# Multiple Loops in STP topology

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So Below is the setup I was thinking about. As you can see I have a PC on one end, and 2 routers running HSRP on the other end.

Now I'm trying to think of the the links that will be blocked with STP in this scenario. If it was four switches, no problem, but adding the 5th switch in this manner creates more than one possible loop in the topology.

I have placed the root bridge as follows in the diagram. My question is, are the links marked with X the correct links to be blocked and thus the PC would take the upper path. And if the root fails and the backup takes over, would the PC then take the lower path?
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millworx wrote: »
So Below is the setup I was thinking about. As you can see I have a PC on one end, and 2 routers running HSRP on the other end.

Now I'm trying to think of the the links that will be blocked with STP in this scenario. If it was four switches, no problem, but adding the 5th switch in this manner creates more than one possible loop in the topology.

I have placed the root bridge as follows in the diagram. My question is, are the links marked with X the correct links to be blocked and thus the PC would take the upper path. And if the root fails and the backup takes over, would the PC then take the lower path?

Is all the depends of the least cost path. If the link that it showing block has a 1gb speed link and the link that you are showing forwarding has a 4mb speed link, then it will block the 4mb link and forward the 1gb. Take a look at Spanning Tree Protocol - Wikipedia, the free encyclopedia
David G.
http://gomezd.com <
My Tshoot test Blog
• Member Posts: 408
So take a look at the image that I did. The least cost path is choose.

Data rate STP Cost (802.1D-199 STP Cost (802.1t-2001)

4 Mbit/s 250 5,000,000
10 Mbit/s 100 2,000,000
16 Mbit/s 62 1,250,000
100 Mbit/s 19 200,000
1 Gbit/s 4 20,000
2 Gbit/s 3 10,000
10 Gbit/s 2 2,000
STP.jpg 60.7K
David G.
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My Tshoot test Blog
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Hi Millworx,
You haven't specified any bandwidths for the links so I will have to assume that they are all the same.I have also assumed that the links are Gigabit when doing my calculations for the root path cost.

I put some names on the switches so I can reference them in my post.

As we all know the root bridge will only have designated ports.

Switch B and C will have their root port on the link directly connected to the root. These switches will have the designated port for the
segment since they have a lower root path cost than switch D.

Switch D has the same cost on both links leading to the root so one of these ports will be root and the other will be blocking.

Switch C will have the designated port on the segment between C and E since it has a lower root path cost.

Depending on the priority of D and E or the MAC address either D or E will have the designated port on the link between them and the other side will be blocking since E has its root port on the other port.

Your traffic flow is correct but it is not certain that the link between C and D will be blocked. I have marked the link between B and D with an X to mark it as possibly blocked.

In short, link between D and E will definately be blocked. Link between B and D or C and D will be the other link that is blocked.
Daniel Dib
CCIE #37149
• Member Posts: 290
reaper81 wrote: »
Hi Millworx,
You haven't specified any bandwidths for the links so I will have to assume that they are all the same.I have also assumed that the links are Gigabit when doing my calculations for the root path cost.

I put some names on the switches so I can reference them in my post.

As we all know the root bridge will only have designated ports.

Switch B and C will have their root port on the link directly connected to the root. These switches will have the designated port for the
segment since they have a lower root path cost than switch D.

Switch D has the same cost on both links leading to the root so one of these ports will be root and the other will be blocking.

Switch C will have the designated port on the segment between C and E since it has a lower root path cost.

Depending on the priority of D and E or the MAC address either D or E will have the designated port on the link between them and the other side will be blocking since E has its root port on the other port.

Your traffic flow is correct but it is not certain that the link between C and D will be blocked. I have marked the link between B and D with an X to mark it as possibly blocked.

In short, link between D and E will definately be blocked. Link between B and D or C and D will be the other link that is blocked.

Thank you reaper, you are correct in assuming all links will be of the same cost.

I'm trying to make it so the PC will take the upper path, and only take the lower path if the root fails, this is primarily because I want to run ZBFW on both routers, and if traffic comes in one ingresses through one router and tries to egress through the other ZBFW will block it (at least in my setup). I appreciate the input, I guess I will have to lab this up today and see how it goes!