STP question....

here is a topology i created in packet tracer. i have some questions about how stp works. all switches are using default settings.

switch 1 (root)

since priorities tie, root goes to the lowest MAC ID.
all ports forwarding (root ports)

switch 2

FA0/2 is forwarding (root port)
FA0/1 - WHY IS IT FORWARDING?....it should be in a blocking state because it has a higher MAC ID than Switch3 (refer to diagram)...?

switch 3

FA0/2 - WHY IS IT BLOCKING?....shouldnt it be forwarding because it has a lower MAC ID than Switch2...?
FA0/1 - WHY IS IT FORWARDING?...shouldnt it be blocking state because it has a higher MAC ID than Switch4..?

switch 4

FA0/1 - forwarding (root port)
FA0/2 - forwarding (designated port)

can anyone explain this to me. thanks.

stp.png

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Comments

vinbuck

Think of it this way...you've created a 4 point loop...all STP is designed to do is prevent loops. How many links need to be blocked in this network to prevent a loop from forming.

Lowest Bridge ID (MAC + STP Priority - default 32768 ) wins so if no other changes are made, which switch has the lowest mac? which is the next best switch? STP will keep going until it finds the lowest priority switch and that's where it will determine which port to block to prevent a loop.

Also bear in mind that Spanning tree works link by link to build a network topology. Each end of the link "competes" with the other end to see which has a higher priority and if it is a path back to the root bridge.

This is the order a port will go through when it receives a BPDU from another switch to determine what role to play in spanning-tree

Lowest root bridge ID
Lowest root path cost
Lowest sender bridge ID
Lowest port ID

Check out these sources and use them for STP and RSTP

http://lockienotlucky.wordpress.com/category/ccnp-switch/page/3/
http://www.cisco.com/en/US/tech/tk389/tk621/technologies_white_paper09186a0080094cfa.shtml

ricky8

vinbuck wrote: »

Think of it this way...you've created a 4 point loop...all STP is designed to do is prevent loops. How many links need to be blocked in this network to prevent a loop from forming.

Lowest Bridge ID (MAC + STP Priority - default 32768 ) wins so if no other changes are made, which switch has the lowest mac? which is the next best switch? STP will keep going until it finds the lowest priority switch and that's where it will determine which port to block to prevent a loop.

Also bear in mind that Spanning tree works link by link to build a network topology. Each end of the link "competes" with the other end to see which has a higher priority and if it is a path back to the root bridge.

This is the order a port will go through when it receives a BPDU from another switch to determine what role to play in spanning-tree

Lowest root bridge ID
Lowest root path cost
Lowest sender bridge ID
Lowest port ID

Check out these sources and use them for STP and RSTP

CCNP | SWITCH « each connection makes us stronger
Understanding Rapid Spanning Tree Protocol (802.1w) [Spanning Tree Protocol] - Cisco Systems

thanks...i understand STP concept and and how it works but can you explain to me the ports that i bolded as to WHY they are in that port state. like...
FA0/1 - WHY IS IT FORWARDING?....it should be in a blocking state because it has a higher MAC ID than Switch3?

and the others as well...thanks

vinbuck

Hate to break it to ya, but if you don't understand why Fa0/1 on Switch 2 is forwarding then you're not understanding the concept of Spanning Tree, at least not fully yet

Blocking has nothing to do with MAC addresses, at least not directly - Bridge ID plus the four steps a port goes through during the election process are what determines whether a port is Forwarding or Blocking.

ricky8

vinbuck wrote: »

Hate to break it to ya, but if you don't understand why Fa0/1 on Switch 2 is forwarding then you're not understanding the concept of Spanning Tree, at least not fully yet

Blocking has nothing to do with MAC addresses, at least not directly - Bridge ID plus the four steps a port goes through during the election process are what determines whether a port is Forwarding or Blocking.

its because switch 2 is advertising a lower cost to the root on the lan segment.?

vinbuck

Not gonna give up the answer yet, but I will help get you there, because you will learn more about STP...

If you think it's due to cost then do a show spanning-tree on packet tracer and calculate the cost over each path from Switch3 to the root bridge.

ricky8

vinbuck wrote: »

Not gonna give up the answer, but I will help get you there, because you will learn more about STP...

If you think it's due to cost then do a show spanning-tree on packet tracer and calculate the cost over each path from Switch3 to the root bridge.

CHOOSING DESIGNATED PORT (straight from my notes)

DP on each LAN segment is switch port that advertises the lowest-cost Hello onto a LAN segment. When nonroot switch forwards a Hello, the nonroot switch sets the cost field in the Hello to that switch’s cost to reach the root.

As a result, switch with the lower cost to reach the root, among all switches connected to a segment, becomes the DP on that segment.

If cost is equal then port switch with lower MAC ID becomes DP.

=====================================================================================
switch 2 fa0/2 has a cost of 19 to the root and will advertise that on the lan segment between switch2-switch3.
switch 3 fa0/1 has a cost of 38 to the root and will advertise that on the lan segment between switch2-switch3.

as a result switch2 fa0/2 is the DP....correct?

vinbuck

It depends on the port cost assignment...paste the show spanning-tree from packet tracer into the thread so we can get a better idea. Switches do not exchange MAC addresses in a spanning tree BPDU, they exchange Bridge IDs which is Priority+MAC and whichever is lower wins...it may seem like splitting hairs when everything is in a default state, but once priorities get changed on all switches, then the MAC address pretty much goes out the window as a tiebreaker.

ricky8

vinbuck wrote: »

It depends on the port cost assignment...paste the show spanning-tree from packet tracer into the thread so we can get a better idea. Switches do not exchange MAC addresses in a spanning tree BPDU, they exchange Bridge IDs which is Priority+MAC and whichever is lower wins...it may seem like splitting hairs when everything is in a default state, but once priorities get changed on all switches, then the MAC address pretty much goes out the window as a tiebreaker.

cost and priority values are default....i posted the bridge ID's in the diagram. and the priority is default 32768..thanks.

ricky8

any thoughts?....thanks.

m3zilla

The decision between blocking and forwarding is based on the cost to the root. The priority will only be looked at if there's a tie on the cost. Switch2 cost to root is 19, switch 3 cost to root is 38.

kurosaki00

you need to understand how STP works a bit more
Root is designated by either lowest ID or Manual designation

STP determines the root ports by calculating the least path cost to root bridge
Now the designated bridge, on each lan segment (this is on each lan segment, contrary of selecting root ports which is cumulative with the whole network) calculate through what switch it has the least cost, that port becomes a designated port

All designated ports and root ports are forwarding ports, else = blocked

Only the root bridge is assigned mainly with the bridge ID
when designated ports as root or designated it uses more the cost and cumulative cost not the lowest ID.