Wednesday, November 18, 2009

STP Root and a simple trick

Hi all,
today I was playing with some switches and I realized this strange STP output:

3560-48#sh spann vlan 10

VLAN0010
Spanning tree enabled protocol ieee
Root ID Priority 24586
Address 0015.facf.0000
This bridge is the root
Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec

Bridge ID Priority 24586 (priority 24576 sys-id-ext 10)
Address 0015.facf.0000
Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec
Aging Time 15

Interface Role Sts Cost Prio.Nbr Type
---------------- ---- --- --------- -------- --------------------------------
Fa0/8 Desg FWD 19 128.10 P2p
Fa0/10 Desg FWD 19 128.12 P2p
Fa0/14 Desg FWD 19 128.16 P2p
Fa0/16 Desg FWD 19 128.18 P2p
Fa0/47 Desg LBK 19 128.51 P2p

This switch is the root bridge for Vlan10, but note that port Fa0/47 is in blocking state.

Here the same output after enabling RSTP, nothing changed:

3560-48# sh spanning-tree vlan 10

VLAN0010
Spanning tree enabled protocol rstp
Root ID Priority 24586
Address 0015.facf.0000
This bridge is the root
Hello Time 2 sec Max Age 20 sec Forward Delay 15 secstyle="font-family: verdana;"

Bridge ID Priority 24586 (priority 24576 sys-id-ext 10)
Address 0015.facf.0000
Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec
Aging Time 300

Interface Role Sts Cost Prio.Nbr Type
---------------- ---- --- --------- -------- --------------------------------
Fa0/8 Desg FWD 19 128.10 P2p Peer(STP)
Fa0/10 Desg FWD 19 128.12 P2p Peer(STP)
Fa0/14 Desg FWD 19 128.16 P2p Peer(STP)
Fa0/16 Desg FWD 19 128.18 P2p Peer(STP)
Fa0/47 Back BLK 19 128.51 P2p

3560-48#


Well the question is ... why a STP root bridge has a blocked port?
as you can see from the second output, it's classified as "Backup" ... but here we are the root, so doesn't make it sense a backup port to reach the root...

If you want to know how it's possible ....Click HERE and laugh ;-) [+/-]



3560-48#sh run int fa 0/47
Building configuration...

Current configuration : 147 bytes
!
interface FastEthernet0/47
switchport access vlan 10
switchport mode access
switchport nonegotiate
no keepalive
end


woops and a L1 loopback inserted into fa 0/47 ;-)



With this physical loopback and keepalive disabled, the port goes up and every bpdu sent is also received, so if I have correctly understood, the root bridge is convincted to have an additional port to reach the root ;-)

... I have several Layer 8 problems, I know ;-)))))

3 comments:

Oliver said...

sei un matto (you're crazy!!)

Marco Rizzi said...

ehehehe I know ;-)

Luca Gervasi said...

CCIE experiments, i presume :)

ehehheahhah