Question about STP

auos

Hi,
I have network as show in attachment file, all switchs are connected together with access link not truck link.

My question is the STP available in this network, or just when I configure a truck link.

luke_bibby

STP is enabled by default and will start transitioning ports into STP states the instant you connect another device to the switch regardless of the switchports operational mode.

This is what happens if you tried to see the spanning tree on a completely isolated switch (nothing connected):
Sw2#show span

No spanning tree instance exists.

Now I have connected two 2950 switches together (the default switchport mode on 2950 is dynamic desirable so they will kick off the DTP exchange by default) using two crossover cables so the topology between the switches is looped, and changed the administrative mode of the two ports (on both switches) to be a static access port rather than negotiate trunks.
Sw2(config)#int range fa 0/1 - 2
Sw2(config-if-range)#switch mode access
Sw2(config-if-range)#do show span

VLAN0001
Spanning tree enabled protocol ieee
Root ID Priority 24577
Address 0014.69da.ee40
Cost 19
Port 1 (FastEthernet0/1)
Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec

Bridge ID Priority 32769 (priority 32768 sys-id-ext 1)
Address 0011.2120.ba40
Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec
Aging Time 300

Interface Role Sts Cost Prio.Nbr Type

---

---- ---

---

---

---

Fa0/1 Root FWD 19 128.1 P2p
Fa0/2 Altn BLK 19 128.2 P2p

Sw2(config-if-range)#do show int trunk

Sw2(config-if-range)#do show int fa 0/1 switch | begin Operational
Operational Mode: static access
...


This shows that no trunks exist between the switches and STP has still been used to provide a loop free layer 2 network (look at the STP state of fa0/1).

Hope that helped!

auos

luke_bibby wrote: »

STP is enabled by default and will start transitioning ports into STP states the instant you connect another device to the switch regardless of the switchports operational mode.

This is what happens if you tried to see the spanning tree on a completely isolated switch (nothing connected):
Sw2#show span

No spanning tree instance exists.

Now I have connected two 2950 switches together (the default switchport mode on 2950 is dynamic desirable so they will kick off the DTP exchange by default) using two crossover cables so the topology between the switches is looped, and changed the administrative mode of the two ports (on both switches) to be a static access port rather than negotiate trunks.
Sw2(config)#int range fa 0/1 - 2
Sw2(config-if-range)#switch mode access
Sw2(config-if-range)#do show span

VLAN0001
Spanning tree enabled protocol ieee
Root ID Priority 24577
Address 0014.69da.ee40
Cost 19
Port 1 (FastEthernet0/1)
Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec

Bridge ID Priority 32769 (priority 32768 sys-id-ext 1)
Address 0011.2120.ba40
Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec
Aging Time 300

Interface Role Sts Cost Prio.Nbr Type

---

---- ---

---

---

---

Fa0/1 Root FWD 19 128.1 P2p
Fa0/2 Altn BLK 19 128.2 P2p

Sw2(config-if-range)#do show int trunk

Sw2(config-if-range)#do show int fa 0/1 switch | begin Operational
Operational Mode: static access
...


This shows that no trunks exist between the switches and STP has still been used to provide a loop free layer 2 network (look at the STP state of fa0/1).

Hope that helped!

Many Thanks.

kryolla

as soon as you enter a vlan # spanning tree starts for that vlan that is why transit switches need that vlan turned up or else it wont pass traffic for that vlan even though the transit switch does not have any access ports belonging to that vlan. Also why SVI will remain down until you turn up the vlan, once spanning tree is forwarding then the SVI will come up.