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What is an STP Carrier vlan?

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TitleWhat is an STP Carrier vlan?
What is an STP Carrier VLAN?
  • An STP carrier VLAN defines the scope of the STP Domain.  The Carrier VLAN must include all the ports that will be protected by the STP domain regardless of their VLAN membership, and if configured, the 802.1Q tag used to transport Extreme Multiple Instance Spanning Tree Protocol (EMISTP) or Per VLAN Spanning Tree (PVST+) encapsulated bridge protocol data units (BPDUs)
  • By default, the Default untagged VLAN is used as the Carrier VLAN for the s0 STP domain.  However, once ports are removed from the Default VLAN or the VLAN is disabled, a tagged Carrier VLAN must be defined if STP will be enabled in the network
  • Only one carrier VLAN can exist in a given STP domain, although some of its ports can be outside the control of any STPD at the same time
  • If you configure EMISTP or PVST+, the STP domain tag must be configured to be identical to the VLAN ID of the carrier VLAN in that STP domain
  • If you configure Multiple Spanning Tree (MSTP), you do not need carrier VLANs for MSTP operation
Defining the Carrier VLAN
  • Create a VLAN that will serve as the carrier VLAN and assign it an 802.1Q VLAN tag
  • Add all the ports that will be protected
  • Create an STP domain
  • Associate the Carrier VLAN to the STP domain along with the ports that will be protected
  • Configure the tag of the STP domain to be identical to the 802.1Q VLAN tag of the carrier VLAN
create vlan <vlan-name> tag <tag>
configure vlan <vlan-name> add ports <port-list> tagged

create stpd <stp-domain-name>
configure stpd <stp-domain-name> add vlan <vlan-name> ports <port-list> <encapsulation-mode>
configure stpd <stp-domain-name> tag <carrier-vlan-tag>
enable stpd <stp-domain-name>
Once the STP domain has been created and enabled, any additional VLANs can be auto-bound to the STP domain using the following command:
enable stpd <stp-domain-name> auto-bind vlan <vlan-name>
Viewing and verifying the configuration can be used using the following commands
# show s0

Stpd: s0                Stp: ENABLED            Number of Ports: 34
Rapid Root Failover: Disabled
Operational Mode: 802.1W                        Default Binding Mode: PVST+
802.1Q Tag: 4000
Ports: 1,2,3,4,5,6,7,8,9,10,
Participating Vlans: Carried
Auto-bind Vlans: Default
Bridge Priority            : 32768
Operational Bridge Priority: 32768
BridgeID                   : 80:00:00:04:96:98:7d:96
Designated root            : 80:00:00:04:96:83:52:21
RootPathCost: 20000     Root Port     : 1
MaxAge      : 20s       HelloTime     : 2s      ForwardDelay     : 15s
CfgBrMaxAge : 20s       CfgBrHelloTime: 2s      CfgBrForwardDelay: 15s
Topology Change Time           : 35s            Hold time        : 1s
Topology Change Detected       : FALSE          Topology Change  : FALSE
Number of Topology Changes     : 0
Time Since Last Topology Change: 0s
Topology Change initiated locally on Port 1
Topology Change last received on Port none from none
Backup Root               : Off         Backup Root Activated  : FALSE
Loop Protect Event Window : 180s        Loop Protect Threshold : 3
New Root Trap             : On          Topology Change Trap   : Off
Tx Hold Count             : 6
To view the current status of the ports, you can use the following command.  In the example below, the port of interest is Port 1, which leads to the peer switch:
# show stpd "s0" ports
Port   Mode   State      Cost  Flags     Priority Port ID Designated Bridge
1      EMISTP FORWARDING 20000 eRapaw---- 128      8001    80:00:00:04:96:83:52:21
2      EMISTP DISABLED   20000 e?ab-w---- 128      8002    00:00:00:00:00:00:00:00
3      EMISTP DISABLED   20000 e?ab-w---- 128      8003    00:00:00:00:00:00:00:00
4      EMISTP DISABLED   20000 e?ab-w---- 128      8004    00:00:00:00:00:00:00:00
5      EMISTP DISABLED   20000 e?ab-w---- 128      8005    00:00:00:00:00:00:00:00


Total Ports: 34
 ------------------------- Flags: ----------------------------
1:                e=Enable, d=Disable
2: (Port role)    R=Root, D=Designated, A=Alternate, B=Backup, M=Master
3: (Config type)  b=broadcast, p=point-to-point, e=edge, a=auto
4: (Oper. type)   b=broadcast, p=point-to-point, e=edge
5:                p=proposing, a=agree
6: (partner mode) d = 802.1d, w = 802.1w, m = mstp
7:                i = edgeport inconsistency
8:                S = edgeport safe guard active
                  s = edgeport safe guard configured but inactive
8:                G = edgeport safe guard bpdu restrict active in 802.1w and mstp
                  g = edgeport safe guard bpdu restrict active in 802.1d
9:                B = Boundary, I = Internal
10:               r = restricted role, t = active role

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