ccie review…

one of the things that i work on as part of studying for ccie r&s is rewriting the ccie r&s quick review…

here is a sample page:



IEEE Standard developed to allow for multiple vlan’s to be grouped to one, or more instances, versus a tree for every vlan. With fewer trees, resources are conserved. Other advantages are load sharing and path optimization. The primary disadvantage is involved configuration.

MST is backwardly compatible due to it’s implementation of a common tree.


Globally enable

spanning-tree mode mst

Enter MST configuration mode

spanning-tree mst configuration

Create the region name

name name

Create a revision number

revision number

Map vlans to instances

instance x vlan range



show spanning-tree mst configurations

show spanning-tree mst vlan_id


see also:


Loop Guard

Loop guard is, naturally, a loop prevention mechanism. It’s role is to detect the lack of BPDU reception on a Loop Guard enabled port. BPDU’s may suddenly go missing on a port that has been rendered unidirectional or that is experiencing congestion, for instance.

Loop Guard is functionally equivalent to UDLD with the primary difference being Loop Guard is per vlan whereas UDLD is per port. Loop Guard must be configured on a p2p port that is currently bidirectional.

Loop Guard, if enabled, will place an alternate or backup port into loop-inconsistent state upon the loss of BPDU’s instead of the port progressing to the other STP states (Listening/Learning/Forwarding)

Loop Guard is enabled globally with:

spanning-tree loopguard default


see also:



Unidirectional Link Detection. As its name states UDLD identifies ports that have become unidirectional. In plain language, when traffic sent by a local device is received, but not returned, the port is shut down if in aggressive mode, and considered undetermined if in normal mode. By default on fiber interfaces, UDLD is enabled and for UDLD to be effective both sides of the link have to be supported. UDLD can be enabled globally or on a port basis.


see also: