Logical Topology

Hi,

I wonder if anyone out there can answer me on the following. It is my understanding that if u are using a physical star topology with an unintelligent hub, messages are broadcasted to every node in that broadcast area, and this means the Logical topology is a 'Bus'.
However these days Physical Star setups use intelligent switches that allows messages to be routed to the appropriate destination port so no broadcasting occurs (unless the switch is not aware of the destination address). In this situation what is the Logical topology (or what protocol is behind the switch) ?

Thanks to whoever can clear up my confusion on this.

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Chris Knight

Switches are layer 2 so think MAC address. This is how a switch determines which port to send to.

Layer3 switches are an entirely different beast, and they incorportate routing.

Hope this helps.

rasherboy

Thanks for that, makes it clearer. I am designing a netowork as part of an assignment for college and need to include my logical topology, it would seem that from what you said I need to discuss the different protocols used. My lectures have been about 'Spanning Tree' and 'OSPF' so just trying to get it clear where these protocols tie in.

I understand that the 'logical topology' is not about the physical layout but rather the routes that signals take (the stuff you can see), so also wondering if I need to discuss CSMA CD & Ethernet.

Any further advice would be great thanks!

snadam

alanoreilly wrote:

I understand that the 'logical topology' is not about the physical layout but rather the routes that signals take (the stuff you can see), so also wondering if I need to discuss CSMA CD & Ethernet.

Any further advice would be great thanks!

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Chris Knight

rasherboy wrote:

Thanks for that, makes it clearer. I am designing a netowork as part of an assignment for college and need to include my logical topology, it would seem that from what you said I need to discuss the different protocols used. My lectures have been about 'Spanning Tree' and 'OSPF' so just trying to get it clear where these protocols tie in.

I understand that the 'logical topology' is not about the physical layout but rather the routes that signals take (the stuff you can see), so also wondering if I need to discuss CSMA CD & Ethernet.

Any further advice would be great thanks!

Just remember all 802.E use csma/cd...

Wifi use csma/ca

Chris Knight

rasherboy wrote:

Thanks for that, makes it clearer. I am designing a netowork as part of an assignment for college and need to include my logical topology, it would seem that from what you said I need to discuss the different protocols used. My lectures have been about 'Spanning Tree' and 'OSPF' so just trying to get it clear where these protocols tie in.

I understand that the 'logical topology' is not about the physical layout but rather the routes that signals take (the stuff you can see), so also wondering if I need to discuss CSMA CD & Ethernet.

Any further advice would be great thanks!

Since your incoorperating spanning tree and OSPF, your thinking layer 3. So think along the lines of that.

Dont forget about RIP,OSPF,BGP, etc...

malcybood

plague22 wrote:

Since your incoorperating spanning tree and OSPF, your thinking layer 3. So think along the lines of that.

Dont forget about RIP,OSPF,BGP, etc...

Spanning Tree uses messages called BPDU's to avoid a switched network creating switching loops. This protocol operates at Layer 2 not layer 3

RIP, OSPF, IGRP, EIGRP, IS-IS, BGP operate at layer 3. You don't need to know them in any great detail for the Net+ as I remember, just what they are. Remember that these are routing protocols and protocols such as IP, IPX/SPX and Appletalk are routed protocols

Routing protocols select the best path to a destination network based on routing table information/metrics. Different protocols use different metrics to determine the best path to a destination, which can include hop count, bandwidth, delay, load, reliability, cost are the most common terms used.

Like STP routing protocols have methodologies built into them to avoid routing loops being created. Some terms to look into in regard to avoiding routing loops are

max hop count
Split Horizon
Hold Down Timers
Route poisoning

You definately need to know the difference between routing and routed protocols and their functions for the exam.

Good luck

rasherboy

Take a bow! All your advice has been very helpful, and has confirmed what I was thinking, as well as making me aware of stuff I didn't know