Spanning Tree Protocol, from a future CCNA´s perspective.

Spanning Tree Protocol, from a feature CCNA´s Perspective.
written by Gerald C. Paciello
Jan. 29, 2015
A little bit of history.
Before we talk about Spanning Tree Protocol, let's organize the different variants of STP. The original STP was
developed by Radia Perlman while working for DCE back in 1990. At this time there were no switches yet, only
bridges, but because bridges and switches technically do the same job -only switches do it more efficiently and
have more features- they suffer from the same issues. Also, because STP was developed during a time when
there were only bridges, the terminology used in STP, even today, makes reference to bridges a lot (Root
Bridge, Bridge-ID, etc.). So, when you read about STP from different sources, remember that the terms Switch
and Bridge might be used interchangeably.
The original STP, developed by Mrs. Perlman, was later standardized by the IEEE as the 802.1D standard. This
two variants, Perlman´s STP and 802.1D, implemented only one STP instance for the whole network (This was
sometimes referred to as Common Spanning Tree-CST), because Vlans had not been invented yet, so one
instance of STP was enough.
When Vlans were introduced, Cisco made some improvement on the 802.1D standard to accommodate Vlans
and called it Per-Vlan ST Plus (PVST+) or sometimes PVSTP. This improvements allowed for multiple STP
instances (one for each Vlan on the network). This means that each Vlan could have its own STP topology. With
this Cisco improvement, a few other extension were also introduce such as PortFast.
Later, the IEEE made its own improvement on its standard 802.1D, which was considered to have a very slow
convergence time, it was called Rapid STP (RSTP) and its standard is 802.1W and, as the first part of the
name indicates, it was much faster (rapid) to converge than its predecessor.
Then Cisco took the 802.1W standard and made some proprietary changes as well and called it Rapid Per-
Vlan ST+ or RPVST+.
Note: Cisco switches support PVST+, RPVST+ and MST and because by default cisco switches use PVST+, we are going
to talk about that STP variant on this paper.
What STP does and why.
STP was developed to prevent loops (aka, Switching loops or layer 2 loops) at layer 2 in a switched network
with parallel links between switches, thus creating more than one path to a single destination. These loops, in
turn, will create a much worst condition called a Broadcast Storm which is a result of broadcast, multi-cast
and/or unknown-destination frames looping endlessly around the network, and this happens because of two
main reasons; First, because switches will forward these frames out of every port except the receiving port (This
is called Flooding) every single time they receive them. Second, at layer 2 (as opposed to layer 3) there is no
Time To Live (TTL) field in the frames header, so frames will not “expire”. They will go around and around in a
network until the network is shutdown or it fails.

So, when broadcast storms happen, frames will start crowding the network´s bandwidth leaving very little room
for "good" frames, so the network will start slowing down. Also, PCs will need to process an enormous amount of
broadcasts, so their performance will be negatively impacted as well.
Even though these loops happen when there are multiple paths between switches, having multiple paths to a
single destination, called Redundancy, is not such a bad thing to have in a network. In fact, redundancy is
convenient, very often essential for a network, because it provides alternative paths in case one path fails. In
fact, STP was developed so that we can have redundancy in our networks.
STP also prevents another switching issue called MAC table corruption, which happens when a switch learns
about the same destination MAC address from duplicate frames received on more than one of its ports.
Protocol Operation.
Before we talk about the specifics of STP operations, I want you think of an STP topology as an INVERTED
tree (look at the image bellow) with its roots being the Root Bridge (for now, let´s say it is the main switch) on
top, and the branches reaching downstream and spanning towards the non-root bridge switches. In turn, these
other switches will reach upstream, towards the Root Bridge (RB).
I can hear you guys saying “Root what, non-root what?!... We´ll get to the good stuff soon enough, for now just
know that on STP, there´s a “boss” switch that is called the Rood Bridge and the other switches will find the
best path to get to it.
STP CONVERGENCE
STP prevents issues created by parallel links on switched networks and allow us to have redundant networks, by
selecting the best path to a destination and blocking the remaining paths until the best path originally
selected, for some reason becomes unavailable.

To do this, STP uses what is called the Spanning Tree Algorithm (STA), this algorithm will come up with a loop
free STP topology by selecting first, a switch that will act as a "central point of reference" called a Root Bridge
(RB) and then it will calculate best paths to this switch from all other switches in the STP domain. This is
accomplished by selectively putting some ports in a forwarding state and other ports on a blocking state,
following specific rules from a process call the Election Process, we´ll see this process in detail soon.
The process just described (very briefly) is referred to as Convergence and we say that something has
converged once it has finished its initiation process an it is ready to do what it´s supposed to do.
Port States.
In order for STP to converge and do what it is supposed to do (which is to create a loop free topology), among
other things, it has to systematically control the ports and elect which port will forward data and which one will
block data, as we stated before. Ports start at the Blocking state and then they start transitioning through the
different states until they get to the Forwarding state.
Before we learn about how STP does this, let´s learn about the port states and their roles:
1. Blocking: The port does not learn MAC addresses nor participate in the switching of
data frames, it will discard these frames when received. But it does receive and process
STP frames (called Bridge Protocol Data Units or BPDUs).
2. Listening: In this state the port does not learn MAC addresses nor participates in the
switching of data frames but it does process BPDUs. Also, it removes old unused MAC
table entries.
3. Learning: During this state, the port still does not forward data frames, but it does
forward BPDUs and re-populates its MAC table. In other words, it learns MAC address
from data frames received.
4. Forwarding: This is the “normal” port state on a switch. When a port is forwarding, it is
fully participating in the switching of every frame accordingly.
5. Disabled: When a port is disabled, it does not participate on any frame switching, it is
shutdown, either administratively (Manually) or because of failure.
Here´s a table that summarizes port states and shows you the LED color on the switch port during on each state:

Timers.
These timers play a big role as they control how often or for how long STP performs its chores. The values,
which are configurable, are carried inside BPDUs sent only by the Root Bridge, and have a direct impact on
convergence time, as we´ll see just ahead. For now, these are the default values:
Convergence Times.
Convergence time is defined by the total time it takes for a port to transition from either, Listening to
Forwarding or Blocking to Forwarding. We can think about this as Convergence Time= Listening to
Forwarding transitions and Re-Convergence Time= Blocking to Forwarding transitions.
Convergence Time: When the switch first comes online (turns on):
1. Listening state – Transitional state. (15secs by default or Forward Delay)
2. Learning state – Transitional state (15secs by default or Forward Delay)
3. Forwarding state - Stable state.
As you can see takes a total of 30 seconds (15 on Listening + 15 on Learning). This is because the ports were
not Blocking to start with so it saves 20 seconds (default value). In other words, it did not have to go through the
Max Age timer.
Re-Convergence: If a process needs to re-converge, it means that it had already converged at least once
before, and the most common reason for STP to have to re-converge is a topology change (i.e. a link failure).
Even though a failure is a failure, when it comes to STP, this same failure will have different effects on STP
Convergence time, depending on where, in the STP topology, the failure occurred. However, it´s not a matter of
physical location, it is a matter of perspective. STP refers to this as Direct Failure or Indirect Failure.
Let´s elaborate; say we have SW1 (RB), SW2 and SW3 connected to each other, and the link between SW1 and
SW2 fails. From SW1 and SW2´s perspective, this will be a Direct Failure causing a 30 seconds re-
convergence, because as soon as the link goes down, SW2´s port will move to the Listening state (15secs
Forward delay) and start sending BPDUs advertising itself as RB. Then, it´ll move to the Learning state (again,

15secs Forward delay) and start learning MAC address and then, after 30 seconds, it transitions to the
forwarding state.
Meanwhile, for SW3 this will be an Indirect Failure (it did not happen on any of its links) and this will cause a 50
seconds re-convergence because as soon as SW3 receives the BPDU from SW2 advertising itself as RB, SW3
will start its Max Age timer (20 secs). After Max Age, if it doesn't receive any BPDUs from SW1, it will erase SW1
´s as RB and enter SW2 in its place. However, in this case, this will not happen because SW3 will still receive a
better BPDUs from SW1 so it´ll send BPDUs out to SW2 saying, “hey I have a better RB than you and I can get
to in with a cost of 19”.
Direct Failure: Failure occurred on one of the switch´s links:
1. No Blocking State, directly to Listening.
2. Listening - Transitional state. (15secs by default or Forward Delay)
3. Learning - Transitional state. (15secs by default or Forward Delay)
4. Forward - Stable state.
Indirect Failure: Failure did not occurred on one of the switch´s links:
1. Blocking - Stable state. (20secs by default or 10 x Hello Timer)
2. Listening - Transitional state. (15secs by default or Forward Delay)
3. Learning - Transitional state. (15secs by default or Forward Delay)
4. Forward - Stable state.
Port Roles.
We are going to get into the details of the Election Process just ahead, for now let´s just say that during this
process, 1st
the RB is elected and then the ports that are going to participate on the STP topology, are elected
as one of three STP Port Roles, these are the Root Port (RP), Designated Port (DP) or non-Designated Port
(non-DP).
Here they are:
1. Root Bridge.
The RB is the center of the universe as far as STP is concerned. The RB sends out a BPDU
called the Configuration BPDU or CBPDUs and also controls the various STP timers.
2. Root Port.
Root Ports (RP) are the ports on non-RB switches (1 per switch) that have the best path to
the RB itself. Think of the RPs as the upstream ports reaching up towards the RB. These
ports will be in the forwarding state.
3. Designated Ports.
A Designated Port (DP) is the port on each network segment that connects its segment to

best segment to the RB. Think of DPs as downstream ports forwarding CBPDUs from the
RB. These ports will be in the forwarding state.
4. Non-Designated Ports.
Any remaining port that was not elected RP nor DP will be a non-DP. These ports will be
Blocking.
They will only receive and process STP BPDUs but will discard any other type of
frames.
In the picture below, the Election Process has ended and all ports are functioning as either, Root Ports (RP),
Designated Ports (DP) or non-Designated Ports (non-DP). Notice that RPs and DPs are Forwarding data
and non-DPs are Blocking, as stated before.
Check your understanding, compare the picture above with the information in the following table:
Port Cost.
Every link In an STP topology has a cost, and this cost is associated with each link´s speed. After the Root
Bridge has been elected, which is the 1st step in the Election Process, the process needs to determine which

are going to be the RPs and DPs respectively. This is done by adding the link´s cost (aka Port Cost) values
along every individual link from non-RBs to the RB, and the path with the lowest path-cost wins the
election.
The STP Ports cost is a configurable value, but its default values are shown below.
Port costs is provably the most easy to understand concept in STP because it is pretty straight forward, but
here is a picture that makes it easier:
Election Process.
Ok, so after mentioning the infamous “Election Process” several times now, let´s see what it is about. I
remember when I started reading about the election process, I was really confused about the values the STA
uses to make each election. Well, it wasn't the values exactly, those are pretty straight forward, it was the
tiebreakers I couldn't find a place for and I think it was because of the way it was explained in the literature I was
reading that time. So before we get into the details about the process itself, let me show you what those values
are and when they are used.
The values listed above are the only 4 values that STA considers for RB election and for RPs / DPs election:
Now, the 1st entry, Lowest Bridge ID, is the only value the STA uses to elect the Root Bridge and it is called
Bridge-ID or just BID. We will see how and what the Bridge-ID is in just a second, for now remember that the
Bridge-ID is the value used to choose the RB.
The 2nd entry is the Lowest Path-Cost to RB and it is used when the STA elects RPs and also DPs. The 3rd

and 4th
entries are ONLY used for RP/DP if and ONLY if there is a tie, specifically, a path-cost tie for RP and DP
election.
So, summarizing:
Let´s elaborate; if, and only if, there is a tie for RP or DP election (the path-costs are equal), the process will
examine the next value looking for a tie breaker, and this value is Lowest Sender´s BID, which is the Bridge ID
of the switch that sent the BPDU. If the tie persists, the process moves on to the second and final tiebreaker; the
Lowest Sender´s Port ID, which is the lowest port number on the switch that sent the BPDU.
Let´s examine the topology below:
As you can see, the STP topology above has already converged and we can see that the RB elected is SW1
and it´s easy to see why, it´s because its BID, 327690007.xxxx.xxxx.xxxx is lower than SW2´s BID. Remember
the first value on our list a minute ago? It was Lowest Bridge-ID and SW1 has the lowest BID. I´ll explain why a
little bit ahead..
We can also see that SW2 has already elected its RP which is Fa0/1, but let´s see why:
The STA will look for Lowest Path-Cost to RB, but this was not possible because both paths to the RB have a
path-cost of 19.
The 1st
tiebreaker calls for Lowest Sender´s BID, again this is not possible because there is only one neighbor
(the RB) and the BPDUs received by SW2 from SW1 will have the same BID, so tied again.
So we come to the 2nd
tiebreaker, Lowest Sender´s Port ID, the lowest port on the RB is F0/1, so this is the

port that will be elected RP or DP, depending on what was being elected. The process for RP or DP election
uses the same value, lowest path-cost, the difference is that for RP the STA looks for lowest cost to RB from the
non-RB switch to the RB and for DP it´s from the network segment to the RB... it is just about perspective.
Bridge Protocol Data Unit - BPDU.
We've also mentioned through out this document that switches, in an STP domain, use a special frame to
exchange STP data between them, so let´s talk about the BPDU. There are 2 types of Bridge Protocol Data
Unit - BPDUs; Configuration BPDUs (CBPDU) and Topology Change Notification BPDUs (TCN).
The CBPDUs, which is sent out ONLY by the RB, carries the necessary information downstream for switches to
make their decisions during the Election Process for Root Bridge (RB), Root Port (RP), Designated Port
(DP) and Non-Designated Port (non-DP) among other things. It also holds the timers mentioned earlier as the
RB sets all the different timers as well.
Every non-RB switch will receive every 2 seconds by default (or Hello Timer), a CBPDU from the RB on its
Root Port (RP), notice that this is downstream. The non-RB switch will only send a BPDU, the TCN BPDU out
of its RP (upstream) when it has to inform about a topology change (i.e. a link went down). When the other
switches receive the TCN, they will forward the TCN upstream and reply with a TCAck downstream to the switch
that sent the TCN, and the process will repeat until the RB is reached.
Note: Think of RPs and DPs like this: RPs send BPDUs UPSTREAM towards the Root Bridge (hence the name, Root Port)
and DPs send BPDUs DOWNSTREAM towards non-RB switches.
The picture below (from http://www.cisco.com/c/en/us/support/docs/lan-switching/spanning-tree-protocol/12013-
17.html#topic2 ) demonstrates this.
When the RB finally receives the TCN, it will reply with CBPDU that has its Configuration Change (TC) bit set
to 1 and NON-RBs will forward this BPDU downstream so every switch knows about the topology change event
(switches receive and process this BPDU, even on blocking ports) and they can start the re-convergence
process to accommodate the topology change.

The picture below (from http://www.cisco.com/c/en/us/support/docs/lan-switching/spanning-tree-protocol/12013-
17.html#topic2 ) demonstrates this.
BRIDGE ID - BID.
As I said before, the CBPDU is the BPDUs used during convergence, it contains, among other fields, the
Bridge-ID or BID which is used for RB election.
Let´s see how STP come up with the Bridge Id. It consists of the switch Priority value, this value is
configurable (in multiples of 4069, we´ll see why later) and by default is 32768, plus the System ID
Extension, which is the Vlan number and finally the MAC address.
Notice that the priority went up from 32768 to 32868, as a result of adding the default priority + sys-id-ext.
Again, the sys-id-ext. Is the Vlan value, in this case 100.
With STP 802.1D, the BID consisted of 8 bytes divided in two, as follows: the first 2 bytes (16bits) corresponds
to the Bridge Priority and the remaining 6 bytes corresponds to the MAC address.

As the following picture from http://www.firewall.cx/networking-topics/protocols/spanning-tree-protocol/1054-spanning-tree-
protocol-root-bridge-election.html demonstrates.
As I stated before, STP 802.1D ran a single instance of STP for the entire network, but as networks started
getting bigger, more complex and then Vlans were introduced, it was necessary to run multiple instances of STP,
and so it was necessary to include Vlan information in the BID. This was accomplished by using 12 out of the
16bits from the Priority field in the 802.1D BID to include the Vlan number. This subfield was called System
ID Extension and notice that 2 to the 12th (12 is the number of bits borrowed) is 4096, exactly the maximum
number of Vlans allowed.
As the following picture from http://www.firewall.cx/networking-topics/protocols/spanning-tree-protocol/1054-spanning-tree-
protocol-root-bridge-election.html indicates.
So, from the 2 bytes Priority field, the first 12 least significant bits (the first 12 bits on the right) were
borrowed to accommodate the Vlan number. This means, obviously, that the Priority field has been reduce
to only 4bits. This is why, where before (on 802.1D) it was possible to have any Priority value from 0 - 65,536
(2 to 16th), now (on any Per Vlan variant) we can only get from 0 - 61440 but in multiples of 4096 only, this
gives us a total of 16 possibilities. But this is not a limitation because there are no scenarios (not that I can think
of, at least) where more than 16 different Priority values would be needed.
If you look at the picture below, specifically the place values on the Bridge Priority field, you´ll see that the
least significant bit has a place value of 4096. This means that the Bridge Priority field cannot use the place
values 1, 2, 4, 8, 16, 32, … and so on. It only has 4096, 8192, 16384 and 32768 to work with and therefor we
can only have bridge priority values from 0 to 61440, expressed in multiples of 4069.
Picture from http://www.firewall.cx/networking-topics/protocols/spanning-tree-protocol/1054-spanning-tree-protocol-root-
bridge-election.html

ROOT BRIDGE.
OK, so now that you've read about what is needed for the Election process, let´s talk about it in detail. The
Root Bridge is the main switch in the topology and every non-RB will have a port called a Root Port pointing
upstream through the lowest path-cost to the RB.
RBs have a couple of very specifics tasks:
 It is the only switch that sends CBPDUs.
 Imposes all the STP timers
Root Bridge Election Process.
When switches come online, they do not know yet if they are the only switch on the network or if there are
hundreds of switches, and so they will start flooding the network with their own CBPDUs advertising their
Bridge-ID and themselves as Root Bridge. In other words, in the CBPDUs that they are flooding onto the
network, the Root BID, which is the BID of the switch that switches believe is the RB and the Bridge ID, which
is the BID of the switch sending the CBPDU, will have identical values.
As soon as they receive a CBPDU from another switch, they realize that they are not alone, and at that
moment the election process will begin.
Election process:
1. The first part of the BID, the priority field, will be compared.
2. If one of the priority value in the CBPDUs is the lowest, the corresponding switch will be elected RB
and the RB election process will stop.
3. If two or more priority values are tied for RB (their Priority values are identical), the MAC address will
be compared (the MAC address is considered only in case a tie).
4. The switch that has the BID with the lowest MAC among the switches that are tied for Priority, will be
elected RB.
ROOT PORTS.
A Root Port is a port on a non-root bridge switch (only 1 per non-RB switch and there is NO RP on the Root

Bridge, only DPs remember) that has the lowest path-cost to the Root Bridge. Remember, on the inverted tree
analogy, the RP sends BPDUs upstream towards the RB.
Root Port Election Process.
The RB floods its CBPDU advertising a Cost to RB value of 0, because it is the RB so there is no cost to get to
the RB, of course. The other switches will receive this CBPDU and add the cost of the port it received the
CBPDU on, to the advertised Cost to RB value on the CBPDU received, which is 0.
For example, if the CBPDU comes in Fa0/1, the switch will ad 19 to 0 (if you look at the Port Costs table you
can see that the cost for a 100Mbps link is 19) so, 19 + 0=19. So this switch will forward a copy of the CBPDU
received advertising a Cost to RB value of 19. The next switch will receive this BPDU and again add the cost
value of the port it received it in (let´s say that it is a 1Gbps link) to the Cost to RB value advertised in the
BPDU received, which is 19, so 4 + 19=23. When this switch send its own BPDU, it will advertise a Cost to RB
value of 23. This same process will repeat until the port with the lowest -cumulative- path-cost on each non-RB
switch is elected Root Port.
Let´s follow this same example step by step:
1. RB floods a CBPDU advertising a Cost to RB of 0 (zero). This makes sense because there is a cost of
0 for the RB to get to itslef.
2. Non-RBs receive this CBPDU and they add the cost of its port to the advertised Cost to RB in the
CBPDU. So Cost to RB(0) + cost of 100Mbps(19) = 19.
3. The non-RB sends its own copy of the CBPDU advertising a Cost to RB of 19
4. The next non-RB downstream receives the BPDU advertising a Cost to RB of 19. The port in which this
BPDU came into is a 1Gbps port, which has a cost of 4. The switch adds 19 + 4 = 23 and sends out a
BPDU with an advertised Cost to RB of 23.
5. The process repeats until all the RPs have been elected.
DESIGNAED PORTS.
A Designated Port is the port forwarding BPDUs to non-RB switches downstream. These BPDUs will be coming
into the non-RBs RPs OR non-DPs (Remember, non-DP are blocking ports but they receive and process
BPDUs still). There is 1 DP per network segment, for example, if there are 7 network segments in total, there
will be 7 DP total. You can also think of a DP as the port that connects the segment it belongs to, to the lowest
patch-cost segment to the RB.
Also, the switch that contains the DP for a particular segment, is referred to as the designated switch for that
segment.
Designated Port Election Process.
The process to elect Designated Ports starts where the RP process ends and, just like the RP process, looks
for the lowest path-cost to the RB. During the DP election process we need to look at it in a “per segment basis”.

Let´s look at the process from the beginning and you´ll see what I mean:
1. Root Bridge: The process looks at the whole topology to find a Root Bridge: Lowest Priority
value or lowest BID if there is a Priority tie, wins.
2. Root Port: Now that everyone knows who is the RB, the STP process looks at each non-RB switch for
the 1 port that has the lowest cost to get to the RB.
3. Designated Port: In this step, the process looks at each network segments and elects the 1 port that
has the lowest path cost to the RB.
4. The process no longer has to calculate anything. Every port remaining that hasn't been elected RP nor
DP, become non-DP.
You might get a bit confused on RP and DP election process because both look for the lowest path-cost to the
bridge. It seems that the process is looking for two different things considering the same value... and that´s
correct!. The exact same value is considered, what makes the difference is the point of reference; For RP, is the
lowest path-cost from the switch and for DP, is the lowest path-cost from the network segment.
Additional STP Features.
Being that this technology has been around for a long time, different updates were introduced over the years.
Among these updates, there are a few features that we need to talk about, and those features are
Etherchannel, PortFast and BPDU Guard.
Etherchannel.
Please observe the picture bellow for a moment. As you can see, they are the same topology but something is
different about the ports states, isn't it? STP is running on both these topologies yet some ports on the top
topology are blocked and some are not. But this is OK, STP does this in order to prevent loops, right? So, if
STP is running on both topologies, how come the one on the bottom has all of its ports in the Forwarding state?
Here is the answer; the topology at the bottom has EtherChannel configured. Each pair of links from switch to

switch (in our example) are logically bundle together and are treated as one single link called Port-Channel
and since the links are treated as a single interface, all the ports are forwarding.
With EtherChannel (aka Link Aggregation) we can bundle up to eight, same speed, parallel links together, and
this is beneficial in many ways; it provides a higher bandwidth between the switches, it can load-balance traffic
across the links involved and it provides redundancy still because even though the Port-Channel is treated as a
single interface, the links inside can still work independently.
Because of the fact that the links can still work independently, EtherChannel eliminates the necessity for STP to
re-converge in case of a failure. This is because, if one of the links inside the Port-Channel goes down, STP
will not re-converge and it will continue working as long as one of the links is still up.
EtherChannel´s Load-balancing.
Let´s talk a little bit about load-balancing; when we hear that traffic is going to load-balanced, we imagine bits
being distributed evenly through all the links involved, right? Well, that might not be the case here, this is
because EtherChannel´s load-balancing has a few methods to distribute the load, that we can pick from and
configure on our switch using the port-channel load-balance (method value) command.
Without going too deep into the details (not a CCNA topic), here´s a table from https://ls-a.org/doku.php?
id=school:ccnp_1a_chapter6_cert showing these methods:
Link Aggregation Protocols.
There are two different protocols for Link Aggregation supported by Cisco switches; these are PAgP (Port
Aggregation Protocol, Cisco proprietary) and LACP (Link Aggregation Control Protocol) and this is the
industry standard, IEEE 802.3AD.
PAgP and LACP Port Negotiation.
When we configure EtherChannel on the switches, all ports on each side of the links need to have the same
values on; speed, duplex and Vlan assignments. If that is the case, the ports will be able to negotiate a Port-

Channel between them if, the negotiation options (referred to as channel modes), are correct at each end.
Depending on the channel mode the ports are in, they will either send out negotiation requests, wait for a
negotiation request, both or neither. If a port on one side is waiting for a request, and the port on the other side
is sending out requests, a Port-Channel will be formed. Since there are two Port Aggregation protocols
supported by Cisco, PAgP and LACP, there are also two similar sets of channel modes for each of them.
PAgP:
The channel modes for PAgP are ON, AUTO and DESIRABLE.
ON: does not send requests nor does it listens for them.
AUTO: does not send requests but it listens for them.
DESIREBLE: sends and listens for requests.
LACP:
The only difference between PAgP and LACP are the modes names, but they work the same way as PAgP.
The channel modes for LACP are ON, PASSIVE and ACTIVE.
ON: does not send requests nor does it listens for them.
PASSIVE: does not send requests but it listens for them.
ACTIVE: sends and listens for requests.
PortFast and BPDU Guard.
As you already know, when a port is participating in STP, before it is ready to forward data, it needs to go
through the Listening and Learning states and this will take 30 seconds by default. What if there is an end
device, such as a PC, connected to this port? Well... some of the more modern PCs are able to boot up in less
then 30 seconds. If this is the case, the PC might not be able to get an IP address through DHCP, or it´ll
simply be siting there waiting to be able to communicate, wasting time.
There is a way we can go around this, there is a feature introduced by Cisco called PortFast and the way it

works is very simple; by configuring PortFast on a port, we are telling it that no BPDUs will be coming through it,
so it should become active and ready to forward frames right away. In other words, the port will NOT go
through the 30 seconds delay from the STP´s Forward Delay timer.
Now, imagine that PortFast is configured on a port that runs to an office cubicle. What if someone comes along
and plugs in a switch into this port? If that happens, BPDUs will start going through this port (because PortFast
does not block any frames), so something is wrong, correct?... well, we can do something about that also.
By also configuring the feature BPDU Guard on the same port that we´ve configured PortFast, we are telling
the port that IF it receives a BPDU, intermediately put the port in a error disabled state. A port that is in an error
disabled state, will not process any frames until the commands shutdown / no shutdown are issued on the port.
--------------------------------------------------------------------------------------------------------------------
References.
Users of The Cisco Learning Network - https://learningnetwork.cisco.com/welcome
Cisco CCNA R&S 200-120 Official Cert Guide by Wendell Odom
Cisco CCNA R&S ICND2 200-101 Official Cert Guide by Wendell Odom
Cisco CCNP SWITCH Simplified Vol.1 by Paul Browning

Spanning Tree Protocol, from a future CCNA´s perspective.

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