06-10-2003 02:01 PM - edited 03-02-2019 08:02 AM
My understanding of MSTP and STP in general is that if a switch has two equal paths (read same port priority and path cost) the port with the lower interface number forwards and the port with the higher interface number blocks.
Here's the layout: 1 Catalyst 6513 at the core and a stack of 3 Catalyst 3550s at the edge. The top 3550 connects to the core via gig fiber on int g0/1; the bottom 3550 connects to the core via gig fiber on int g0/2; the middle 3550 connects to the top and bottom 3550s via gig copper on int g0/1 and g0/2.
I've spot checked approximately 20 stacks in this configuration, and the middle switch was always blocking int g0/2. Today, however, I've got one switch that insists on blocking int g0/1...even if the port priority of g0/1 is set to 0 or 16. The only way I can get it to forward is by changing the path cost. Am I missing something basic here? Is this a bug in IOS 12.1(13)EA1a?
For what it's worth, I took the 6513 out of the picture and placed another 3550 in its place, set the new 3550 as the root bridge...yielded same results.
Thanks in advance for any help or advice.
06-10-2003 02:32 PM
Assumption #1: the 6513 is the Root Bridge for all VLANs.
Assumption #2: in the stack in question, the middle 3550's Gig0/1 interface is connected to the top 3550's Gig0/2 interface.
Question: Do you have UplinkFast enabled on the top 3550, and disabled on the bottom 3550?
The reason why I ask is, that the UplinkFast feature changes STP-related settings in two ways:
1. It sets the Bridge Priority of a switch to a high value (49152), which almost always keeps the switch from becoming Root Bridge for any VLAN; and
2. It adds 3000 to the path costs of all ports and VLAN trunks on the switch, which makes the switch less attractive as a pathway to the Root Bridge for any VLAN.
If you have UplinkFast enabled on the top 3550 and not on the bottom 3550, then the middle 3550 should block interface Gig0/1 and favor Gig0/2, because it costs less to go through the bottom 3550 to reach the Root Bridge.
Hope this helps.
06-11-2003 04:36 AM
Yes, the 6513 is the root bridge for all VLANs. And yes, the top g0/2 is connected to the middle g0/1, the middle g0/2 is connected to the bottom g0/1. Uplinkfast is not enabled. All 3550s are configured identically (less hostname, IP, etc.) as follows:
!
no service pad
service timestamps debug uptime
service timestamps log uptime
service password-encryption
!
hostname xxxxxxxxxxxxx
!
aaa new-model
aaa authentication login default group tacacs+ line
aaa authentication login console line
aaa authorization exec default group tacacs+ if-authenticated
aaa authorization network default group tacacs+
enable secret xxxxxxxxxxxxx
!
clock timezone CST -6
clock summer-time CST recurring
ip subnet-zero
no ip domain-lookup
!
mls qos map cos-dscp 0 8 16 26 32 46 48 56
mls qos min-reserve 5 170
mls qos min-reserve 6 10
mls qos min-reserve 7 65
mls qos min-reserve 8 26
mls qos
udld aggressive
!
!
spanning-tree mode mst
spanning-tree extend system-id
spanning-tree backbonefast
!
spanning-tree mst configuration
name xxxxxx
revision 2
instance 1 vlan 1-4094
!
!
!
interface FastEthernet0/1
switchport access vlan 124
switchport mode dynamic desirable
switchport trunk encapsulation dot1q
switchport trunk native vlan 124
switchport voice vlan 201
no ip address
mls qos trust device cisco-phone
mls qos trust cos
storm-control broadcast level 75.00
auto qos voip cisco-phone
wrr-queue bandwidth 20 1 80 1
wrr-queue min-reserve 1 5
wrr-queue min-reserve 2 6
wrr-queue min-reserve 3 7
wrr-queue min-reserve 4 8
wrr-queue cos-map 1 0 1 2 4
wrr-queue cos-map 3 3 6 7
wrr-queue cos-map 4 5
priority-queue out
spanning-tree portfast
!
!
!All FastEthernet interfaces configured the same
!
!
interface FastEthernet0/24
switchport access vlan 124
switchport mode dynamic desirable
switchport trunk encapsulation dot1q
switchport trunk native vlan 124
switchport voice vlan 201
no ip address
mls qos trust device cisco-phone
mls qos trust cos
storm-control broadcast level 75.00
auto qos voip cisco-phone
wrr-queue bandwidth 20 1 80 1
wrr-queue min-reserve 1 5
wrr-queue min-reserve 2 6
wrr-queue min-reserve 3 7
wrr-queue min-reserve 4 8
wrr-queue cos-map 1 0 1 2 4
wrr-queue cos-map 3 3 6 7
wrr-queue cos-map 4 5
priority-queue out
spanning-tree portfast
!
interface GigabitEthernet0/1
switchport trunk encapsulation dot1q
switchport mode trunk
no ip address
mls qos trust cos
udld port aggressive
storm-control broadcast level 75.00
auto qos voip trust
wrr-queue bandwidth 20 1 80 1
wrr-queue queue-limit 80 1 20 1
wrr-queue cos-map 1 0 1 2 4
wrr-queue cos-map 3 3 6 7
wrr-queue cos-map 4 5
priority-queue out
!
interface GigabitEthernet0/2
switchport trunk encapsulation dot1q
switchport mode trunk
no ip address
mls qos trust cos
udld port aggressive
storm-control broadcast level 75.00
auto qos voip trust
wrr-queue bandwidth 20 1 80 1
wrr-queue queue-limit 80 1 20 1
wrr-queue cos-map 1 0 1 2 4
wrr-queue cos-map 3 3 6 7
wrr-queue cos-map 4 5
priority-queue out
!
interface Vlan1
no ip address
shutdown
!
interface Vlan124
ip address 10.xxx.xxx.x 255.255.254.0
!
ip default-gateway 10.xxx.xxx.1
ip classless
no ip http server
!
!
logging 10.xxx.xxx.xxx
access-list 10 permit 10.xxx.xxx.0 0.0.15.255
snmp-server community xxxxxxxxx RW
snmp-server chassis-id xxxxxxxxxx
snmp-server host 10.xxx.xxx.xxx xxxxxxx envmon syslog snmp
tacacs-server host 10.xxx.xxx.xxx key xxxxxxxxx
!
line con 0
password xxxxxxxxx
logging synchronous
login authentication console
line vty 0 4
access-class 10 in
password xxxxxxxxx
logging synchronous
login authentication default
line vty 5 15
access-class 10 in
password xxxxxxxxx
logging synchronous
login authentication default
!
ntp server 10.xxx.xxx.xxx
!
end
Here is the output from a show spantree detail:
MST00 is executing the mstp compatible Spanning Tree protocol
Bridge Identifier has priority 32768, sysid 0, address 000b.be3d.7a80
Configured hello time 2, max age 20, forward delay 15
Current root has priority 24576, address 000b.4522.45c0
Root port is 26 (GigabitEthernet0/2), cost of root path is 0
Topology change flag not set, detected flag not set
Number of topology changes 2 last change occurred 00:01:13 ago
from GigabitEthernet0/2
Times: hold 1, topology change 35, notification 2
hello 2, max age 20, forward delay 15
Timers: hello 0, topology change 0, notification 0
Port 25 (GigabitEthernet0/1) of MST00 is alternate blocking
Port path cost 20000, Port priority 128, Port Identifier 128.25.
Designated root has priority 24576, address 000b.4522.45c0
Designated bridge has priority 32768, address 000b.bea4.9f00
Designated port id is 128.26, designated path cost 0
Timers: message age 4, forward delay 0, hold 0
Number of transitions to forwarding state: 0
Link type is point-to-point by default, Internal
BPDU: sent 13, received 45
Port 26 (GigabitEthernet0/2) of MST00 is root forwarding
Port path cost 20000, Port priority 128, Port Identifier 128.26.
Designated root has priority 24576, address 000b.4522.45c0
Designated bridge has priority 32768, address 000b.bea3.b280
Designated port id is 128.25, designated path cost 0
Timers: message age 4, forward delay 0, hold 0
Number of transitions to forwarding state: 0
Link type is point-to-point by default, Internal
BPDU: sent 13, received 43
MST01 is executing the mstp compatible Spanning Tree protocol
Bridge Identifier has priority 32768, sysid 1, address 000b.be3d.7a80
Configured hello time 2, max age 20, forward delay 15
Current root has priority 24577, address 000b.4522.45c0
Root port is 26 (GigabitEthernet0/2), cost of root path is 40000
Topology change flag not set, detected flag not set
Number of topology changes 5 last change occurred 00:01:17 ago
from GigabitEthernet0/2
Times: hold 1, topology change 35, notification 2
hello 2, max age 20, forward delay 15
Timers: hello 0, topology change 0, notification 0
Port 25 (GigabitEthernet0/1) of MST01 is alternate blocking
Port path cost 20000, Port priority 128, Port Identifier 128.25.
Designated root has priority 24577, address 000b.4522.45c0
Designated bridge has priority 32769, address 000b.bea4.9f00
Designated port id is 128.26, designated path cost 20000
Timers: message age 4, forward delay 0, hold 0
Number of transitions to forwarding state: 1
Link type is point-to-point by default, Internal
BPDU: sent 13, received 47
Port 26 (GigabitEthernet0/2) of MST01 is root forwarding
Port path cost 20000, Port priority 128, Port Identifier 128.26.
Designated root has priority 24577, address 000b.4522.45c0
Designated bridge has priority 32769, address 000b.bea3.b280
Designated port id is 128.25, designated path cost 20000
Timers: message age 4, forward delay 0, hold 0
Number of transitions to forwarding state: 2
Link type is point-to-point by default, Internal
BPDU: sent 13, received 45
Thanks for any help or suggestions.
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