Term
| what must match on all switches within an mst region and how is it communicated? |
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Definition
1. region name
2. revision
3. vlan to instance mapping
communicated via hash in bpdu. |
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Term
| describe how info for each instance is sent in a bpdu. |
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Definition
1. bpdu contains mrecord for each instance, other than the IST.
2. ist does not use an mrecord and is always sent in the bpdu.
3. mrecord will not be included for an instance, if no vlans from the instance are live on the interface. |
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Term
| what info does an mrecord contain? |
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Definition
1. root bid for instance.
2. sending bid for instance.
3. root cost for instance. |
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Term
| what bpdu version is used for each flavour of stp? |
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Definition
0 - 802.1D
2 - RSTP
3 - MST |
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Term
| list the rstp bpdu flags. |
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Definition
0 - TC
1 - Proposal
2-3 - port role
4 - learning
5 - forwarding
6 - agreement
7 - TCA |
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Term
| what are the possible values for the rstp Port Role flag? |
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Definition
00 - unknown
01 - alternate port
10 - root port
11 - designated port |
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Term
| what is the default max-hop count for an mst region and what happens when it is exceeded? |
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Definition
| the default is 20. a bpdu is discarded if the max-hop count is exceeded. |
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Term
| describe the long path-cost calculation method and define some common values. |
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Definition
a 32-bit number is used to compute the path cost values. Some common values are as follows:
10Mbps - 2,000,000
100Mbps - 200,000
1Gbps - 20,000
10Gbps - 2,000
100Gbps - 200 |
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Term
| command to change path-cost calculation. |
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Definition
|
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Term
| how many instances can each mst region support? |
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Definition
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Term
| what will setting the diameter for mst instance 0 achieve. |
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Definition
| the switch will automatically calculate the optimal hello, forward-delay and max-age timers. |
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Term
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Definition
|
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Term
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Definition
| the spanning-tree for any of the configured instances i.e. MST 1-64 |
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Term
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Definition
1. The internal spanning tree instance for a particular region i.e. for MST0.
2. it is the only instance with timer related parameters.
3. ist info is sent with every MST bpdu. |
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Term
| what are the 2 types of ports on an mst switch? |
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Definition
1. intra-mst.
2. boundary. |
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Term
| when are boundary ports defined? |
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Definition
1. mst details don't match (misconfig or separate region).
2. legacy bpdu received.
3. MST might be trying to peer with prestandard mst. |
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Term
| what does mst use to communicate outside the region? |
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Definition
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Term
| why will adding vlans to an mst instance likely result in disruption? |
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Definition
| because if it is configured on 1 switch at a time, the vlan to instance mappings within the region won't match and this is likely to affect the logical topology. Use VTPv3 to overcome. |
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Term
| what is the main disadvantage of having the CST root outside of the mst region in mixed environments? |
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Definition
| The mst region won't be able to load balance across multiple ports. This is because the region is seen as a virtual switch and will have to block on alternate ports. |
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Term
| how do switches outside of an mst region view the region? |
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Definition
the region is seen as a single virtual switch.
1. won't know the real cost to the root (inside mst). the internal cost will be hidden from switches on boundary ports.
2. can't see non-root bridge IDs in the region.
3. sending BID is same as root ID (virtual switch assumes root's details). |
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Term
| describe the 2 root bridges that are created by the mst virtual bridge? |
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Definition
1. CST root is elected for the entire topology. It is ususally beneficial for this to reside within the mst region.
2. CIST root (AKA IST master) is elected within the region. |
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Term
| How is the CIST root (IST master) elected when the CST root is outside the mst region? |
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Definition
1. CIST root is the switch with lowest cost to root.
2. within the region, switches select IST root port based on location of IST master. CST root is not really relevant within the region (from a regular switch perspective). |
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Term
| How is the CIST root (IST master) elected when the CST root is inside the mst region? |
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Definition
| if CST root is within mst region, CST root = IST master. |
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Term
| how can boundary ports be verified on a Cisco switch? |
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Definition
| "Show spanning-tree" - note that boundary ports that are designated will not be labelled. |
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Term
| what issue does pvst simulation overcome? |
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Definition
| it allows mst to be compatible with pvst and rapid-pvst. Standard 802.1s was only designed to be compatible with 802.1d i.e. a single spanning-tree for all vlans. |
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Term
| explain how an mst region communicates with each pvst vlan over a boundary port. |
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Definition
| it replicates each IST bpdu for each vlan and ensures they are tagged correctly on a boundary port. |
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Term
| when migrating from rstp to mst, why does the root need to be in the mst region? |
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Definition
| if the root is in the rstp domain, the root port from the mst region will enter the PVST Inconsistent state if the PVST simulation rules are not met. |
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Term
| commands for a view of instance to vlan mappings |
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Definition
show spann mst con - best
show spann mst |
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Term
| TRUE/FALSE - all inter-region traffic will need to be forwarded via the regional root? |
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Definition
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Term
| 2 critical rules for PVST simulation to work seamlessly. |
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Definition
1. If the root bridge for CIST is within a non-MST region, the spanning-tree priority of VLANs 2 and above within that domain must be better (lesser) than that of VLAN 1.
2. If the root bridge for CIST is within a MST region, VLANs 2 and above defined in the non-MST domains must have their spanning-tree priorities worse (greater) than that of the CIST root. |
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Term
| In an environment with PVST and MST, where the CST root is located within the MST region, explain how the PVST switches will calculate cost to the CST root. |
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Definition
| the PVST switches will see the mst region as a logical switch, cost to the CST root will be calculated as the cost to the nearest switch within the MST region. This is based on the normal stp selection process. |
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Term
| In an environment with PVST and MST, where the CST root is located outside of the MST region, explain how all switches within the MST region calculate cost to the CST Root. |
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Definition
| The cost for all switches is the regional root's cost to the CST master. |
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Term
| How is the CIST Regional Root (IST Master) elected in an MST region? |
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Definition
if the CST root is in the mst region, the regional root is elected using the usual process, e.g. lowest bridge ID etc.
If the CST root is outside of the mst region, the regional root will be the closest mst switch to the CST root. |
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