answer by Giuseppe Larosa about 8 years 3 months ago

Hello Sarah,

you can create multiple MST instances but in MST configuration mode, and by declaring what Vlans are associated to each of them.

Then, at the interface level you can modify the port cost for a specific MST instance, I'm not sure you can change port priority only for one MST instance.

Different MST instances can have the same root bridge and with different per port STP costs you will get different topologies.

All MST config must be done in mst config mode and in all switches that are in the same region.

About Max-Age: MST 802.1s is based on Rapid STP 802.1W so questions about max-age are of less interest : where is no TTL on STP BPDUs and the original settings of max-age have been thought to accomodate the flow of STP BPDUs every 2 seconds from root bridge to branch switches.

Max-age is used in 802.1D to decide when to react to missing BPDUs received on the root port: it is taylored for the network size.

In 802.1w a switch can move an alternate port to forwarding state by doing an handshake with its neighbor (on the alternate link) after 6 seconds so max-age is not used here.

Hope to help

Giuseppe

Correct Answer by Giuseppe Larosa about 8 years 3 months ago

Hello Sarah,

thanks for your good remarks.

Before in PVSTP scenario each Vlan had its own STP instance running.

Now, the switch has to perform an additional lookup to understand to what MST instance is the Vlan associated to.

Active on a port = configured on the port as the data or voice vlan in an access port or as a Vlan permitted on the trunk in a trunk port.

in PVST you can disable STP on a specific vlan this doesn't stop MAC address learning and frame switching.

I'm not sure but in MST there are even less reasons to disable STP on a specific Vlan and may be it is impossible ( all vlans at the beginning are associated to IST instance 0).

If a Vlan is not specified in the range for all MST instances 1 to 15 it is with IST.

Hope to help

Giuseppe

Correct Answer by Giuseppe Larosa about 8 years 3 months ago

Hello Sarah,

MISTP is simply a pre-standard Cisco version of MST 802.1s and probably is not supported anymore on new releases that support MST.

http://www.cisco.com/en/US/docs/switches/lan/catalyst6500/ios/12.2SX/configuration/guide/spantree.html

Hope to help

Giuseppe

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sarahr202 Mon, 09/01/2008 - 11:09

Thanks for your Reply. The link you forwarded says" The switch forwarding logic for vlan was changed a little bit. In order for a frame to be forwarded out of a port,two conditions must be met:

first, vlan must be active on this port,secondly the stp instanc the vlan maps to , must be in non-discarding state for this port"

My question is what we mean by active on the port?

My other question is it not always the case for frame to be forwarded out of port that port should in forwading state and vlan to which frame destined to must be active on the port?

thanks a lot !

Correct Answer
Giuseppe Larosa Mon, 09/01/2008 - 11:29

Hello Sarah,

thanks for your good remarks.

Before in PVSTP scenario each Vlan had its own STP instance running.

Now, the switch has to perform an additional lookup to understand to what MST instance is the Vlan associated to.

Active on a port = configured on the port as the data or voice vlan in an access port or as a Vlan permitted on the trunk in a trunk port.

in PVST you can disable STP on a specific vlan this doesn't stop MAC address learning and frame switching.

I'm not sure but in MST there are even less reasons to disable STP on a specific Vlan and may be it is impossible ( all vlans at the beginning are associated to IST instance 0).

If a Vlan is not specified in the range for all MST instances 1 to 15 it is with IST.

Hope to help

Giuseppe

sarahr202 Mon, 09/01/2008 - 14:37

hi Giuseppe!

I am just wondering if it is possible to create MST instances by modifying path cost or port priorty while keeping the same switch as root for all instances?

My second question is about the Max-age in Mst.

The max-hop should be configured to accommodate the edge switch. For example the farthest switch from the root switch is 6 hops away.If we configure max-hop as 5, then that farthest switch will receive bpdu with max-age=0, which will cause this switch to declare itself as root in its bpdu.That means Max hop should be chosen in such manner that farthest switch from root can receive it without max-age=0.

Am i right?

thanks a lot!

Correct Answer
Giuseppe Larosa Mon, 09/01/2008 - 22:48

Hello Sarah,

you can create multiple MST instances but in MST configuration mode, and by declaring what Vlans are associated to each of them.

Then, at the interface level you can modify the port cost for a specific MST instance, I'm not sure you can change port priority only for one MST instance.

Different MST instances can have the same root bridge and with different per port STP costs you will get different topologies.

All MST config must be done in mst config mode and in all switches that are in the same region.

About Max-Age: MST 802.1s is based on Rapid STP 802.1W so questions about max-age are of less interest : where is no TTL on STP BPDUs and the original settings of max-age have been thought to accomodate the flow of STP BPDUs every 2 seconds from root bridge to branch switches.

Max-age is used in 802.1D to decide when to react to missing BPDUs received on the root port: it is taylored for the network size.

In 802.1w a switch can move an alternate port to forwarding state by doing an handshake with its neighbor (on the alternate link) after 6 seconds so max-age is not used here.

Hope to help

Giuseppe

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