SPANNING TREE PROTOCOL Flashcards
What is Broadcast Radiation?
the accumulation of broadcast and multicast traffic on a computer network.
What is a Broadcast Storm?
Extreme amounts of broadcast traffic constitute a broadcast storm.
. A storm occurs when
broadcasts are endlessly forwarded through the loop. Eventually, the storm
will choke off all other network traffic.
Why is redundancy an important part of the hierarchical design? (2 marks)
By eliminating single points of failure and preventing disruption of network
services to users.
Why do Ethernet LANs require a loop-free topology with a single path between any two devices? (1 mk).
A loop in an Ethernet LAN can cause continued propagation of Ethernet frames until a link is disrupted and breaks
the loop.
How does STP compensate for a failure in the network? (1 mk)
by recalculating and opening up previously blocked ports.
What effects can a layer 2 loop result in making the network unusable? (3
marks).
MAC address table instability, link saturation, and high CPU
utilization on switches and end-devices
How are switches running STP able to compensate for failures? (2 mks)
by dynamically unblocking the previously blocked ports and permitting traffic to
traverse the alternate paths.
- List the 4 steps of how STA creates a loop-free topology? (4 mks)
Selecting a Root Bridge, Block Redundant Paths, Create a Loop-Free Topology,
Recalculate in case of Link Failure
As networks begun to grow and become more complex, VLANs were
introduced, allowing the creation of multiple logical and physical networks.
It was then necessary to run multiple instances of STP in order to
accommodate each network - VLAN. Which are they? (4 mks).
These multiple instances are called Multiple Spanning Tree (MST), Per-VLAN
Spanning Tree (PVST) and Per-VLAN Spanning Tree Plus (PVST+).
Building the STP topology is a multistep convergence process: What are the steps
- A Root Bridge is elected
- Root ports are identified
- Designated ports are identified
- Ports are placed in a blocking state as required, to eliminate loops
How is a root bridge elected?
A Root Bridge is elected based on its Bridge ID, comprised of two
components in the original 802.1D standard:
* 16-bit Bridge priority
* 48-bit MAC address
The default priority is 32,768, and the lowest priority wins. If there is a tie
in priority, the lowest MAC address is used as the tie-breaker.
*See page 3 for an example
How are root ports identified
The root port of each switch has the lowest root path cost to get to the Root
Bridge.
Each switch can only have one root port. The Root Bridge cannot have a
root port, as the purpose of a root port is to point to the Root Bridge.
Path cost is a cumulative cost to the Root Bridge, based on the bandwidth of
the links. The higher the bandwidth, the lower the path cost:
The lowest cumulative path cost is considered superior, thus the port
directly connecting to SwitchA will become the root port. A BPDU
advertising a higher path cost is often referred to as an inferior BPDU.
*See page 4 for example
How are designated ports identified
. A single designated port is identified for each network segment. This
port is responsible for forwarding BPDUs and frames to that segment.
If two ports are eligible to become the designated port, then there is a loop.
One of the ports will be placed in a blocking state to eliminate the loop.
Similar to a root port, the designated port is determined by the lowest
cumulative path cost leading the Root Bridge. A designated port will never
be placed in a blocking state, unless there is a change to the switching
topology and a more preferred designated port is elected.
Note: A port can never be both a designated port and a root port.
Remember, every network segment must have one designated port,
regardless if a root port already exists on that segment.
As with electing the Root Bridge, if there is a tie in priority, the lowest
MAC address is used as the tie breaker.
Remember: Any port not elected as a root or designated port will be placed
in a blocking state
*See page 6 and 7 for example
When electing root and designated ports, it is possible to have a tie in both
path cost and Bridge ID. What happens in this case
Port ID is used as the final tiebreaker, and consists of two components:
* 4-bit port priority
* 12-bit port number, derived from the physical port number
By default, the port priority of an interface is 128, and a lower priority is
preferred. If there is a tie in priority, the lowest port number is preferred.
The sender port ID determines the tie break, and not the local port ID.
*See page 8
Describe the three versions of STP
CST utilizes a single STP instance for all VLANs, and is sometimes referred
to as mono spanning tree. All CST BPDU’s are sent over the native VLAN
on a trunk port, and thus are untagged.
PVST employs a separate STP instance for each VLAN, improving
flexibility and performance. PVST requires trunk ports to use ISL
encapsulation. PVST and CST are not compatible.
The enhanced PVST+ is compatible with both CST and PVST, and supports
both ISL and 802.1Q encapsulation. PVST+ is the default mode on many
Cisco platforms.
