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.
Leave a Comment » | ICND2 Topics, Networking, Protocols, Switches | Tagged: 801.2d, bid, bpdu, ccna, cisco switch, designated port, hello timer, icnd2, maxage, PVSTP, root port, stp, stp timers, switch | Permalink
Posted by Josh Gahan
Etherchannels
October 20, 2013To finish up the first topic under LAN Switching Technologies, identifying enhanced switching technologies. I will give you an overview of Etherchannels.
What they are, what they do and how to configure one for yourself.
Firstly an Etherchannel is a configuration that allows a switch to treat two parallel links as one physical link. This also allows STP to treat the entire channel as one and either block or forward traffic over this link as per vlan.
As a result of this, the switch can now load balance traffic over the channel. Without Etherchannel one of the links would be blocked by STP, as is shown in the example below.
In the above example, SW0 and SW1 are connected via two links. Fa0/1 to Fa0/1 and Fa0/2 to Fa0/2, but due to STP it has blacked Fa0/2 on SW1 from forwarding frames. STP at this point see’s these two links as they are, separate from one another.
When we run a show spanning-tree command on SW1 we discover that, port Fa0/1 is in a forwarding state as the RP, Port Fa0/2 is blocked and Fa0/3 is the DP. Running the show spanning-tree command on SW0 would show us that it is the Root Bridge and all three ports are DP’s or Designated Ports.
To make STP use both the Fa0/1 and Fa0/2 links on SW0 and SW1 we need to configure the interfaces in the same Channel-group or Etherchannel. Doing so will change how STP see’s those two links. From two separate links to one link which can be used to load balance. We are not actually making one physical link but like a VLAN where we can make many Virtual LAN’s we are creating one Virtual Link using 2 parallel links.
For this network the configuration is virtually the same, just remember you will need to add the correct interfaces in the same channel group for an Etherchannel to become active.
The following image shows you how to correctly configure an Etherchannel, using the command channel-group 1 mode on, you can configure upto 8 parallel links on the one Etherchannel.
Below you can see the new show spanning-tree output displayed on SW1 after the Etherchannel has been configured. Notice the Po1, that indicates that all the configuration parameters checked and that is the new channel group 1, which was configured earlier.
The new diagram, with the two links now bundled in the new Etherchannel. Notice how Fa0/2 has come out of blocking mode to become a RP
Leave a Comment » | ICND2 Topics, IOS Commands, Networking, Switches | Tagged: 200-101, ccna, cisco, cisco switches, etherchannels, icnd2, networking, port channels, switch, switching | Permalink
Posted by Josh Gahan
PVSTP – Per VLAN Spanning Tree Protocol
October 19, 2013The second item under the topics in the new ICND2 200-101 exam, requires that you can identify PVSTP.
PVSTP or Per VLAN Spanning Tree Protocol does just that, each VLAN will have a different STP setup or topology, as it is done on per VLAN basis.
As the engineer this provides you with a great tool to load balance traffic over the network, using some configuration options you can decide what switches will become root.
Note: All of the interfaces on the Root switch will be DP’s
The above example show you how the two VLANs have a different topology.
Leave a Comment » | ICND2 Topics, Networking, Protocols, Switches, VLANs | Tagged: 2001-101, ccna exam, cicsco, cisco switches, cisco swittching, icnd2, per vlan spanning tree protocol, PVSTP, switches, topology, vlan | Permalink
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
Leave a Comment » | ICND2 Topics, Networking, Protocols, Switches | Tagged: 801.d, 801.w, blocked, blocked ports, cisco switching, convergence, difference between RSTP and STP, discarding, forwarding, icnd2, Lan switching technologies, learning, listening, RSTP, stp, switched, switching | Permalink
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
Leave a Comment » | Networking, Protocols, Switches, Troubleshoot | Tagged: broadcast storms, ccna, ccna exam, cisco, cisco switching, computer networks, frames, icnd2, icnd2 exam, it networking, mac, MAC table, networking, stp, switches | Permalink
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
Spanning Tree Protocol Port Roles
October 1, 2013The diagram below is the network in use:
Switch 3
FA0/1 – DP
FA1/1 – DP
Switch 1
FA1/1 – RP
FA0/1 DP
Switch 2
FA1/1 – RP
FA0/1 – DP
Switch 0
FA0/1 – RP
FA1/1 – ALTN
The lab for this example can be downloaded by clicking here
Leave a Comment » | ICND2 Topics, IOS Commands, Networking, Protocols, Switches, Troubleshoot | Tagged: ccna, ccna exam, cisco, cisco ios, cisco networking, cisco switches, hints n tips, icnd2, iOS, ios commands, labs, lan, Microsegmentation, networking, study, switch, switches | Permalink
Posted by Josh Gahan
Setting A Switch’s Revision Number To Zero
September 1, 2013- configure the new switch for VTP transparent mode and then back to VTP client or server mode, which resets the VTP revision number to 0
- Erase the new switch’s vlan.dat file in flash and reload the switch. This file contains the switch’s VLAN database, including the revision number
Leave a Comment » | IOS Commands, Networking, Switches, VTP | Tagged: ccna, ccna exam, cisco, cisco switch, icnd2, icnd2 exam, revision number, switch, vlan, vlan database, Vlan.dat, vtp configuration, VTP server | Permalink
Posted by Josh Gahan
VLAN & VTP Switch Defaults (CISCO)
August 26, 2013- VTP server mode
- No VTP domain name
- VLAN 1 and VLAN’s 1002-1005 are automatically configured (cannot be deleted)
- All access interfaces are assigned to VLAN 1 (an implied switchport access vlan 1 command)
Leave a Comment » | IOS Commands, Networking, Protocols, Switches, VLANs, VTP | Tagged: 4094 vlans, access port, ccna defaults, ccna exam, ccna study, cisco defaults, cisco ios, default vlan, icnd2, iOS, sccess switch, switches, trunking, virtual lan, vlan, vlan 1002, vlan 1003, vlan 1004, vlan 1005, VLAN Trunking Protocol, vlan1, vlans, vtp domain name, vtp server mode | Permalink
Posted by Josh Gahan