1. LAN Segmentation
1.1 In a collision domain, a frame sent by a device can cause collision with a frame sent by
another device in the same collision domain. Moreover, a device can hear the frames
destined for any device in the same collision domain.
1.2 In a broadcast domain, a broadcast frame sent by a device can be received by all other
devices in the same broadcast domain.
1.3 A LAN segment or an Ethernet network segment consists of the devices connected with
a coaxial cable or a hub. The devices are in the same collision domain.
1.4 Ethernet congestion problem occurs when too many devices are connected to the same
Ethernet network segment, such that the high network bandwidth utilization increases the
possibility of collision, which causes degradation of network performance.
1.5 LAN segmentation solves the congestion problem by breaking the network into separate
segments or collision domains using bridges, switches or routers (but not hubs or
repeaters). LAN segmentation can reduce the number of collisions in the network and
increase the total bandwidth of the network (e.g. 10 Mbps for one segment, 20 Mbps for
two segments, 30 Mbps for three segments, and so on).
1.6 The 80/20 rule should be used when designing how to segment a network, i.e. 80% or
more data traffic should be on the local network segment while 20% or less data traffic
should cross network segments.
2. LAN Switching
2.1 LAN switching (or Layer 2 switching) refers to the switching of a frame from the
source computer to the destination computer across network segments. It consists of
three major functions:
CCNA 640-801 Exam Notes - Fundamentals of Switching 2
Address learning - learning the MAC addresses of the connected devices to build
the bridge table.
Forward and filter decision - forwarding and filtering frames based on the bridge
table entries and the bridge logic.
Loop avoidance - avoiding network loop by using Spanning Tree Protocol.
2.2 A bridge or switch maintains a forwarding table (also known as bridge table or MAC
address table) which maps destination physical addresses with the interfaces or ports
to forward frames to the addresses.
2.3 A bridge or switch builds a bridge table by learning the MAC addresses of the
connected devices. The process is as follows:
1. When a bridge is first powered on, the bridge table is empty.
2. The bridge listens to the incoming frames and examines the source MAC addresses
of the frames. For example, if there is an incoming frame with a particular source
MAC address received from a particular interface, and the bridge does not have an
entry in its table for the MAC address, an entry will be created to associate the MAC
address with the interface.
3. An entry will be removed from the bridge table if the bridge has not heard any
message from the concerned host for a certain time period (default aging time = 300
seconds or 5 minutes).
2.4 A bridge or switch forwards or filters a frame based on the following logic:
1. If the destination MAC address of the frame is the broadcast address (i.e.
FFFF.FFFF.FFFF) or a multicast address, the frame is forwarded out all interfaces,
except the interface at which the frame is received.
2. If the destination MAC address is an unicast address and there is no associated entry
in the bridge table, the frame is forwarded out all interfaces, except the interface at
which the frame is received.
3. If there is an entry for the destination MAC address in the bridge table, and the
associated interface is not the interface at which the frame is received, the frame is
forwarded out that interface only.
4. Otherwise, drop the frame.
2.5 There are three types of switching method:
Store-and-forward switching
The entire frame is received and the CRC is computed and verified before
forwarding the frame.
If the frame is too short (i.e. less than 64 bytes including the CRC), too long (i.e.
more than 1518 bytes including the CRC), or has CRC error, it will be discarded.
It has the lowest error rate but the longest latency for switching. However, for
high-speed network (e.g. Fast Ethernet or Gigabit Ethernet network), the latency
is not significant.
It is the most commonly used switching method, and is supported by most
switches.
Cut-through switching (also known as Fast Forward switching or Real Time
switching)
A frame is forwarded as soon as the destination MAC address in the header has
been received (i.e. the first 6 bytes following the preamble).
It has the highest error rate (because a frame is forwarded without verifying the
CRC and confirming there is no collision) but the shortest latency for switching.
Fragment-free switching (also known as Modified Cut-through switching)
A frame is forwarded after the first 64 bytes of the frame have been received.
Since a collision can be detected within the first 64 bytes of a frame (collision
window size of Ethernet), fragment-free switching can detect a frame corrupted
by a collision and drop it. Therefore, fragment-free switching provides better
error checking than cut-through switching.
The error rate of fragment-free switching is above store-and-forward switching
and below cut-through switching.
The latency of fragment-free switching is shorter than store-and-forward
switching and longer than cut-through switching.
2.6 Bridges only support store-and-forward switching. Most new switch models also use
store-and-forward switching. However, it should be noted that Cisco 1900 switches use
fragment-free switching by default.
3. Spanning Tree
3.1 In a switched network with redundant paths (i.e. with loops), the following problems will
occur:
Broadcast Storm - A broadcast or multicast frame will be forwarded by a switch out
all its active ports except the source port. The resulted frames will then be
forwarded by the other switches in the network similarly. Some of the frames will
be forwarded around the network loop and back to the original switch. The process
then repeats. Therefore, the frames will loop indefinitely in the network and
eventually exhaust the processing power of the switches and the bandwidth of the
network.
Receiving multiple copies of a frame - When a switch receives an unicast frame to
a destination device that it does not have an entry in its bridge table, it will forward
the frame out all its active ports except the source port. Therefore, the destination
device may receive multiple copies of the frame through the redundant links.
Bridge Table Thrashing - A switch may receive frames from a source device at
more than one ports if there are redundant links. It needs to update its bridge table
whenever a frame from the source device arrives at a port differs from the last time.
If the arrival frequency of such frames is high, the processing power of the switch
will be exhausted.
Spanning Tree Protocol Basics
3.2 Spanning Tree Protocol or STP (IEEE 802.1d) is used to solve the looping problem.
It runs on bridges and switches in a network. It implements a Spanning Tree
Algorithm (STA), which calculates a loop-free topology for the network.
3.3 STP ensures that there is only one active path between any two network segments by
blocking the redundant paths. A redundant path is used only when the corresponding
active path failed. It is not used for load-balancing.
3.4 Because STP solves the looping problem by blocking one or more links in a network, the
frames traveling between some source / destination devices may not be able to use the
shortest physical path.
3.5 Bridges exchange STP information using messages called Bridge Protocol Data Units
(BPDUs) through Layer 2 multicast.
3.6 A port of a bridge running STP can be in one of the following 5 states:
State Handling of
BPDUs
Learning MAC
addresses
Handling of
frames
Disabled (administratively down) Does not receive
BPDUs
Does not learn
addresses
Discards frames
received
Blocking (default state when a
bridge is powered on)
Receives
BPDUs
Does not learn
addresses
Discards frames
received
Listening (a blocking port goes
through this state before entering
the learning state)
Receives and
forwards
BPDUs
Does not learn
addresses
Discards frames
received
Learning (a listening port goes
through this state before entering
the forwarding state)
Receives and
forwards
BPDUs
Learns addresses Discards frames
received
Forwarding (all ports in the
forwarding state belong to the
current spanning tree)
Receives and
forwards
BPDUs
Learns addresses Receives and
forwards frames
By default, the transition from the blocking state to the listening state takes 20 seconds
(MaxAge time), from the listening state to the learning state takes 15 seconds (FwdDlay
time), and from the listening state to the forwarding state takes another 15 seconds
(FwdDlay time). The whole process takes 50 seconds.
3.7 In a network without any network topology change, all bridge ports should be either in
the forwarding state or the blocking state. When there is a change in the status of a port
(e.g. a port is brought up), the spanning tree topology may change and some ports may
transit from the blocking state to the forwarding state (through the listening state and the
learning state) or vice versa.
3.8 Convergence refers to the condition that all bridge ports in a network have transitioned
to either the forwarding state or the blocking state after a network topology change.
3.9 A spanning tree consists of a root bridge, which likes the root of a living tree. There
is only one root bridge in the whole switched network. There is a single path from the
root bridge (root) to each network segment (leaf). The paths form the spanning tree of
the network. The bridges place the interfaces on the spanning tree in the forwarding
state, and the interfaces not on the spanning tree in the blocking state.
3.10 Each bridge has an 8-byte Bridge ID, which is the concatenation of the priority (2-byte)
and the MAC address (6 byte) of the bridge. The default priority of a device is 32,768.
3.11 The bridge with the lowest bridge ID is elected as the root bridge.
3.12 The root path cost of a bridge (i.e. cost of the path from the bridge to the root bridge) is
the accumulated cost of the links along the root path. The cost of a link is determined
by its bandwidth. The following default costs are used for different types of links:
Link Speed New IEEE Cost Original IEEE Cost
10Gbps 2 1
1Gbps 4 1
100Mbps 19 10
10Mbps 100 100
3.13 In a spanning tree, the ports of a non-root bridge can be classified as follows:
Root port - The root port of a bridge is the port that is the closest to the root bridge
in terms of path cost. The path cost can be calculated based on the information
stored in the BPDUs sent by the root bridge (to be explained later in this Section).
Designated port - For each physical network segment, the bridge with the lowest
cost to the root bridge is elected as the designated bridge of that segment. If two
or more bridges have the same cost to the root bridge, the bridge with the lowest
bridge ID is elected. The designated bridge puts the port connected to that segment
in the forwarding state. This port is known as a designated port. For those
segments that are directly connected to the root bridge, the root bridge is their
designated bridge.
3.14 In determining which is the root port of a non-root bridge, if there are two or more ports
with equal root path cost, the following factors are used as the tiebreaker in sequence:
Sender Bridge ID, i.e. the bridge ID of the next bridge in the path to the root bridge
(the lowest one is preferred).
Sender Port ID (the lowest one is preferred).
3.15 The Port ID of a port is 2 bytes long, and is the concatenation of the port priority (1-byte)
and the physical port number (1 byte).
3.16 Other than the ports of the root bridge, the root port of each non-root bridge, and the
designated port of each LAN segment, all ports in the network are put in the blocking
state.
BPDU & STP Logic
3.17 There are two types of BPDUs. They are:
Configuration BPDU
Topology Change Notification (TCN) BPDU
7 CCNA 640-801 Exam Notes - Fundamentals of Switching
3.18 The root bridge sends a Configuration BPDU (or Hello BPDU) out each interface
periodically (every 2 seconds, by default). Each bridge forwards the BPDU to the other
bridges downstream after updating several fields in the BPDU, including the cost from
this bridge to the root bridge. As long as such BPDUs are received periodically, a
bridge knows that the path to the root bridge is still working. Otherwise, it needs to
update its spanning tree.
3.19 A Configuration BPDU is 35 bytes long and contains the following information:
Protocol ID (2 bytes) and Version (1 byte).
Message type (1 byte) - Configuration BPDU or TCN BPDU.
Flag (1 byte) - It contains a topology change (TC) bit and a topology change
acknowledgement (TCA) bit.
Root bridge ID (8 bytes) - Bridge ID of the root bridge.
Root path cost (4 bytes) - Cost of the path from the sender bridge (the bridge
forwarding the BPDU) to the root bridge.
Sender bridge ID (8 bytes).
Port ID (2 bytes) of the port forwarding the BPDU.
Message Age (2 bytes) in 1/256 second.
The time elapsed since the root bridge sent the original BPDU that this BPDU is
based on.
Hello time (2 bytes) in 1/256 second.
The time interval between BPDUs are sent from the root bridge.
The default Hello interval is 2 seconds.
MaxAge time (2 bytes) in 1/256 second.
If a new BPDU is not received before the MaxAge timer expires, the BPDU
information is considered invalid and the bridge will try to update the STP
topology.
In other words, it is the time interval required for a port (on the alternate path) to
transit from the blocking state to the listening state.
The default MaxAge is 20 seconds.
Forward Delay time (fwddlay) (2 bytes) in 1/256 second.
The time interval for a port to move from the listening state to the learning state.
It is also the time interval for a port to move from the learning state to the
forwarding state.
The time interval for moving from the listening state to the learning state allows
for re-election of the root bridge, if necessary.
The time interval for moving from the learning state to the forwarding state
avoids looping of frames before the bridges have converged and learned the new
MAC addresses.
The default forward delay time interval is 15 seconds.
3.20 The values of the Hello timer, MaxAge timer, and Forward Delay timer affect the time
required for the bridges to agree on the new spanning tree when there is a change in the
topology of the network (i.e. the time required for the bridges to converge, or the STP
convergence time). The maximum STP convergence time using the default timer
values is 50 seconds (MaxAge + FwdDlay + FwdDlay).
3.21 A Topology Change Notification (TCN) BPDU is sent out when a bridge detects that a
port in the forwarding state is going down or a port is moving to the forwarding state (e.g.
the port is enabled by the administrator). The bridge will send TCN BPDUs out of its
root port towards the root bridge at every Hello interval until it is acknowledged. A
TCN BPDU is only 4 bytes long, which includes protocol ID, version field, and message
type field. It virtually contains no information.
3.22 When a non-root bridge receives a TCN BPDU, it will forward the BPDU upstream
towards the root bridge. It will also set the TCA bit in the next Configuration BPDU
going downstream. The Configuration BPDU notifies the downstream bridge that the
TCN BPDU has been received so that it can stop sending out TCN BPDUs.
3.23 When the root bridge receives a TCN BPDU, it will send out a Configuration BPDU
with the TCA bit set, just like a non-root bridge. In addition, the TC bit of the BPDU
will also be set to notify all the bridges in the network that there is a topology change.
The TC bit will be set by the root bridge for a certain period of time (MaxAge +
Fwddlay).
3.24 When a bridge receives a BPDU with the TC bit set, it will shorten the aging time of its
bridge table entries from the default of 300 seconds to the Forward Delay time.
Therefore the entries will be timed out quickly and the bridge will learn the topology of
the new spanning tree.
3.25 In summary, STP works as follows:
Election of the root bridge
1. When a bridge is powered up, it claims to be the root bridge by sending Hello
BPDUs with its bridge ID as the root bridge's ID and the cost to the root bridge
equals 0.
2. The bridge with the lowest bridge ID is elected as the root bridge.
3. The root bridge puts all its ports in the forwarding state.
9 CCNA 640-801 Exam Notes - Fundamentals of Switching
Selection of the root port for each non-root bridge
4. The root bridge continually sends Hello BPDUs out all its ports every Hello time
interval.
5. When a non-root bridge receives a Hello BPDU, it modifies the packet by
incrementing the cost field, and then forwards the packet out all its ports (except the
port at which the packet is received).
6. Each non-root bridge compares the cost value of the BPDUs received from different
ports. The port that receives the lowest-cost BPDU is the root port of the bridge.
The bridge then puts the root port in the forwarding state.
Election of the designated bridge for each LAN segment
7. For each physical network segment, the bridge with the lowest cost to the root
bridge is elected as the designated bridge of that segment. The designated bridge
then puts the port connected to that segment in the forwarding state. This port is
known as the designated port.
Blocking of redundant links for loops removal
8. Other than the ports on the spanning tree, i.e. ports of the root bridge, the root port
of each non-root bridge, and the designated port of each LAN segment, all ports are
put in the blocking state.
3.26 For example, in the following network, Switch X has the lowest bridge ID and is elected
as the root bridge. Its ports are in the forwarding state. The root ports of Switch Y
and Z are also in the forwarding state. Both Switch Y and Z have the same cost to the
root bridge (Switch X), but Switch Y has a lower bridge ID. Therefore, Switch Y is
elected as the designated bridge for the network segment between Switch Y and Z. The
non-designated port of Switch Z is put in the blocking state.
3.27 Now, if the link between Switch X and Switch Z failed, the following changes will
happen:
1. When Switch Z detects the link failure or it has not received any Hello BPDU from
Switch X for a time period of MaxAge (worst case), it either advertises itself as the
Sw itc h X
( ro o t b r id ge )
Sw it c h Z
ro o t p o r t
ro o t p o r t
d e sign a te d p o r t
Sw it c h Y
b lo c k e d p o r t
root for re-election of the root bridge, or selects another port as its root port. Since
it still receives BPDUs from Switch Y and knows that the bridge ID of Switch X is
lower than itself, it selects the port to Switch Y as its new root port.
2. Switch Z puts the port to Switch Y in the listening state (from the blocking state).
It also sends a TCN BPDU out the port to Switch Y.
3. Switch Y forwards the TCN BPDU towards the root bridge, i.e. Switch X, and
acknowledges the TCN BPDU (by setting the TCA bit of the next Configuration
BPDU received from the root bridge and forwarding it to Switch Z).
4. Switch X sends a Configuration BPDU downstream to Switch Y, with the TC bit set.
Switch Y forwards the BPDU to Switch Z. Both Switch Y and Z then change the
aging time of their bridge table entries from 300 seconds to the forward delay time.
Therefore an entry will be aged out if no frame is received from the host specified in
the entry within the forward delay time.
5. When the forward delay timer expires, Switch Z puts the port to Switch Y in the
learning state (from the listening state), and learns MAC addresses based on
received frames.
6. When the forward delay timer expires again, Switch Z puts the port to Switch Y in
the forwarding state (from the learning state), and starts forwarding frames through
this interface.
4. Virtual LAN (VLAN) and VLAN Trunking
Virtual LAN
4.1 A Virtual LAN (VLAN) is a broadcast domain created based on the functional,
security, or other requirements, instead of the physical locations of the devices, on a
switch or across switches. With VLANs, a switch can group different interfaces into
different broadcast domains. Without VLANs, all interfaces of a switch are in the same
broadcast domain; switches connected with each other are also in the same broadcast
domain, unless there is a router in between.
4.2 Different ports of a switch can be assigned to different VLANs. A VLAN can also span
multiple switches (i.e. have members on multiple switches).
4.3 The advantages of implementing VLAN are:
It can group devices based on the requirements other than their physical locations.
It breaks broadcast domains and increases network throughput.
It provides better security by separating devices into different VLANs.
11 CCNA 640-801 Exam Notes - Fundamentals of Switching
Since each VLAN is a separate broadcast domain, devices in different VLANs
cannot listen or respond to the broadcast traffic of each other.
Inter-VLAN communication can be controlled by configuring access control lists
on the router or Layer 3 switch connecting the VLANs.
4.4 VLANs can be configured using one of the following two methods:
Static VLAN
Assigning VLANs to switch ports based on the port numbers.
It is easier to set up and manage.
Dynamic VLAN
Assigning VLANs to switch ports based on the MAC addresses of the devices
connected to the ports.
A VLAN management application is used to set up a database of MAC addresses,
and configure the switches to assign VLANs to the switch ports dynamically
based on the MAC addresses of the connected devices. The application used by
Cisco switches is called VLAN Management Policy Server (VMPS).
4.5 Cisco switches support a separate instance of spanning tree and a separate bridge table
for each VLAN.
4.6 A VLAN is different from an IP subnet (details about IP subnet can be found in Chapter
5) in concept. However, there is a one-to-one relationship between a VLAN and an IP
subnet. It means that devices in the same VLAN are also in the same IP subnet, devices
in different VLANs are also in different IP subnets.
4.7 Conventional switching (i.e. Layer 2 switching) cannot switch frames across VLANs.
4.8 To forward packets between VLANs, a router or a Layer 3 switch is required.
4.9 A router can route traffic between different VLANs by having a physical interface
connected to the switch for each VLAN. Each interface is connected to an access link
of the switch (details about access links will be explained later in this Section). The
default gateway of the hosts of each VLAN should be configured as the interface of the
router connected to that VLAN.
4.10 For example, if Host A in VLAN 8 sends a packet to Host B in VLAN 9, the packet will
be forwarded to the router's interface for VLAN 8 because it is the default gateway of
Host A (and other hosts in VLAN 8). The router will then route the packet out the
interface for VLAN 9 (the VLAN of Host B) based on IP routing (details about IP
CCNA 640-801 Exam Notes - Fundamentals of Switching 12
routing can be found in Chapter 5). The switch will then forward the packet to Host B.
4.11 If a router supports VLAN trunking, it can route traffic between different VLANs by
having only one physical interface connected to the switch. The interface should be
connected to a trunk link of the switch carrying traffic for all the VLANs (details about
trunk links will be explained later in this Section). This type of configuration is
sometimes called "router on a stick".
4.12 A Layer 3 switch is a switch with routing features. It uses specialized hardware
(Application Specific Integrated Circuits, ASICs) to route packets between VLANs or IP
subnets. Therefore, it is more efficient than routers. Moreover, VLAN routing does
not involve processing of the Layer 3 header of the packets.
VLAN Trunking
4.13 There are two different types of links in a switched network:
Access link - a link that is part of only one VLAN. Therefore, a port connected to
an access link can be a member of only one VLAN.
Trunk link - a 100 Mbps or 1000 Mbps point-to-point link that connects switches or
routers, and carries frames of different VLANs. Therefore, a port connected to a
trunk link can be a member of multiple VLANs. All VLANs are configured on a
trunk link by default.
4.14 VLAN Trunking, by making use of frame tagging, allows traffic from different VLANs
to transmit through the same Ethernet link (trunk link) across switches.
4.15 VLAN Trunking identifies the VLAN from which a frame is sent by tagging the frame
Host A Host B
Router
Switch
VLAN 8 VLAN 9
VLAN 8 VLAN 9
13 CCNA 640-801 Exam Notes - Fundamentals of Switching
with the source VLAN ID (12-bit long). This feature is known as frame tagging or
frame identification.
4.16 With frame tagging, a switch knows which ports it should forward a broadcast frame
(forward out the ports which have the same VLAN ID as the source VLAN ID). It also
knows which bridge table it should use for forwarding an unicast frame (since a separate
bridge table is used for each VLAN).
4.17 A frame tag is added when a frame is forwarded out to a trunk link, and is removed when
the frame is forwarded out to an access link. Therefore, any device attached to an
access link is unaware of its VLAN membership.
4.18 Cisco switches support two trunking protocols:
Inter-switch Link (ISL)
It is a Cisco proprietary VLAN trunking protocol and can only be used between
Cisco switches or switches supporting ISL.
It encapsulates a frame by an ISL header and trailer.
An ISL header is 26 bytes long and contains the 12-bit VLAN ID, MAC
addresses of the sending and the receiving switch, and some other information.
An ISL trailer is 4 bytes long and contains the CRC of the frame.
It supports a separate instance of spanning tree for each VLAN by using a Cisco
proprietary feature called Per-VLAN Spanning Tree (PVST+). Different
instances of spanning tree allow the STP parameters of different VLANs to be
configured independently. For example, we can break a network loop by
blocking different links for different VLANs instead of blocking the same link for
all VLANs, so that the available bandwidth can be used more efficiently.
IEEE 802.1q
It is the IEEE standard trunking protocol.
It inserts a 4-byte header to the middle of the original Ethernet header. The
802.1q header contains the 12-bit VLAN ID and some other information.
Ethernet frame without
802.1q header
Dest. addr.
(6 bytes)
Source addr.
(6 bytes)
Type
(2 bytes)
Data
(46-1500 bytes)
FCS
(4 bytes)
With 802.1q header Dest. addr.
(6 bytes)
Source addr.
(6 bytes)
802.1q header
(4 bytes)
Type
(2 bytes)
Data
(46-1500 bytes)
FCS
(4 bytes)
Recalculation of the FCS is required after the insertion of the 802.1q header as
the original header has been changed.
It did not support a separate instance of spanning tree for each VLAN originally.
However, Cisco switches can use PVST+ with 802.1q to support this feature.
IEEE has also defined a new specification called 802.1S, which can be used with
CCNA 640-801 Exam Notes - Fundamentals of Switching 14
802.1q to support multiple instances of spanning tree.
It defines one VLAN as the native VLAN. It does not insert 802.1q header into
the frames sent from the native VLAN over a trunk link. The default native
LAN is VLAN 1.
Since 802.1q is defined as a type of Ethernet frame, it does not require that every
device on a link understands 802.1q. By defining a trunk port as a member of
the native VLAN, any Ethernet device (even if it does not understand 802.1q)
connected to the trunk port can read frames for the native VLAN.
Both sides of a trunk link must agree on which VLAN is used as the native
VLAN. Otherwise, the trunk will not operate properly.
VLAN Trunking Protocol (VTP)
4.19 VLAN Trunking Protocol (VTP) is a Cisco proprietary protocol for switches to
exchange VLAN configuration information (e.g. VLAN membership). It ensures the
configuration information is consistent across a VTP domain. For example, you only
need to create a VLAN (or define the name of a VLAN, etc.) on one switch, and VTP
will distribute that information to all switches in the VTP domain automatically. There
is no need to repeat the configuration works manually on each switch. All devices that
need to share VLAN information must use the same VTP domain name.
4.20 Switches exchange VLAN configuration information using VTP advertisements.
Some of the information included in a VTP advertisement are:
Configuration revision number.
Configuration information for each VLAN, e.g. the VLAN ID, VLAN name, and
switches which have ports that are members of the VLAN.
4.21 A device running VTP operates in one of the following three modes:
Server mode
A VTP server can create, update, and delete VLANs and VTP information in a
VTP domain.
A VTP server increments the configuration revision number when there is a
change to the VLAN configuration information.
A VTP server floods VTP advertisements to the VTP servers and clients in the
VTP domain every 5 minutes or when there is a change to the VLAN
configuration information.
When a VTP advertisement with a higher revision number is received, the stored
VLAN configuration information is updated.
A VTP server saves the VLAN configuration information in NVRAM (Non15
CCNA 640-801 Exam Notes - Fundamentals of Switching
volatile RAM).
Client mode
A VTP client cannot create, update, or delete the VLAN configuration
information in a VTP domain.
A VTP client receives and forwards VTP advertisements from the other VTP
switches in the same VTP domain, and learns the VLAN configuration
information from the advertisements.
A VTP client does not save the VLAN configuration information in NVRAM.
Transparent mode
A switch in transparent mode does not participate in any VTP domain. It can
create, update, and delete the VLAN configuration information with only local
significance, i.e. the changes only affect that switch and will not be sent to other
switches.
A switch in transparent mode receives and forwards VTP advertisements from
other VTP switches but ignore the information in the advertisements.
A switch in transparent mode saves the locally defined VLAN configuration
information in NVRAM.
4.22 There must be at least one VTP server in a VTP domain.
4.23 To configure a switch as a VTP server, first configure it as a VTP client such that it can
receive the updated VLAN configuration information from the other VTP switches.
After that, it can be configured as a VTP server.
4.24 VTP Pruning prevents broadcast frames and unicast frames for a particular VLAN from
being forwarded to the switches that do not have any port with membership of that
VLAN. It allows more efficient use of network bandwidth.
CCNA 640-801 Exam Notes - Extending Switched Networks with VLANs 16
Extending Switched Networks with VLANs
1. Cisco Switches - Basic Configuration
1.1 A Cisco 2950 switch can work out-of-the-box without any additional configuration. As
no user password or enable password has been set, you can enter the user mode
immediately by connecting through the console port, and enter the enable mode by using
the command ">enable".
1.2 The passwords of a Cisco switch can be set just like a Cisco router running IOS
software:
Switch>enable
Switch#configure terminal
Switch(config)#line console 0
Switch(config-line)#login
Switch(config-line)#password <new console password>
Switch(config-line)#exit
Switch(config)#line vty 0 4
Switch(config-line)#login
Switch(config-line)#password <new telnet password>
Switch(config-line)#exit
Switch(config)#enable secret <new enable secret>
Set the password for
console port 0.
Set the telnet password
for ports 0-4.
Set the enable secret.
1.3 VLAN 1 is the default VLAN of an interface. It is also the default management
VLAN of a switch. The IP address of a switch should be defined on the management
VLAN virtual interface as follows:
Switch(config)#interface vlan 1
Switch(config-if)#ip address <ip address> <netmask>
Switch(config-if)#exit
Switch(config)#
In addition, BPDUs, CDP and VTP advertisements are also sent on VLAN 1.
1.4 The interface configuration commands for a Cisco switch are similar to that for a
17 CCNA 640-801 Exam Notes - Extending Switched Networks with VLANs
Cisco router. Some of the configuration commands are summarized below:
Command Description
(config)#interface vlan
<management vlan-id>
Enter the interface configuration mode,
"(config-if)#", to configure the IP address of
the switch under the management VLAN.
The default management vlan-id is 1.
(config-if)#[no] ip address
<ip address> <netmask>
Set (or remove) the IP address of the switch
under the management VLAN. This
command should be issued after the command
"(config)#interface vlan <management vlanid>".
(config)#interface <int-type>
<int-num>
Select the interface for configuration and enter
the interface configuration mode "(configif)#".
Examples of interface type parameter values
are "ethernet", "fastethernet",
"gigabitethernet", etc.
(config-if)#description <description>
(config-if)#no description
Add a description to the interface (or remove
the description).
The description will be shown in the output of
the show configuration commands.
There is no description by default.
(config-if)#duplex {full | half | auto}
(config-if)#no duplex
Configure the duplex operation of the
interface as full-duplex, half-duplex (default),
or auto-negotiation.
The "no" form of the command restores the
default value, i.e. half-duplex.
(config-if)#speed {10 | 100 |
1000 | auto | nonegotiate}
(config-if)#no speed
Configure the speed of the interface as
10Mbps, 100Mbps (default), 1000Mbps, autonegotiation,
or no negotiation (i.e. autonegotiation
is disabled and the interface runs
at 1000 Mbps. This option is valid and
visible only on Gigabit Ethernet interfaces).
The "no" form of the command restores the
default value, i.e. 100Mbps.
(config-if)#[no] shutdown Administrative shutdown (or bring up) the
interface.
1.5 Some commands for displaying switch configuration information are summarized
below:
CCNA 640-801 Exam Notes - Extending Switched Networks with VLANs 18
Command Description
#show startup-config Show the startup configuration file.
#show running-config Show the running configuration file.
#show interfaces
[<int-type> <int-num>]
Show information of all interfaces configured
on the switch, or the specified interface.
Some of the configuration information that
will be displayed are:
Interface status (up/down).
Line protocol status (up/down).
MAC address.
MTU, duplex (full-duplex or half-duplex),
speed (10Mbps, 100Mbps, etc.).
Bandwidth, delay, keepalive interval, etc.
Some of the statistics that will be displayed
are:
5-minute input rate and output rate in bps
and packets per second.
#input packets, #output packets
#input error packets, #output error packets
#collisions, etc.
#show interfaces vlan
<management vlan-id>
Show the IP address of the switch configured
under the management VLAN.
#show interfaces status List all interfaces with the following
information for each interface:
Port name.
Status (notconnect, connected, disabled or
err-disabled).
VLAN ID.
Duplex (auto or half or full).
Speed (auto, 10, 100, etc.).
Interface type (e.g. 10/100BaseTX).
#show mac-address-table
[vlan <vlan-id>]
[address <mac address>]
[interface <int-type> <int-num>]
[dynamic | static]
Show all entries in the MAC address table (i.e.
the forwarding table), or only the entries of a
particular VLAN-ID, MAC address, interface,
or only dynamic (i.e. dynamically learned) /
static entries.
For each entry, the following information are
shown:
VLAN ID.
MAC address.
Port name.
Entry type - dynamic or static.
19 CCNA 640-801 Exam Notes - Extending Switched Networks with VLANs
1.6 The port security feature of a Cisco 2950 switch allows the access interfaces (not trunk
interfaces) of the switch to be configured to restrict the devices that can be connected to
the interfaces by MAC addresses.
1.7 A port can be configured with a maximum number of secure MAC addresses (i.e. the
MAC addresses of the devices that can be connected to the port). The secure MAC
addresses can be added in the following ways:
Manually configured.
Dynamically learned from the source addresses of the frames received by the port.
Some addresses are manually configured and the rest are dynamically learned.
1.8 An example for configuring the port security feature is as follows:
Switch(config)#interface fastethernet0/2
Switch(config-if)#switchport mode access
Switch(config-if)#switchport port-security
Switch(config-if)#switchport port-security
mac-address 0422.1043.0a11
Switch(config-if)#exit
Switch(config)#
Define the interface as an access
interface rather than a trunk interface.
Enable port security on the interface.
Specify the MAC address of the device
that can be connected to this interface.
1.9 The commands for configuring port security settings are summarized below:
Command Description
Basic Configuration
(config-if)#switchport mode access Configure the interface as an access interface.
The port security feature is supported on
access interfaces only.
(config-if)#[no] switchport
port-security
Enable (or disable) port security on the
interface.
Port security is disabled by default.
(config-if)#[no] switchport
port-security
maximum <maximum value>
Specify the maximum number (1-132) of
secure MAC addresses for the interface.
The "no" form of the command resets the
maximum value to the default value.
The default maximum value is 1.
(config-if)#switchport port-security
mac-address <mac-address>
Configure a secure MAC address for the
interface.
CCNA 640-801 Exam Notes - Extending Switched Networks with VLANs 20
(config-if)#[no] switchport
port-security mac-address sticky
Enable (or disable) the interface for sticky
learning, i.e. add all dynamically learned
secure MAC addresses into the running
configuration file, including those that were
learned before this command is issued.
If you save the configuration file, the interface
does not need to relearn the sticky addresses
after the switch is restarted.
If you disable sticky learning, the sticky
secure MAC addresses are converted to
dynamic secure MAC addresses and are
removed from the running configuration file.
Sticky learning is disabled by default.
(config-if)#switchport
port-security mac-address
sticky [<mac-address>]
Configure a sticky secure MAC address.
#clear port-security
{all | configured | dynamic | sticky}
[{address <mac-address>} |
{interface <int-type> <int-num>}]
Remove all, configured, dynamically learned,
or sticky secure MAC addresses.
The parameter "address" can be used for
removing the specified secure MAC address.
The parameter "interface" can be used for
removing the secure MAC addresses on the
specified port.
(config-if)#[no] switchport
port-security violation
{protect | restrict | shutdown}
Define the action to be taken if the port
security setting of the interface has been
violated (i.e. a frame is received of which the
source MAC address is not on the list of
secure MAC addresses):
protect - drop the frame without logging.
restrict - drop the frame and log the
violation (i.e. an SNMP trap is sent, a
syslog message is logged, and the violation
counter increments).
shutdown - change the status of the
interface to error-disabled immediately, turn
off the port LED, and log the violation.
The interface can be re-enabled by using the
interface commands "shutdown" and then
"no shutdown".
The default action is shutdown.
Aging of Secure MAC Addresses
(config-if)#switchport port-security
aging time <time>
(config-if)#no switchport
port-security aging time
Specify the aging time (0-1440) in minutes for
the interface. A secure address will be
removed from the secure address list after the
aging time.
If the time is 0 (the default aging time), aging
21 CCNA 640-801 Exam Notes - Extending Switched Networks with VLANs
is disabled for this interface.
The "no" form of the command resets the
aging time to the default value (i.e. 0 min, or
aging is disabled).
(config-if)#switchport port-security
aging type {absolute | inactivity}
(config-if)#no switchport
port-security aging type
Set the aging type (or reset to the default type)
for the interface as follows:
Absolute - all the secure addresses will age
out after the time period specified by the
command "(config-if)#switchport portsecurity
aging time".
Inactivity - a secure address ages out only if
there is no data traffic from the address for
the specified time period.
The default aging type is "absolute".
(config-if)#[no] switchport
port-security aging static
Enable (or disable) aging for statically
configured secure addresses on the interface.
This feature is disabled by default.
1.10 The commands for displaying port security settings are summarized below:
Command Description
#show port-security Show the following information for each
interface with port security enabled:
Interface type and number.
Maximum number of secure MAC
addresses allowed.
Current number of secure MAC addresses.
Violation count, i.e. number of violations.
Violation mode, i.e. action to be taken for a
violation.
#show port-security interface
<int-type> <int-num>
Show the port security settings and statistics
of the specified interface, including the
following information:
Port security (Enabled or Disabled).
Port status (SecureUp or Err-Disabled).
Maximum number of secure MAC
addresses allowed.
Current number of secure MAC addresses.
Number of statically configured secure
MAC addresses.
Violation mode and Violation count.
Aging time, aging type, and if aging for
statically configured secure addresses is
enabled.
#show port-security
[interface <int-type> <int-num>]
address
Show the secure addresses for each interface
or the specified interface. The following
information will be displayed for each
address:
VLAN ID.
MAC address.
Type (Dynamic or Configured).
Interface type and number.
Remaining age.
2. Cisco Switches - VLAN Configuration
2.1 On a 2950 switch, configuration information for the normal-range VLANs (i.e. VLAN
1 - 1005) is stored in the VLAN database. Normal-range VLANs configuration can
be performed using the commands under the VLAN configuration mode:
Command Description
#vlan database Enter the VLAN configuration mode
"(vlan)#".
Changes made in the VLAN configuration
mode are recorded in the proposed database.
They are not effective until the command
"(vlan)#exit" or "(vlan)#apply" is issued.
(vlan)#[no] vlan <vlan-id>
[name <vlan name>]
[media {ethernet | fddi | fddi-net |
tokenring | tr-net}]
[mtu <mtu in bytes>]
[state {suspend | active}]
Add a VLAN.
Assign a name to the VLAN (default =
"VLAN<vlan-id in 4 digits>"). The name of
VLAN 1 is "default", which cannot be
changed.
Define the media type for the VLAN (default
= ethernet).
Define the MTU for the VLAN (default =
1500 bytes for ethernet).
Specify the state for the VLAN (default =
active).
The "no" form of the command resets the
parameters to their default values.
(vlan)#no vlan <vlan-id> Delete the VLAN.
(vlan)#apply Apply the changes made in the VLAN
configuration mode and increment the VLAN
database revision number.
(vlan)#exit Apply changes, increment revision number,
and exit to the privileged exec mode.
(vlan)#abort Exit to the privileged exec mode without
applying the changes.
(vlan)#reset Reset the proposed database (i.e. abandon the
changes) and remain in the VLAN
configuration mode.
Remark: Cisco is phasing out the vlan configuration mode (i.e. vlan database mode).
2.2 Alternatively, VLAN configuration can be performed using the following commands
under the config-vlan mode:
Command Description
(config)#[no] vlan <vlan-id> Add a VLAN and enter the config-vlan mode
"(config-vlan)#".
The VLAN will be created when you exit the
config-vlan mode.
Except the command "(config-vlan)#
shutdown", all commands under the configvlan
mode take effect when you exit the
config-vlan mode.
The "no" form of the command deletes the
VLAN.
(config-vlan)#[no] name
<vlan name>
Assign a name to the VLAN (or reset it to the
default value).
The default value is "VLAN<vlan-id in 4
digits>".
(config-vlan)#media {ethernet |
fddi | fd-net | tokenring | tr-net}
Define the media type of the VLAN.
The default value is "ethernet".
(config-vlan)#state {suspend |
active}
Specify the state of the VLAN.
The default value is "active".
(config-vlan)#mtu <mtu in bytes> Define the MTU of the VLAN.
The default value is "1500".
(config-vlan)#[no] shutdown Shutdown (or bring up) VLAN switching on
the VLAN. This command takes effect
immediately.
(config-vlan)#exit Apply changes and exit the config-vlan mode.
CCNA 640-801 Exam Notes - Extending Switched Networks with VLANs 24
2.3 VLAN membership of an interface can be set using the following commands:
Command Description
(config-if)#switchport mode
{access | trunk |
dynamic {auto | desirable}}
Configure the VLAN membership mode of the
port as follows:
access - configure as a static-access or
dynamic-access port depending on the
setting of the command "(config-if)#
switchport access vlan". An access port
can be assigned to only one VLAN.
trunk - configure as a trunk port
unconditionally, and negotiate with the
connected device (i.e. the device on the
other end of the link) to decide whether to
use 802.1Q or ISL.
dynamic auto - configure as a trunk port
only if the connected device is in dynamic
desirable or trunk state.
dynamic desirable - configure as a trunk
port only if the connected device is in
dynamic auto, dynamic desirable, or trunk
state.
The default mode is dynamic desirable.
(config-if)#switchport access vlan
{<vlan-id> | dynamic}
Define the port as:
a static-access port and specify the VLAN
membership; or
a dynamic-access port, i.e. VLAN
membership assignment is defined in a
VLAN Membership Policy Server (VMPS)
based on the MAC address of the host
connected to the port.
The default VLAN membership type of an
access port is static and the default VLAN is
1.
(config-if)#no switchport access Reset the port to the default VLAN for the
switch.
2.4 A trunk interface can be configured using the following commands:
Command Description
(config-if)#switchport trunk
encapsulation {isl | dot1q |
negotiate}
(config-if)#no switchport trunk
encapsulation
On switches that support both ISL and 802.1q,
set the trunk encapsulation format to ISL,
802.1q, or negotiation.
The "no" form of the command resets the
encapsulation format to the default value.
A 2950 switch does not support ISL, but only
802.1q. This command is also not support.
25 CCNA 640-801 Exam Notes - Extending Switched Networks with VLANs
(config-if)#[no] switchport trunk
allowed vlan <vlan-list>
On a trunk interface, define the VLANs that
can send or receive traffic on this interface in
tagged format, i.e. tagged frames (or reset to
the default value).
<vlan-list> can be "all", or "add | remove |
except <vlan-id>".
By default, all VLANs are allowed.
(config-if)#[no] switchport trunk
native vlan <vlan-id>
On a trunk interface, define the native VLAN
for sending and receiving untagged traffic
when the interface is in 802.1q trunking mode
(or reset to the default value).
Both sides of a trunk link must agree on which
VLAN as the native VLAN.
The default native VLAN is 1.
(config-if)#[no] switchport trunk
pruning vlan <vlan-list>
On a trunk interface, define the VLANs that
are enabled for VTP pruning, i.e. the pruningeligible
VLAN list (or reset the list to the
default value).
VLAN 1, VLANs 1002 to 1005, and
extended-range VLANs (i.e. VLANs 1006 to
4094) cannot be pruned.
<vlan-list> can be "none", or "add | remove |
except <vlan-id>".
By default, all VLANs (except VLAN 1 and
1002-4094) are enabled for VTP pruning.
Remark: You also need to enable VTP pruning
by the command "(config)#vtp pruning"
(details about the command can be found in
the next section).
2.5 An example for configuring the VLAN settings on a 2950 switch is as follows:
Using the vlan configuration mode (i.e. vlan database mode):
Switch#vlan database
Switch(vlan)#vlan 3 name hr_vlan
VLAN 3 added:
Name: hr_vlan
Switch(vlan)#exit
APPLY completed.
Exiting….
Switch(config)#interface fastethernet 0/5
Switch(config-if)#switchport mode access
Switch(config-if)#switchport access vlan 3
Create VLAN 3 and assign the name
"hr_vlan" to it.
Exit the VLAN configuration mode and
apply the change.
Configure the interface as an access
interface.
Assign the interface as a member of
CCNA 640-801 Exam Notes - Extending Switched Networks with VLANs 26
Switch(config-if)#exit
Switch(config)#interface fastethernet 0/6
Switch(config-if)#switchport mode trunk
Switch(config-if)#exit
VLAN 3.
Configure the interface as a trunk
interface. Use 802.1q trunking,
VLAN 1 as the native VLAN, and allow
traffic of all VLANs by default.
Using the config-vlan mode:
Switch(config)#vlan 3
Switch(config-vlan)#name hr_vlan
Switch(config-vlan)#exit
Switch(config)#interface fastethernet 0/5
Switch(config-if)#switchport mode access
Switch(config-if)#switchport access vlan 3
Switch(config-if)#exit
Switch(config)#interface fastethernet 0/6
Switch(config-if)#switchport mode trunk
Switch(config-if)#exit
Create VLAN 3.
Assign the name "hr_vlan" to VLAN 3.
Configure the interface as an access
interface.
Assign the interface as a member of
VLAN 3.
Configure the interface as a trunk
interface. Use 802.1q trunking,
VLAN 1 as the native VLAN, and allow
traffic of all VLANs by default.
2.6 The following commands can be used for showing VLAN configuration information:
Command Description
(vlan)#show Show information in the VLAN database.
For each VLAN, the following information
will be displayed:
VLAN ID.
VLAN name.
Status (e.g. operational or suspended).
Media type.
MTU.
(vlan)#show {current | proposed |
changes} [<vlan-id>]
Show information in the current database / in
the proposed database / about the difference
between the current and the proposed database
for all VLANs or the specified VLAN.
#show vlan [{id <vlan-id>} |
{name <vlan name>}]
Show the following information for each
VLAN or the specified VLAN(s):
VLAN ID.
VLAN name.
Status (active or suspended).
Ports that belong to the VLAN.
Media type, MTU, etc.
27 CCNA 640-801 Exam Notes - Extending Switched Networks with VLANs
<vlan-id> can be a specific VLAN ID, a range
of VLANs separated by a hyphen (e.g. "3-7"),
or a series of VLANs separated by commas
(e.g. "3,7,11"). This format also applies to
the other VLAN-related commands where
<vlan-id> can be used to specify multiple
VLANs.
#show vlan brief Show the following information for each
VLAN in a table format:
VLAN ID.
VLAN name.
Status (active or suspended).
Ports that belong to the VLAN.
#show interfaces
[<int-type> <int-num>] switchport
Show the switchport information of all
interfaces or the specified interface.
The following information for each interface
will be displayed:
Administrative mode (access, dynamic auto,
dynamic desirable, or trunk).
Operational mode (access or trunk).
Administrative trunking encapsulation (isl,
802.1q, or negotiate).
Operational trunking encapsulation (isl or
802.1q).
Negotiation of trunking (on or off).
Access mode VLAN.
Trunking native mode VLAN.
VLANs allowed on the trunk.
VLANs that are pruning-eligible.
etc.
#show interfaces
[<int-type> <int-num>] trunk
Show the trunk information of all active trunk
interfaces or the specified interface.
The following information for each interface
will be displayed:
Administrative mode (access, dynamic auto,
dynamic desirable, or trunk)
Trunking encapsulation (isl or 802.1q).
Status.
Native VLAN (default = 1).
VLANs allowed on the trunk.
VLANs allowed and active in the domain.
VLANs in spanning tree forwarding state
and not pruned.
CCNA 640-801 Exam Notes - Extending Switched Networks with VLANs 28
3. Cisco Switches - VTP Configuration
3.1 On a 2950 switch, VTP configuration can be performed under the VLAN
configuration mode using the following commands:
Command Description
(vlan)#vtp domain <domain name> Set the VTP domain name (case sensitive) that
identifies the VTP administrative domain for
the switch.
There is no domain name by default.
(vlan)#vtp {server | client |
transparent}
(vlan)#no vtp {client | transparent}
Set the VTP mode (or reset to the default
mode) for the switch.
The default mode is VTP server.
(vlan)#vtp password <password>
(vlan)#no vtp password
Set (or remove) the domain password (case
sensitive) for the generation of the secret value
to be sent in VTP advertisements and to
validate received VTP advertisements.
There is no password by default.
(vlan)#[no] vtp pruning Enable (or disable) pruning in the VTP
domain (default = disabled).
If VTP pruning is enabled, information about
each pruning-eligible VLAN (defined by the
command "(config-if)#switchport trunk
pruning") will be removed from the VTP
updates if there are no stations belonging to
that VLAN.
(vlan)#[no] vtp v2-mode Enable (or disable) VTP version 2 in the
domain (default = disabled).
To use VTP version 2, all switches in the
network must support version 2. You only
need to enable version 2 on one switch, and
the version number will then be propagated to
the other switches in the domain.
(config)#vtp file <filename>
(config)#no vtp file
Specify the system file where the VTP VLAN
database is stored (or reset to the default
value).
The default value is "flash:vlan.dat".
3.2 Alternatively, VTP configuration can be performed under the global configuration
mode using the following commands:
29 CCNA 640-801 Exam Notes - Extending Switched Networks with VLANs
Command Description
(config)#vtp domain
<domain-name>
Set the VTP domain name.
There is no domain name by default.
(config)#vtp mode {client | server |
transparent}
(config)#no vtp mode
Set the VTP mode for the switch (or reset to
the default value).
The default mode is VTP server.
(config)#vtp password <password>
(config)#no vtp password
Set (or reset) the VTP domain password.
There is no password by default.
(config)#vtp pruning
{enable | disable}
(config)#no vtp pruning
Enable or disable pruning for the VTP domain
(or reset to the default value).
Pruning is enabled by default.
(config)#vtp version {1 | 2}
(config)#no vtp version
Set VTP version to 1 or 2 (or reset to the
default value).
The default version is 1.
3.3 The following commands can be used for displaying VTP configuration information:
Command Description
#show vtp status Show general information about the VTP
management domain status, including:
VTP domain name.
VTP operating mode of the switch (server,
client, or transparent).
VTP pruning mode of the domain (enabled
or disabled).
VTP version operating on the switch.
VTP V2 mode for the domain (enabled or
disabled).
Number of existing VLANs.
Maximum number of VLANs supported
locally (1005 if standard software image is
used).
Current configuration revision number on
the switch.
Date and time of the last configuration
modification, and the IP address that caused
the configuration change.
etc.
#show vtp counters Show the VTP statistics such as:
Number of summary advertisements, subset
advertisements, and request advertisements
CCNA 640-801 Exam Notes - Extending Switched Networks with VLANs 30
received and sent.
Number of configuration revision errors or
digest errors.
VTP pruning statistics (e.g. number of
pruning messages received and sent), etc.
