STP Designated switch/port

bmauro

Hey guys - trying to completely understand the election of the designated port on non-root bridges, and everytime I think I've got it, I find a scenario that messes me up.

I understand that for each lan segment there is a designated port for that segment, and by default the root bridge considers all it's forwarding ports to be designated ports

Here's the output from my home lab:

**ROOT**
2950B#show spanning-tree

VLAN0001
Spanning tree enabled protocol ieee
Root ID Priority 24577
Address 000f.24cb.f540
This bridge is the root
Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec

Bridge ID Priority 24577 (priority 24576 sys-id-ext 1)
Address 000f.24cb.f540
Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec
Aging Time 15

Interface Role Sts Cost Prio.Nbr Type

---

---- ---

---

---

---

Fa0/24 Desg FWD 19 128.24 P2p
Gi0/1 Desg FWD 4 128.25 P2p

**SWITCH 2 **

3550A#show spanning-tree

VLAN0001
Spanning tree enabled protocol ieee
Root ID Priority 24577
Address 000f.24cb.f540
Cost 8
Port 49 (GigabitEthernet0/1)
Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec

Bridge ID Priority 32769 (priority 32768 sys-id-ext 1)
Address 000a.b7eb.e000
Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec
Aging Time 15

Interface Port ID Designated Port ID
Name Prio.Nbr Cost Sts Cost Bridge ID Prio.Nbr

---

---

---

---

---

---

---

Fa0/46 128.42 19 BLK 4 32769 000f.f72b.42c0 128.24
Fa0/47 128.43 19 BLK 0 24577 000f.24cb.f540 128.24
Fa0/48 128.44 19 FWD 8 32769 000a.b7eb.e000 128.44
Gi0/1 128.49 4 FWD 4 32769 000f.f72b.42c0 128.26



** SWITCH 3 **
2950A#show spanning-tree

VLAN0001
Spanning tree enabled protocol ieee
Root ID Priority 24577
Address 000f.24cb.f540
Cost 4
Port 25 (GigabitEthernet0/1)
Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec

Bridge ID Priority 32769 (priority 32768 sys-id-ext 1)
Address 000f.f72b.42c0
Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec
Aging Time 300

Interface Role Sts Cost Prio.Nbr Type

---

---- ---

---

---

---

Fa0/24 Desg FWD 19 128.24 P2p
Gi0/1 Root FWD 4 128.25 P2p
Gi0/2 Desg FWD 4 128.26 P2p

Switch 3 is whats making me scratch my head. Why are all three int's forwarding? I understand that Gi0/1 is the RP, which should forward. Gi0/2 is attached to SWITCH 2 RP, which would make it the designated port for that segment - but why is Fa0/24 also forwarding? One designated port for that segment has already been elected.

I've read my spanning-tree chapters several times, read Cisco's 2950 STP configuration PDF, and did a search on the forums for designated switch information. Any help would be great!

EdTheLad

Maybe it might be a good idea to provide info on where the ports are connected, am i supposed to guess?

bmauro

Sorry - posted that a little too early this morning. Come on - can't ya just guess???


OK - the root switch has two interfaces

2950B:
Fa0/24 ---->Fa0/47 (3550)
Gig01

---

>Gig0/1(2950A)

3550:
Gig0/1

---

>Gig0/2(2950A)
Fa0/46

---

>Fa0/24(2950A)
Fa0/47

---

>Fa0/47(2950B)

2950A:
Gig0/1

---

>Gig0/1(2950B)
Gig0/2

---

>Gig0/1(3550)
Fa0/24

---

>Fa0/46(3550)

Hope that isn't too confusing. I'll try to draw a crude diagram

............2950B *root*
Fa0/24..................Gig0/1
......... /..............\
........ /................\
....... /..................\
Fa0/47...................Gig0/1
3550 ======= 2950A
Fa0/46................. Fa0/24
Gig0/1................. Gig0/2

bmauro

I'm such a noob - that little crude diagram took me forever to get the formatting correct :P

That's how bad I want to understand STP completely!!!

agustinchernitsky

Nice Diagrams!

between 3550 and 2950A, STP will block fa0/46 because the cost compared to the gig0/1 is lower and since you have multiple connections between both routers, STP will block the port with lower priority or the port in the bridge with lower priority. It seems that 3550 has lower priority (ID+MAC).

From the 2950A perspective, all ports would be FWD... including the fa0/24, since only the fa0/46 on the 3550 will be blocking.

Do this: change the priority of the SW 3550 and 2950... still one of the FA ports will be disabled but which one will be inverted.

bmauro

Thanks for the props!!

Ok - so I changed the priority as asked and this is what I got.

2950B - This was the root, and was NOT touched this time. It has a user-defined priority 24577

3550 - Had the default 32768 (+1) priority - did not touch this time.

2950A - Had the default 32768 (+1) priority - but I changed the priority to 36865 - and the ports did not change.

2950A#show spanning-tree

VLAN0001
Spanning tree enabled protocol ieee
Root ID Priority 24577
Address 000f.24cb.f540
Cost 4
Port 25 (GigabitEthernet0/1)
Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec

Bridge ID Priority 36865 (priority 36864 sys-id-ext 1)
Address 000f.f72b.42c0
Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec
Aging Time 300

Interface Role Sts Cost Prio.Nbr Type

---

---- ---

---

---

---

Fa0/24 Desg FWD 19 128.24 P2p
Gi0/1 Root FWD 4 128.25 P2p
Gi0/2 Desg FWD 4 128.26 P2p

---

3550A#show spanning-tree

VLAN0001
Spanning tree enabled protocol ieee
Root ID Priority 24577
Address 000f.24cb.f540
Cost 8
Port 49 (GigabitEthernet0/1)
Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec

Bridge ID Priority 32769 (priority 32768 sys-id-ext 1)
Address 000a.b7eb.e000
Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec
Aging Time 300

Interface Port ID Designated Port ID
Name Prio.Nbr Cost Sts Cost Bridge ID Prio.Nbr

---

---

---

---

---

---

---

Fa0/46 128.42 19 BLK 4 36865 000f.f72b.42c0 128.24
Fa0/47 128.43 19 BLK 0 24577 000f.24cb.f540 128.24
Fa0/48 128.44 19 FWD 8 32769 000a.b7eb.e000 128.44
Gi0/1 128.49 4 FWD 4 36865 000f.f72b.42c0 128.26



So what I don't understand is

1.) The 3550 originally had the SAME priority as the 2950 (default 3276 - so doesn't that mean the next item is the MAC address? Which again the 3550 should win that election. (for designated switch)



THANKS!!!

EdTheLad

You must follow the BPDUs to understand whats happening.
Switch 1 is the Root bridge, it will flood BPDUs out its interfaces to switches 2 and 3,which will in turn send the bpdus out their ports.

Switch 2 will receive bpdus on 3 ports fa0/47,fa0/46 and gi0/1.Ths cost via switch 3 is 8 which is less than 19 through the fa0/47,so the root will be the gi0/1 .The other two ports that received bpuds i.e. fa0/47 and fa0/46 will be blocked as only one port is allowed to receive bpdus per switch i.e. the root.

Switch 3 will receive bpdus on gi0/1 the root port, it will forward these out both fa0/24 and gi0/2.Since both these port are not receiving bpus from switch 2 both will be forwarding as there is no reason to block.

bmauro

Thanks ed - it that cleared it up a bit more. I think I'm going to grab a co-workers CCNP book on switching to see if I can find some more info.

Humper

bmauro wrote:

Thanks ed - it that cleared it up a bit more. I think I'm going to grab a co-workers CCNP book on switching to see if I can find some more info.

A really good book I highly recommend for STP,VTP,VLAN's.etc is "LAN Switching" by Kennedy Clark / Kevin Hamilton. Some of the material is legacy now, but does a really good job explaining the fundamentals.

bmauro

ModemHumper - thx!! I was going to post another question on any good reference books for switching.

I got TCP/IP Routing Vol 1 - as a interior routing reference book - and it ROCKS. I won't need the material for testing for quite some time, but its great it I need more info on a subject.

I was wondering if there is a comparable book for switching.

I'll be looking into the Clark/Hamilton book

Thanks again.

Humper

bmauro wrote:

ModemHumper - thx!! I was going to post another question on any good reference books for switching.

I got TCP/IP Routing Vol 1 - as a interior routing reference book - and it ROCKS. I won't need the material for testing for quite some time, but its great it I need more info on a subject.

I was wondering if there is a comparable book for switching.

I'll be looking into the Clark/Hamilton book

Thanks again.

NP, read chapter 6 for the basics of STP (covered in CCNA) and chapter 7 is advanced topics for STP like load balancing, bpdu types and how they operate. It is definitely out of the scope for the CCNA exam but I recommend the read anyways. It is CCIE Professional Development, but any ccna can read it.