Thursday, January 27, 2011

CCNP - SWITCH - Spanning-tree Protocol notes

- Once a root bridge is elected, Configuration BPDU's are sent only by the Root Bridge. All the other switches must forward the BPDU's (or relay them) adding their own send Bridge ID to the Message.

- On a config output if you see 'Spanning tree enabled protocol ieee' it indicates ieee 802.1d (Common SPanning-Tree) is in use REGARDLESS of the ROLES listed.
- Enable Loop Guard on root ports and alternate ports
- If a BPDU is received on a Loop Guard port that is in an inconsistant state the port will transition to the appropriate state as determined by the normal function of the spanning tree verion/type in place
- Root Guard is enabled on a per-port basis, it re-enables a switch port once it stops receiving superior BPDU's
- When you have a link that is configured as Half-duplex on one end and Full on the other the following situations could arise:
 i) The switch with the Full Duplex setting could unblock it's port potentially creating a loop
 ii) The switch with the Full Duplex port will not be performing CSMA/CD
 iii) BPDU's amy not successfully negotiate port states on the link between the two switches.
- A switch running 802.1d (cst) will use the Forward delay timer to age out entries in the MAC address table when a tology change is received.
- MST extends 802.1w to multiple spanning trees.

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