STP root election + trunk load balance

[BSpendlove]

I have a question/demonstration? (example, I guess to see if I understand the basic concept)

 

When we trunk between 2 switches, STP will elect the root bridge in a step process comparing bpdu information such as root bridge id, path cost, sending bridge id and port id...

 

If we have a switch trunked with 2 ports to another switch with ( the 2 ports will pretty much have the same information such as root bridge ID, path cost and sending bridge) It then really all comes down to port ID which is made from:

 

1) Port priority (which is default 32? although it seems the default value of 128 on my 2950's)

2) Port Index (which is unique, I guess the lower index will win example: FastEthernet port 23 vs Fast Ethernet port 24... 23 will win because of port index?)

 

If we trunk ports fa0/23 and fa0/24, port 23 will automatically win the process since the port index? Which we can make fa0/24 win by changing the port-priority of fa0/24 to a lower value than 128 (default value)... This seems to work and responded in the way I expected. Although say we have 2 VLANS:

 

VLAN100

VLAN101

 

They will respectfully share the 100Mbps trunk, which is recommended to change priority of one trunk for the specific vlan so that fa0/23 is responsible for VLAN100 and fa0/24 is responsible for VLAN101. What will actually happen if fa0/24 is broke, will VLAN101 fall over to fa0/23 from STP or would it completely stop working without additional configuration?

[brwainer]

It should fail over, but it is always best to test your failure responses.

[BSpendlove]

1 minute ago, brwainer said:

It should fail over, but it is always best to test your failure responses.

So this failover is automatic ?

 

[Lurick]

Failover should be automatic, yes

[BSpendlove]

What if you set up:

 

3 trunks (VLAN100 = fa0/22, VLAN101 = fa0/23, VLAN102 = fa0/24) and for instance fa0/24(VLAN102) fails. What information will switch process so it knows which link to choose? (I guess it would choose the link with the most available bandwidth or just follows the same process with port-priority) but what if I know that VLAN 102 generates more traffic than the others requiring the full speed of the link?

 

Would I be correct in stating: I could actually determine the next port that the VLAN will use with the spanning tree port-priority?

 

Say if fa0/24 port priority = 16

fa0/23 = 32

fa0/22 = 48

 

It is literally that simple and when fa0/24 fails, it will compare BPDU's and fa0/23 would win?

 

Sorry, it seemed like a confusing topic at first but it looks very straight forward now that I keep going over it...

 

[leadeater]

Will always use the standard calculations as you understand, if the ports have the same speed then priority will be used.

 

Quote

Data rate	STP cost (802.1D-1998)	RSTP cost (802.1W-2004, default value)[7]:154
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

https://en.wikipedia.org/wiki/Spanning_Tree_Protocol#Data_rate_and_STP_path_cost