So now we need to address what we've talked about before with port priorities
and how this ties into everything.
So we've said that cost is an inbound concept. If I have an advertised cost,
whatever that is, let's say it's 100, and this port here is 150 cost.
Well then when I receive it, I consider my inbound cost here, and therefore my
cost to get to the root this way is going to be 250.
I add the 100 to the 150 because again it's an inbound concept. WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000
I add the 100 to the 150 because again it's an inbound concept.
But now we're going to talk about port priorities. And we're talking about port
priority.
What we'll find is that it's actually an outbound concept.
I'm not going to configure port priorities where I'm receiving the BPDU.
So if I'm receiving the BPDU this way, we might expect that if I want to
configure cost, I would put it here on the inbound interface, and therefore if WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000
configure cost, I would put it here on the inbound interface, and therefore if
I want to configure port priority, then I would also put it on the same
interfaces.
And that would be wrong.
Port priority is an outbound concept, meaning that it's going to have to go
here at the interfaces that are sending the BPDU because it's an outbound, or
in other words, an egress counting method.
Now we recall that the port priority is actually just part of the equation. WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000
Now we recall that the port priority is actually just part of the equation.
Ultimately we're going to form a port ID, which consists of the priority value
plus a port number.
And as we explained in the past, this number is an internal value. Usually we
can say that for example gig 01 is less than gig 02.
At that point we can just use basic math, but it gets a little trickier when we
say okay that's great and all but how does gig 01 fit into all of this.
I mean it'd be one thing if it was one slash zero, but maybe the switch has a
gig one or a gig zero, and then we have to go and look and find out what that WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000
gig one or a gig zero, and then we have to go and look and find out what that
port number is.
So it's an internal concept.
But in a lot of cases we can just look at the values here and compare them and
say gig 01 would be less than gig 012.
Now the priority again, this is actually eight bit, and so the ranges from zero
to 255.
And Cisco's default value, if we remember, is 128.
And furthermore, we must configure the priorities in increments of 16. WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000
And furthermore, we must configure the priorities in increments of 16.
So it's similar to how we can, or we have to configure our bridge priorities in
increments of 4096, even though we have a reason for that with the VLANs.
The port priority is also going to be in increments of 16 here.
So we usually don't need more than just really two values, maybe one that's
higher than 128, one that's lower than 28.
And so certainly we have plenty we don't need to worry about running out of
port priorities or anything like that. WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000
port priorities or anything like that.
So again, let's go ahead and zoom up here and take a look at a situation where
we've got, let's just say two interfaces here.
And so when we look at the port number, let's just say that we're sending the P
2Us by the way in this direction.
And so our port numbers are going to be zero and one.
Maybe that's a gig zero slash zero and zero slash one, but regardless, we just
have numbers on the ports.
And let's flip it around.
Let's say we've got port five down here. WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000
Let's say we've got port five down here.
So this is the higher number and the lower number port three here on the right
on this switch.
So here's the question.
If this switch here has to block one of these ports, which port does it end up
blocking and which one becomes the root port?
So the answer here is to again compare the port IDs and the port ID, we prefer
the lower number, just like with everything else in spanning tree.
And so when I look at this and say, okay, well, I prefer the lower number, WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000
And so when I look at this and say, okay, well, I prefer the lower number,
which is three, and therefore I block this, except again, it's an outbound
concept.
We're not looking at the inbound interfaces.
We don't care what these are.
We only care what the outbound is.
And the outbound says that this port is the lower one and should be favored and
therefore we will block this even though it's actually the lower ID locally.
So this is an interesting concept because the control actually rests on the
switch that isn't having to make the decisions. WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000
switch that isn't having to make the decisions.
So if I wanted to, I could then get on here and say, well, let's change the
priority.
Maybe we want to move the blocked port over here or something like that.
So by default, it's 128 and 128.
And so we have to go with the lower port number.
But now I could change this to say, you know, instead of this being 128, let's
make it 80, another multiple of 16.
Well, now 80 compared to 128, just like again with bridge IDs and such, we're WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000
Well, now 80 compared to 128, just like again with bridge IDs and such, we're
not even going to look at the MAC address when the bridge priority tells us
which one is better.
Same thing here.
The port priority tells us which one's better.
This is now the superior BPDU.
Therefore, I'm going to bring this port online and end up blocking the left
port.
At this point, it has nothing to do with the port numbers as everything to do
with the port priority.
Now, in the real world here, as we often like to bring this back to reality, in WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000
Now, in the real world here, as we often like to bring this back to reality, in
the real world, we almost never worry about port priorities.
There's just not a lot of reason to worry about which link is getting blocked
because we're usually going to use ether channels.
It would take a pretty special situation to say, first of all, we can't bundle
these connections together and make them both online.
And then second of all, that we care which one is being chosen.
Probably the closest possibility might be a situation where we have two
switches with an ether channel here and an ether channel here, at which point WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000
switches with an ether channel here and an ether channel here, at which point
both of these links can't be online at the same time, one of them would have to
get blocked.
So we are leveraging ether channels, but we still need to worry about blocking
one.
And then, if we maybe one is 10 gig and one is one gig ether channel, well, at
that point, spanning tree cost would just favor the higher bandwidth.
And so we don't even necessarily need to worry about bandwidth at this point, WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000
And so we don't even necessarily need to worry about bandwidth at this point,
but we might have to come up with some other reason why we'd really care which
one of these ends up getting blocked, I guess, is what we're getting at.
So chances are in the real world, we're never going to have to worry about this
.
However, when we're preparing for an exam like this, when we're trying to
understand the basic mechanisms of spanning tree, we do need to understand this
.
We do need to understand how to manipulate this.
And in the end, understand that if I want to change which port gets blocked,
the control lies on the upstream switch, not down here.
If I want to change the cost, then I can do it down here. If I want to change WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000
If I want to change the cost, then I can do it down here. If I want to change
the port priority, it's going to be up here.
Either way, I can control which port gets blocked, but I'm going to have to do
it differently depending on which switch I'm configuring.