Question regarding output of show spanning-tree detail command ?

chandra_rc16 · ‎10-19-2013

Please take a look at my topology attached. All the values: brdige priority and port priorities on both switches are default.

Here switch1 is ROOT switch because it has the lower MAC.

The O/p of show spanning-tree detail from switch2 is here:

-------------------

switch2#show spanning-tree detail

VLAN0001 is executing the ieee compatible Spanning Tree Protocol

Bridge Identifier has priority of 32768, sysid 1, 00E0.F9AB.AE05

Configured hello time 2, max age 20, forward delay 15

Current root has priority 32769

Root port is 1 (FastEthernet0/1), cost of root path is 19 // Since there is only one port connectd to root switch. Fa0/1 became ROOT port//

Topology change flag not set, detected flag not set

Number of topology changes 0 last change occurred 00:00:00 ago

from FastEthernet0/1

Times: hold 1, topology change 35, notification 2

hello 2, max age 20, forward delay 15

Timers: hello 0, topology change 0, notification 0, aging 300

Port 1 (FastEthernet0/1) of VLAN0001 is designated forwarding //why it is showing designated forwarding whereas it as to show ROOT forwarding//

Port path cost 19, Port priority 128, Port Identifier 128.1 // I believe it is showing here the Fa0/1 Port details //

Designated root has priority 128, address 000A.4110.BE01 // What does it mean here. Does it mean ROOT port priority and address. If yes, why it couldn't be like this "Root port has priority 128, address 000A.4110.BE01//

Designated bridge has priority 32769, address 0001.97C8.DC03 // I think it is showing the designated bridge for this segment is so and so..//

Designated port id is 128.1, designated path cost 19 // Again which port details are here. Because Fa0/1 port details are already given at second line//

Timers: message age 16, forward delay 0, hold 0

Number of transitions to forwarding state: 1

Link type is point-to-point by default

Port 2 (FastEthernet0/2) of VLAN0001 is designated forwarding

Port path cost 19, Port priority 128, Port Identifier 128.2 // I believe it is showing here details of current port i.e., Fa0/2 //

Designated root has priority 128, address 000A.4110.BE01 // What does it mean here. Does it mean ROOT port priority and address. If yes, why it couldn't be like this "Root port has priority 128, address 000A.4110.BE01//

Designated bridge has priority 32769, address 0001.97C8.DC03

Designated port id is 128.2, designated path cost 19 // Again to which desingated port it is refering to. Because already the details of current port are shown at Second line //

Timers: message age 16, forward delay 0, hold 0

Number of transitions to forwarding state: 1

Link type is point-to-point by default

Port 3 (FastEthernet0/3) of VLAN0001 is designated forwarding

Port path cost 19, Port priority 128, Port Identifier 128.3

Designated root has priority 128, address 000A.4110.BE01

Designated bridge has priority 32769, address 0001.97C8.DC03

Designated port id is 128.3, designated path cost 19 //Again to which desingated port it is refering to. Because already the details of current port are shown at Second line //

Timers: message age 16, forward delay 0, hold 0

Number of transitions to forwarding state: 1

Link type is point-to-point by default

------------------------------------------------

Can someone please explain me ?

Regards,

Chandu

Regards, Chandu

InayathUlla Sharieff · ‎10-20-2013

Chandra,

Dont expect many things out of Cisco Packet tracer I have used a lot and many things which are expected to see doenst work well on CPT.

You can use the real switches to have good understanding of STP this is my opinion.

Regards

Inayath

Rolf Fischer · ‎10-20-2013

Hi Chandu,

I agree on what Inayath says about the limitations of Packet Tracer.

However, perhaps we can clarify at least some of your questions.

The interfaces Fa0/2 and /3 are the easy part: They are edge-ports, meaning that the connected devices (PCs) are not spanning-tree aware, so this ports never receive a BPDU and the switch will always be designated for the segement.

Now the not-so-easy part:

Your topology doesn't have any physical loops, and the spanning-tree topology couldn't be simpler.

As the "detail" keyword implies, you get a lot of information but in such a simple topology with default values for bridge-IDs, port-cost and port priorities, this information may appear redundant.

So let's take a look at the output of a "show spanning-tree brief", which is much more clear:

Interface Designated

Name Port ID Prio Cost Sts Cost Bridge ID Port ID

-------------------- ------- ---- ----- --- ----- -------------------- -------

FastEthernet1/0 128.41 128 19 BLK 19 32768 0000.0033.3333 128.41

FastEthernet1/1 128.42 128 19 FWD 0 32768 0000.0011.1111 128.46

FastEthernet1/2 128.43 128 19 BLK 19 32768 0000.0022.2222 128.46

FastEthernet1/15 128.56 128 19 BLK 19 32768 0000.0033.3333 128.41

You can see that half of the information is reported by the designated switches (red) and the other half is local.

One could think that it's always the same but it isn't. Take a look at the next output, I changed the bridge-IDs, port costs and port priorities to make them unique in this domain:

Interface Designated

Name Port ID Prio Cost Sts Cost Bridge ID Port ID

-------------------- ------- ---- ----- --- ----- -------------------- -------

FastEthernet1/0 176.41 176 500 BLK 301 32768 0000.0033.3333 144.41

FastEthernet1/1 184.42 184 704 BLK 0 8192 0000.0011.1111 96.46

FastEthernet1/2 192.43 192 502 FWD 201 16384 0000.0022.2222 120.46

FastEthernet1/15 200.56 200 515 BLK 301 32768 0000.0033.3333 144.41

As you already know, there is a decision process to build a loop-free topology and this works without configuring any additional spanning-tree parameters by the fixed parts of the bridge- and port-IDs and default costs. However, there are also configurable parts in order to change to topology depending on your requirements.

HTH

Rolf

chandra_rc16 · ‎10-21-2013

I thanks for both of your suggestion. But now i have a doubt, can i pass CCNA R&S exam by practicing only with PT ?

Because due to my financial status i can't offer switches or any other hardware.

Regards,

Chandu

Regards, Chandu

InayathUlla Sharieff · ‎10-21-2013

Chandra,

Its been very long time that I had completed the CCNA hence not 100% sure on whats going onwith the paper.

But I can suggest from my side is that once you are good with your concept then I believe you should be good to go with this if you dont have physical access to the devices. But dont expect this to work 100% as you want.(For example: CPT doesnt go well with Layer 3 port-channel and some outputs are not exactly the same as we read on the books).

Hence I would suggest that understanding of concept and technology is the first preference then go and implement the same on the boxes.

Hope this helps.

Regards

Inayath

devils_advocate · ‎10-22-2013

Personally no, I don't think packet tracer is enough to help you pass the exam but other may disagree.

I would be looking at something like GSN3 for the routing and PT for the switching element.

Nothing can compete with working on real equipment though.

You can pickup a couple of 1700 routers for £10-15 each, 2x 2950 swtiches for similar prices and then just a cheap 3550 switch for a little more so a CCNA lab can be had for under £100 easily.

s.lachica · ‎10-22-2013

Hi Chandra,

Yes, the root port has to have "Root" state, not Designated. I also believe this is one of the PT bugs. I've teached CCNA over the past few years, and whenever I'm using packet tracer on STP labs, it almost always encounter bugs related to STP convergence like this. I do not believe that using PT only for CCNA will not help you prepare for the exam. In fact, everything you need to know and understand are in the current version of PT.

CCIE (R&S) #27666
CCSI
HP MASE

CCIE (R&S) #27666 CCSI HP MASE

sbhadrav@cisco.com · ‎05-02-2018

An STP port can be in forwarding state if:

a) it is the root port (the port to the root bridge)

b) it is the designated port for the network segment to which it is connected

The designated port is elected using the same rules that are used to choice the root port.

So the designated cost should be the lowest cost heard or sent out of an STP port.

It is so, see the following link:

http://www.cisco.com/en/US/docs/ios/12_3t/lanswitch/command/reference/lan_s1gt.html#wp1022593

Designated Cost

Designated cost for the path

Designated Bridge ID

Bridge identifier of the bridge assumed to be the designated bridge for the LAN associated with the port.