Switches - SpaceAgeTimes.com

hellhollowreadingΔίκτυα και Επικοινωνίες

26 Οκτ 2013 (πριν από 4 χρόνια και 13 μέρες)

76 εμφανίσεις

Sybex CCNA 640
-
802

Chapter 8: Layer
-
2 Switching

Instructor

& Todd Lammle

Chapter 8 Objectives

The
CCNA Topics
Covered in this chapter include:


What is
layer
-
2 switching


Switching
services


Bridges

vs.
LAN switching


Three switch functions


MAC table


Switching loops
Spanning
-
Tree Protocol
(
STP
)





2

Layer 2 Switching


Purposes for using
switching

1.
Breaks up collision domains

2.
Cost
-
effective, resilient
internetwork



Purpose for
Spanning
-
Tree Protocol

(
STP
)


Stops
loops

in layer 2 switched networks


Before

Layer 2 Switching

Switched LANs

Typical Switched Designs

One link
to the
server
!

Switching Services

Layer 2 switching provides:


Hardware
-
based

bridging

(
ASIC
)


Wire speed


Low latency


Low cost

Limitations

of Layer 2 Switching


Must break up the collision domains
correctly
.


Rule of Thumb:
Make sure that users spend
80 percent of their time on the
local segment
.


Switches
do not

break up
broadcast domains
by
default
.


Or at
all

, unless they are
layer 3 switches.

Bridging

vs.
LAN switching


Bridges are
software

based
, switches are
hardware

based
using
ASIC chips
to filter decisions.


A switch can be viewed as a
multiport bridge
.


Bridges

can only have
one spanning
-
tree instance

per
bridge,
and
switches

can have
many
.


Switches have a
higher number
of
ports

than most bridges.


Both

of them
forward

layer 2 broadcasts
.


Both of them
learn MAC addresses
by
examining

the
source

address

of
each frame received
.


Both of them make
forwarding

decisions
based on
layer 2
addresses.


Three Switch Functions

at
Layer
-
2


Address learning


Layer 2 switches and bridges
remember

the
source hardware address
of
each frame received
on an interface, and they enter this information
into a
MAC database
called a
forward/filter table.


Forward/filter decisions


When a frame is received on an interface, the switch looks at the
destination hardware address
and finds the
exit interface
in the
MAC
database
. The frame is
only

forwarded
out the
specified

destination
port.


Loop avoidance


If
multiple connections
between switches are created for
redundancy

purposes,
network loops
can occur.
Spanning Tree Protocol
(
STP
) is
used to
stop network loops
while still
permitting redundancy
.

Empty MAC table

Nothing here at this point

How Switches Learn Hosts’ Locations
(p 501)

Switching Loops

Switching Loop Problems

The MAC address filter table could be totally confused

about the device’s location because the switch can receive
the frame from more than one link.

Spanning
-
Tree Protocol (STP)

Solves Switching loops at layer 2


STP


Root Bridge


BPDU


Bridge ID


Nonroot Bridge


Root port


Designated port


Port cost


Nondesignated
port


Forwarding port


Block port

Spanning
-
Tree Terms


Root bridge
:
The
root bridge is the bridge with the best
bridge ID
.


With STP, the key is for all the switches in the network to elect a root
bridge that becomes the focal point in the network.


All other decisions in the network

such as which port is to be blocked
and which port is to be put in forwarding mode

are made from the
perspective of this root bridge.


BPDU
:
All the switches exchange information to use in the
selection of the root switch as well as in subsequent
configuration of the network.


Each switch compares the parameters in the
Bridge Protocol Data Unit
(BPDU)
that it sends to one neighbor with the one that it receives
from
another neighbor.


Bridge ID
:
The bridge ID is
how STP keeps track
of
all the switches
in
the network. It is
determined

by a
combination

of the
bridge priority
(
32,768

by
default

on
all Cisco switches
) and the
base MAC address
.


The bridge with the
lowest bridge ID
becomes the
root bridge
.

Spanning
-
Tree Terms


Nonroot bridges
:
All bridges that are
not

the
root bridge.


Port cost
:
Port cost determines the
best path

when
multiple
links

are used
between two switches and none of the links
is a root port.


The cost of
a link is determined by the bandwidth of a link.


Root port
:
The root port is always the
link directly connected
to the root bridge
, or the
shortest path
to the
root bridge
.


If
more than one link
connects to the root bridge, then a
port cost
is determined by checking the
bandwidth

of
each
link.


The
lowest
-
cost port
becomes the
root port
.


If
multiple links
have the
same cost
, the bridge with the
lower advertising bridge ID

is used.


Since
multiple links
can be from the
same device
, the
lowest port number
will be used.

Spanning
-
Tree Terms


Designated port
:
A designated port is one that has been
determined as having the
best (lowest) cost
.


A
designated

port
will be marked as a
forwarding

port
.


Nondesignated

port
:
A
nondesignated

port is one with a
higher cost
than the
designated

port.


Nondesignated

ports are put in
blocking

mode

they are
not forwarding ports.


Forwarding port
:
A forwarding port
forwards frames
.


Blocked port
:
A blocked port is the port that, in order to
prevent loops
, will
not forward frames
.



However, a
blocked port
will always
listen

to
frames
.

Spanning
-
Tree Operations


Selecting the root bridge


Selecting the designated port


Spanning
-
Tree Port States


Blocking
:
A blocked port
won’t forward frames
; it just
listens to BPDUs
. The purpose of the blocking state is to
prevent

the use of
looped paths.


Listening
:
The port
listens

to BPDUs to make sure no loops
occur on the network
before passing data frames
. A port in
listening state
prepares

to
forward data frames
without
populating

the
MAC address table
.


Learning
:
The switch port
listens

to BPDUs and
learns

all the
paths

in the switched network. A port in learning state
populates

the
MAC address table
but
doesn’t forward data frames
.


Forwarding:
The port
sends

and
receives

all data frames
on the bridged port. If the port is still a designated or root port at
the end of the learning state, it enters the forwarding state.


Disabled:
port in the disabled state (administratively) does not
participate in the frame forwarding or STP.

Spanning
-
Tree Example

Written Labs and Review Questions


Open your books and go through all the written labs and the
review questions.


Review the answers in class.

23

Post
-
Class Material: Collapsed Backbone,
(pp 494ff)

The figure below shows the old model of a network, prior to switches. It was called a
collapsed backbone
because all hosts would need to go to the
corporate backbone
to
reach any
network services

both
LAN

and
mainframe
.

Root Bridge Elections:
(pp 508ff)


Each switch will have a
Bridge ID Priority
value, more
commonly referred to as a
BID
.


This
BID

is a
combination

of a
default priority value
and the
switch's
MAC address
, with the
priority value listed first
!


For example, if a Cisco switch has the
default priority value
of
32,768

and a
MAC address
of
11
-
22
-
33
-
44
-
55
-
66
, the
BID

would be
32768:11
-
22
-
33
-
44
-
55
-
66
. This is the
KEY!


Therefore, if the
switch priority
is left at the
default
, the
MAC
address

is the
deciding factor
.


Switches are a lot like people
-

when they first arrive, they
announce that they are the center of the universe! Unlike some
people, the switches will soon get over it. BPDUs will be
exchanged until one switch is elected Root Bridge, and it's the
switch with the
lowest
BPDU that will end up being the Root
Bridge.

Root Bridge Elections

VLAN 10

VLAN 20

VLAN 30

BID Election 2


If
STP

is
left totally alone
, a
single switch
is going to be the
root bridge
for
every single VLAN
in your
network!



Worse, that single
switch

is going to be
selected

because

it has
a
lower MAC address
than
every other switch
, not the
criteria

you want to use to
select

a
single root bridge.


You will prefer to determine a
particular switch
to be the
root
bridge

for your VLANs, or you may want to
spread the root
bridge workload around
.


[There are 2 ways to change the BID. The first is below:]


You can do this with the cmd:
spanning
-
tree vlan root
SW1(config)#
spanning
-
tree vlan 1 ?

forward
-
time

Set the forward delay for the spanning tree

hello
-
time

Set the hello interval for the spanning tree

max
-
age

Set the max age interval for the spanning tree

priority
Set the bridge priority for the spanning tree

root
Configure switch as root



In
this example
, we've got
two switches


SW1

has been elected the
root bridge
for VLANs
10, 20,
&

30.
We'll use
the spanning
-
tree vlan root command on
SW2

to make it the
root bridge
for VLANs
20

and
30
.


SW2(config)#spanning
-
tree vlan 20 root primary

SW2(config)#spanning
-
tree vlan 30 root primary


SW2#
show spanning vlan 20


VLAN0020

Spanning tree enabled protocol ieee

Root ID Priority 24596

Address 000f.90e2.1300

This bridge is the root


SW2#
show spanning vlan 30


VLAN0030

Spanning tree enabled protocol ieee

Root ID Priority 24606

Address 000f.90e2.1300

This bridge is the root


SW 2 is now the root bridge for both VLAN 20 and 30. Notice that the
priority value has changed from the default of 32768.



Second way
to
change

the
BID


Change the
priority of the switch

that you
want to be the root bridge. For example:


SW2(
config
)#
spanning
-
tree vlan 3
priority

8192


Remember, the priority can be anything from
0
to 65535.


The priority must be in increments of 4096, starting
from 0


Because the BID is read from left to right, the
priority
is read
first!


So if the priority of one switch is lower than that of
another switch, the switch with the lower BID wins
and the MAC address is never considered!

Convergence
:
Point 3
,
pg 511


Switches

are converged when they are all
in either a
blocking

or
forwarding
state.


Also note: if a switch is blocking and then
must become the root bridge because of a
topology change, it must end up in the
forwarding
state …


But it must go through the
listening

and the
learning
states on the way.


Redundant Link Convergence:
pg 512 ff


The typical spanning
-
tree topology’s time to convergence from
blocking to forwarding on a switch port is 50 seconds. This
could create time
-
out problems on your servers or hosts

for
example, when you reboot them.


To address this hitch, you can disable spanning tree on
individual ports using PortFast (or similar commands).


Portfast



enables fast connectivity to be
established on access layer switch ports to
workstations


UplinkFast

---

enables fast uplink failover on an
access layer switch when dual uplinks are
connected to distribution layer



BackboneFast



enables fast convergence in
network backbone (core) after STP change