CCIE Routing & Switching v5 Workbook -
CCIE R&S v5 Advanced Technology Labs -
LAN Switching
MST Path Selection with Port Cost
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 Multiple Spanning-Tree on SW1 - SW4 as follows:
Use region named MST and a revision of 1.
Instance 1 should service VLANs 1 - 100.
Instance 2 should service VLANs 101 - 200.
Configure SW1 as the STP Root Bridge for instance 1, using the lowest
priority value.
Using Spanning-Tree cost, modify the Layer 2 transit network so that traffic for MST
instance 1 from SW2 to SW1 uses the last link between SW2 and SW4.
If this link goes down, traffic should fall over to the remaining link between
SW2 and SW4.
Configuration
SW1 - SW4:
spanning-tree mst configuration
name MST
revision 1
instance 1 vlan 1-100
instance 2 vlan 101-200
!
spanning-tree mode mst
SW1:
spanning-tree mst 1 priority 0
SW2:
interface range FastEthernet0/23 - 24
spanning-tree mst 1 cost 500000
!
interface FastEthernet0/19
spanning-tree mst 1 cost 2
!
interface FastEthernet0/20
spanning-tree mst 1 cost 1
Verification
Similar to CST and PVST, MST uses a cost value derived from the inverse
bandwidth of the interface (higher bandwidth means lower cost). The root port is
chosen based on the lowest end-to-end cost to the root bridge. The
show spanning-
tree mst
command shows the local cost values of the outgoing ports on the local switch.
Verify that initially, SW2’s Root Port for instance 1 is the direct Fa0/23 link to SW1:
SW2#show spanning-tree mst 1
##### MST1vlans mapped: 1-100
Bridgeaddress 000a.b832.3a80
priority
4097 (4096 sysid 1)
Rootaddress 0013.605f.f000
priority
1(0 sysid 1)
portFa0/23cost200000 rem hops 19
InterfaceRole Sts CostPrio.Nbr Type
---------------- ---- --- --------- -------- --------------------------------
Fa0/19Desg FWD 200000
128.21 P2p
Fa0/20Desg FWD 200000
128.22 P2p Fa0/23Root FWD 200000 128.25 P2p
Fa0/24Altn BLK 200000128.26 P2p
Configure the STP cost on SW2 and verify now that SW2’s Root Port changed to its
Fa0/20 link towards SW4:
SW2#show spanning-tree mst 1
##### MST1vlans mapped: 1-100
Bridgeaddress 000a.b832.3a80
priority
4097 (4096 sysid 1)
Rootaddress 0013.605f.f000
priority
1(0 sysid 1)
portFa0/20cost400001 rem hops 17
InterfaceRole Sts CostPrio.Nbr Type
---------------- ---- --- --------- -------- --------------------------------
Fa0/19Altn BLK 2
128.21 P2p Fa0/20Root FWD 1128.22 P2p
Fa0/23Altn BLK 500000
128.25 P2p
Fa0/24Altn BLK 500000
128.26 P2p
To see the entire end-to-end cost of a path, the
show spanning-tree mst 1 detail
command should be used. The end-to-end cost is made up of the upstream
(designated) cost plus the local port cost. In this output, the alternate ports Fa0/23 –
Fa0/24 have a total cost of 500.000 because of the manual cost change. Fa0/19 has
a total cost of 400.002, which is 2 to SW4, 200.000 from SW4 to SW3, and 200.000
from SW3 to SW1. Fa0/20 wins the Root Port election because it has a total cost of
400.001.
SW2#show spanning-tree mst 1 detail
##### MST1vlans mapped: 1-100
Bridgeaddress 000a.b832.3a80
priority
4097 (4096 sysid 1)
Rootaddress 0013.605f.f000
priority
1(0 sysid 1)
portFa0/20cost400001 rem hops 17
FastEthernet0/19 of MST1 is alternate blocking
Port infoport id128.21 priority 128 cost2
Designated root
address 0013.605f.f000
priority1 cost400000
Designated bridge
address 001a.a174.2500
priority 32769 port id 128.21
Timers: message expires in 5 sec, forward delay 0, forward transitions 2
Bpdus (MRecords) sent 147, received 142
FastEthernet0/20 of MST1 is root forwarding
Port infoport id128.22 priority 128 cost1
Designated root
address 0013.605f.f000
priority1 cost400000
Designated bridge
address 001a.a174.2500
priority 32769 port id 128.22
Timers: message expires in 5 sec, forward delay 0, forward transitions 2
Bpdus (MRecords) sent 146, received 142
FastEthernet0/23 of MST1 is alternate blocking
Port infoport id128.25 priority 128 cost500000
Designated root
address 0013.605f.f000
priority
1 cost0
Designated bridge
address 0013.605f.f000
priority
1 port id 128.25
Timers: message expires in 5 sec, forward delay 0, forward transitions 2
Bpdus (MRecords) sent 146, received 145
FastEthernet0/24 of MST1 is alternate blocking
Port infoport id128.26 priority 128 cost500000
Designated root
address 0013.605f.f000
priority
1 cost0
Designated bridge
address 0013.605f.f000
priority
1 port id 128.26
Timers: message expires in 5 sec, forward delay 0, forward transitions 0
Bpdus (MRecords) sent 147, received 144
When SW2’s port Fa0/20 is down, the next lowest cost path is 400.002 through
Fa0/19:
SW2#configure terminal
Enter configuration commands, one per line. End with CNTL/Z.SW2(config)#interface fastEthernet0/20
SW2(config-if)#shutdown
!
!SW2#show spanning-tree mst 1
##### MST1vlans mapped: 1-100
Bridgeaddress 000a.b832.3a80
priority
4097 (4096 sysid 1)
Rootaddress 0013.605f.f000
priority
1(0 sysid 1)
portFa0/19cost400002 rem hops 17
InterfaceRole Sts CostPrio.Nbr Type
---------------- ---- --- --------- -------- --------------------------------Fa0/19Root FWD
2128.21 P2p
Fa0/23Altn BLK 500000
128.25 P2p
Fa0/24Altn BLK 500000
128.26 P2p