v5 Written and Lab: Optional Spanning-Tree Features Notes
v5 Written:
2.1.f Implement and troubleshoot spanning-tree
2.1.f [i] PVST+ / RPVST+ / MST
2.1.f [ii] Switch priority, port priority, path cost, STP timers
2.1.f [iii] port fast, BPDUguard, BPDUfilter
2.1.f [iv] loopguard, rootguard
v5 Lab:
1.1.f Implement and troubleshoot spanning-tree
1.1.f [i] PVST+ / RPVST+ / MST
1.1.f [ii] switch priority, port priority, path cost, STP timers
1.1.f [iii] port fast, BPDUguard, BPDUfilter
1.1.f [iv] loopguard, rootguard
Documentation:
Catalyst 3750-X and 3560-X Software Configuration Guide, Release 15.0(1)SE,
Chapter 22, Configuring Optional Spanning-Tree Features, pgs 22-1 to 22-19
Books:
CCIE Routing and Switching Exam Certification Guide 4th Ed; Chapter
INE:
Port Fast
– immediately brings an interface configured as an access or trunk port to the forwarding
state
– bypasses the listening and learning states
– interfaces should not receive BPDUs
conf t
int fa0/1
spanning-tree portfast
conf t
int fa0/1
spanning-tree portfast trunk
conf t
spanning-tree portfast default
show spanning-tree int fa0/1 portfast
BPDU Guard
– shuts down ports that are in the Port Fast operational state when BPDUs are received
– port is placed in an err-disabled state
– can be configured to shutdown the the VLAN configured on the port instead of the port
– when configured globally, only affects ports in the Port Fast operational state
– can be configured on the interface without enabling Port Fast
conf t
spanning-tree portfast bpduguard default
int fa0/1
spanning-tree portfast
conf t
int fa0/1
spanning-tree bpduguard enable
conf t
errdisable detect cause bpduguard shutdown vlan
BPDU Filter
– prevents interfaces that are in a Port Fast operational state from sending or receiving
BPDUs
– if a BPDU is received, the interface loses its Port Fast operational state and BPDU
filtering is disabled
– when configured globally, only affects ports in the Port Fast operational state
– can be configured on the interface without enabling Port Fast
– enabling BPDU filtering on an interface is the same as disabling spanning tree and can
result in spanning tree loops
conf t
spanning-tree portfast bpdufilter default
int fa0/1
spanning-tree portfast
conf t
int fa0/1
spanning-tree bpdufilter enable
Uplink Fast
– accelerates the choice of a new root port when a link or switch fails or when spanning
tree reconfigures
– the root port transitions to the forwarding state immediately
– does not go through the listening or learning state
– when spanning tree reconfigures the new root port, the other interfaces flood the
network with multicast packets
– the multicast packets include each address learned on the interface
– the multicast traffic can be reduced by reducing the max-update-rate parameter
– Uplink Fast is most useful at the access layer or the edge or the network
– Uplink Fast is not appropriate for backbone devices
conf t
spanning-tree uplinkfast
conf t
spanning-tree uplinkfast max-update-rate
show spanning-tree summary
Backbone Fast
– detects indirect failures in the core of the backbone
– complementary technology to the Uplink Fast feature
– optimizes the maximum-age timer, which controls the amount of time the switch stores
protocol information received on the interface
conf t
spanning-tree backbonefast
show spanning-tree summary
EtherChannel Guard
– detects and EtherChannel misconfiguration between the switch and the connected device
– places the interface in the error-disabled state
conf t
spanning-tree etherchannel guard misconfig
show spanning-tree summary
show interfaces status err-disabled
show etherchannel summary
Root Guard
– configured on switches that are not supposed to be the root switch
– if spanning tree calculations select the switch as the root switch, root guard places
the interface in the root-inconsistent (blocked) state, which prevents the switch from
becoming the root switch or being in the path to the root
conf t
int fa0/1
spanning-tree guard root
Loop Guard
– prevents alternate or root ports from becoming designated ports because of a failure
that leads to a unidirectional link
– the feature is most effective when it is enable on the entire switching network
– prevents alternate and root ports from becoming designated ports
– BPDUs are not sent on alternate or root ports
– in MST mode, BPDUs are not sent on nonboundary ports only if the interface is blocked by
loop guard in all MST instances
– on a boundary port, loop guard blocks the interface in all MST instances
conf t
spanning-tree loopguard default
show spanning-tree active
show spanning-tree mst
Default Optional Spanning-Tree configuration
– Port Fast: globally disabled
– BPDU filtering: globally disabled
– BPDU guard: globally disabled
– Uplink Fast: globally disabled
– Backbone Fast: globally disabled
– EtherChannel Guard: globally ENABLED
– Root Guard: disabled on all interfaces
– Loop Guard: disabled on all interfaces
show spanning-tree active
show spanning-tree detail
show spanning-tree interface fa0/1
show spanning-tree mst interface fa0/1
show spanning-tree summary
show spanning-tree summary totals
