Term
| what are the rstp port states? |
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Definition
discarding - dropping frames
learning - building cam table
forwarding |
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Term
| what additional port roles are used in rstp? |
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Definition
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Term
| what happens when a non-edge port flaps? |
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Definition
a TCN is sent
the CAM table for the affected vlan is flushed on each switch. |
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Term
| what happens if bpdus are detected on a portfast enabled edge port? |
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Definition
| it loses its portfast status. |
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Term
| what are the 2 types of non-edge ports? |
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Definition
point-to-point - rstp sync process works on these ports.
Shared - must use legacy stp as rstp sync won't work. |
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Term
| how are the 2 types of non-edge port selected. |
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Definition
point-to-point is assumed if link is full duplex.
Shared is assumed if link is half duplex. |
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Term
| briefly describe the rstp sync process. |
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Definition
uses request/response mechanism to achieve convergence.
Should achieve sub-second convergence in properly designed network.
Will establish root port first (except root switch), assume designated and block all other non-edge ports.
Proposals will be sent downstream.
Downstream switches agree/disagree.
topology is built from the root. |
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Term
| on what type of links can the sync process occur? |
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Definition
| point-to-point links only (not shared). |
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Term
| what command is used to enable an 802.1w switch to immediately use an alternate port when the root port fails? |
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Definition
| No commands are required, this functionality is built-in. |
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Term
| if a root port has been determined and all designated ports blocked, when will they be unblocked? |
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Definition
1. When an agreement is received from the downstream switch.
2. If the downstream switch sets its port to alternate/blocking, it sends no agreement and ignores the proposals. The designated switch continues to send proposals and waits for the forward delay timers to expire before the port transitions to forwarding. |
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Term
| when does reconvergence begin in rstp? |
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Definition
| each bridge generates its own hellos, reconvergence begins when 3 hellos are missed from a neighbor. |
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Term
| define an rstp topology change. |
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Definition
generated when link becomes forwarding.
originated by switch that detects change.
uses special bpdu bit.
flooded by all switches using RPF. |
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Term
| how can excessive unknown unicasts be avoided when using rstp? |
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Definition
| ensure all edge ports are defined as such using the portfast command. This will limit TCs. |
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Term
| who generates bpdus in rstp. |
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Definition
1. generated by each switch for designated ports.
2. Generated based on the root's timers.
3. upstream switch is presumed dead if no bpdu is received in 3xhello time. |
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Term
| What triggers a bpdu with the TC flag set? |
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Definition
| when a non-edge port transitions to the forwarding state. |
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Term
| when a TC is detected, on which ports will the switch send bpdus with the TC flag set? |
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Definition
| all non-edge forwarding ports. |
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Term
| when a 802.1w switch detects a TC event, desribe what happens to the CAM table. |
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Definition
flush most of the cam entries for the affected vlan.
1. edge ports are not flushed.
2. MACs on ports receiving TC-BPDU are not flushed. |
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Term
| what will happen if a bpdu is received on an edge port? |
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Definition
| the port will lose its edge status, but it won't trigger a TC-BPDU if the port was already up. |
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Term
| what will happen if a switch is connected to an edge port that was down? |
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Definition
the port will come up as a non-edge port.
A TC BPDU will be triggered. |
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Term
| how do the port costs of rstp compare with regular stp? |
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Definition
By default, costs are the same as legacy stp:
10M - 100
100M - 19
1G - 4
10G - 2 |
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Term
| in what state is an "alternate" port in rstp? |
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Definition
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Term
| in what state is a "backup" port in rstp? |
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Definition
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Term
| command to configure a single rstp "edge" port. |
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Definition
spanning-tree portfast
interface level command - works for access port only.
"trunk" keyword required for trunk edge ports. |
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Term
| if an rstp switch is connected to a switch running stp, how long will it take before traffic can be forwarded on the link? Assume default timers. |
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Definition
30 secs - for the stp switch to transition its port to forwarding.
2 x forward delay. |
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Term
| why are all designated ports blocked during the rstp sync process? |
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Definition
| to prevent transient loops. |
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Term
| what is the role of max age in rstp? |
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Definition
1. hop count - incremented by each switch
2. backwards compatibility on shared ports |
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Term
| a switch loses its root port and has no alternate port, what does it do? |
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Definition
1. declares itself root.
2. syncs this decision.
3. may adapt to superior BPDUs. |
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Term
| methods to avoid "count to infinity" in rstp. |
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Definition
1. keep topology small - placement of L3 boundary.
2. avoid excessive redundancy.
3. rely on physical link failure, not hello bpdus. |
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Term
| command to implement rstp. |
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Definition
| spanning-tree mode rapid-pvst. |
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Term
| how is path selection determined in rstp? |
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Definition
same as stp.
1. lowest root ID.
2. lowest root path cost.
3. lowest sending bridge ID
4. lowest port ID. |
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Term
| quick method to ensure all switches have rstp convergence for vlan 3. |
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Definition
| "show spann vl 3" - stp is converged if all switches agree on root. |
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Term
| when issuing the "show spann" command, what does a port type of "p2p peer (stp)" indicate. |
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Definition
| the switch on the other end is running legacy stp. |
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Term
| what single command will configure all access ports as edge ports? |
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Definition
| spanning-tree portfast default |
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Term
| command to view rstp sync process in action. |
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Definition
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