STP

Say you had 3 switches each connected to the other two thru fast ethernet ports using default costs of 19, SW1 is the root, SW2 has a higher bridge ID than SW3. If you changed the port cost to 1 for SW2's port connecting it to SW3, wouldn't SW2 become the designated bridge on that segment?

Crude diagram:

SW1 - - SW2
| / |
| /
SW3

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Comments

Netstudent

you would need to change the port cost on switch2's port to switch1 to ensure that switch2 became the designated bridge on the segment between 2 and 3.

Switch3 would advertise 19 if this is a fast ethernet link and switch2 would advertise 1. Therefore switch2 would have the designated port on the segment.

Which book are you reading?

Mikdilly

Netstudent wrote:

you would need to change the port cost on switch2's port to switch1 to ensure that switch2 became the designated bridge on the segment between 2 and 3.

Thanks, so the port that's connected to the segment is not considered when figuring the cost of the hello, correct?

Mikdilly

Ciscopress, i got this from a question on the practice exam cd.

Netstudent

Mikdilly wrote:

Netstudent wrote:

you would need to change the port cost on switch2's port to switch1 to ensure that switch2 became the designated bridge on the segment between 2 and 3.

Thanks, so the port that's connected to the segment is not considered when figuring the cost of the hello, correct?

correct. It's the port that the BPDU came in on. IF you have the cisco press ICND book, look on page 41.

The cost is calculated by adding the cost of the recieved BPDU(0 in this case) to the cost of the interface on which the BPDU was recieved.

If you would have changed the cost on switch2's port to switch3, then switch3's port would have been the designated port because the BPDU's would have had the same cost(19) and the bridgeID's would have been used to contend for designated bridge. IN that case switch3 would have won

Mikdilly

Netstudent wrote:

Mikdilly wrote:

Netstudent wrote:

you would need to change the port cost on switch2's port to switch1 to ensure that switch2 became the designated bridge on the segment between 2 and 3.

Thanks, so the port that's connected to the segment is not considered when figuring the cost of the hello, correct?

correct. It's the port that the BPDU came in on. IF you have the cisco press ICND book, look on page 41.

The cost is calculated by adding the cost of the recieved BPDU(0 in this case) to the cost of the interface on which the BPDU was recieved.

If you would have changed the cost on switch2's port to switch3, then switch3's port would have been the designated port because the BPDU's would have had the same cost(19) and the bridgeID's would have been used to contend for designated bridge. IN that case switch3 would have won

Actually had that exact page open, it just didn't click in until I read your first reply, thanks again.

EdTheLad

Mikdilly wrote:

Thanks, so the port that's connected to the segment is not considered when figuring the cost of the hello, correct?

Not exactly true, the local port connected to the segment will be considered in one case.If you have two switches connected together through a hub and the local switch has two ports connectd to the hub while the neighbor switch has one port connected to the hub, the election will be as follows.

1) Path cost, both ports are fastethernet so have a cost to the neighbor switch via the hub of default 19.
2) Since the neighbor switch is seen on both ports the senders bridge id is the same.
3) Path cost is a combination of priority and port number of the sending switch, since the neighbor switch has only one port connected to the hub, the same path cost will be seen on both local ports.
4)The lowest port id on the local switch will take priority and become root.So port 1 will have priority
over port 2.

Netstudent

hmmm interesting....but thats one crazy scenario that I never would have thought of unless for some unknown reason I would have used a hub.

Since a hub only extends the physical media, it's almost like having 2 segments attached to the root bridge. Each would have the same cost and priority, so port# is used.

EdTheLad

Netstudent wrote:

hmmm interesting....

Here's something even more interesting.
If you have a root bridge connected to a hub via two ports and the downsteam switch has one port connected to one hub port, which of the two root bridge ports will be used to exchange data between the two switches since both the root bridge ports will be forwarding.

Netstudent

If we can't use the lowest port rule, then maybe one port is used for upstream traffic and the other port is used downstream traffic?

Damn I need hub. Didn't think I would ever be saying that.

Maybe the even and odd ports are differentiated some how???

EdTheLad

Netstudent wrote:

If we can't use the lowest port rule, then maybe one port is used for upstream traffic and the other port is used downstream traffic?

Damn I need hub. Didn't think I would ever be saying that.

Maybe the even and odd ports are differentiated some how???

NAH actually it will use the lowest port rule and one of the ports on the root bridge will be blocking.
So nothing is set in stone, the statement that all ports on the root bridge and designated and forwarding is not always true.

Netstudent

Cool! well I've said it before and I'll say it again. The more you know, the more you realize you don't know. Just when you thought you had a firm grasp on a concept, you go deeper and the bottom just falls out from underneath you. When does it stop?

I guess all you can do is keep on diggin.