Can A ring Topology Function Without STP

hitmen

Assuming that I have a router connected to a ring of 7 switches. The switches are in then connected to hosts of course.

There is only 1 subnet between the router and the first switch. Can the ring topology run successfully without STP? Will there be any loopback?

I dont understand. Why is a ring topology recommended in cisco textbooks if routing loops may occur?

networker050184

Routing loops are not the same thing as a L2 loop.

Do you know how switches forward unknown unicasts or broadcasts?

iamme4eva

There is a difference between routing loops and layer 2 loops.

PC's send out layer two broadcasts all the time, such as ARP requests. Each switch will flood the broadcast out of all ports except the one it was received on. If your switches are connected in a way that the broadcast can end up back at a switch that has already seen it (basically anything except a straight line), that's a loop. The switch has no idea that it has seen that broadcast before, and will flood it out of all ports except the one it came in on.

Routing loops are different. A routing loop would occur where one router has a next hop that has already been in the chain somewhere. Routers don't (by default) forward broadcast, so a routing loop only occurs where a router actively makes the decision to point the packet to a next hop that it has already been through. Routing protocols are pretty good at avoiding them though.

Basically, a layer 2 loop creates a broadcast storm. Your switches have so many broadcasts to deal with that network congestion is high and your network gradually gets ground to a halt. A routing loop isn't as devestating - it just ends up that you can't reach your destination.

Switches don't decrement the TTL, so a layer 2 broadcast will go on forever.

Spanning-tree is essential, and is enabled by default, and with the aid of spanning tree a ring topology is perfectly safe. A ring topology provides a level of redundancy, as one single link failure can be overcome by an alternate path.

Hope you can make sense of this - it's waffled on a bit!

hitmen

So you are saying that in a ring topology, STP must be enabled. If STP is not enabled then a broadcast storm will occur.

Then if a broadcast storm occurs, what will happen to the host other than congestion?

networker050184

Have you actually tried researching this information? I'd suggest you grab a good book and get to learning. It's very simple stuff you are asking here.

NetworkVeteran

hitmen wrote: »

So you are saying that in a ring topology, STP must be enabled. If STP is not enabled then a broadcast storm will occur.

No. More accurately, in a L2 ring topology, some loop-prevention mechanism should be in place, and STP is one choice.

hitmen

ok. let me get this straight. Loop prevention mechanism is only used in ring topology but NOT star OR bus topology. Am I correct?

In that case why didnt the ccna textbook elaborate that the ring topology is "special" compared to the others?

Souljacker

hitmen wrote: »

ok. let me get this straight. Loop prevention mechanism is only used in ring topology but NOT star OR bus topology. Am I correct?

In that case why didnt the ccna textbook elaborate that the ring topology is "special" compared to the others?

Because it's not "special". If you have a circle of anything and you don't want it to loop, you need to have some way to prevent the loop. Getting hung up on terminology when the issue is conceptual in nature is probably what is confusing you.