Below is a short overview of STP and how to navigate it in the CISCO IOS enjoy.
[youtube=http://www.youtube.com/watch?v=zapAZRP7mEk&w=560&h=315]
STP Overview Video
December 12, 2013
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IOS Commands, Networking, Protocols, Switches, Videos | Tagged: 802.1w, blocking port, ccent, ccent exam, ccent study, ccna, cisco icnd2, cisco packet tracer, designated port, fast ethernet, GNS3, icnd1 study, icnd2, icnd2 exam, interface cost, Microsegmentation, packet racer, root bridge, stp, stp overview, stp port roles, testking | 
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Posted by Josh Gahan
STP Root Switch Election
November 25, 2013The Root switch is elected in the following order:
- Switch with the lowest Bridge ID (BID)
- Switch with the lowest MAC address
Once the root switch is elected either by lowest BID or MAC address, this switch will be responsible for sending the periodic Hello Bridge Protocol Data Unit‘s (BPDU). These Hello BPDU’s are status messages which all switches on the LAN will receive from the root switch to indicate that the connected links are still working. This sort of works like a heartbeat for the link.
The Hello BPDU contains the following pieces of information:
- The Roots BID
- Sending Device’s BID
- Sending Devices Cost to reach the Root
The hello timer defaults to 2 seconds with a MaxAge time of 10secs.
The Hello timer is the amount of time which the Root waits to send a Hello, while the MaxAge timer is used to tell the listening device how long it should wait before it converges to a new STP topology, after not receiving Hello’s for the indicated amount of time.
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Posted by Josh Gahan
RSTP Overview
October 16, 2013Rapid Spanning Tree Protocol or RSTP (801.W) is an improved version of STP (801.D). RSTP works alot like STP does but it has improved convergence times. The difference between STP and RSTP is the discarding role.
RSTP has three port states:
- Discarding
- Learning
- Forwarding
Learning and forwarding correspond to the same function just like STP but discarding is the name given for STP’s Disabled, Blocking and Listening States.
Election of the root switch, Root ports, Designated ports and tiebreakers all work exactly the same in RSTP when compared to STP
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Posted by Josh Gahan
Problems avoided using STP
October 15, 2013If you are going to setup a LAN with redundant paths, it is important that STP is running. If not you will be opening yourself up to these few problems
- Broadcast Storms
- MAC Table instability
- Multiple frame transmission
While the issue of broadcast storms may seem obvious, the other two may not be the things you think about at all. In the case of MAC table instability your switches will experience frequent updates to their MAC tables with wrong entires, from the looped frames which are being sent around the LAN.
The second issue of Multiple frame transmission can be somewhat, a disastrous side affect of not running STP. Duplicate copies of the looping frame can end up at the end host, in turn completely confusing it, resulting in discarded frames.
Luckily though on CISCO switches STP is on by DEFAULT
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Posted by Josh Gahan
Spanning Tree Protocol (STP)
October 14, 2013For those unfamiliar with this protocol, you will find that it is a very important part of switching. It is defined by IEEE 802.1D. The purpose of STP is to allow you to create a redundant topology utilising switches and bridges without creating loops. STP will dynamically elect a Root switch for which all traffic must pass through and set the appropriate interfaces in a blocking mode. This is how STP prevents frames from forever looping around the LAN.
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Posted by Josh Gahan
ISL and 802.1Q Overview
August 21, 2013ISL and 802.1Q Similarities
- Both define a VLAN header with a VLAN ID field
- Both support 4094 VLANS
- Both use a 12bit VLAN header to number VLANS
- Both support separate instances of STP for each VLAN
ISL and 802.1Q Differences
- ISL is proprietary and 802.1Q is standard
- Each use a different header
- 802.1Q uses the native VLAN concept
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Posted by Josh Gahan
Reasons For Using VLANs
August 15, 2013- Create designs that allow you to group users more easily by either those working together or department
- Allows you to segment the LAN which has the positive affect of reducing the overhead on each LAN segment
- Provide Spanning Tree Protocol (STP) with less work to do by limiting a VLAN to an access switch
- By keeping hosts that work together on a single VLAN, you are able to enforce stronger security by keeping the data on a individual VLAN
- Seperate CISCO IP Phone traffic from the PC traffic
You could simplify this further:
- Security
- Performance
- Design
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Posted by Josh Gahan
Spanning Tree Protocol (STP)
April 27, 2013Switching loops form when multiple paths exist, when a frame is sent and the the frame travels between the switches and never ends up reaching it’s final destination.
STP is used to prevent switching loops and is on by default, it determines a loop free path and ports that are not on the path are put into a blocking mode. If the best path is no longer available STP will calculate a new “path” and ports on that path that where blocked are brought out of the blocking mode.
NB. the shortest physical path is not the best, STP looks at the speed of the links
STP Quick Overview
- On by default
- Used to prevent switching loops
Leave a Comment » | Networking, Protocols, Switches | Tagged: blocking mode, ccent, ccent exam, ccna, ccna exam, cisco switches, default, l2pdu, networking, packet, router, security, segmentation, spanning tree, spanning tree protocol, stp, switch protocols, TCP, technology, UDP | Permalink
Posted by Josh Gahan