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20 Cards in this Set
- Front
- Back
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Definition:
Loop |
can be defined as two or more switches that are interconnected by two or more physical links.
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Switching Loops create 3 major problems:
Broadcast Storms |
Broadcast storms—Switches must flood broadcasts, so a looped topology will create multiple copies of a single broadcast and perpetually cycle them through the loop.
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Switching Loops create 3 major problems:
MAC table instability |
MAC Table Instability: Loops make it appear that a single MAC
address is reachable on multiple ports of a switch, and the switch is constantly updating the MAC table. |
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Switching Loops create 3 major problems:
Duplicate frames |
Duplicate frames:
Because there are multiple paths to a single MAC, it is possible that a frame could be duplicated to be flooded out all paths to a single destination MAC. |
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Definition:
Spanning Tree Protocol (STP) |
802.1D
STP is a protocol that runs on bridges and switches to find and block redundant looped paths during normal operations. |
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STP eliminates Layer 2 loops in switched networks with redundant paths.
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STP’s basic function is tocreate a loop-free path to a root bridge
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The Root is the switch
with the lowest Bridge ID |
the ID is determined by a combination of an admin-
istrative Priority and the MAC address of the switch. |
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What is the default for administrative priority?
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32,768
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What is the bridge ID?
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Priority prepended to the MAC:
32768,MAC = Bridge ID |
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The Root is elected based on the Bridge ID and the Priority. The Switch with the lowest priority will always be the Root.
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The Root is elected based on the Bridge ID and the Priority. The Switch with the lowest priority will always be the Root.
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What is BPDU
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Bridge Protocol Data Units.
Multicasted every 2 seconds. |
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Some of the important fields in a BPDU:
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Root Priority, Root ID, Root Path Cost, Bridge Priority, Bridge ID, Message Age, Max Age, Hello Time, Forward Delay
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STP path costs:
10Gbps: 2 1Gbps: 4 100Mbps: 19 10Mbps: 100 |
Each LAN segment must have one Designated Port.
The designated port is the port that has the least cost path to the Root from the LAN segment |
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The Root switch has only Designated ports. Because it isthe root, it won’t have
a Root port, and it can’t block any of the ports that connect to other switches |
Designated ports. Because it isthe root, it won’t have
a Root port, and it can’t block any of the ports that connect to other switches |
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The order of criteria a switch goes through when deciding its Root and
Designated ports is as follows: |
1.The port with the lowest cumulative Root Path Cost will be the Root
port/Designated port. |
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The order of criteria a switch goes through when deciding its Root and
Designated ports is as follows: |
2. If tied between multiple ports, the port that connects to the neighboring
switch with the lowest Bridge ID becomes the Root port/Designated port. |
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The order of criteria a switch goes through when deciding its Root and
Designated ports is as follows: |
3. If there are multiple connections to that same switch, the port with the
lowest assigned STP priority will be the Root port/Designated port. |
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The order of criteria a switch goes through when deciding its Root and
Designated ports is as follows: |
4. If tied, the port with the lowest hardware number (Fa0/1 is lower than
Fa0/2) will be the Root port/Designated port. |
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Blocked Port:
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A Blocked port is neither the Root port nor the Designated port, but is part of
the redundant links between switches. A Blocked port does not send data; it only receives BPDUs. |
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Convergence
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Convergence is the term used to describe the process STP goes through to achieve
a stable, loop-free network |
