Spanning Tree Protocol question

Dilan77

Hi Experts

I have a home lab with a couple of Cisco 2924 XL switches that I'm practising Spanning tree on. I've made 2 connections from each switch to the other to provide a redundant connection and see ST in action. On the 2924_A switch, there are also connections to my laptop and a Cisco PIX firewall, which are labelled.

Here is the output of sh spanning-tree from both switches (omitting unused ports for clarity)

Switch: 2924_A:

Spanning tree 1 is executing the IEEE compatible Spanning Tree protocol
Bridge Identifier has priority 32768, address 00d0.ba2f.ae00
Configured hello time 2, max age 20, forward delay 15
Current root has priority 32768, address 0002.fdf6.f380
Root port is 23, cost of root path is 19
Topology change flag not set, detected flag not set, changes 10
Times: hold 1, topology change 35, notification 2
hello 2, max age 20, forward delay 15
Timers: hello 0, topology change 0, notification 0

Interface Fa0/1 (port 13) in Spanning tree 1 is FORWARDING
Port path cost 100, Port priority 128
Designated root has priority 32768, address 0002.fdf6.f380
Designated bridge has priority 32768, address 00d0.ba2f.ae00
Designated port is 13, path cost 19
Timers: message age 0, forward delay 0, hold 0
BPDU: sent 5558, received 0

**link to Cisco PIX Firewall**

Interface Fa0/9 (port 22) in Spanning tree 1 is FORWARDING
Port path cost 19, Port priority 128
Designated root has priority 32768, address 0002.fdf6.f380
Designated bridge has priority 32768, address 00d0.ba2f.ae00
Designated port is 22, path cost 19
Timers: message age 0, forward delay 0, hold 0
BPDU: sent 5506, received 0

**link to laptop**

Interface Fa0/10 (port 23) in Spanning tree 1 is FORWARDING
Port path cost 19, Port priority 128
Designated root has priority 32768, address 0002.fdf6.f380
Designated bridge has priority 32768, address 0002.fdf6.f380
Designated port is 13, path cost 0
Timers: message age 2, forward delay 0, hold 0
BPDU: sent 64, received 770

**link to 2924_B**

Interface Fa0/17 (port 31) in Spanning tree 1 is BLOCKING
Port path cost 19, Port priority 128
Designated root has priority 32768, address 0002.fdf6.f380
Designated bridge has priority 32768, address 0002.fdf6.f380
Designated port is 22, path cost 0
Timers: message age 1, forward delay 0, hold 0
BPDU: sent 41, received 765

**link to 2924_B**

---

Switch: 2924_B

Spanning tree 1 is executing the IEEE compatible Spanning Tree protocol
Bridge Identifier has priority 32768, address 0002.fdf6.f380
Configured hello time 2, max age 20, forward delay 15
We are the root of the spanning tree
Topology change flag not set, detected flag not set, changes 7
Times: hold 1, topology change 35, notification 2
hello 2, max age 20, forward delay 15
Timers: hello 0, topology change 0, notification 0

Interface Fa0/1 (port 13) in Spanning tree 1 is FORWARDING
Port path cost 19, Port priority 128
Designated root has priority 32768, address 0002.fdf6.f380
Designated bridge has priority 32768, address 0002.fdf6.f380
Designated port is 13, path cost 0
Timers: message age 0, forward delay 0, hold 0
BPDU: sent 722, received 11

**link to 2924_A**

Interface Fa0/9 (port 22) in Spanning tree 1 is FORWARDING
Port path cost 19, Port priority 128
Designated root has priority 32768, address 0002.fdf6.f380
Designated bridge has priority 32768, address 0002.fdf6.f380
Designated port is 22, path cost 0
Timers: message age 0, forward delay 0, hold 0
BPDU: sent 769, received 1

**link to 2924_A**

---

I can see that 2924_B has become the Root Bridge because it has the lower MAC address.

On the 2924_A switch, there are two connections to the root bridge, fa0/10 and fa0/17. Based on the information above, it seems that fa0/10 has been placed in Forwarding because it has the lower Port ID.

I do have a couple of questions, however;

i) How is the Port path cost for an interface calculated? I know that 100Mbs is 19 and 10Mbs is 100, but what is actually measured? For instance, for fa0/1 on the 2924_A switch, the port path cost is 100 [because the PIX inside interface is set to 10BaseT]. But is this supposed to represent the cost from an extra imaginary cable on that port to the root bridge?

ii) The interface number, e.g. fa0/10, and the port number (e.g. port 23), for some ports do not corelate. On fa0/10 on 2924_A, the interface is fa0/10 but the port number is 23. Where is this port number from?

iii) All ports also have a 'Designated Port x, path cost y' in their information. What does this represent?


Many tia

cisco_trouble

Path costs are locally significant. The switch uses paths costs and the BID to determine whether its the root and which ports should be forwarding.