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Switch spanning tree root bridge purpose

                   Hi guys,

Me and colleague at work had a long discussion today in regards to role of root bridge, Can anyone explian and provide some documentation on role of cisco root bridge purpose. colleauge says data trffic doesnt have to hit root bridge all the time and can starigh go to destination and i believe that all the data and bpdu goes to root bridge and from there it goes to destination.....also i am seeing switch are learning mac address of devices through two diff vlans and mac are unique and can be on one port only and should e only one path to it is this need correcting? Help and gueidance is appreciated.....

2 ACCEPTED SOLUTIONS

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Advocate

Mohammad,

Your colleague is correct.

The root bridge function is only for the spanning tree protocol.

The other switches/bridges refer to the root bridge to find

redundant paths so as to ensure NO Layer 2 Loops exist.

See this link.

http://www.cisco.com/univercd/cc/td/doc/product/rtrmgmt/sw_ntman/cwsimain/cwsi2/cwsiug2/vlan2/stpapp.htm

Regards,
Alex.
Please rate useful posts.

Regards, Alex. Please rate useful posts.

View solution in original post

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Hall of Fame Cisco Employee

Hello Mohammad,

Your colleague is correct. Consider the following topology (taken from Cisco documentation):

http://www.cisco.com/en/US/i/000001-100000/70001-75000/74001-75000/74623.jpg

The dotted lines are installed links that are blocked by STP while the bold lines are installed links in Forwarding state.

The root bridge is located at the top right corner. Notice now the two leftmost 2950 access layer switches. If the stations connected to them (not present in the exhibit) need to communicate, the traffic will go through the ingress 2950, go up to the 3550 switch in the distribution layer, and then proceed down to the next 2950. The traffic will not go through the root bridge. The same is true for the communication of the two rightmost 2950 access layer switches. Only if, say, the leftmost and rightmost 2950 want to send traffic to each other, the traffic will need to go through the root bridge because there is no other (Forwarding) way that connects these two 2950 together.

In general, traffic goes through the root bridge only if the root bridge lies in the path towards the destination. If the root bridge is not on the path towards a particular destination, the traffic will not go through it. When delivering frames, do not think too much on STP - simply take into consideration only those links that are Forwarding, and then apply basic Ethernet switching principles.

The root bridge does not perform any specific task with respect to data flow or delivery. STP needs a root bridge because it essentially behaves like RIP - a distance-vector routing protocol running on each switch that tries to find the shortest path to a single specific destination - the root bridge. That is why the root bridge must exist, otherwise STP would not have anything to compute shortest paths to.

bpdu goes to root bridge

With respect to BPDUs, BPDUs originate at the root bridge and flow "down" the spanning tree, from the root bridge through other bridges in the network, till they reach the network's edge. More technically, each switch sends out BPDUs from its Designated ports.

switch are learning mac address of devices through two diff vlans and  mac are unique and can be on one port only and should e only one path to  it is this need correcting

I am sorry, I do not understand this question. Would you mind saying it differently?

Best regards,

Peter

View solution in original post

6 REPLIES 6
Highlighted
Advocate

Mohammad,

Your colleague is correct.

The root bridge function is only for the spanning tree protocol.

The other switches/bridges refer to the root bridge to find

redundant paths so as to ensure NO Layer 2 Loops exist.

See this link.

http://www.cisco.com/univercd/cc/td/doc/product/rtrmgmt/sw_ntman/cwsimain/cwsi2/cwsiug2/vlan2/stpapp.htm

Regards,
Alex.
Please rate useful posts.

Regards, Alex. Please rate useful posts.

View solution in original post

Highlighted
Hall of Fame Cisco Employee

Hello Mohammad,

Your colleague is correct. Consider the following topology (taken from Cisco documentation):

http://www.cisco.com/en/US/i/000001-100000/70001-75000/74001-75000/74623.jpg

The dotted lines are installed links that are blocked by STP while the bold lines are installed links in Forwarding state.

The root bridge is located at the top right corner. Notice now the two leftmost 2950 access layer switches. If the stations connected to them (not present in the exhibit) need to communicate, the traffic will go through the ingress 2950, go up to the 3550 switch in the distribution layer, and then proceed down to the next 2950. The traffic will not go through the root bridge. The same is true for the communication of the two rightmost 2950 access layer switches. Only if, say, the leftmost and rightmost 2950 want to send traffic to each other, the traffic will need to go through the root bridge because there is no other (Forwarding) way that connects these two 2950 together.

In general, traffic goes through the root bridge only if the root bridge lies in the path towards the destination. If the root bridge is not on the path towards a particular destination, the traffic will not go through it. When delivering frames, do not think too much on STP - simply take into consideration only those links that are Forwarding, and then apply basic Ethernet switching principles.

The root bridge does not perform any specific task with respect to data flow or delivery. STP needs a root bridge because it essentially behaves like RIP - a distance-vector routing protocol running on each switch that tries to find the shortest path to a single specific destination - the root bridge. That is why the root bridge must exist, otherwise STP would not have anything to compute shortest paths to.

bpdu goes to root bridge

With respect to BPDUs, BPDUs originate at the root bridge and flow "down" the spanning tree, from the root bridge through other bridges in the network, till they reach the network's edge. More technically, each switch sends out BPDUs from its Designated ports.

switch are learning mac address of devices through two diff vlans and  mac are unique and can be on one port only and should e only one path to  it is this need correcting

I am sorry, I do not understand this question. Would you mind saying it differently?

Best regards,

Peter

View solution in original post

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Thanks alot guys i havent checked the link yet but will read once i get home .....Peter to clarify my second question basically i have core switch and when i check arp and mac address mapping it tells me that my distribution layer is learning about core from two diff vlans below is example.... thanks guys

show mac-address-table

same mac address learnt from two different vlans/svi...shoudnt there be only one svi forwarding trafic to to ore instead two

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Disclaimer


The  Author of this posting offers the information contained within this  posting without consideration and with the reader's understanding that  there's no implied or expressed suitability or fitness for any purpose.  Information provided is for informational purposes only and should not  be construed as rendering professional advice of any kind. Usage of this  posting's information is solely at reader's own risk.

Liability Disclaimer

In  no event shall Author be liable for any damages whatsoever (including,  without limitation, damages for loss of use, data or profit) arising out  of the use or inability to use the posting's information even if Author  has been advised of the possibility of such damage.

Posting

I hope I don't confuse the original poster, but in Peter's diagram, if the middle distribution switch was made the root bridge, all data traffic (between switches) would cross the root bridge switch.  I mention this as one possible selection criteria for selecting a root bridge might be fewest hops between switches.  If the original poster encountered such a topology, the fact that all data does cross the root bridge, isn't just because it's the root bridge.

Highlighted

Joseph good point and just to clarify you mean if middle distribution switch is made a root bridge all traffic will go through it because then it has the shortest distance to reach all the other switches, since it is directly connected to all the switches, am I on the right track with that statement?

Highlighted

Disclaimer

The Author of this posting offers the information contained within this posting without consideration and with the reader's understanding that there's no implied or expressed suitability or fitness for any purpose. Information provided is for informational purposes only and should not be construed as rendering professional advice of any kind. Usage of this posting's information is solely at reader's own risk.

Liability Disclaimer

In no event shall Author be liable for any damages whatsoever (including, without limitation, damages for loss of use, data or profit) arising out of the use or inability to use the posting's information even if Author has been advised of the possibility of such damage.

Posting

Correct, shortest path from the tree's root.

PS:

More interesting about Peter's diagram, non-blocked links would be expected from the top-right root and the other backbone switch and the 3 distribution switches, as all are shortest path, but what about from distribution switches to the access switches?

In the diagram, we see left and right distribution switches have the non-blocked paths, but distance wise, middle distribution switch has connections to all 4 access switches.  I.e., from a distance basis, they all are tied, but STP uses other attributes to decide the "best" port.

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