Chapter 5 - Network Engineering@CAVC

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LAN Switching & Wireless


Chapter
5


STP
-

Key concepts

5.1.2

When operating a network of switches
with

redundant connections
, what is the effect of turning off
STP?

An endless loop is created.

5.2.1

The Spanning Tree algorithm operates with the
following rules:



One root bridge (switch) per network



One root port on
every

non
-
root bridge



Root ports face the root bridge (exit port of the least
-
cost path to root bridge)



Every

port on a root bridge is a designated port



There is one designated port for

each

LAN segment

5.2.2

BPDUs (BIP
-
ē
-
doos) are layer 2 frames that identify the least
-
cost path to the root bridge and determine
which ports will forward frames in the Spanning Tree.

5.2.4

The Bridge ID (BID) is used to elect a root bridge. It is made up of
a
bridge priority

and
MAC address
.

What are the three port roles of STP?



Root


Exit port

with the best path to the Root bridge
;

one per non
-
root bridge



Designated


Receives and forwards

traffic

to the Root Bridge; one per LAN segment



Non
-
Designated


Blocked to prevent loops in redundant networks

What are the three STP timers and what is there default length?



Hello


2 sec.



Max. Age


20 sec.



Forward Delay


15 sec. (x2)

5.2.5

What are
the five states of STP?



Blocking


STP election process takes place



Listening


Transmits BPDUs but does not forward



Learning


Records MAC addresses but does not forward



Forwarding


Considered part of the active topology and now forwards frames



Disabled


Administratively shut down, does not participate in STP


5.3.1

What is the first step in the STP process?

Election of a root bridge

5.3.2

“Tie Breakers”

(
lowest

number takes precedence)

1.

Root bridge ID

2.

Bridge priority

3.

MAC address

4.

Port priority

5.4.2

The
version of STP that we use on Cisco equipment is PVST+, a proprietary version that supports
spanning tree instances on a per
-
VLAN basis.

5.4.3

RSTP vs. STP



RSTP utilizes edge ports to transition immediately from a blocked state to forwarding,
whereas STP requires all participating ports to transition through the max age and forward delay timers
(50
sec.

default).

Many commands are identical and RSTP BPDUs
are backwards compatible with STP.

5.4.4

What happens if an edge port receives a BPDU?

It immediately loses its edge status and becomes a normal Spanning Tree port in a blocking state.

5.4.5

What are the three types of links in RSTP?

Shared, edge
-
type, a
nd point
-
to
-
point.