=====================================================================
- 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.
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