Dear Anne,
I can answer this question. :)
Case Scenario: (You may want to draw as you go along...)
Swith A is the root bridge, and Switch B is the non-root bridge
Switch A is connected to Switch B via to links of equal cost.
We will call the ports on Switch A: W and X, and the ports on Switch B:Y and
Z
W is connected to Y and X is connected to Z
Changing the priority of port Y or Z will have NO effect on the spanning
tree.
However changing the the priority of W or X will cause the following:
1) If W is greater than X, then Z will be blocked
2) If X is greater than W, then Y will be blocked
I tried many other case scenarios yesterday evening. I can summurize them as
follows:
1) If the cost of link WY and the cost of link XZ are not equal, then cost
IS the criteria for deciding which port will be blocked
and no change of priority (on W, X , Y ,Z) is going to change the spanning
tree
2) If the cost of link WY and the cost of link XZ are equal, then the
priorities set by ports X and W of the root bridge will be the determining
factors.
3) If the priority of link WY and the priority of link XZ are the same, the
non root bridge will choose as the root port, the sending port that had the
lowest mac address (If we assume that W has a lower mac address than X,
then port Y will be the root port and port Z will be blocked)
Did that fulfill some of your thurst for knowledge? :)
Pierre-Alex
-----Original Message-----
From: Leigh Anne Chisholm [mailto:[EMAIL PROTECTED]]
Sent: Thursday, February 15, 2001 10:38 AM
To: Pierre-Alex
Subject: RE: DISTURBING: Spanning Tree Protocol Refuses to Cooperate!
By the way... if you've got a few moments, would you like to try something
for me? At the bottom of my email, I wanted to know why setting the port
priority didn't influence which port becomes blocked. If the path costs of
0/26 and 0/27 were identical, which port priority values need to be altered
to influence spanning tree path selection? Is it port priority on the 1912
or the 2924XL?
-- Leigh Anne
-----Original Message-----
From: Pierre-Alex [mailto:[EMAIL PROTECTED]]
Sent: February 15, 2001 9:22 AM
To: [EMAIL PROTECTED]
Cc: [EMAIL PROTECTED]
Subject: RE: DISTURBING: Spanning Tree Protocol Refuses to Cooperate!
"Why did yours reset to its original value while mine retained its new path
cost? We're both running 9.00.04 code so I wouldn't suspect a bug to be the
cause of the problem... One possibility may be that if you set your port
path cost and then immediately reset your switch"
This is precisely the mistake I made. While I figured it out, everything
worked OK.
Thanks again.
Pierre-Alex
-----Original Message-----
From: Leigh Anne Chisholm [mailto:[EMAIL PROTECTED]]
Sent: Wednesday, February 14, 2001 6:58 PM
To: Pierre-Alex
Cc: [EMAIL PROTECTED]
Subject: RE: DISTURBING: Spanning Tree Protocol Refuses to Cooperate!
"Once the root bridge has been selected, all bridges and switches in the
network calculate the shortest path distance between itself and the root
bridge. Each port sends a configuration BPDU message to determine the cost
between itself and the root bridge. The port with the lowest cost is
designated as the root port for the bridge. "
On my Catalyst 1924-F-EN, I set the port cost of Ethernet 0/1 to 50 using
the "spantree cost 50" command. I then typed reload. Once the switch was
back up, I issued the command "show span". Results were:
Port Ethernet 0/1 of VLAN1 is Forwarding
Port path cost 50, Port priority 128
Why did yours reset to its original value while mine retained its new path
cost? We're both running 9.00.04 code so I wouldn't suspect a bug to be the
cause of the problem... One possibility may be that if you set your port
path cost and then immediately reset your switch, your changes may not have
been saved in NVRAM. Verify that you used the appropriate command and that
you waited at least 20 seconds before issuing a "reload" command. Give it
another try...
"Port ID: Each port on a bridge or switch is assigned a unique 2-byte
identifier. The Port ID is composed of a one-byte priority value
configurable by an administrator, followed by a 1-byte value assigned by the
device to the port." Port priority does influence the selection of the
designated or root port when two or more ports on the bridge are connected
the same remote bridge/switch.
So, why won't setting the port priority influence which port becomes
blocked? Give this a whirl and then let me know what happens--try
equalizing port priorities on 0/26 and 0/27 on the 1912 (restore to default
settings). Now alter the port priorities on the 2924XL's 0/20 and 0/21
ports. Does it make a difference?
-- Leigh Anne
-----Original Message-----
From: [EMAIL PROTECTED] [mailto:[EMAIL PROTECTED]]On Behalf Of
Pierre-Alex
Sent: February 14, 2001 12:31 PM
To: [EMAIL PROTECTED]
Cc: [EMAIL PROTECTED]
Subject: DISTURBING: Spanning Tree Protocol Refuses to Cooperate!
Here is another challenging problem I have. Anyone?
Switch 1912-EN is connected to Switch 2924XL via two trunks (Port A and port
B.)
The spanning tree has disabled port B to prevent a loop.
I am trying to force B to go in fowarding mode and have A be in blocking
mode.
I tried changing the cost of the path, but the swith rectified it to 10. So
I thought:
"well, I can change the priority of the interfaces and reload the switch
and that should do the trick".
Well it did not (See below). I can only think of two things:
A. I have not understood proprely the use of a port priority and I am
changing the wrong parameter
B. I have changed the right parameter but there is something else that I am
missing in the configuration.
Port FastEthernet 0/26 of VLAN1 is Forwarding
Port path cost 10, Port priority 100<---------- I changed this from 128
to 100
Designated root has priority 32768, address 0050.3EF0.3580
Designated bridge has priority 32768, address 0050.3EF0.3580
Designated port is 22, path cost 0
Timers: message age 20, forward delay 15, hold 1
--More--
Port FastEthernet 0/27 of VLAN1 is Blocking
Port path cost 10, Port priority 0 <------------------------------I
changed this from 128 to 0
Designated root has priority 32768, address 0050.3EF0.3580
Designated bridge has priority 32768, address 0050.3EF0.3580
Designated port is 23, path cost 0
Timers: message age 20, forward delay 15, hold 1
Pierre-Alex
As several people have found out recently, I **love** understanding all the
nuances of why technology works the way it does. When things go wrong, it's
so much easier to identify a problem when you can compare what SHOULD be
happening to what IS occurring. Took me a while to pick through everything,
but my spanning tree troubleshooting skill-set increased significantly as a
result of this exercise! Thanks for the opportunity!
-- Leigh Anne
-----Original Message-----
From: Pierre-Alex [mailto:[EMAIL PROTECTED]]
Sent: February 14, 2001 10:41 AM
To: [EMAIL PROTECTED]; Cisco Groupstudy (E-mail)
Cc: Dale Cunningham; [EMAIL PROTECTED]; [EMAIL PROTECTED];
[EMAIL PROTECTED]
Subject: RE:PROBLEM FIXED: ----------------------> DISTURBING: Spanning
Tree Protocol Does not Work.
HI LEIGH ANNE and everyone,
The problem was fixed by allowing VLANs out of the trunk on port f0/21. (See
below).
I don't have an amber light on the blocked port but since the show span
indicates that the port is blocked I can live with that. (Maybe TAC should
be the next step!)
I would like now to document the rules used to solve this problem: Leigh
Anne, please correct me if I am wrong!
1) The spanning protocol includes trunk ports in its calculation.
2) If a trunk is not allowed to trunk any VLAN, the port is considered
inactive
3) Since an inactive port does not forward traffic it does not create a loop
4) A port that does not create a loop will not be put in blocking mode
(which explains my earlier problem)
So LEIGH ANNE, when are you going to claim your CCIE? (My apology if you
already have it!)
A thousand thanks to Leigh Anne and all for the learning experience,
Pierre-Alex
**************************************CONFIGURATION
PROBLEM**************************************
C2924XL#sh int f 0/20 sw
Switchport: Enabled
Administrative mode: trunk
Operational Mode: trunk
Administrative Trunking Encapsulation: isl
Operational Trunking Encapsulation: isl
Negotiation of Trunking: Disabled
Access Mode VLAN: 0 ((Inactive))
Trunking Native Mode VLAN: 1 (default)
Trunking VLANs Enabled: ALL
Trunking VLANs Active: 1-3
Pruning VLANs Enabled: NONE
C2924XL#sh int f 0/21 sw
Name: Fa0/21
Switchport: Enabled
Administrative mode: static access
Operational Mode: static access
Administrative Trunking Encapsulation: isl
Operational Trunking Encapsulation: isl
Negotiation of Trunking: Disabled
Access Mode VLAN: 1 (default)
Trunking Native Mode VLAN: 1 (default)
Trunking VLANs Enabled:
NONE<--------------------------------------------------
Pruning VLANs Enabled: NONE
******************************I AM FIXING THE
PROBLEM***********************************************
On interface f0/21 I did:
sw trunk allowed vlan all
sw trunk allowed vlan add 1
sw trunk allowed vlan add 2
sw trunk allowed vlan add 3
******************************PROBLEM
FIXED!!*****************************************************
C2924XL#sh int f 0/21 sw
Name: Fa0/21
Switchport: Enabled
Administrative mode: trunk
Operational Mode: trunk
Administrative Trunking Encapsulation: isl
Operational Trunking Encapsulation: isl
Negotiation of Trunking: Disabled
Access Mode VLAN: 0 ((Inactive))
Trunking Native Mode VLAN: 1 (default)
Trunking VLANs Enabled:
ALL<--------------------------------------------------
Trunking VLANs Active:
1-3<--------------------------------------------------
Pruning VLANs Enabled: NONE
**************SPANNING PROTOCOL WITH ONE PORT
BLOCKED!******************************
Interface Fa0/20 (port 22) in Spanning tree 1 is FORWARDING
Port path cost 19, Port priority 128
Designated root has priority 32768, address 0050.3ef0.3580
Designated bridge has priority 32768, address 0050.3ef0.3580
Designated port is 22, path cost 0
Timers: message age 0, forward delay 0, hold 0
BPDU: sent 78555, received 6
Interface Fa0/21 (port 23) in Spanning tree 1 is FORWARDING
Port path cost 19, Port priority 128
Designated root has priority 32768, address 0050.3ef0.3580
Designated bridge has priority 32768, address 0050.3ef0.3580
Designated port is 23, path cost 0
Timers: message age 0, forward delay 0, hold 0
BPDU: sent 78551, received 4
Port FastEthernet 0/26 of VLAN1 is Forwarding
Port path cost 10, Port priority 128
Designated root has priority 32768, address 0050.3EF0.3580
Designated bridge has priority 32768, address 0050.3EF0.3580
Designated port is 22, path cost 0
Timers: message age 20, forward delay 15, hold 1
--More--
Port FastEthernet 0/27 of VLAN1 is
Blocking<--------------------------------------------------
Port path cost 10, Port priority 128
Designated root has priority 32768, address 0050.3EF0.3580
Designated bridge has priority 32768, address 0050.3EF0.3580
Designated port is 23, path cost 0
Timers: message age 20, forward delay 15, hold 1
Pierre-Alex
-----Original Message-----
From: Leigh Anne Chisholm [mailto:[EMAIL PROTECTED]]
Sent: Wednesday, February 14, 2001 9:57 AM
To: Pierre-Alex; Cisco Groupstudy (E-mail)
Cc: Dale Cunningham
Subject: RE: DISTURBING: Spanning Tree Protocol Does not Work.
Port 0/27 on the Catalyst 1912 doesn't seem to be sending anything to Port
0/21 on the Catalyst 2924XL. If you look at my message from last night,
you'll notice that Fa0/21 hasn't been receiving input for 3 hours. If
BPDU's are sent every 2 seconds, there's some sort of communication fault
occurring with the port.
Since the Catalyst 1912's 0/27 port is set to trunk on, it is likely not
communicating with Port 0/21 because port 0/21 hasn't been set to trunk.
Try trunking the 2924XL's port and see what happens. In your configuration,
port 0/27 on the Catalyst 1912 is the one that Spanning Tree will block.
Check its status once you've completed setting up the trunking...
-- Leigh Anne
-----Original Message-----
From: [EMAIL PROTECTED] [mailto:[EMAIL PROTECTED]]On Behalf Of
Pierre-Alex
Sent: February 14, 2001 7:31 AM
To: [EMAIL PROTECTED]; Cisco Groupstudy (E-mail)
Cc: Dale Cunningham
Subject: RE: DISTURBING: Spanning Tree Protocol Does not Work.
Hi Leigh Anne and others:
Leigh Anne, I hope you did not loose sleep over this problem.... At 8:30 PM
after a full day on this problem I went to sleep and crashed ....
So here we again:
You discovered correctly that PORT A is connected to f0/20 and PORT B to f
0/21
ALL those ports are part of VLAN 1 (see output bellow)
And all the ports are in fowarding mode and the lights on the switch are
glowing GREEN! (see below the span tree)
Someone suggested the presence of an etherchannel configured by default. I
will look into this
and will let you know ....
Pierre-Alex
Interface Fa0/20 (port 22) in Spanning tree 1 is FORWARDING
Port path cost 19, Port priority 128
Designated root has priority 32768, address 0050.3ef0.3580
Designated bridge has priority 32768, address 0050.3ef0.3580
Designated port is 22, path cost 0
Timers: message age 0, forward delay 0, hold 0
BPDU: sent 73253, received 5
Interface Fa0/21 (port 23) in Spanning tree 1 is FORWARDING
Port path cost 19, Port priority 128
Designated root has priority 32768, address 0050.3ef0.3580
Designated bridge has priority 32768, address 0050.3ef0.3580
Designated port is 23, path cost 0
Timers: message age 0, forward delay 0, hold 0
BPDU: sent 73251, received 3
--More--
VLAN Name Status Ports
---- -------------------------------- --------- ----------------------------
---
1 default active Fa0/2, Fa0/3, Fa0/4, Fa0/5,
Fa0/6, Fa0/7, Fa0/10,
Fa0/11,
Fa0/12, Fa0/13, Fa0/14,
Fa0/15,
Fa0/17, Fa0/18, Fa0/19,
Fa0/21,
Fa0/22, Fa0/23
2 VLAN_A active Fa0/9, Fa0/16, Fa0/24
3 VLAN_B active Fa0/1, Fa0/8
___________________________________________
Port FastEthernet 0/26 of VLAN1 is Forwarding
Port path cost 10, Port priority 128
Designated root has priority 32768, address 0050.3EF0.3580
Designated bridge has priority 32768, address 0050.3EF0.3580
Designated port is 22, path cost 0
Timers: message age 20, forward delay 15, hold 1
--More--
Port FastEthernet 0/27 of VLAN1 is Forwarding
Port path cost 10, Port priority 128
Designated root has priority 32768, address 0050.3EF0.3580
Designated bridge has priority 32768, address 0050.50E2.42C0
Designated port is 27, path cost 10
Timers: message age 20, forward delay 15, hold 1
Pierre-Alex
-----Original Message-----
From: Leigh Anne Chisholm [mailto:[EMAIL PROTECTED]]
Sent: Wednesday, February 14, 2001 1:29 AM
To: Pierre-Alex; Cisco Groupstudy (E-mail)
Cc: Dale Cunningham
Subject: RE: DISTURBING: Spanning Tree Protocol Does not Work.
Okay, here's the jist of things.
The Catalyst 2924XL is the root bridge:
> C2924XL#sh span
>
> Spanning tree 1 is executing the IEEE compatible Spanning Tree protocol
> Bridge Identifier has priority 32768, address 0050.3ef0.3580
> Configured hello time 2, max age 20, forward delay 15
> We are the root of the spanning tree
Port 0/26 on the Catalyst 1912 is identifying "Port 22" as the "designated
port":
> Port FastEthernet 0/26 of VLAN1 is Forwarding
> Port path cost 10, Port priority 128
> Designated root has priority 32768, address 0050.3EF0.3580
> Designated bridge has priority 32768, address 0050.3EF0.3580
> Designated port is 22, path cost 0
> Timers: message age 20, forward delay 15, hold 1
Port 22 is, port 0/20 on the Catalyst 2924XL switch:
> Interface Fa0/20 (port 22) in Spanning tree 1 is FORWARDING
> Port path cost 19, Port priority 128
> Designated root has priority 32768, address 0050.3ef0.3580
> Designated bridge has priority 32768, address 0050.3ef0.3580
> Designated port is 22, path cost 0
> Timers: message age 0, forward delay 0, hold 0
> BPDU: sent 46897, received 5
We can deduce that FastEthernet 0/26 on the 1912 switch is directly
connected to FastEthernet 0/20 on the 2924XL switch.
Note that FastEthernet 0/26 on the Catalyst 1912 is identified as the root
port as seen below:
> C1912#sh span
>
> VLAN1 is executing the IEEE compatible Spanning Tree Protocol
> Bridge Identifier has priority 32768, address 0050.50E2.42C0
> Configured hello time 2, max age 20, forward delay 15
> Current root has priority 32768, address 0050.3EF0.3580
> Root port is FastEthernet 0/26, cost of root path is 10
> Topology change flag not set, detected flag not set
> Topology changes 5, last topology change occured 0d02h42m54s ago
> Times: hold 1, topology change 8960
> hello 2, max age 20, forward delay 15
> Timers: hello 2, topology change 35, notification 2
So what we have is a LAN segment connection where one end of the connection
is identified as the root port while the other is identified as the root
port for the non-root-bridge Catalyst 1912 switch. Everything seems to be
"kosher".
All ports on the root bridge are designated ports by default. During the
execution of the Spanning Tree Algorithm, a root port (the port with the
lowest cost path to the root bridge) is identified, and each LAN segment
must identify one port as a designated port. If the LAN segment contains
one or more ports not identified as the root port for the switch or a
designated port, the port is identified as non-designated and is placed in
the blocking state.
If spanning tree was working properly in this topology, and if a looped
topology actually existed, another port on the Catalyst 1912 should show a
port on the Catalyst 2924XL as its designated port with that port on the
Catalyst 1912 as set in the blocking state -- but we don't see that. What
MAY be the case, because on the Catalyst 1912 FastEthernet port 0/27 is set
to trunking but hasn't established a physical connection, is that the port
is operating normally as a member of VLAN 1. If the port in which it is
connected to on the 2924XL is a member of a VLAN other than VLAN 1,
communication shouldn't occur properly -- and a looped topology wouldn't
actually exist. Only port 0/21 on the 2924XL is operating at 100 Mbps in
Full-Duplex mode - so I suspect the 1912 0/27 port IS in fact connected to
the 0/21 port. HOWEVER... if you do a "show interface fastethernet 0/21,
you will see that last input was 3:49:16 - which means it's been quite some
time since this port received anything from what it's connected to... It's
not receiving BPDU's every 2 seconds from 0/27 if it is in fact connected to
0/27...
> C2924XL#sh int f 0/21
> FastEthernet0/21 is up, line protocol is up
> Hardware is Fast Ethernet, address is 0050.3ef0.3597 (bia
0050.3ef0.3597)
> MTU 1500 bytes, BW 100000 Kbit, DLY 100 usec, rely 255/255, load 1/255
> Encapsulation ARPA, loopback not set, keepalive not set
> Full-duplex, 100Mb/s, 100BaseTX/FX
> ARP type: ARPA, ARP Timeout 04:00:00
> Last input 03:49:16, output 00:00:01, output hang never
So... do we have an active looped topology? I doubt it. Likely, Spanning
Tree's working just fine. Check the LED's on both of your switches to see
if you see a solid orange glow...
-- Leigh Anne
-----Original Message-----
From: [EMAIL PROTECTED] [mailto:[EMAIL PROTECTED]]On Behalf Of
Leigh Anne Chisholm
Sent: February 13, 2001 10:22 PM
To: Cisco@Groupstudy. Com
Subject: RE: DISTURBING: Spanning Tree Protocol Does not Work.
Which ports on the 2924XL are trunking? The only port I can see configured
for trunking is FA0/20. I'm not clear on exactly where you've connected the
1912's 0/26 and 0/27 ports on the 2924XL...
Also, you indicate that you've set up the 2924XL to be the root bridge --
you might want to check what criteria it is that you use when ensuring a
device is deemed to be the root bridge. I suspect you've played with the
wrong parameters to ensure that the 2924XL is the root bridge. Yes, the
2924XL IS the root bridge--but not because of anything you've configured.
-- Leigh Anne
(My apologies if this appears twice in GroupStudy -- I originally sent this
four hours ago, but have yet to see it appear on the list...)
-----Original Message-----
From: Pierre-Alex [mailto:[EMAIL PROTECTED]]
Sent: February 13, 2001 5:22 PM
To: Rik Guyler; Cisco Groupstudy (E-mail)
Cc: Lachisho; [EMAIL PROTECTED]; Dale Cunningham
Subject: RE: DISTURBING: Spanning Tree Protocol Does not Work.
<snip>
C2924XL# sh run
Building configuration...
Current configuration:
!
version 11.2
no service pad
no service udp-small-servers
no service tcp-small-servers
!
hostname C2924XL
!
enable secret 5 $1$OixX$sb.r.W9jrZfT7VfMQdtgF/
!
!
no ip domain-lookup
!
interface VLAN1
ip address 192.168.0.10 255.255.255.0
no ip route-cache
!
interface FastEthernet0/1
switchport access vlan 3
!
interface FastEthernet0/2
!
interface FastEthernet0/3
!
interface FastEthernet0/4
!
interface FastEthernet0/5
!
interface FastEthernet0/6
!
interface FastEthernet0/7
!
interface FastEthernet0/8
switchport access vlan 3
!
interface FastEthernet0/9
switchport access vlan 2
!
interface FastEthernet0/10
!
interface FastEthernet0/11
!
interface FastEthernet0/12
!
interface FastEthernet0/13
!
interface FastEthernet0/14
!
interface FastEthernet0/15
!
interface FastEthernet0/16
switchport access vlan 2
!
interface FastEthernet0/17
!
interface FastEthernet0/18
!
interface FastEthernet0/19
!
interface FastEthernet0/20
switchport mode trunk
!
interface FastEthernet0/21
!
interface FastEthernet0/22
!
interface FastEthernet0/23
!
interface FastEthernet0/24
switchport access vlan 2
!
snmp-server community private RW
snmp-server community public RO
snmp-server chassis-id 0x0E
!
line con 0
exec-timeout 0 0
stopbits 1
line vty 0 4
exec-timeout 0 0
password cisco
logging synchronous
login
line vty 5 14
exec-timeout 0 0
logging synchronous
login
!
end
<snip>
Pierre-Alex
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