Daily Archives: October 11, 2015

SWITCH 300-115 1.6 Configure and verify spanning tree

1.6.a PVST+, RPVST+, MST


In order to facilitate a loop free topology a root switch is elected as a reference point for the entire tree. This is accomplished by establishing a BID (bridge id) for every switch in the diameter. A bridge ID is an 8 byte construct composed of 2 bytes of priority, and 6 bytes MAC address. Further, the priority is segmented into 4 bits priority and 12 bits extended system id, where the extended system id is the VLAN ID.


Spanning tree enabled protocol rstp

Root ID Priority 32778

Address 0009.b73f.ce80

Cost 12

Port 64 (Port-channel2)

Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec

Bridge ID Priority 32778 (priority 32768 sys-id-ext 10)

32768 16384 8192 4096 2048 1024 512 256 128 64 32 16 8 4 2 1

here is the binary math with the example vlan 10

1000 0000 0000 1010

32768 + 8 + 2


SWITCH 300-115: 1.3 Configure and verify VLANs

1.3.c Normal, extended VLAN, voice VLAN

Normal, extended, voice vlan VIDEO


Normal Vlans

The range of vlans between 1 and 1005 is considered normal, however vlan 1 is obviously special, and 1002 through 1005 are reserved for ancient technologies like FDDI and token ring that nobody uses anymore and these  cannot be modified.
There are quite a few parameters that can be set on the vlan as shown here:

config t
vlan 10

But mostly you will give the vlan an id and name and little else. Private-vlan and remote-span might be an occasional use case.

Extended Vlans

Extended VLANs are VLANs that fall in the range 1006 to 4094. They are mainly used in service provider networks to allow for the provisioning of large numbers of customers.
Extended Vlans differ from normal vlans because they have higher numbers.
Extended Vlans must be configured in VTP transparent mode
Extended Vlans are saved to the running-config

If you are actually in need of that many Vlans on your enterprise network you should have your head examined just before rewriting your resume.

There is one interesting note here.

Getting ahead a little, if you make a switchport a routed port with:

int f0/22
no switchport
ip add
no shut

You will see that the routed port will use up the next available extended vlan id, in this case 1006:
show vlan internal usage.
Something to be aware of in case you suddenly go off the deep end and make a thousand vlans.

Voice Vlan.

As we discussed in the Vlan database video, an access port may be provisioned for both data and voice with:
config t
vlan 10
name data
vlan 25
name voice
int f0/21
sw mode acc
sw acc vlan 10
sw voice vlan 25
show vlan brief