STP question again - Designated bridge vs Root bridge...

Is the term designated bridge synonymous with root bridge? Do they refer to the same bridge/switch? Is the root bridge the designated bridge?

I understand what the root bridge is, but then I see this statement..."The bridge with the lowest administrative distance to the root bridge is called the designated bridge."...and all hell breaks loose!!

The bridge with the lowest admin distance (zero) to the root bridge, is the root bridge itself right?

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mikej412

r_durant wrote:

Is the term designated bridge synonymous with root bridge? Do they refer to the same bridge/switch? Is the root bridge the designated bridge?

No. No. No.

Maybe the example (and picture) here will help.

From after Figure 23-3

Finally, designated bridges and their designated ports are determined. A designated bridge is the bridge on each LAN that provides the minimum root path cost. A LAN's designated bridge is the only bridge allowed to forward frames to and from the LAN for which it is the designated bridge. A LAN's designated port is the port that connects it to the designated bridge.

In some cases, two or more bridges can have the same root path cost. In Figure 23-3, for example, Bridges 4 and 5 can both reach Bridge 1 (the root bridge) with a path cost of 10. In this case, the bridge identifiers are used again, this time to determine the designated bridges. Bridge 4's LAN V port is selected over Bridge 5's LAN V port.

The theory (and definition) in the first paragraph is demonstrated referencing the figure in that 2nd paragraph -- so hopefully it makes more sense.

r_durant

Thanks for the response Mike, it may be simple and trivial, but the concept(s) do have me a bit confused....

One other question...in the statement..."A designated bridge is the bridge on each LAN that provides the minimum root path cost."

Does "LAN" refer to the entire network or the segment between the switches?

rjbarlow

r_durant wrote:

Thanks for the response Mike, it may be simple and trivial, but the concept(s) do have me a bit confused....

One other question...in the statement..."A designated bridge is the bridge on each LAN that provides the minimum root path cost."

Does "LAN" refer to the entire network or the segment between the switches?

Hi durant, I'm sure that it's a lacking word in that You report: what is lacking is segment.

Than, Your phrase would have to look like follows:

"A designated bridge is the bridge on each LAN segment that provides the minimum root path cost."

Mean "segment" like a communication line between two switches (and switches itself).

r_durant

rjbarlow wrote:

r_durant wrote:

Thanks for the response Mike, it may be simple and trivial, but the concept(s) do have me a bit confused....

One other question...in the statement..."A designated bridge is the bridge on each LAN that provides the minimum root path cost."

Does "LAN" refer to the entire network or the segment between the switches?

Hi durant, I'm sure that it's a lacking word in that You report: what is lacking is segment.

Than, Your phrase would have to look like follows:

"A designated bridge is the bridge on each LAN segment that provides the minimum root path cost."

Mean "segment" like a communication line between two switches (and switches itself).

You're right RJ...that's why I asked the last question there, because I was thinking of a single root bridge or designated bridge on the entire network and not on a segment...

I did understand Mike's example, but it was just to understand what "LAN" refered to in this case...

Thanks guys...(Mike & RJ)

dtlokee

Traditionally a LAN was a single broadcast domain, comprised of switches, hubs, bridges, repeaters and other layer 1/2 technologies. Once you introduced a router (or other L3 device) it became an "Internetwork", when I first started in networking people called then "internets" but then the Internet grew so the name seemed to change to avoid confusion. The term segment on Ethernet using switches or hubs would refer to the connection between switches, or from switch to host. On older Ethernet networks (10 base 2 and 10 base 5) the term segment referred to the entire coax segment between repeaters or bridges, and LAN was again the entire broadcast domain. This is where the 5-4-3 rule came in, 5 segments seperated by 4 repaters with only 3 of them populated with hosts.

Now with the introduction of VLANs the idea of a LAN has changed, but a segment is still a segment. A VLAN is still a single broadcast domain, but it can be part of a switch instead of the entire switch.

r_durant

Ok granted, I understand that now...well, not that I don't understand what a LAN or a segment is, but what confused me was how the term LAN (which I deemed to mean the entire network, all switches included) was used in what I read, and even in Mike's example...I thought well, the LAN has already elected a root bridge, where or what is the designated bridge...So now my confusion is gone and I realize that in this case, LAN refers to the segment between two switches....It's just as RJ said, segment could have possibly been used - "LAN segment"...