hubs to vlans cisco networking academy program © cisco systems, inc. 2000 from hubs to vlans

Cisco Networking Academy Program

From Hubs to VLANs

Using Hubs

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

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

172.30.1.21255.255.255.0

172.30.1.22255.255.255.0

172.30.1.23255.255.255.0

172.30.1.24255.255.255.0

Single HubŸ One Network (IP Network Address - usually)Ÿ One Collision DomainŸ One Broadcast Domain

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

Single Hub - Two subnetsŸ Two subnetsŸ One Collision DomainŸ One Broadcast Domain

172.30.1.21255.255.255.0

172.30.1.22 172.30.1.23

172.30.1.24255.255.255.0

255.255.255.0 255.255.255.0

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

All HubsŸ One Network AddressŸ One Collision DomainŸ One Broadcast Domain

172.30.1.21255.255.255.0

172.30.1.22255.255.255.0

172.30.1.23255.255.255.0

172.30.1.24255.255.255.0

172.30.1.25255.255.255.0

172.30.1.26255.255.255.0

172.30.1.27255.255.255.0

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

Switch and Hub NetworkŸ One NetworkŸ Several Collision Domains

Ÿ One per switch portŸ One for the entire Hub

Ÿ One Broadcast Domain

172.30.1.21255.255.255.0

172.30.1.22255.255.255.0

172.30.1.23255.255.255.0

Switch

172.30.1.24255.255.255.0

172.30.1.25255.255.255.0

172.30.1.26255.255.255.0

172.30.1.27255.255.255.0

Two virtual circuits: (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

172.30.1.21255.255.255.0

172.30.1.22255.255.255.0

172.30.1.23255.255.255.0

Switch

172.30.1.24255.255.255.0

172.30.1.25255.255.255.0

172.30.1.26255.255.255.0

172.30.1.27255.255.255.0

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!

172.30.1.21255.255.255.0

172.30.1.22255.255.255.0

172.30.1.23255.255.255.0

Switch

172.30.1.24255.255.255.0

172.30.1.25255.255.255.0

172.30.1.26255.255.255.0

172.30.1.27255.255.255.0

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

172.30.1.21255.255.255.0

172.30.1.22255.255.255.0

172.30.1.23255.255.255.0

Switch

172.30.1.24255.255.255.0

172.30.1.25255.255.255.0

172.30.1.26255.255.255.0

172.30.1.27255.255.255.0

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

Other Switching Features

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

Ports between switches and server ports are good candidates for higher bandwidth ports (100 Mbps) and full-duplex ports.

All Switched NetworkŸ One NetworkŸ Several Collision Domains

Ÿ One per switch portŸ One Broadcast Domain

Switch 1

172.30.1.21255.255.255.0

172.30.1.22255.255.255.0

172.30.1.23255.255.255.0

Switch 2

172.30.1.25255.255.255.0

172.30.1.26255.255.255.0

172.30.1.27255.255.255.0

172.30.1.28255.255.255.0172.30.1.24

255.255.255.0

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.

All Switched Network - Two NetworksŸ Two SubnetsŸ Several Collision Domains

Switch 1

172.30.1.21255.255.255.0

172.30.2.10255.255.255.0

172.30.1.23255.255.255.0

Switch 2

172.30.1.25255.255.255.0

172.30.2.14255.255.255.0

172.30.1.27255.255.255.0

172.30.2.16255.255.255.0172.30.2.12

255.255.255.0

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

All Switched Network - Two NetworksŸ Two SubnetsŸ Several Collision Domains

Switch 1

172.30.1.21255.255.255.0

172.30.2.10255.255.255.0

172.30.1.23255.255.255.0

Switch 2

172.30.1.25255.255.255.0

172.30.2.14255.255.255.0

172.30.1.27255.255.255.0

172.30.2.16255.255.255.0172.30.2.12

255.255.255.0

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.

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?

Two Switched NetworksŸ Two SubnetsŸ Several Collision Domains

Ÿ One per switch portŸ Two Broadcast Domain

Switch 1

172.30.1.21255.255.255.0

172.30.1.23255.255.255.0

172.30.1.25255.255.255.0

Switch 2

172.30.2.10255.255.255.0

172.30.2.12255.255.255.0

172.30.2.14255.255.255.0

172.30.2.16255.255.255.0172.30.1.26

255.255.255.0

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.

Routed NetworksŸ Two SubnetsŸ Several Collision Domains

Ÿ One per switch portŸ Communication between subnets

Switch 1

172.30.1.21255.255.255.0

172.30.1.23255.255.255.0

172.30.1.25255.255.255.0

Switch 2

172.30.2.10255.255.255.0

172.30.2.12255.255.255.0

172.30.2.14255.255.255.0

172.30.2.16255.255.255.0172.30.1.26

255.255.255.0

Router

172.30.1.1255.255.255.0

172.30.2.1255.255.255.0

Routed Network:

Two separate broadcast domains, because the router will not forward the layer 2 broadcasts such as ARP Requests.

Switches with multiple subnets

• So far this should have been a review.• Let’s see what happens when we have two

subnets on a single switch and we want to route between the two subnets.

Routed NetworksŸ Two SubnetsŸ Communication between subnets

Switch 1

172.30.1.21255.255.255.0

172.30.2.10255.255.255.0

172.30.1.23255.255.255.0

172.30.2.12255.255.255.0

Router172.30.1.1

172.30.2.1 sec255.255.255.0

Router-on-a-stick:

When a single interface is used to route between subnets or networks, this is known 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

Router-on-a-stickAdvantages• 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.

Gotcha’s1. 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

Switch 1172.30.1.21

255.255.255.0

172.30.2.10255.255.255.0

172.30.1.23255.255.255.0

172.30.2.12255.255.255.0

Router172.30.1.1255.255.255.0

172.30.2.1255.255.255.0

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.

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.

Switch 1

172.30.1.21255.255.255.0

172.30.2.10255.255.255.0

172.30.1.23255.255.255.0

172.30.2.12255.255.255.0

Router 172.30.1.1172.30.2.1 sec255.255.255.0

ARP Request

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.

Switch 1

172.30.1.21

172.30.2.10255.255.255.0

172.30.1.23255.255.255.0

172.30.2.12255.255.255.0

Router 172.30.1.1172.30.2.1 sec255.255.255.0

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.

Two VLANsŸ Two Subnets

Switch 1

172.30.1.21255.255.255.0

VLAN 1

172.30.2.10255.255.255.0

VLAN 2

172.30.1.23255.255.255.0

VLAN 1

172.30.2.12255.255.255.0

VLAN 2

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.

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.

1 2 3 4 5 6 .1 2 1 2 2 1 .

PortVLAN

Catalyst 1900 - VLAN Membership Configuration

[M] Membership type [V] VLAN assignment [R] Reconfirm dynamic membership [X] Exit to previous menu Enter Selection:

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

Layer 2 broadcast control:

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

No VLANs

Ÿ Same as a single VLAN

Ÿ Two Subnets

172.30.1.21255.255.255.0

172.30.2.10255.255.255.0

172.30.1.23255.255.255.0

172.30.2.12255.255.255.0

Switch 1

172.30.1.21255.255.255.0

VLAN 1

172.30.2.10255.255.255.0

VLAN 2

172.30.1.23255.255.255.0

VLAN 1

172.30.2.12255.255.255.0

VLAN 2

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.

1 2 3 4 5 6 .1 2 1 2 2 1 .

PortVLAN

Switch Port: VLAN ID

Switch 1

172.30.1.21255.255.255.0

VLAN 1

172.30.2.10255.255.255.0

VLAN 2

172.30.1.23255.255.255.0

VLAN 1

172.30.2.12255.255.255.0

VLAN 2

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

Gotcha’s

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

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.

VLANsŸ Two SubnetsŸ Communication between VLANsŸ NOTE: VLANs assigned only to the

Switch 1172.30.1.21255.255.255.0

VLAN 1

172.30.2.10255.255.255.0

VLAN 2

172.30.1.23255.255.255.0

VLAN 1

172.30.2.12255.255.255.0

VLAN 2

Router

172.30.1.1255.255.255.0

VLAN 1

172.30.2.1255.255.255.0

VLAN 2

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

Gotcha’s1. 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

Switch Port: VLAN ID

Router

172.30.1.1255.255.255.0

(VLAN 1)

172.30.2.1255.255.255.0

(VLAN 2)

(VLAN ID not set at router.)

So, what’s 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.

Here is an ARP Request example without VLANs.

Switch 1172.30.1.21

255.255.255.0

172.30.2.10255.255.255.0

172.30.1.23255.255.255.0

172.30.2.12255.255.255.0

Router172.30.1.1

255.255.255.0172.30.2.1

255.255.255.0

ARP Request

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.

VLANsŸ Two SubnetsŸ Communication between VLANsŸ NOTE: VLANs assigned only to the

Switch 1172.30.1.21

255.255.255.0VLAN 1

172.30.2.10255.255.255.0

VLAN 2

172.30.1.23255.255.255.0

VLAN 1

172.30.2.12255.255.255.0

VLAN 2

Router172.30.1.1

255.255.255.0VLAN 1

172.30.2.1255.255.255.0

VLAN 2

ARP Request

• Can I use the Router-on-a-stick method with multiple VLANs?

• Can you remind me what Router-on-a-stick is?

Switch 1172.30.1.21

255.255.255.0

172.30.2.10255.255.255.0

172.30.1.23255.255.255.0

172.30.2.12255.255.255.0

Router 172.30.1.1172.30.2.1 sec255.255.255.0

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

With Router-on-a-stick, ISL or 802.1Q trunking is needed. We will talk about tagging and trunking in the next section.

VLANsŸ Two SubnetsŸ Communication between VLANs using trunkingŸ NOTE: VLANs assigned only to the ports

Switch 1172.30.1.21255.255.255.0

VLAN 1

172.30.2.10255.255.255.0

VLAN 2

172.30.1.23255.255.255.0

VLAN 1

172.30.2.12255.255.255.0

VLAN 2

Router

172.30.1.1172.30.2.1 secondary

255.255.255.0Trunking ISLor 802.1Q

Trunking ISL or 802.1Q

Non-tagging Switches

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

100BaseT Ports

Port 1 = VLAN 1 & Port 2 = VLAN 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.

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.

Port 1 = VLAN 1 & Port 2 = VLAN 2

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.

hubs to vlans cisco networking academy program © cisco systems, inc. 2000 from hubs to vlans

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