BPDU Filter

• Quite possibly the most misunderstood STP feature
• Two feature with the same same

• Locally (per-port) configured BPDU Filter
• Unconditional
• Filters all incoming and all out going BPDUs
• Equivalent of turning off STP on the port

Cmd:  spanning-tree bpdufilter enable

• Globally configured BPDU Filter
• Conditional on Portfast state
• Filters most of outgoing BPDUs
• Does not filter incoming BPDUs

• Configure:
• Mode Interface:
• Filter all incoming and outgoing BPDU, traffic can pass.

• Cho topology:
• Run spanning-tree vlan 10 all IOU.
• Config interface e0/1 IOU1 enable BPDU Filter

• Check int e0/1 truoc khi enable bpdufilter

IOU1#show span int e0/1 de

 Port 2 (Ethernet0/1) of VLAN0010 is designated forwarding

   Port path cost 100, Port priority 128, Port Identifier 128.2.

   Designated root has priority 20490, address aabb.cc00.0200

   Designated bridge has priority 24586, address aabb.cc00.0100

   Designated port id is 128.2, designated path cost 100

   Timers: message age 0, forward delay 0, hold 0

   Number of transitions to forwarding state: 1

   Link type is shared by default

    BPDU: sent 332, received 1

IOU1#

interface Ethernet0/1

 switchport access vlan 10

 switchport mode access

 spanning-tree bpdufilter enable

IOU1#show span int e0/1 de

 Port 2 (Ethernet0/1) of VLAN0010 is designated forwarding

   Port path cost 100, Port priority 128, Port Identifier 128.2.

   Designated root has priority 20490, address aabb.cc00.0200

   Designated bridge has priority 24586, address aabb.cc00.0100

   Designated port id is 128.2, designated path cost 100

   Timers: message age 0, forward delay 0, hold 0

   Number of transitions to forwarding state: 1

   Link type is shared by default

   Bpdu filter is enabled

   BPDU: sent 332, received 1

• Goi tin BPDU nhận vào và gửi ra không đổi vì đã bị filter.

• Nhược điểm: gây ra loop khi filter BPDU ở block port

• BPDU filter (Pert-port) Có thể kết hợp các featuer khác theo nguyên tắc ở hình sau đây:

• Chiều out filter BPDU
• Chiều In hoạt động kết hợp theo nguyên tắc như trên:
• BPDU filter + BPDU guard --> BPDU guard disable
• BPDU filter + Portfast ---> OK con dê :v
• ….

• Mode Global.
• Conditional on Portfast state  (rất quan trọng).

=> có nghĩa là portfast enable -> bpdufilter enable và ngược lại.

Cmd: spanning-tree porfast bpdufilter

Enable trên các interface operating porfast.

• Hoạt động theo hình sau.

•  Chiều "out" Filter Most (not all of them) BPDU

• Chiều "in" (bỏ qua BPDU guard).
• Khi có 1 gói tin BPDU đi vào -> gặp portfast -> portfast disable -> BPDU filter disable -> interface running normal STP.

Port 2 (Ethernet0/1) of VLAN0010 is designated forwarding

   Port path cost 100, Port priority 128, Port Identifier 128.2.

   Designated root has priority 20490, address aabb.cc00.0200

   Designated bridge has priority 24586, address aabb.cc00.0100

   Designated port id is 128.2, designated path cost 100

   Timers: message age 0, forward delay 0, hold 0

   Number of transitions to forwarding state: 1

   Link type is shared by default

   BPDU: sent 332, received 1

• ƯU ĐIỂM: CHỐNG LOOP NHƯ TRƯỜNG HỢP CẤU HÌNH PER-PORT

BPDU Guard

• Prevents connection of "rogue switches"
• … or any device that generates BPDU (Wireless AP)
• Global or per-port configuration
• Global depends on portfast
• Cmd: spanning-tree porfast bpduguard default
• Per-port is unconditional
• Cmd: spanning-tree bpduguard enable
• When triggered
• Port is err-disabled when head incoming BPDU
• Can recovery by command:
errdisable recovery interval 30
errdisable recovery cause bpduguard




• Config BPDU guard.
• Mode Interface:







• Cho topology:
• Chay spanning-tree all IOU
• IOU2 root spanning-tree vlan 10


• Config BPDU guard on interface e0/1 IOU1.
IOU1#
interface Ethernet0/1
 switchport access vlan 10
 switchport mode access
 spanning-tree bpduguard enable


IOU1#show span int e0/1 detail
 Port 2 (Ethernet0/1) of VLAN0010 is designated forwarding
   Port path cost 100, Port priority 128, Port Identifier 128.2.
   Designated root has priority 20490, address aabb.cc00.0200
   Designated bridge has priority 24586, address aabb.cc00.0100
   Designated port id is 128.2, designated path cost 100
   Timers: message age 0, forward delay 0, hold 0
   Number of transitions to forwarding state: 1
   Link type is shared by default
   Bpdu guard is enabled
   BPDU: sent 102, received 0


• Tien hanh "no shut" interface e0/0 IOU3. => IOU1 nhan BPDU tu interface e0/1
IOU1#    
*Jan 26 14:34:01.275: %SPANTREE-2-BLOCK_BPDUGUARD: Received BPDU on port Et0/1 with BPDU Guard enabled. Disabling port.
*Jan 26 14:34:01.275: %PM-4-ERR_DISABLE: bpduguard error detected on Et0/1, putting Et0/1 in err-disable state
*Jan 26 14:34:01.276: STP: VLAN0010 sent Topology Change Notice on Et0/0
*Jan 26 14:34:01.276: STP[10]: Generating TC trap for port Ethernet0/1
*Jan 26 14:34:02.277: %LINEPROTO-5-UPDOWN: Line protocol on Interface Ethernet0/1, changed state to down
*Jan 26 14:34:03.280: %LINK-3-UPDOWN: Interface Ethernet0/1, changed state to down



IOU1#show span int e0/1 de no spanning tree info available for Ethernet0/1 IOU1#



• IOU1 nhan BPDU tu interface e0/1 enable bpduguard --> interface err-disable state
De enable port up khi interface khong nhan duojc BPDU ta cau hinh cmd:


IOU1(config)#do show run | se errdisable
errdisable recovery interval 30
errdisable recovery cause bpduguard


IOU3(config)#int e0/0
IOU3(config-if)#shut


*Jan 26 14:41:45.264: %SYS-5-CONFIG_I: Configured from console by console
IOU1#
*Jan 26 14:42:14.268: %PM-4-ERR_RECOVER: Attempting to recover from bpduguard err-disable state on Et0/1
*Jan 26 14:42:14.268: set portid: VLAN0010 Et0/1: new port id 8002
*Jan 26 14:42:14.268: STP: VLAN0010 Et0/1 -> listening
*Jan 26 14:42:16.268: %LINK-3-UPDOWN: Interface Ethernet0/1, changed state to up
*Jan 26 14:42:17.272: %LINEPROTO-5-UPDOWN: Line protocol on Interface Ethernet0/1, changed state to up


• Mode Global:
• Cấu hình bằng command:
spanning-tree porfast bpduguard default
• Enable bpduguard trên nhưng interface operating portfast.


• Nguyên tắc hoạt động ở mode global giống với mode interface + portfast:

• Như hình vẽ, khi port enable -> chưa có gói tin BPDU -> portfast hoạt động -> port chuyển từ trạng thái disable sang forwarding.
• Khi thấy có gói tin BPDU nhận vào gặp bpduguard -> port chuyển sang trạng thái err-disable.
-> Nói chung là hoạt động bình thường.

Portfast

• Enabled globally or per port
• Global applies to all operational access ports
• Cmd: spanning-tree portfast default

• Per-port applies to a local port, if it's  operational access
• Cmd: spanning-tree porfast
• Can be enable for operational trunk port
• Cmd: spanning-tree portfast trunk

• By pass the listening and learcing States.  ( Bản chất khi enable portfast -> Switch sẽ k sinh ra bản tin TCN gửi tới root khi port đó thay đổi trạng thái ).
• Only when transitions from a Disabled state

• Conditional on incoming BPDUs
• Incoming BPDU resets the operational portfast state
• Pertfast state is maintained on a per-Vlan basic

• Dependent features
• BPDU guard
• BPDU Filter

• Portfast la feature rat manh, giup giam thoi gian runing spanning tree khi port ket noi voi end host.
• Vi du nhu truong hop host cau hinh nhan dia chi DHCP, neu cho thoi gian hoi tu cua STP thi phai mat it nhat 30s de nhan duoc dia chi IP.

•  

• TEST1: config and verify portfast.

+ cho topology: IOU1, IOU2 run spanning tree vlan 10.

Config portfast:

IOU1#

interface Ethernet0/1

 switchport access vlan 10

 switchport mode access

 spanning-tree portfast

Shut down

Tien hanh "no shutdown" interface e0/1 IOU1:

IOU1#spa

*Jan 26 12:59:47.446: setting bridge id (which=3) prio 32778 prio cfg 32768 sysid 10 (on) id 800A.aabb.cc00.0100

*Jan 26 12:59:47.446: set portid: VLAN0010 Et0/1: new port id 8002

*Jan 26 12:59:47.446: STP: VLAN0010 Et0/1 ->jump to forwarding from blocking

IOU1#show span int e0/1 portfast

VLAN0010            enabled

• TEST2: Conditional Portfast incoming BPDU
• Cau hinh IOU1 root vlan 10
• Interface e0/0 enable portfast.

IOU1#show span int e0/0 port

VLAN0010            enabled

• Vi IOU1 la root nen cong e0/0 luon send BPDU

IOU1#show span int e0/0 de

 Port 1 (Ethernet0/0) of VLAN0010 is root forwarding

   Port path cost 100, Port priority 128, Port Identifier 128.1.

   Designated root has priority 20490, address aabb.cc00.0200

   Designated bridge has priority 20490, address aabb.cc00.0200

   Designated port id is 128.1, designated path cost 0

   Timers: message age 2, forward delay 0, hold 0

   Number of transitions to forwarding state: 1

   The port is in the portfast mode

   Link type is shared by default

   BPDU: sent 50, received 0

• Cau hinh IOU2 la root vlan 10
• IOU2#spanning-tree vlan 10 root primary

• Interface e0/0 IOU1 se nhan BPDU

IOU1#show span int e0/0 de

 Port 1 (Ethernet0/0) of VLAN0010 is root forwarding

   Port path cost 100, Port priority 128, Port Identifier 128.1.

   Designated root has priority 20490, address aabb.cc00.0200

   Designated bridge has priority 20490, address aabb.cc00.0200

   Designated port id is 128.1, designated path cost 0

   Timers: message age 2, forward delay 0, hold 0

   Number of transitions to forwarding state: 1

   The port is in the portfast mode

   Link type is shared by default

   BPDU: sent 50, received 51

• => Portfast se disable

IOU1#show span int e0/0 port

VLAN0010          disable

• Shut down port e0/0 IOU1. va no sh thi port se chuyen ngay sang trang thai forwarding. Co nghia la portfast chi bi disable operting  khi nhan duoc goi tin BPDU incoming

• TEST3:  Only when transitions from a Disabled state

• Cau hinh nhu sau:
• IOU2 root vlan 10
• Interace e0/0 , e0/2 IOU1 enable portfast
• Tat ca cac port cau hinh mode access vlan 10

IOU1(config-if-range)#shut

*Jan 26 13:47:18.922: %LINK-5-CHANGED: Interface Ethernet0/0, changed state to administratively down

*Jan 26 13:47:18.931: %LINK-5-CHANGED: Interface Ethernet0/2, changed state to administratively down

*Jan 26 13:47:19.929: %LINEPROTO-5-UPDOWN: Line protocol on Interface Ethernet0/0, changed state to down

*Jan 26 13:47:19.938: %LINEPROTO-5-UPDOWN: Line protocol on Interface Ethernet0/2, changed state to down

IOU1(config-if-range)#no sh

*Jan 26 13:47:24.015: setting bridge id (which=3) prio 24586 prio cfg 24576 sysid 10 (on) id 600A.aabb.cc00.0100

*Jan 26 13:47:24.015: set portid: VLAN0010 Et0/0: new port id 8001

*Jan 26 13:47:24.015: STP: VLAN0010 Et0/0 ->jump to forwarding from blocking

*Jan 26 13:47:24.015: set portid: VLAN0010 Et0/2: new port id 8003

*Jan 26 13:47:24.015: STP: VLAN0010 Et0/2 ->jump to forwarding from blocking

IOU1(config-if-range)#do show span vlan 10 VLAN0010   Spanning tree enabled protocol ieee   Root ID    Priority    20490              Address     aabb.cc00.0200              Cost        100              Port        1 (Ethernet0/0)              Hello Time   2 sec  Max Age 20 sec  Forward Delay 15 sec   Bridge ID  Priority    24586  (priority 24576 sys-id-ext 10)              Address     aabb.cc00.0100              Hello Time   2 sec  Max Age 20 sec  Forward Delay 15 sec              Aging Time  15  sec Interface           Role Sts Cost      Prio.Nbr Type ------------------- ---- --- --------- -------- -------------------------------- Et0/0               Root FWD 100       128.1    Shr Et0/2               Root FWD 100       128.3    Shr

• STEP1: khi "no shut" port e0/0 và e0/2 enable portfast sẽ chuyển sang trạng thai forwarding vai trò ROOT ngay lập tức.

IOU1(config-if-range)#

*Jan 26 13:47:25.561: STP: VLAN0010 heard root 20490-aabb.cc00.0200 on Et0/0

*Jan 26 13:47:25.561:     supersedes 24586-aabb.cc00.0100

*Jan 26 13:47:25.561: STP: VLAN0010 new root is 20490, aabb.cc00.0200 on port Et0/0, cost 100

*Jan 26 13:47:25.567: STP: VLAN0010 sent Topology Change Notice on Et0/0

*Jan 26 13:47:25.567: STP[10]: Generating TC trap for port Ethernet0/2

*Jan 26 13:47:25.567: STP: VLAN0010 Et0/2 -> blocking

*Jan 26 13:47:26.017: %LINK-3-UPDOWN: Interface Ethernet0/0, changed state to up

*Jan 26 13:47:26.017: %LINK-3-UPDOWN: Interface Ethernet0/2, changed state to up

*Jan 26 13:47:27.023: %LINEPROTO-5-UPDOWN: Line protocol on Interface Ethernet0/0, changed state to up

*Jan 26 13:47:27.023: %LINEPROTO-5-UPDOWN: Line protocol on Interface Ethernet0/2, changed state to up

IOU1(config-if-range)#do show span vlan 10 VLAN0010   Spanning tree enabled protocol ieee   Root ID    Priority    20490              Address     aabb.cc00.0200              Cost        100              Port        1 (Ethernet0/0)              Hello Time   2 sec  Max Age 20 sec  Forward Delay 15 sec   Bridge ID  Priority    24586  (priority 24576 sys-id-ext 10)              Address     aabb.cc00.0100              Hello Time   2 sec  Max Age 20 sec  Forward Delay 15 sec              Aging Time  15  sec Interface           Role Sts Cost      Prio.Nbr Type ------------------- ---- --- --------- -------- -------------------------------- Et0/0               Root FWD 100       128.1    Shr Et0/2               Altn BLK 100       128.3    Shr

• STEP2: Vì IOU2 là root nên IOU1 sẽ nhận được gói tin BPDU từ 2 e0/0 và e0/2.--> IOU1 biết loop xảy ra chuyển ngay trạng thái port e0/2 sang block

• Đồng thời disable operating portfast ở port e0/0 và e0/2.

• Tiến hành "shut down" interface e0/0 IOU1.

IOU1(config)#int e0/0

IOU1(config-if)#shut

*Jan 26 13:54:47.146: STP: VLAN0010 new root port Et0/2, cost 100

*Jan 26 13:54:47.146: STP: VLAN0010 Et0/2 -> listening

*Jan 26 13:54:47.146: STP[10]: Generating TC trap for port Ethernet0/0

*Jan 26 13:54:49.145: %LINK-5-CHANGED: Interface Ethernet0/0, changed state to administratively down

*Jan 26 13:54:49.155: STP: VLAN0010 sent Topology Change Notice on Et0/2

*Jan 26 13:54:50.146: %LINEPROTO-5-UPDOWN: Line protocol on Interface Ethernet0/0, changed state to down

*Jan 26 13:55:02.152: STP: VLAN0010 Et0/2 -> learning

*Jan 26 13:55:17.152: STP[10]: Generating TC trap for port Ethernet0/2

*Jan 26 13:55:17.152: STP: VLAN0010 Et0/2 -> forwarding

=> Port e0/2 tiến hành chạy STP normal (vì portfast đã bị disable khi nhận BPDU incoming ở trên) trải qua các quá trình STP -> forwarding. Vậy portfast operating khi và chỉ khi ở trạng thái disable

Loop Guard

• Prevent loops by detecting the sudden loss of BPDUs
• Non-designated port cannot transistion to designated
• Does not work when transitioning from disabled state
• Only superior BPDUs are considered
• Imcompatible with RootGuard

• Global or per-port configuration
• Blogbal configuration enables LoopGuard on all port

• Protects from
• Unidirectional links
• Switches/links that block BPDUs

• LoopGuard-enable ports may send BPDUs
• When inferior BPDUs are received

Loop guard dùng trong trường hợp unidirectional links, ngăn cản Non-Designated port trở thành Designated port tránh loop có thể xảy ra.

- Cho topo: 

+ IOU1 và IOU3 chạy STP vlan 1 (IOU2 không tham gia STP vlan1 )

+ link giữa port e0/1 (IOU3) và e0/0 (IOU1) unidirectional links.

GIẢ SỬ: e0/1 IOU3 trạng thái block.

Do nguyên nhân nào đó trên IOU2 (như e0/0, e0/1 shutdown, filter BPDU...) làm IOU3 không nhận được BPDU ở port e0/1 --> port e0/1 chuyển từ trạng thái block -> Designated port (sau STP time).

=> Loop có thể xảy ra.

Để tránh trường hợp đó, loop guard sẽ được cấu hình trên e0/1 IOU3. Khi port e0/1 không nhận được BPDU, port sẽ chuyển sang trạng thái Loop Inconsistent (giống với trạng thái block) => tránh loop xảy ra.

Root Guard

=====================================================================

• Protects the topology from "rogue" root switches
• Switch sending superior BPDUs
• Lower priority or MAC than current root
• Better root path cost than the local port

• Configured per-port
• Not vlan-aware

• Block the port upon receiving a superior BPDU
• "Root Inconsistent" state in Spanning-tree
• State cleard after these BPDUs stop arring
• Unlike blocking state, BPDUs are sent out

SW3#show spanning-tree vlan 1

VLAN0001

  Spanning tree enabled protocol ieee

  Root ID    Priority    32769

             Address     aabb.cc00.0100

             Cost        100

             Port        3 (Ethernet0/2)

             Hello Time   2 sec  Max Age 20 sec  Forward Delay 15 sec

  Bridge ID  Priority    32769  (priority 32768 sys-id-ext 1)

             Address     aabb.cc00.0300

             Hello Time   2 sec  Max Age 20 sec  Forward Delay 15 sec

             Aging Time  15  sec

Interface           Role Sts Cost      Prio.Nbr Type

------------------- ---- --- --------- -------- --------------------------------

Et0/1               Altn BLK 100       128.2    Shr

Et0/2               Root FWD 100       128.3    Shr

Et0/3               Desg FWD 100       128.4    Shr

=> SW1 is root with priotiy = 32768

SW1# *Sep 28 08:01:56.165: STP: VLAN0001 Et0/0 tx BPDU: config protocol=ieee

    Data : 0000 00 00 00 8001AABBCC000100 00000000 8001AABBCC000100 8001 0000 1400 0200 0F00

================================================================

Khi them SW4 vao , voi gia tri priority = 4096 is the best. SW3 nhan superior BPDU  = > STP se chay lai toan bo mang va binh chon SW4 lam vai tro root.

De tranh truong hop nay tren e0/3 SW3 tien hanh cau hinh root guard . Khi e0/3 SW3 nhan superior BPDU -> interface e0/3 se o trang thai Root inconsistent ( tuong tu nhu trang thai block) se khong gui BPDU, nhung van nhan. Port se chuyen sang trang thai khac khi khong con nhanh superior BPDU ( no se so sanh 2 BPDU giu SW1 va SW 4 de nhan biet superior BPDU)

 *Sep 28 08:11:14.165: STP: VLAN0001 sent Topology Change Notice on Et0/2

*Sep 28 08:11:14.165: STP[1]: Generating TC trap for port Ethernet0/3

*Sep 28 08:11:17.172: set portid: VLAN0001 Et0/3: new port id 8004

*Sep 28 08:11:17.172: STP: VLAN0001 Et0/3 -> listening

*Sep 28 08:11:17.570: STP: VLAN0001 heard root  4097-aabb.cc00.0400 on Et0/3

*Sep 28 08:11:17.570:     supersedes 32769-aabb.cc00.0100

*Sep 28 08:11:17.570: %SPANTREE-2-ROOTGUARD_BLOCK: Root guard blocking port Ethernet0/3 on VLAN0001.

*Sep 28 08:11:17.570: STP: VLAN0001 Et0/3 -> blocking

*Sep 28 08:11:19.172: STP: VLAN0001 heard root  4097-aabb.cc00.0400 on Et0/3

*Sep 28 08:11:19.172:     supersedes 32769-aabb.cc00.0100

*Sep 28 08:11:21.174: STP: VLAN0001 heard root  4097-aabb.cc00.0400 on Et0/3

*Sep 28 08:11:21.174:     supersedes 32769-aabb.cc00.0100

===============================================================

SW3#show spanning-tree vlan 1

VLAN0001

  Spanning tree enabled protocol ieee

  Root ID    Priority    32769

             Address     aabb.cc00.0100

             Cost        100

             Port        3 (Ethernet0/2)

             Hello Time   2 sec  Max Age 20 sec  Forward Delay 15 sec

  Bridge ID  Priority    32769  (priority 32768 sys-id-ext 1)

             Address     aabb.cc00.0300

             Hello Time   2 sec  Max Age 20 sec  Forward Delay 15 sec

             Aging Time  300 sec

Interface           Role Sts Cost      Prio.Nbr Type

------------------- ---- --- --------- -------- --------------------------------

Et0/1               Altn BLK 100       128.2    Shr

Et0/2               Root FWD 100       128.3    Shr

Et0/3               Desg BKN*100       128.4    Shr *ROOT_Inc

=========================================================================

SW3#show span int e0/3 de

 Port 4 (Ethernet0/3) of VLAN0001 is broken  (Root Inconsistent)

   Port path cost 100, Port priority 128, Port Identifier 128.4.

   Designated root has priority 32769, address aabb.cc00.0100

   Designated bridge has priority 32769, address aabb.cc00.0300

   Designated port id is 128.4, designated path cost 100

   Timers: message age 1, forward delay 0, hold 0

   Number of transitions to forwarding state: 0

   Link type is shared by default

   Root guard is enabled on the port

   BPDU: sent 1, received 42    // chi nhan BPDU cua SW4 nhung k gui BPDU SW1 ra

============================================================

Cau hinh priority SW4 = 40960 (not superior BPDU)

*Sep 28 08:14:13.604: STP: VLAN0001 heard root 40961-aabb.cc00.0400 on Et0/3

*Sep 28 08:14:31.603: %SPANTREE-2-ROOTGUARD_UNBLOCK: Root guard unblocking port Ethernet0/3 on VLAN0001.

*Sep 28 08:14:31.603: STP: VLAN0001 Et0/3 -> listening

*Sep 28 08:14:31.607: STP: VLAN0001 heard root 40961-aabb.cc00.0400 on Et0/3

*Sep 28 08:14:31.612: STP: VLAN0001 Topology Change rcvd on Et0/3

*Sep 28 08:14:31.612: STP: VLAN0001 sent Topology Change Notice on Et0/2

*Sep 28 08:14:46.603: STP: VLAN0001 Et0/3 -> learning

*Sep 28 08:15:01.605: STP[1]: Generating TC trap for port Ethernet0/3

*Sep 28 08:15:01.606: STP: VLAN0001 sent Topology Change Notice on Et0/2

*Sep 28 08:15:01.606: STP: VLAN0001 Et0/3 -> forwarding

=========================================================

SW3#show spanning-tree vlan 1

VLAN0001

  Spanning tree enabled protocol ieee

  Root ID    Priority    32769

             Address     aabb.cc00.0100

             Cost        100

             Port        3 (Ethernet0/2)

             Hello Time   2 sec  Max Age 20 sec  Forward Delay 15 sec

  Bridge ID  Priority    32769  (priority 32768 sys-id-ext 1)

             Address     aabb.cc00.0300

             Hello Time   2 sec  Max Age 20 sec  Forward Delay 15 sec

             Aging Time  300 sec

Interface           Role Sts Cost      Prio.Nbr Type

------------------- ---- --- --------- -------- --------------------------------

Et0/1               Altn BLK 100       128.2    Shr

Et0/2               Root FWD 100       128.3    Shr

Et0/3               Desg FWD 100       128.4    Shr

=========================================================================

SW3#show span int e0/3 de

 Port 4 (Ethernet0/3) of VLAN0001 is designated forwarding

   Port path cost 100, Port priority 128, Port Identifier 128.4.

   Designated root has priority 32769, address aabb.cc00.0100

   Designated bridge has priority 32769, address aabb.cc00.0300

   Designated port id is 128.4, designated path cost 100

   Timers: message age 0, forward delay 0, hold 0

   Number of transitions to forwarding state: 1

   Link type is shared by default

   Root guard is enabled on the port

   BPDU: sent 61, received 91

Note : chi co root moi gui BPDU voi time = 2s. Con nhung SW khac se gui goi tin TCN, config khi topo co su thay doi.