STP & EtherChannel Flashcards
(27 cards)
What are the benefits of network redundancy?
Keeps the network reliable by preventing service disruptions.
Eliminates single points of failure with backup equipment and links.
Redundant paths ensure traffic can reroute instantly if the main path fails.
What are the risks of network redundancy without STP?
Redundant paths can cause physical and logical Layer 2 loops.
MAC Address Table Instability – Copies of the same frame being received on different ports. MAC table instability causes switches to make wrong forwarding decisions.
Broadcast Storms – Broadcasts endlessly circulate, using up bandwidth and disrupting network traffic. Broadcast storms overwhelm the network.
Both result in network congestion, dropped frames, and communication failure.
What is MAC Address Table Instability?
Ethernet has no TTL, so frames can loop endlessly.
Broadcasts (like ARP) loop through redundant links.
Switches keep relearning the same MAC address on different ports.
This constant change confuses switches, so they can’t correctly forward traffic.
Eventually, even unicast traffic starts looping, making the host unreachable.
What is a Broadcast Storm?
A broadcast storm happens when too many broadcast frames flood the network, often due to switching loops, overwhelming devices and causing network slowdowns or failure.
What is Spanning Tree Protocol (STP)?
STP is a protocol used to prevent network loops in Layer 2 networks.
It ensures redundancy by allowing only one active path between devices, blocking other paths that could cause loops.
This creates a loop-free topology while still maintaining backup paths for reliability.
How does STP build a loop-free topology?
STP builds a loop-free topology in a four-step process:
- Elect the root bridge.
- Elect the root ports.
- Elect designated ports.
- Elect alternate (blocked) ports.
What is a Bridge Protocol Data Unit (BPDU)?
BPDUs are special frames used by switches to share STP information.
They help elect the root bridge and decide port roles (root, designated, alternate).
What is the root bridge? How is it elected?
The root bridge serves as a reference point for all spanning-tree calculations to determine which redundant paths to block.
STP elects the root bridge based on the lowest Bridge ID (BID).
Priority is checked first (default is 32768, lower is better).
If priorities match, the lowest MAC address wins.
How is the root path cost calculated?
It is the sum of the port costs of all the links from a switch to the root bridge in the Spanning Tree Protocol (STP).
What are the different STP port roles?
Root Port - One port on a switch that has the lowest cost to reach the root bridge.
Designated Port - One port per network segment (link) chosen because it has the lowest cost to the root bridge on that segment. It forwards traffic for that segment.
Alternate and Backup Port - Alternate ports are selected only on links where neither end is a root port. Both ports are in blocking state, to prevent loops.
Disabled Port - Port that is shut down.
How are port roles decided?
On the root bridge, all ports are designated ports.
On other switches:
- The port with the lowest-cost path to the root bridge becomes the root port (also forwarding traffic).
For each link between two switches:
- One port is a designated port.
- If one end is a root port, the other end is a designated port.
- If neither end is root, the switch with the lowest cost to the root bridge has the designated port on that link.
All other ports that don’t fit these roles become alternate ports (blocked to prevent loops).
Ports connected to end devices are also always designated ports.
How are port roles decided when there are multiple equal-cost paths?
When a switch has multiple equal-cost paths to the root bridge, the switch will determine a port using the following criteria:
- Lowest sender BID.
- Lowest sender port priority.
- Lowest sender port ID.
What are the different STP port states?
Disabled: Manually turned off, no activity.
Blocking: Doesn’t forward data, only listens to BPDUs.
Listening: Still not forwarding data, but sends and receives BPDUs.
Learning: Learns MAC addresses (updates MAC table) but still doesn’t forward data; sends and receives BPDUs.
Forwarding: Forwards data and sends/receives BPDUs.
What are the different types of STP?
STP = Original STP standard.
- Only one path is used for the whole network no matter how many VLANs — no load balancing.
- Slow convergence.
- Low resource usage, but not flexible
RSTP = Faster version of STP.
- Still one tree for the whole network.
- Fast convergence.
- Good for faster recovery, but not VLAN-specific.
MSTP = RSTP for groups multiple VLANs to share the same spanning tree
- Groups multiple VLANs to share the same spanning tree.
PVST+ = One STP instance per VLAN.
- Cisco proprietary.
- Different VLANs can use different paths, allowing load balancing.
- More flexible, but uses more CPU, especially with many VLANs.
Rapid PVST+ = RSTP (which is improved STP) also one STP instance per VLAN.
- Runs a separate RSTP instance per VLAN.
- Combines faster convergence with per-VLAN flexibility.
What do PortFast and BPDU Guard do?
PortFast: Makes an access port go straight to forwarding state to connect devices faster. Only use on ports connected to end devices, not switches.
BPDU Guard: Automatically disables a PortFast port if it receives any BPDUs, preventing loops. The port must be manually re-enabled by the admin.
Does link speed affect STP path cost?
Yes, link speed affects STP via path cost, which can influence which ports are root/designated. But if all links are the same speed, it won’t matter, and STP will fall back on other criteria to decide port roles.
A quicker (faster) link means lower cost. Path cost is evaluated first in the STP decision-making process.
Why is EtherChannel required?
Sometimes one link isn’t enough for bandwidth or redundancy.
But STP blocks extra links to avoid loops.
EtherChannel solves this by bundling multiple physical links into one logical link.
This lets switches use all links together for more speed and backup, without STP blocking them.
What is EtherChannel? What are the advantages?
Combines multiple ports into one logical link called a port channel.
Configurations are done on the port channel, keeping all links consistent.
Uses existing ports—no hardware upgrades needed.
Balances traffic evenly across all links for better performance and reliability.
Provides redundancy by staying active even if one link fails.
What are the restrictions of using EtherChannel?
You cannot mix interface types (e.g., Fast Ethernet and Gigabit Ethernet) in the same EtherChannel.
Up to 8 physical ports can be bundled together.
Port settings must match on both ends: all member ports must have the same speed, duplex, VLAN settings, and trunk/native VLAN configurations.
What are the two Auto Negotiation Protocols?
EtherChannels can be formed through negotiation using one of two protocols, Port Aggregation Protocol (PAgP) or Link Aggregation Control Protocol (LACP).
These protocols allow ports with similar characteristics to form a channel through dynamic negotiation with adjoining switches.
On Cisco devices, both PAgP and LACP are supported.
What is Port Aggregation Protocol (PAgP)?
Cisco-proprietary protocol for EtherChannel negotiation.
Exchanges PAgP packets between compatible ports to form an EtherChannel.
When links match, it bundles them into one EtherChannel. The EtherChannel appears as a single port in the spanning tree.
What is Link Aggregation Control Protocol (LACP)?
Used to facilitate EtherChannels in multi-vendor environment.
What are the Port Aggregation Protocol (PAgP) modes?
On: Channel member without negotiation (no protocol).
Desirable: Actively asking if the other side can or will participate.
Auto: Passively waiting for the other side.
What are the Link Aggregation Control Protocol (LACP) modes?
On: Channel member without negotiation (no protocol).
Active: Actively asking if the other side can or will participate.
Passive: Passively waiting for the other side.
