CCIE Routing & Switching v5 Workbook -
CCIE R&S v5 Advanced Technology Labs -
LAN Switching
MST Root Bridge Election
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.
Instance 3 should service all other VLANs.
Ensure the following Root Bridge selection:
Configure SW1 as the STP Root Bridge for instance 1.
Configure SW4 as the STP Root Bridge for instance 2.
If SW1 goes down, SW2 should become the STP Root Bridge for instance 1.
If SW4 goes down, SW3 should become the STP Root Bridge for instance 2.
Use the lowest priority values to achieve task requirements.
Configuration
SW1:
spanning-tree mst configuration
name MST
revision 1
instance 1 vlan 1-100
instance 2 vlan 101-200
instance 3 vlan 201-4094
!
spanning-tree mode mst
spanning-tree mst 1 priority 0
SW2:
spanning-tree mst configuration
name MST
revision 1
instance 1 vlan 1-100
instance 2 vlan 101-200
instance 3 vlan 201-4094
!
spanning-tree mode mst
spanning-tree mst 1 priority 4096
SW3:
spanning-tree mst configuration
name MST
revision 1
instance 1 vlan 1-100
instance 2 vlan 101-200
instance 3 vlan 201-4094
!
spanning-tree mode mst
spanning-tree mst 2 priority 4096
SW4:
spanning-tree mst configuration
name MST
revision 1
instance 1 vlan 1-100
instance 2 vlan 101-200
instance 3 vlan 201-4094
!
spanning-tree mode mst
spanning-tree mst 2 priority 0
Verification
Multiple Spanning-Tree (MST) is an IEEE standard defined in 802.1s which allows
user-defined STP instances to be mapped to multiple VLANs. Unlike the Cisco
proprietary Per-VLAN Spanning-Tree (PVST), MST can be used to eliminate the
overhead of redundant STP instances in topologies where multiple VLANs, but not
all VLANs, follow the same layer 2 forwarding path, while at the same time allowing
for flexible failure domain separation and traffic engineering. MST essentially takes
the best features of IEEE 802.1D Spanning-Tree, AKA Common Spanning-Tree,
and the Cisco extensions to STP, PVST, PVST+, Rapid PVST+, and combines
them.
For example, in this design STP instances are created for VLANs 1-4094. In
Common Spanning-Tree, all 4094 VLANs would map to one instance. This has very
little overhead but does not allow for detailed traffic engineering. With PVST, there
would be 4094 separate instances of STP, which allows for detailed traffic
engineering but creates immense overhead in the control-plane. With MST, three
user-defined instances are created that map different portions of the VLAN space
into separate instances with a similar forwarding path.
Like CST and PVST, MST uses the lowest Bridge-ID (BID) in the network to elect
the Root Bridge. The BID is made up of the priority value and the MAC address. The
lower priority wins the election, and if there is a tie in priority the lowest MAC
address is the tie breaker. In PVST, there is one root bridge election per VLAN,
because there is one STP instance per VLAN, but in MST there is one election per
user-defined instance.
From the
show spanning-tree mst
output, we can see which VLANs are mapped to
the particular MST instance, who the root bridge is, and how the root port election
has occurred. In this case, SW1 is the root for instance 1, whereas SW4 is the root
for instance 2. SW1 is the root for instance 1 because it has a priority value of 1,
which is made up of the configured priority of 0 plus the sytem-id extension of 1. In
MST the sysid field is the instance number, whereas in PVST the sysid is the VLAN
number.
Verify the MST configuration and that switches run in MST mode, for example on
SW1:
SW1#show spanning-tree mst configuration
Name[MST]
Revision 1Instances configured 4
Instance
Vlans mapped
--------
---------------------------------------------------------------------
0
none
1
1-100
2
101-200
3
201-4094
-------------------------------------------------------------------------------
!
!SW1#show spanning-tree summary
Switch is in mst mode (IEEE Standard)
Root bridge for: MST1
Extended system IDis 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
UplinkFast
is disabled
BackboneFast
is disabled
Configured Pathcost method used is short (Operational value is long)
NameBlocking Listening Learning Forwarding STP Active
---------------------- -------- --------- -------- ---------- ----------
MST010045
MST100055
MST230014
---------------------- -------- --------- -------- ---------- ----------
3 msts4001014
Verify that SW1 is Root Bridge for instance 1 and SW4 is Root Bridge for instance 2,
thus both have all ports as Designated Forwarding for the respective instance:
SW1#show spanning-tree mst 1
##### MST1vlans mapped: 1-100
Bridgeaddress 0013.605f.f000 priority1(0 sysid 1)
Rootthis switch for MST1
InterfaceRole Sts CostPrio.Nbr Type
---------------- ---- --- --------- -------- --------------------------------
Fa0/1Desg FWD 200000
128.3 P2p
Fa0/19Desg FWD 200000
128.21 P2p
Fa0/20Desg FWD 200000
128.22 P2p
Fa0/23Desg FWD 200000
128.25 P2p
Fa0/24Desg FWD 200000
128.26 P2p
!
!SW4#show spanning-tree mst 2
##### MST2vlans mapped: 101-200
Bridgeaddress 001a.a174.2500 priority2(0 sysid 2)
Rootthis switch for MST2
InterfaceRole Sts CostPrio.Nbr Type
---------------- ---- --- --------- -------- --------------------------------
Fa0/19Desg FWD 200000
128.21 P2p
Fa0/20Desg FWD 200000
128.22 P2p
Fa0/23Desg FWD 200000
128.25 P2p
Fa0/24Desg FWD 200000
128.26 P2p
MSTP does not generate per-instance BPDUs as RSTP did with per-VLAN BPDUs.
Instead, it uses a single BPDU which has additional records to advertise STP data
for all configured instances. Even though with our configuration no VLANs are
assigned to the default MST instance of 0 (to which by default all VLANs are
assigned), STP still runs for this instance, as STP actually runs over instance zero
which forms the IST, or CIST in the case of multiple MST regions. As we did not
modify the default MST priority for instance zero, in this case SW2 was selected as
the CIST Root Bridge and also the Regional Root as MST is configured for a single
region, but this may depend on your rack assignment:
SW2#show spanning-tree mst 0
##### MST0vlans mapped: none
Bridgeaddress 000a.b832.3a80 priority32768 (32768 sysid 0)
Rootthis switch for the CIST)
Operational hello time 2 , forward delay 15, max age 20, txholdcount 6
Configuredhello time 2 , forward delay 15, max age 20, max hops 20
InterfaceRole Sts CostPrio.Nbr Type
---------------- ---- --- --------- -------- --------------------------------
Fa0/19Desg FWD 200000
128.21 P2p
Fa0/20Desg FWD 200000
128.22 P2p
Fa0/23Desg FWD 200000
128.25 P2p
Fa0/24Desg FWD 200000
128.26 P2p
!
!SW1#show spanning-tree mst
##### MST0vlans mapped: none
Bridgeaddress 0013.605f.f000
priority
32768 (32768 sysid 0)
Rootaddress 000a.b832.3a80
priority
32768 (32768 sysid 0)
portFa0/23path cost0
Regional Root address 000a.b832.3a80 priority32768 (32768 sysid 0)
internal cost 200000 rem hops 19
Operational hello time 2 , forward delay 15, max age 20, txholdcount 6
Configuredhello time 2 , forward delay 15, max age 20, max hops 20
InterfaceRole Sts CostPrio.Nbr Type
---------------- ---- --- --------- -------- --------------------------------
Fa0/1Desg FWD 200000
128.3 P2p
Fa0/19Desg FWD 200000
128.21 P2p
Fa0/20Desg FWD 200000
128.22 P2p
Fa0/23Root FWD 200000
128.25 P2p
Fa0/24Altn BLK 200000
128.26 P2p
##### MST1vlans mapped: 1-100
Bridgeaddress 0013.605f.f000 priority1(0 sysid 1)Rootthis switch for MST1
InterfaceRole Sts CostPrio.Nbr Type
---------------- ---- --- --------- -------- --------------------------------
Fa0/1Desg FWD 200000
128.3 P2p
Fa0/19Desg FWD 200000
128.21 P2p
Fa0/20Desg FWD 200000
128.22 P2p
Fa0/23Desg FWD 200000
128.25 P2p
Fa0/24Desg FWD 200000
128.26 P2p
##### MST2vlans mapped: 101-200
Bridgeaddress 0013.605f.f000 priority32770 (32768 sysid 2)
Rootaddress 001a.a174.2500 priority2(0 sysid 2)
portFa0/19cost400000 rem hops 18
InterfaceRole Sts CostPrio.Nbr Type
---------------- ---- --- --------- -------- --------------------------------
Fa0/19Root FWD 200000128.21 P2p
Fa0/20Altn BLK 200000
128.22 P2p
Fa0/23Altn BLK 200000
128.25 P2p
Fa0/24Altn BLK 200000
128.26 P2p
Test that if SW1 is offline, SW2 becomes the Root Bridge for instance 1:
SW1#configure terminal
Enter configuration commands, one per line. End with CNTL/Z.
SW1(config)#interface range fastEthernet0/19 - 20, fastEthernet0/23 - 24
SW1(config-if-range)#shutdown
!
!SW2#show spanning-tree mst 1
##### MST1vlans mapped: 1-100
Bridgeaddress 000a.b832.3a80 priority4097 (4096 sysid 1)
Rootthis switch for MST1
InterfaceRole Sts CostPrio.Nbr Type
---------------- ---- --- --------- -------- --------------------------------
Fa0/19Desg FWD 200000
128.21 P2p
Fa0/20Desg FWD 200000
128.22 P2p