CCIE Routing & Switching v5 Workbook -
CCIE R&S v5 Advanced Technology Labs -
LAN Switching
STP BackboneFast
A Note On Section Initial Configuration Files: You must load the
initial configuration files for the section, named LAN Switching Initial
Spanning Tree, which can be found in
. Reference the Virtual Routers &
Physical Switches Diagram to complete this task.
Task
Configure Spanning-Tree BackboneFast so that if the links between SW1 and SW2
go down, SW4 immediately expires its maxage timer and begins Spanning-Tree
reconvergence.
Configuration
SW1 - SW4:
spanning-tree backbonefast
Verification
The Cisco proprietary BackboneFast feature is used to speed up convergence when
an indirect failure occurs upstream in the network by immediately expiring the
max_age timer. The feature is useful when running legacy 802.1d STP, because
RSTP and MSTP standards were designed to have a built-in functionality to resolve
the max_age timer expiry issue. It must be enabled on all switches in the Layer 2
topology to be functional.
Based on configured STP priorities from initial configurations, in this design, SW4’s
Root Port is toward SW2 on Fa0/19. If Fa0/19 goes down, Fa0/20 toward SW2 will
be selected as the new Root Port.
SW4#show spanning-tree vlan 2
VLAN0002
Spanning tree enabled protocol ieeeRoot ID Priority 4098
Address0013.605f.f000
Cost38Port21 (FastEthernet0/19)
Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec
Bridge ID Priority8194 (priority 8192 sys-id-ext 2)
Address001a.a174.2500
Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec
Aging Time 300 sec
InterfaceRole Sts CostPrio.Nbr Type
------------------- ---- --- --------- -------- --------------------------------
Fa0/19Root FWD 19
128.21 P2p
Fa0/20Altn BLK 19
128.22 P2p
Fa0/23Altn BLK 19
128.25 P2p
Fa0/24Altn BLK 19
128.26 P2p
Before configuring BackboneFast, enable conditional STP events debugging for
VLAN 2 on SW4 and shut down both Ethernet ports between SW2 and SW1. Check
the log messages on SW4 and note that it ignores the inferior BPDU messages
received from SW2 until the max_age timer of 20 seconds expires, allowing SW4 to
age out the old root bridge stored BPDU on the respective port. SW2 sends inferior
BPDU messages claiming itself as root bridge because it is isolated as it lost its
Root Port.
SW4#debug spanning-tree events
Spanning Tree event debugging is on
!SW4#debug condition vlan 2
Condition 1 set
!SW4#configure terminal
Enter configuration commands, one per line. End with CNTL/Z.
SW4(config)#service timestamps debug datetime msec
!
!SW2#configure terminal
Enter configuration commands, one per line. End with CNTL/Z.
SW2(config)#interface range fastEthernet0/23 - 24
SW2(config-if-range)#shutdown
!
!*Mar 3 02:45:30.829: STP: VLAN0002 heard root 16386-000a.b832.3a80 on Fa0/19
*Mar 3 02:45:30.829: STP: VLAN0002 heard root 16386-000a.b832.3a80 on Fa0/20
SW4#
*Mar 3 02:45:31.895: STP: VLAN0002 heard root 16386-000a.b832.3a80 on Fa0/19
*Mar 3 02:45:31.895: STP: VLAN0002 heard root 16386-000a.b832.3a80 on Fa0/20
SW4#
*Mar 3 02:45:33.891: STP: VLAN0002 heard root 16386-000a.b832.3a80 on Fa0/19
*Mar 3 02:45:33.891: STP: VLAN0002 heard root 16386-000a.b832.3a80 on Fa0/20
SW4#
*Mar 3 02:45:35.904: STP: VLAN0002 heard root 16386-000a.b832.3a80 on Fa0/19
*Mar 3 02:45:35.904: STP: VLAN0002 heard root 16386-000a.b832.3a80 on Fa0/20
SW4#
*Mar 3 02:45:37.909: STP: VLAN0002 heard root 16386-000a.b832.3a80 on Fa0/19
*Mar 3 02:45:37.909: STP: VLAN0002 heard root 16386-000a.b832.3a80 on Fa0/20
SW4#
*Mar 3 02:45:39.914: STP: VLAN0002 heard root 16386-000a.b832.3a80 on Fa0/19
*Mar 3 02:45:39.914: STP: VLAN0002 heard root 16386-000a.b832.3a80 on Fa0/20
SW4#
*Mar 3 02:45:41.911: STP: VLAN0002 heard root 16386-000a.b832.3a80 on Fa0/19
*Mar 3 02:45:41.911: STP: VLAN0002 heard root 16386-000a.b832.3a80 on Fa0/20
SW4#
*Mar 3 02:45:43.924: STP: VLAN0002 heard root 16386-000a.b832.3a80 on Fa0/19
*Mar 3 02:45:43.924: STP: VLAN0002 heard root 16386-000a.b832.3a80 on Fa0/20
SW4#
*Mar 3 02:45:45.929: STP: VLAN0002 heard root 16386-000a.b832.3a80 on Fa0/19
*Mar 3 02:45:45.929: STP: VLAN0002 heard root 16386-000a.b832.3a80 on Fa0/20
SW4#
*Mar 3 02:45:47.934: STP: VLAN0002 heard root 16386-000a.b832.3a80 on Fa0/19
*Mar 3 02:45:47.934: STP: VLAN0002 heard root 16386-000a.b832.3a80 on Fa0/20
After max_age timer expires, SW4 starts reconverging its STP port states and finally
selects the new Root Port to be Fas0/23, its connection to SW3.
*Mar 3 02:45:48.814: STP: VLAN0002 new root port Fa0/20, cost 38
*Mar 3 02:45:48.814: STP: VLAN0002 Fa0/20 -> listening
*Mar 3 02:45:48.814: STP: VLAN0002 new root port Fa0/23, cost 38
*Mar 3 02:45:48.814: STP: VLAN0002 Fa0/23 -> listening
SW4#
*Mar 3 02:45:49.855: STP: VLAN0002 Topology Change rcvd on Fa0/19
*Mar 3 02:45:49.855: STP: VLAN0002 sent Topology Change Notice on Fa0/23
SW4#
*Mar 3 02:46:03.822: STP: VLAN0002 Fa0/20 -> learning
*Mar 3 02:46:03.822: STP: VLAN0002 Fa0/23 -> learning
SW4#
*Mar 3 02:46:18.829: STP[2]: Generating TC trap for port FastEthernet0/20
*Mar 3 02:46:18.829: STP: VLAN0002 sent Topology Change Notice on Fa0/23
*Mar 3 02:46:18.829: STP: VLAN0002 Fa0/20 -> forwarding
*Mar 3 02:46:18.829: STP[2]: Generating TC trap for port FastEthernet0/23
*Mar 3 02:46:18.829: STP: VLAN0002 Fa0/23 -> forwarding
!
!SW4#show spanning-tree vlan 2
VLAN0002
Spanning tree enabled protocol ieee
Root IDPriority
4098
Address
0013.605f.f000
Cost
38
Port
25 (FastEthernet0/23)
Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec
Bridge ID Priority8194 (priority 8192 sys-id-ext 2)
Address001a.a174.2500
Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec
Aging Time 300 sec
InterfaceRole Sts CostPrio.Nbr Type
------------------- ---- --- --------- -------- --------------------------------
Fa0/19Desg FWD 19
128.21 P2p
Fa0/20Desg FWD 19
128.22 P2p Fa0/23Root FWD 19128.25 P2p
Fa0/24Altn BLK 19128.26 P2p
Re-enable SW2’s Ethernet links to SW1, Fa0/23, and Fa0/24, configure
BackboneFast on all switches, enable STP BackboneFast debugging, and test the
same scenario again. Note that as soon as SW4 receives the inferior BPDU from
SW2, it no longer waits for max_age timer to expire; instead, SW4 sends RLQ (Root
Link Query) messages out on all its non-designated ports.
SW4#debug spanning-tree backbonefast
Spanning Tree backbonefast general debugging is on
!SW4#debug spanning-tree backbonefast detail
Spanning Tree backbonefast detail debugging is on
!SW2#configure terminal
Enter configuration commands, one per line. End with CNTL/Z.
SW2(config)#interface range fastEthernet0/23 - 24
SW2(config-if-range)#shutdown
!
!
*Mar 3 06:40:27.876: STP: VLAN0002 heard root 16386-000a.b832.3a80 on Fa0/19
*Mar 3 06:40:27.876: STP FAST: received inferior BPDU on VLAN0002 FastEthernet0/19.
*Mar 3 06:40:27.876: STP FAST: sending RLQ request PDU on VLAN0002(2) Fa0/20 Vlan2
*Mar 3 06:40:27.876: STP FAST: sending RLQ request PDU on VLAN0002(2) Fa0/23 Vlan2
*Mar 3 06:40:27.876: STP: VLAN0002 heard root 16386-000a.b832.3a80 on Fa0/20
*Mar 3 06:40:27.885: STP FAST: received inferior BPDU on VLAN0002 FastEthernet0/20.
*Mar 3 06:40:27.885: STP FAST: sending RLQ request PDU on VLAN0002(2) Fa0/23 Vlan2
*Mar 3 06:40:27.885: STP FAST: sending RLQ request PDU on VLAN0002(2) Fa0/24 Vlan2
SW4 receives a negative RLQ response from SW2 (the root bridge is not accessible
via SW2) and positive RLQ response from SW3 (the root bridge is still accessible
via SW3).
*Mar 3 06:40:27.901: STP FAST: Received RLQ response PDU on VLAN0002 FastEthernet0/20.
*Mar 3 06:40:27.910: STP FAST: Received RLQ response PDU on VLAN0002 FastEthernet0/23.
*Mar 3 06:40:27.910: STP FAST: received RLQ response PDU was expected on VLAN0002 FastEthernet0/23 - resp root id 4
*Mar 3 06:40:27.910: STP FAST: Received RLQ response PDU on VLAN0002 FastEthernet0/24.
*Mar 3 06:40:27.910: STP FAST: received RLQ response PDU was expected on VLAN0002 FastEthernet0/24 - resp root id 4
*Mar 3 06:40:27.910: STP FAST: received_rlq_bpdu on VLAN0002 FastEthernet0/19 - making FastEthernet0/19 a designate
*Mar 3 06:40:27.910: STP FAST: received_rlq_bpdu on VLAN0002 FastEthernet0/20 - making FastEthernet0/20 a designate
Based on received RLQ responses, SW4 will mark its ports to SW2 as designated
and will select its new Root Port via SW3. All ports, however, still transition through
regular STP port states, listening, learning, and forwarding or blocking.
*Mar 3 06:40:27.910: STP: VLAN0002 new root port Fa0/23, cost 38
*Mar 3 06:40:27.910: STP: VLAN0002 Fa0/23 -> listening
*Mar 3 06:40:27.910: STP: VLAN0002 Fa0/20 -> listening
*Mar 3 06:40:27.910: STP FAST: Received RLQ response PDU on VLAN0002 FastEthernet0/23.
*Mar 3 06:40:27.910: STP FAST: Received RLQ response PDU on VLAN0002 FastEthernet0/24.
*Mar 3 06:40:27.910: STP FAST: Received RLQ response PDU on VLAN0002 FastEthernet0/23.
*Mar 3 06:40:27.910: STP FAST: Received RLQ response PDU on VLAN0002 FastEthernet0/24.
*Mar 3 06:40:27.910: STP FAST: Received RLQ response PDU on VLAN0002 FastEthernet0/23.
*Mar 3 06:40:27.910: STP FAST: Received RLQ response PDU on VLAN0002 FastEthernet0/24.
*Mar 3 06:40:28.405: STP: VLAN0002 Topology Change rcvd on Fa0/19
*Mar 3 06:40:28.405: STP: VLAN0002 sent Topology Change Notice on Fa0/23
*Mar 3 06:40:42.917: STP: VLAN0002 Fa0/23 -> learning
*Mar 3 06:40:42.917: STP: VLAN0002 Fa0/20 -> learning
*Mar 3 06:40:57.924: STP[2]: Generating TC trap for port FastEthernet0/23
*Mar 3 06:40:57.924: STP: VLAN0002 sent Topology Change Notice on Fa0/23
*Mar 3 06:40:57.924: STP: VLAN0002 Fa0/23 -> forwarding
*Mar 3 06:40:57.924: STP[2]: Generating TC trap for port FastEthernet0/20
*Mar 3 06:40:57.924: STP: VLAN0002 Fa0/20 -> forwarding
!
!SW4#show spanning-tree vlan 2
VLAN0002
Spanning tree enabled protocol ieee
Root IDPriority
4098
Address
0013.605f.f000
Cost
38
Port
25 (FastEthernet0/23)
Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec
Bridge ID Priority8194 (priority 8192 sys-id-ext 2)
Address001a.a174.2500
Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec
Aging Time 300 sec
InterfaceRole Sts CostPrio.Nbr Type
------------------- ---- --- --------- -------- --------------------------------
Fa0/19Desg FWD 19
128.21 P2p
Fa0/20Desg FWD 19
128.22 P2p Fa0/23Root FWD 19128.25 P2p
Fa0/24Altn BLK 19128.26 P2p
Verify that BackboneFast is enabled, and also note the exchange of RLQ
messages; for example, on SW4:
SW4#show spanning-tree summary
Switch is in pvst mode
Root bridge for: none
Extended system ID
is enabled
Portfast Default
is disabled
PortFast BPDU Guard Default
is disabled
Portfast BPDU Filter Default is disabled
Loopguard Defaultis disabled
EtherChannel misconfig guard is enabled
UplinkFastis disabledBackboneFastis enabled
Configured Pathcost method used is short
NameBlocking Listening Learning Forwarding STP Active
---------------------- -------- --------- -------- ---------- ----------
VLAN000110045
VLAN000210034
VLAN000510034
VLAN000710034
VLAN000810034
VLAN000910034
VLAN001010034
VLAN002210034
VLAN004310034
VLAN005810034
VLAN006710034
VLAN007910034
VLAN014610034
---------------------- -------- --------- -------- ---------- ----------
13 vlans13004053
BackboneFast statistics
-----------------------
Number of transition via backboneFast (all VLANs): 13
Number of inferior BPDUs received (all VLANs): 26
Number of RLQ request PDUs received (all VLANs): 0
Number of RLQ response PDUs received (all VLANs): 117
Number of RLQ request PDUs sent (all VLANs): 65
Number of RLQ response PDUs sent (all VLANs): 0