Questions about STP

In the Sybex book dealing with STP and port forwarding/blocking, it says that the root bridge always has all its ports in the forwarding state; it also says that - in a switch looping environment - root ports are always forwarding ports. Yet the diagram in the book (p. 360) has the port other than the root port as the forwarding port on all the switches. If switch A is the root bridge, which is connected to switch B, which is connected to switch C, then to switch D, the book says that on switch B - the link to switch A (the root bridge) - is the root port, so it should be the forwarding port? But the link from the root bridge should be the forwarding port, shouldn't it?

Also, if you're using mac addresses to determine priorities, what if you're comparing, say, mac 0000.0101.c777, mac 0000.0101.j444, and mac 0000.0101.2011, which would be the root bridge and number two and three?

And lastly, just for clarification, in a multi-switch network, how do you find which link will have the rex X, so to speak, on the link to blocking it?

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Comments

Netstudent

Tricon7 wrote:

If switch A is the root bridge, which is connected to switch B, which is connected to switch C, then to switch D, the book says that on switch B - the link to switch A (the root bridge) - is the root port, so it should be the forwarding port?

Also, if you're using mac addresses to determine priorities, what if you're comparing, say, mac 0000.0101.c777, mac 0000.0101.j444, and mac 0000.0101.2011, which would be the root bridge and number two and three?

If switch A is the root, then all ports are fwd'ing. If switch B is connected to the root bridge, then that port will be the root port. Both ports will be passing traffic. The designated port on switchB would be the port that is connected to switchC as long as it advertises the lower BPDU or BridgeID/MAC onto the segment . Also i am not looking at the book, I'm just giving an example.

Secondly "j" is not a valid hexidecimal value. BUt out of the other two 0000.0101.2011 would be lower. Hex is 0-9 and A-F A=10 B=11 ect....

FF = 15*16 + 15*1 = 255
CF = 12*16 + 15*1 = 207
4B = 4*16 + 11*1 = 75

IN case you didn't see this little flash show I posted yesterday, here it is. It might help you a little. http://www.cisco.com/warp/public/473/spanning_tree1.swf

networker050184

Ok not only is the root port in forwarding state but designated ports are in a forwarding state also. All ports on the root switch are forwarding since they are all designated ports. I think you are a little confused by what ports go into a forwarding state.

If switch A is the root and switch B is connected to it then A and B ports connected together will both be forwarding. Switch A's port will be a designated port and B's port to A will be its root port.

The ports between B and C the same thing. Switch B's port will be the designated port and switch C's port will be its root port, so they will both be forwarding.

Root port and designated ports go into forwarding state while non root and non designated ports go into blocking state.

Hope this helps.

Tricon7

Netstudent wrote:

Tricon7 wrote:

If switch A is the root bridge, which is connected to switch B, which is connected to switch C, then to switch D, the book says that on switch B - the link to switch A (the root bridge) - is the root port, so it should be the forwarding port?

Also, if you're using mac addresses to determine priorities, what if you're comparing, say, mac 0000.0101.c777, mac 0000.0101.j444, and mac 0000.0101.2011, which would be the root bridge and number two and three?

If switch A is the root, then all ports are fwd'ing. If switch B is connected to the root bridge, then that port will be the root port. Both ports will be passing traffic. The designated port on switchB would be the port that is connected to switchC as long as it advertises the lower BPDU or BridgeID/MAC onto the segment . Also i am not looking at the book, I'm just giving an example.

Secondly "j" is not a valid hexidecimal value. BUt out of the other two 0000.0101.2011 would be lower. Hex is 0-9 and A-F A=10 B=11 ect....

FF = 15*16 + 15*1 = 255
CF = 12*16 + 15*1 = 207
4B = 4*16 + 11*1 = 75

IN case you didn't see this little flash show I posted yesterday, here it is. It might help you a little. http://www.cisco.com/warp/public/473/spanning_tree1.swf

Duh - I should have realized that about hex and not used a "j".

If I'm given a scenario where I need to determine which ports on which switches will be forwarding, you're saying that for the switch connected to the root bridge, both ports will be in a "forwarding" status that are connecting the two?

Also, if I need to determine who has the lower ID/mac of a switch, and I have 0000.0000.2000 and 0000.0000.c000, which will be the lowest one?

Lastly, let's say you have a switching environment with 10 switches. For those switches not connected to the root bridge, you can determine which is lower by ID/mac. But you're only going to have one blocking point to block loops. How is this part determined? The highest mac of them all have the segment where the block will occur?

Netstudent

Tricon7 wrote:

Netstudent wrote:

Tricon7 wrote:

If switch A is the root bridge, which is connected to switch B, which is connected to switch C, then to switch D, the book says that on switch B - the link to switch A (the root bridge) - is the root port, so it should be the forwarding port?

Also, if you're using mac addresses to determine priorities, what if you're comparing, say, mac 0000.0101.c777, mac 0000.0101.j444, and mac 0000.0101.2011, which would be the root bridge and number two and three?

If switch A is the root, then all ports are fwd'ing. If switch B is connected to the root bridge, then that port will be the root port. Both ports will be passing traffic. The designated port on switchB would be the port that is connected to switchC as long as it advertises the lower BPDU or BridgeID/MAC onto the segment . Also i am not looking at the book, I'm just giving an example.

Secondly "j" is not a valid hexidecimal value. BUt out of the other two 0000.0101.2011 would be lower. Hex is 0-9 and A-F A=10 B=11 ect....

FF = 15*16 + 15*1 = 255
CF = 12*16 + 15*1 = 207
4B = 4*16 + 11*1 = 75

IN case you didn't see this little flash show I posted yesterday, here it is. It might help you a little. http://www.cisco.com/warp/public/473/spanning_tree1.swf

Duh - I should have realized that about hex and not used a "j".

If I'm given a scenario where I need to determine which ports on which switches will be forwarding, you're saying that for the switch connected to the root bridge, both ports will be in a "forwarding" status that are connecting the two?

Also, if I need to determine who has the lower ID/mac of a switch, and I have 0000.0000.2000 and 0000.0000.c000, which will be the lowest one?

Lastly, let's say you have a switching environment with 10 switches. For those switches not connected to the root bridge, you can determine which is lower by ID/mac. But you're only going to have one blocking point to block loops. How is this part determined? The highest mac of them all have the segment where the block will occur?

Yes both ports will be FWD;ing because the ports on the root bridge are designated ports and the port hearing the least cost BPDU from the root is a root port. IN this case SwitchB hears a BPDU with a cost of 0 from the root bridge.

IN HEX
C= 12 and 2 = 2
So 0000.0000.2000 is lower.
______________________
0x20
2*16 + 0 = 32 is decimal

0xC0
12*16 + 0 = 192 in decimal
_______________________
I don;t quite understand your last question. But the desgnated port is determined by the port that is advertising the lowest cost BPDU onto the segment. The BPDU's are the same cost, then bridge ID is used.

networker050184

Just remeber root ports and designated ports forward no matter how many switches you have. If you stick with that you will be good.

Root ports are the ports receiving the lowest root path cost.

Designated ports are the ports advertising the lowest root path cost on to the segment.

You only have to worry about priority id there is a tie.

Tricon7

Let me rephrase: If you have multiple segments with high/low mac addresses on switches not connected to the root bridge, how do you know which of the ports will be blocked to prevent switching loops? You're only going to have one "red X", so to speak, in this broadcast domain, and every segment has a switch with a lower/higher mac than the one it's connected to. So which segment will be blocked? This segment over here at the highest mac port, or that segment over there with the highest mac port?

networker050184

Every switch weather its connected directly to the root or not has a root port which will be forwarding. Also Wich ever port has the lowest root path cost (weather its connected to the root or not) becomes the designated port and forwards. As you can tell it doesn't matter if they are connected the same rules apply. If they tie then then the one with the lowest BID becomes designated.

Netstudent

Try to think of it in terms of: Root ports listen to costs, designated ports advertise costs.