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How to Verify the Traffic is going through Root Bridge

Posted on 2014-11-18
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I have the topology shown below. I can ping from R1 to R2 and back. SW1 is the Root Bridge, If I understand all traffic goes through RB, but how do I verify that when pinging between routers ?

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SR
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Question by:jskfan
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by:Neil Russell
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Why would ALL traffic go through the root bridge?  on a large network that would be a huge bottleneck!
The root bridge is just a root for calculating the "Tree".
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by:Don S.
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The short answer is - you can't.  The bigger question is why do you have the switches in a loop?  that is not a great idea.  Yes, spanning tree should be able to take care of disabling the loop so broadcasts don't go crazy, but I think you are asking for trouble with broadcast storms when you setup a loop in a switched network.
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by:Neil Russell
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I did comment on the LOOP in an earlier question but was not answered.
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by:Don Johnston
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First, all traffic does not necessarily go through the root bridge.

For example, R1 - SW3 would not go through the root bridge (unless you've got a REALLY slow link between SW4 and SW3).

Now if the link between SW1 and SW3 or the link between SW3 and SW4 is slow enough, then you may see that the SW2 - SW4 link is not blocking.  Which would mean that traffic between R1 and R2 does not pass through the root bridge.

The way you would determine which path the traffic takes is to map out where the blocking ports are on your diagram.  There will only be one path between R1 and R2.
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by:mikebernhardt
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Pinging has nothing to do with spanning tree. The way to verify if the traffic is going through the root bridge is to look at the ports which face the root and see what is blocking and what is forwarding. Assuming these are Cisco switches, you can type
show spanning-tree vlan [vlan#]
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by:jskfan
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I thought the Root Bridge is the one that traffic goes through....I mean all other switches forward traffic to it...
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by:Don Johnston
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I thought the Root Bridge is the one that traffic goes through...
Not necessarily.
I mean all other switches forward traffic to it...
That's not exactly correct either. All other switches make their decision on which port to block based on the cost to the root.  But that does not mean that all traffic goes through the root.
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by:jskfan
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So if I understand you Guys, Root Bridge will handle Broadcast "Traffic" only ?
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by:Don Johnston
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No.

The root bridge is the source of the configuration BPDU's which allow all other switches to identify loops and block ports to eliminate the loops.  As a result, all switches will have one root port which is the port that has the lowest cost (cost is a function of bandwidth) to the root. If a switch receives BPDU's (with a higher cost) on any other ports, then those ports will be blocked.
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by:jskfan
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What do the BPDUs carry as traffic type? are they used just for keep alive of the switch ports ?
initially I thought you determine the Root Bridge in function of how you want to route the traffic...
for instance in the topology I posted, SW1 is the Root Bridge, and it would be efficient to make SW2 or SW4 as the RB , and it would be the short path from R1 to R2
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by:Don Johnston
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What do the BPDUs carry as traffic type?
I don't understand what you're asking.  BPDU's don't "carry" any traffic.
are they used just for keep alive of the switch ports ?
I guess you could think of them as a type of keepalive. BPDU's are used to allow all other switches to identify loops and block ports to eliminate the loops.
initially I thought you determine the Root Bridge in function of how you want to route the traffic...
That's pretty much true. You locate the root bridge so that traffic will flow over a desired path.
for instance in the topology I posted, SW1 is the Root Bridge, and it would be efficient to make SW2 or SW4 as the RB , and it would be the short path from R1 to R2
For traffic to moving from R1 to R2, making SW2 or SW4 would guarantee that traffic flows through a root.  But that's not the goal. The goal is to provide a good path for traffic. If that means is goes through a root, fine. But that's not the goal.
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by:jskfan
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Ok what I can retain from this discussion is: STP and Root bridge has nothing to do in the decision of which path the traffic will take...they are just there to prevent the Loop..

So if you have a design that has Access switches/Distribution/ and core switches, where would you put the Root Bridge? does it matter where ?
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by:Don Johnston
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Ok what I can retain from this discussion is: STP and Root bridge has nothing to do in the decision of which path the traffic will take...they are just there to prevent the Loop..
Well, by preventing the loop, you are also affecting the traffic path.
So if you have a design that has Access switches/Distribution/ and core switches, where would you put the Root Bridge?
In a location that allows an optimal traffic path for most traffic.
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by:jskfan
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Well, by preventing the loop, you are also affecting the traffic path.

In the topology I posted above, let's say the ICMP traffic going from R1 to R2 (Pinging from R1 to R2), but SW4 has interface E2/2 facing SW2 in Blocking mode(STP). will the ICMP traffic go SW4 then SW3 then SW1 then SW2 to reach R2 ?

OR

ICMP traffic does not care about STP Blocked Port ?
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Don Johnston earned 332 total points
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Other than BPDU's, no traffic goes through a blocked port... period

If SW2 port e2/2 is blocking then the only path for any traffic between R1 and R2 is through SW4-SW3-SW1-SW2.
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by:jskfan
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If SW2 port e2/2 is blocking then the only path for any traffic between R1 and R2 is through SW4-SW3-SW1-SW2.

Good...
I believe Traceroute will verify that ?
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by:Don Johnston
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No.  Traceroute is a layer 3 function.  So only layer 3 devices will respond to traceroute packets.

The only ways to determine the path traffic takes through a layer 2 network is to either:

1) Identify which ports are blocked on a topology diagram. There will only be one path through the network (see attached).

2) Look at the MAC address table for each switch and note which interface is used to forward traffic for a particular MAC address.
SR.JPG
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by:jskfan
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Thanks Don.
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Author Closing Comment

by:jskfan
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Thank you All !!
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