Virtual LAN

1
Virtual LAN
2
Using Hubs

• Layer 1 devices
• Inexpensive
• In one port, out the others
• One collision domain
• One broadcast domain

3

• This is fine for small workgroups, but does not
  scale well for larger workgroups or heavy traffic.

4

• What if the computers were on two different
  subnets? Could they communicate within their own
  subnet? Yes Between subnets? No, need a router.

5

• Same issues as before, with more of an impact on
  the network.

6
Using Switches

• Layer 2 devices
• Moderate expense for common access switches, but
  can be very expensive.
• Layer 2 filtering based on Destination MAC
  addresses and Source Address Table
• One collision domain per port
• One broadcast domain

7
Two parallel paths (complete SAT tables) Data
traffic from 172.30.1.24 to 172.30.1.25 and from
172.30.1.26 to 172.30.1.27
8
As opposed to the Hub Data traffic from
172.30.1.21 to 172.30.1.22 and from 172.30.1.23
to 172.30.1.24
Collision!
9
Collisions and Switches What happens when two
devices on a switch, send data to another device
on the switch. 172.30.1.24 to 172.30.1.25 and
172.30.1.26 to 172.30.1.25
10
The switch keeps the frames in buffer memory, and
queues the traffic for the host 172.30.1.25.
This means that the sending hosts do not know
about the collisions and do not have to re-send
the frames.
Frames in buffer
11
Other Switching Features

• Review
• Asymmetric ports 10 Mbps and 100 Mbps
• Full-duplex ports
• Cut-through versus Store-and-Forward switching

12
Ports between switches and server ports are good
candidates for higher bandwidth ports (100 Mbps)
and full-duplex ports.
13
Introducing Multiple Subnets/Networks without
Routers

• Switches are Layer 2 devices
• Router are Layer 3 devices
• Data between subnets/networks must pass through a
  router.

14
A Switched Network with two subnets What are the
issues? Can data travel within the subnet? Yes
Can data travel between subnets? No, need a
router! What is the impact of a layer 2
broadcast, like an ARP Request?
ARP Request
15
All devices see the ARP Request. One broadcast
domain means the switches flood all broadcast out
all ports, except the incoming port. Switches
have no idea of the layer 3 information contained
in the ARP Request. This consumes bandwidth on
the network and processing cycles on the hosts.
16
One Solution Physically separate the subnets.
But still no data can travel between the subnets.
How can we get the data to travel between the
two subnets?
17
Introducing Multiple Subnets/Networks with Routers

• Switches are Layer 2 devices
• Router are Layer 3 devices
• Data between subnets/networks must pass through a
  router.

18
Routed Network Two separate broadcast domains,
because the router will not forward the layer 2
broadcasts such as ARP Requests.
19
Switches with multiple subnets

• So far this should have been a review.
• Lets see what happens when we have two subnets on
  a single switch and we want to route between the
  two subnets.

20
Router-on-a-stick When a single interface is
used to route between subnets or networks, this
is know as a router-on-a-stick. To assign
multiple ip addresses to the same interface,
secondary addresses or subinterfaces are used.
interface e 0 ip address 172.30.1.1
255.255.255.0 ip address 172.30.2.1 255.255.255.0
secondary
21

• Router-on-a-stick
• Advantages
• Useful when there are limited Ethernet interfaces
  on the router.
• Disadvantage
• Because a single link is used to connect multiple
  subnets, one link is having to carry the traffic
  for multiple subnets.
• Be sure this is link can handle the traffic. You
  may wish to use a high-speed link (100 Mbps) and
  full-duplex.

22

• Gotchas
• 1. Remember to have the proper default gateway
  set for each host.
• 172.30.1.0 hosts - default gateway is 172.30.1.1
• 172.30.2.0 hosts - default gateway is 172.30.2.1
• 2. The router must still route between subnets,
  so you must include
• Router (config) router rip
• Router (config-router) network
  172.30.0.0

23
Multiple interfaces Two Ethernet router ports
may be used instead of one. However this may be
difficult if you do not have enough Ethernet
ports on your router.
E0
E1
24
One switch two subnets Good News Data can
travel between subnets and we have two separate
broadcast domains. Bad News Hosts are on
different subnets but on a single layer 2
broadcast domain.
ARP Request
25
An ARP Request from 172.30.1.21 for 172.30.1.23
will still be seen by all hosts on the switch.
The switch is a layer 2 device and will flood
broadcast traffic out all ports, except the
incoming port.
26
Introducing VLANs

• VLANs create separate broadcast domains
• Routers are needed to pass information between
  different VLANs
• VLANs are not necessary to have separate subnets
  on a switched network, but as we will see they
  give us more advantages when it comes to things
  like data link (layer 2) broadcasts.

27
VLAN Definition

• A logical subgroup within a local area network
  that is created via software rather than manually
  moving cables in the wiring closet. It combines
  user stations and network devices into a single
  unit regardless of the physical LAN segment they
  are attached to and allows traffic to flow more
  efficiently within populations of mutual
  interest.
• VLANs are implemented in port switching hubs and
  LAN switches and generally offer proprietary
  solutions. VLANs reduce the time it takes to
  implement moves, adds and changes.
• VLANs function at layer 2. Since their purpose
  is to isolate traffic within the VLAN, in order
  to bridge from one VLAN to another, a router is
  required. The router works at the higher layer 3
  network protocol, which requires that network
  layer segments are identified and coordinated
  with the VLANs. This is a complicated job, and
  VLANs tend to break down as networks expand and
  more routers are encountered.

28
Layer 2 broadcast control An ARP Request from
172.30.1.21 for 172.30.1.23 will only be seen by
hosts on that VLAN. The switch will flood
broadcast traffic out only those ports belonging
to that particular VLAN, in this case VLAN 1.
ARP Request
Switch Port VLAN ID
29
Port-centric VLAN Switches Remember, as the
Network Administrator, it is your job to assign
switch ports to the proper VLAN. This assignment
is only done at the switch and not at the host.
Note The following diagrams show the VLAN below
the host, but it is actually assigned within the
switch.
30
Catalyst 1900 - VLAN Membership Configuration
Port VLAN Membership Type
----------------------------- 1 1
Static 2 2 Static 3 1
Static 4 2 Static 5
2 Static 6 1 Static 7
1 Static 8 1 Static
9 1 Static 10 1
Static 11 1 Static 12 2
Static AUI 1 Static A
1 Static B 1 Static
M Membership type V VLAN
assignment R Reconfirm dynamic membership
X Exit to previous menu Enter Selection
31
Layer 2 broadcast control Without VLANs, the ARP
Request would be seen by all hosts. Again,
consuming unnecessary network bandwidth and host
processing cycles.
ARP Request
32
With VLANs Data will only travel within the
VLAN. Remember that switches are Layer 2 devices
and they can only pass traffic within the VLAN.
ARP Request
Switch Port VLAN ID
33
Switch Port VLAN ID
34
With VLANs A switch cannot route data between
different VLANs. Example Data from 172.30.1.21
to 172.30.2.12
X
Switch Port VLAN ID
35

• Gotchas
• 1. Remember that VLAN IDs (numbers) are assigned
  to the switch port and not to the host.
  (Port-centric VLAN switches)
• 2. Be sure to have all of the hosts on the same
  subnet belong to the same VLAN, or you will have
  problems.
• Hosts on subnet 172.30.1.0/24 - VLAN 1
• Hosts on subnet 172.30.2.0/24 - VLAN 2
• etc.

36
Routing and VLANs

• In the previous example data could travel within
  the VLAN, but not between VLANs.
• Just like subnets, a router is needed to route
  information between different VLANs.
• The advantage is the switch propagates broadcast
  traffic only within the VLAN.

37
Data between VLANs is routed through the router.
Data from 172.30.1.21 to 172.30.2.12
38

• Gotchas
• 1. Remember to have the proper default gateway
  set for each host.
• 172.30.1.0 hosts - default gateway is 172.30.1.1
• 172.30.2.0 hosts - default gateway is 172.30.2.1
• 2. The router must still route between subnets,
  so you must include
• Router (config) router rip
• Router (config-router) network
  172.30.0.0
• 3. The switch ports to the router must have the
  corresponding VLAN ID to that subnet.
• Switch port to 172.30.1.1 must be on
  VLAN 1
• Switch port to 172.30.2.1 must be on
  VLAN 2

39
Switch Port VLAN ID
(VLAN ID not set at router.)
40
So, whats the difference?

• One of the main differences between subnets with
  VLANs and subnets without VLANs on switched
  networks, is that VLANs offer layer 2 broadcast
  control.

41
Here is an ARP Request example without VLANs.
ARP Request
42
Here is an ARP Request example with VLANs.
Notice that the broadcast is isolated only to the
VLAN that it came from, in this case VLAN 1.
ARP Request
43

• Can I use the Router-on-a-stick method with
  multiple VLANs?
• Can you remind me what Router-on-a-stick is?

44
What is Router-on-a-stick? When a single
interface is used to route between subnets or
networks, this is know as a router-on-a-stick.
To assign multiple ip addresses to the same
interface, secondary addresses or subinterfaces
are used.
interface e 0 ip address 172.30.1.1
255.255.255.0 ip address 172.30.2.1 255.255.255.0
secondary
45
With Router-on-a-stick, ISL or 802.1Q trunking is
needed. We will talk about tagging and trunking
in the next section.
46
VLAN introduction
.

• VLANs provide segmentation based on broadcast
  domains.
• VLANs logically segment switched networks based
  on the functions, project teams, or applications
  of the organization regardless of the physical
  location or connections to the network.
• All workstations and servers used by a particular
  workgroup share the same VLAN, regardless of the
  physical connection or location.

47
VLAN introduction
.

• VLANs are created to provide segmentation
  services traditionally provided by physical
  routers in LAN configurations.
• VLANs address scalability, security, and network
  management. Routers in VLAN topologies provide
  broadcast filtering, security, and traffic flow
  management.
• Switches may not bridge any traffic between
  VLANs, as this would violate the integrity of the
  VLAN broadcast domain.
• Traffic should only be routed between VLANs.

48
Broadcast domains with VLANs and routers
.

• A VLAN is a broadcast domain created by one or
  more switches.
• The network design above creates three separate
  broadcast domains.

49
Broadcast domains with VLANs and routers
2) With or without VLANs
10.0.0.0/8
10.1.0.0/16
1) Without VLANs
10.2.0.0/16
10.3.0.0/16

• 1) No VLANs, or in other words, One VLAN. Single
  IP network.
• 2) With or without VLANs. However this can be
  and example of no VLANS. In both examples, each
  group (switch) is on a different IP network.
• 3) Using VLANs. Switch is configured with the
  ports on the appropriate VLAN.
• What are the broadcast domains in each?

One link per VLAN or a single VLAN Trunk (later)
10.1.0.0/16
1) With VLANs
10.2.0.0/16
10.3.0.0/16
50
Tagging and Trunking
51

• Non-tagging Switches
• Lets first see how multiple VLANs are
  interconnected using switches that do not have
  the tagging capability.

52
Non-tagging Switches
For each VLAN, there must be a link between the
two switches. One link per VLAN. Be sure the
switch ports on the switches are configured for
the proper VLAN.
100BaseT Ports
Port 1 VLAN 1 Port 2 VLAN 2
Moe
1 2
VLAN 1 Port 1 on switch Moe is connected to
Port 1 on Switch Larry. VLAN 2 Port 2 on switch
Moe is connected to Port 2 on Switch Larry.
1 2
Larry
Port 1 VLAN 1 Port 2 VLAN 2
100BaseT Ports
53

• Advantages
• Each VLAN gets its own dedicated link with its
  own bandwidth.
• Disadvantages
• This requires a separate link for each VLAN.
  There may not be enough ports on the switch to
  accommodate a lot of different VLANs.

54

• Introducing Tagging and Trunking

55

• Some quick terminology
• Channel - multiple links that carry a single VLAN
  (I.e. Fast-Etherchannel)
• Trunk - one link that carries multiple VLANs
• Tagging - used to Identify which VLAN a frame
  belongs to

56

• Reminder Switches and Routers
• It is important to remember that hosts on
  different switches, can communicate with hosts
  which belong to their same subnet, without VLANs.
• It is also important to remember that if hosts on
  different subnets wish to communicate, then that
  traffic must be routed via a router.

57

• VLANs and Switches
• However, if you put those hosts that are on
  different subnets, into different VLANs, then the
  switches will need to communicate the VLAN IDs.
• Again, this can be done without VLANs, but as we
  saw one of the benefits to VLANs is layer 2
  broadcast control.

58

• Trunking (or tagging) is needed between switches,
  or a switch and a router, to pass traffic for
  multiple VLANs, if a single link is used.
• Your switches must have ports that can do this
  trunking or tagging.

59

• Advantages
• A single port on a switch or router can be used
  to send and receive traffic for multiple VLANs.
• Disadvantages
• This can put a lot of traffic on a single link,
  so be sure the link has enough bandwidth to
  handle it.
• This also requires the switch and/or router ports
  that are used for tagging to be capable of doing
  the tagging/trunking.

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61
Tagging needed between the switches. Note, that
there is no router here, so there is no
communications between the VLANs. Here is an
example of 172.30.1.20 sending information to
172.30.1.25
62

• VLAN Network - Inter-switch VLANs
• Two separate Broadcast Domains (VLAN 1 and VLAN
  2)
• Communications over the trunk links (i.e. between
  switches) uses Tagging
• 802.1q
• ISL (Inter-Switch Link) - Cisco
• 802.10 - FDDI
• ATM LANE
• Tagging needed between the switches
• No communications between the VLANs, because
  there is not a router
• NOTE VLAN ID is on the switches not on the
  hosts.

63
Catalyst 1900 - VLAN Membership Configuration
Port VLAN Membership Type
----------------------------- 1 1
Static 2 2 Static 3 1
Static 4 2 Static 5
2 Static 6 1 Static 7
1 Static 8 1 Static
9 1 Static 10 1
Static 11 1 Static 12 2
Static AUI 1 Static A
1 Static B 1 Static
M Membership type V VLAN
assignment R Reconfirm dynamic membership
X Exit to previous menu Enter Selection
NOTE This is just an example of a switch
configuration menu and does not show represent
the configuration of the previous example.
64
The router is now connected, so we can see how to
communicate between the VLANs. Because we are
using Router-on-a-stick, the router will also
need to be configured to include the ISL or
802.1Q tagging.
65

• Same Gotchas
• 1. Remember to have the proper default gateway
  set for each host.
• 172.30.1.0 hosts - default gateway is 172.30.1.1
• 172.30.2.0 hosts - default gateway is 172.30.2.1
• 2. The router must still route between subnets,
  so you must include
• Router (config) router rip
• Router (config-router) network
  172.30.0.0
• 3. The switch ports to the router must have the
  corresponding VLAN ID to that subnet.
• Switch port to 172.30.1.1 must be on
  VLAN 1
• Switch port to 172.30.2.1 must be on
  VLAN 2

66

• New Gotchas
• 4. Ports interconnecting switches must be
  capable of doing VLAN trunking, with either ISL
  or 802.1Q.
• 5. If you are using Router-on-a-stick, then the
  switch port and the router interface must be
  capable and configured to do trunking/tagging
  with either ISL or 802.1Q.
• 6. Remember, all traffic between different VLANs
  must be routed via the router.
• Question
• What if the router is not capable of doing the
  tagging or trunking? How can we use the router
  to switch between VLANs?

67

• Thats right! You use two interfaces on the
  router instead of one. One for each VLAN. On
  the switch you will not need to use trunk ports
  for the router. No ISL or 802.1Q tagging is
  needed.

68

• Would you like to see how the router is
  configured, with and without trunking?

69

• Well, we will do it anyways. -)
• Instead of using secondary addresses, we will use
  something more current, know as subinterfaces.
• This allows you to configure multiple interfaces
  on a single physical interface.
• Cisco has said that secondary addresses will
  eventually not be a part of future IOS releases.

70
Router-on-a-stick, the router will also need to
be configured to include the ISL or 802.1Q
tagging. Secondary or subinterfaces can be used.
71

• Using multiple Ethernet interfaces. On the
  switch you will not need to use trunk ports for
  the router. No ISL or 802.1Q tagging is needed.
  Each switch port is on a separate VLAN.

72
Fast Etherchannel
73
Fast Etherchannel
Allows two or four contiguous 100 Mbps ports to
operate as a single link, giving twice the
throughput. (command port-channel mode on)
100BaseT Ports
10BaseT Ports (12)
Moe
A B
Two 100BaseT Full-duplex ports 2 x (100 x 2)
400 Mbps throughput
A B
10BaseT Ports (12)
Larry
100BaseT Ports
74

• Fast Etherchannel is a Cisco proprietary feature,
  although other vendors have a similar solution.
• Fast Etherchannel allows some Cisco switches to
  use either two or four 100 Mbps ports as a
  single, virtual port.
• To the switch the multiple links will look like
  one, single, higher-bandwidth connection,
  combining the bandwidth of the two or four links
  between the two switches.

75
NetFlow Switching

• NetFlow Switching provides network layer
  switching to campus switches at high forwarding
  rates.
• The first packet of the flow is routed via the
  router.
• When a flow is detected, NetFlow switching
  establishes a cut-through path for all remaining
  packets in the flow.
• These can be switched by the switch and not
  routed by the router.

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77
VLAN operation
.

• Each switch port can be assigned to a different
  VLAN.
• Ports assigned to the same VLAN share broadcasts.
  
• Ports that do not belong to that VLAN do not
  share these broadcasts.

78
VLAN operation
.

• Static membership VLANs are called port-based and
  port-centric membership VLANs.
• As a device enters the network, it automatically
  assumes the VLAN membership of the port to which
  it is attached.
• The default VLAN for every port in the switch is
  the management VLAN. The management VLAN is
  always VLAN 1 and may not be deleted.
• This statement does not give the whole story. We
  will examine Management, Default and other VLANs
  at the end.
• All other ports on the switch may be reassigned
  to alternate VLANs.
• More on VLAN 1 later.

79
VLAN operation
.

• Important notes on VLANs
• VLANs are assigned on the switch port. There is
  no VLAN assignment done on the host (usually).
• In order for a host to be a part of that VLAN, it
  must be assigned an IP address that belongs to
  the proper subnet.
• Remember VLAN Subnet

80
VLAN operation
.

• Dynamic membership VLANs are created through
  network management software. (Not as common as
  static VLANs)
• CiscoWorks 2000 or CiscoWorks for Switched
  Internetworks is used to create Dynamic VLANs.
• Dynamic VLANs allow for membership based on the
  MAC address of the device connected to the switch
  port.
• As a device enters the network, it queries a
  database within the switch for a VLAN membership.

81
Benefits of VLANs
If a hub is connected to VLAN port on a switch,
all devices on that hub must belong to the same
VLAN.

• The key benefit of VLANs is that they permit the
  network administrator to organize the LAN
  logically instead of physically.
• Note Can be done without VLANs, but VLANs limit
  the broadcast domains
• This means that an administrator is able to do
  all of the following
• Easily move workstations on the LAN.
• Easily add workstations to the LAN.
• Easily change the LAN configuration.
• Easily control network traffic.
• Improve security.

82
Without VLANs No Broadcast Control
ARP Request

• Without VLANs, the ARP Request would be seen by
  all hosts.
• Again, consuming unnecessary network bandwidth
  and host processing cycles.

83
With VLANs Broadcast Control
Switch Port VLAN ID
ARP Request
84
VLAN Types
85
MAC address Based VLANs
.

• Rarely implemented.

86
Two Types of VLANs
.

• End-to-End or Campus-wide VLANs
• Geographic or Local VLANs

87
End-to-End or Campus-wide VLANs
.
88
Geographic or Local VLANs
.
89
End-to-End or Campus-wide VLANs
.

• End-to-End or Campus-wide VLANs
• Same VLAN/Subnet no matter what the location is
  on the network
• Trunking at the Core
• Usually not recommended by Cisco or other Vendors
• Adds complexity to network administration
• Does not resolve Layer 2 Spanning Tree issues
• Use to be recommended with routing at the Core
  was considered to slow.

90
End-to-End or Campus-wide VLANs
.

• The core layer router is being used to route
  between subnets (VLANs).
• The network is engineered, based on traffic flow
  patterns, to have 80 percent of the traffic
  contained within a VLAN.
• The remaining 20 percent crosses the router to
  the enterprise servers and to the Internet and
  WAN.
• Note This is known as the 80/20 rule. With
  todays traffic patterns, this rule is becoming
  obsolete.

91
Geographic or Local VLANs
.

• Geographic or Local VLANs
• More common
• Routing at the core
• Different VLAN/Subnet depending upon location

92
Geographic or Local VLANs

• As many corporate networks have moved to
  centralize their resources, end-to-end VLANs have
  become more difficult to maintain.
• Users are required to use many different
  resources, many of which are no longer in their
  VLAN.
• Because of this shift in placement and usage of
  resources, VLANs are now more frequently being
  created around geographic boundaries rather than
  commonality boundaries.

93
Geographic or Local VLANs
.

• This geographic location can be as large as an
  entire building or as small as a single switch
  inside a wiring closet.
• In a VLAN structure, it is typical to find the
  new 20/80 rule in effect. 80 percent of the
  traffic is remote to the user and 20 percent of
  the traffic is local to the user.
• Although this topology means that the user must
  cross a Layer 3 device in order to reach 80
  percent of the resources, this design allows the
  network to provide for a deterministic,
  consistent method of accessing resources.