Spanning Tree Protocol

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This is a physical topology of network.

-------------------| | | | layer | | 2 | | Switch 1 |------------------------ -------------------- | | | | | | | | | --------------------|

-------------------- | | | layer

2 | | layer |-----------------| switch (3) | | 2 | | | | Switch 2 | | | --------------------

-------------------- | | | | | | - | | | | | PC1 pc2 pc3

Now , by this topology a loop is formed among 3 of the above switches. So after spanning tree performed a loop free (logical) topology formed which look like......

-------------------| | | root switch | layer | | 2 | | Switch 1 |------------------------ desgnated port -------------------- | | desgnated port | | | | | | X (this path is blocked by STP) --------------------|

-------------------- | | | layer

2 | | layer |-----------------| switch (3) | | 2 |desgnated port | | | Switch 2 | | | --------------------

-------------------- | | | | | | - | | | | | PC1 pc2 pc3

Now my question is , weather the ports for 3 hosts (pc1, pc2 and pc3) of switch 2 are also became designated port. And why????

Vicky,

Please switch to a fixed width font if doing ascii art. It may look good on /your/ screen, I cannot get a good picture in either fixed or proportional font. The recommended fix, indeed the only one, is that everyone uses a fixed width font (or switches to fixed width if needed).

HTH, M4

Yes. Every active port (i.e., a port onto which frames are forwarded by a switch) is either a Designated Port or a Root Port. Each Designated Bridge (switch) has one (and only one) Root Port, plus one or more Designated Ports.

While it is difficult to see your true topology (due to the ASCII art, which is still not aligned in my screen view), Switch 2 in your drawing is apparently a Designated Bridge, with its Root Port connected to Switch 1 (the Root Brodge), and a number of Designated Ports, three of which connect to PC1, PC2, and PC3.

I hope this helps.

-- Rich Seifert Networks and Communications Consulting 21885 Bear Creek Way (408) 395-5700 Los Gatos, CA 95033 (408) 228-0803 FAX

Send replies to: usenet at richseifert dot com

switch 1

switch 2

switch 3 | | | 3 hosts

[The loop formed by the links between the three switches is eliminated.]

The ports you mention are ports that connect only to hosts. Otherwise known as edge ports.

First, you will note in Figure 17-1 of IEEE 802.1D that edge ports are defined to be a subset of designated ports. Why? Because they are active ports and they provide the shortest path from that LAN to the root port. So that fits the definition of designated port.

So the next question might be, why are these edge ports active? Because RSTP sees no BPDUs arriving at these ports, so it can conclude that they are edge ports and therefore cannot create loops. So the state can be changed to forwarding as soon as this is determined.

Or, these edge ports can be administratively set to "edge port" in managed switches. In which case the switch can immediately transition them to forwarding state after power-up (because it knows that these ports cannot create loops).

Bert

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This is a physical topology of network.

switch (1)---------------------------- | | | | | | switch (2)------------------------switch (3) | | | | | | pc1 pc2 pc3

Now , by this topology a loop is formed among 3 of the above switches. So after spanning tree performed a loop free (logical) topology formed which look like......

(root switch) switch (1)---------------------------- desgnated port | desgnated port | | | | desgnated port X (this path is blocked by STP) switch (2)------------------------switch (3) | | | | | | pc1 pc2 pc3

Now my question is , weather the ports for 3 hosts (pc1, pc2 and pc3) of switch 2 are also became designated port. And why????

You have twice been asked to post in a fixed font.

Your schemas are unreadable to most of us. Not taking the basic courtesy of making your post readable for everyone is quite annoying.

I might have helped, but simply CANNOT, as I cannot figure out your schemas.

Please apply some basic nettiquette!

M4

Is there something in Rich Seifert's response, or in mine, that didn't answer your question?

In other words, are these three ports of the switch, to which the three hosts connect, not edge ports? If they are edge ports, then is there something in the replies you received that isn't clear?

To draw ASCII pictures, you have to use a font in which each character is assigned the same amount of space on the line. Try Courier font, for example, and be sure to use a return at the end of each line.

Bert

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Hello,

As some switches support MSTP, Some others support RSTP ,, so is that switches are also compatible with those switches , which support only STP (IEEE 802.1d).

If it is possible then please also tell me ... Is any bits to be set for providing the compatiblity with those switches which support MSTP or RSTP , with switch which support only simple STP. or it done automatically by switch hardware.

Thanks in advance.

Vikrant

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Hello,

As some switches support MSTP, Some others support RSTP ,, so is that switches are also compatible with those switches , which support only STP (IEEE 802.1d).

If it is possible then please also tell me ... Is any bits to be set for providing the compatiblity with those switches which support MSTP or RSTP , with switch which support only simple STP. or it done automatically by switch hardware.

Thanks in advance.

Vikrant

MSTP is described in IEEE 802.1Q Clause 13.

The first important point is that MSTP is layered over RSTP (or STP). It is used in conjunction with these 802.1D protocols. So, for example, there will always be the common spanning tree, as created by STP or RSTP, underlying MSTP.

One stipulation of MSTP is that all of the switches belonging to one of the multiple spanning trees must agree as to the VLAN ID of that VLAN. If they do not agree, then some of the LANs of that VLAN might not communicate with the rest.

So, if some of the switches belonging to a particular VLAN do not know MSTP, then all the switches belonging to that VLAN should not use MSTP.

Bert

(vlan1) (vlan2) pc1 pc2 | | | | | | --------------------- (designated port) | layer 2 |----------------------------------------------------------- | switch (1) | | | | |

--------------------- | | (designated port) | | | | | | (root port) X

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

------------------------ | layer 2 | | layer 2 | | switch (2) |-------------------------------------------------| switch (3) | | | (designated port) | |

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

------------------------ | | | | pc1 pc2 (Vlan 1) (vlan 2)

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Hello Sir

can u plz tell me that

Is STP weather decide (when every time topology change) that which port became trunk port or some other mechanism is used...

Thanks in advance

Vikrant

Once again, this material is thoroughly covered in the relevant standard (IEEE 802.1D), as well as "The Switch Book." You should read one or both and then pose your questions.

That said, if you are using STP, then it is STP that determines the state of each port, both before and after topology changes.

-- Rich Seifert Networks and Communications Consulting 21885 Bear Creek Way (408) 395-5700 Los Gatos, CA 95033 (408) 228-0803 FAX

Send replies to: usenet at richseifert dot com