networks. Although the Internet serves as the
obvious example, this point is true for any type of network,
such as a large campus backbone. Because routers prevent
broadcast propagation and use more intelligent forwarding
algorithms than bridges and switches, routers provide more
efficient use of bandwidth. This simultaneously results in
flexible and optimal path selection. For example, it is very
easy to implement load balancing across multiple paths in most
networks when routing. On the other hand, Layer 2 load
balancing can be very difficult to design, implement, and
maintain. If a VLAN spans across multiple devices a trunk is
used to interconnect the devices. A trunk carries traffic for
multiple VLANs. For example, a trunk can connect a switch to
another switch, a switch to the inter-VLAN router, or a switch
to a server with a special NIC installed that supports
trunking. Remember that when a host on one VLAN wants to
communicate with a host on another, a router must be involved.
Interactive Media Activity Drag and Drop: Inter-VLAN
Routing When the student has completed this activity, the
student will learn the path packets take in a network with
inter-VLAN routing. The student will predict the path a packet
will take given the source host and the destination host.
Content 9.3 Inter-VLAN Routing Overview
9.3.3 Inter-VLAN issues and solutions When VLANs are
connected together, several technical issues will arise. Two of
the most common issues that arise in a multiple-VLAN
environment are: - The need for end user devices to
reach non-local hosts
- The need for hosts on different
VLANs to communicate
When a device needs to make a
connection to a remote host, it checks its routing table to
determine if a known path exists. If the remote host falls into
a subnet that it knows how to reach, then the system checks to
see if it can connect along that interface. If all known paths
fail, the system has one last option, the default route. This
route is a special type of gateway route, and it is usually the
only one present in the system. On a router, an asterisk (*)
indicates a default route in the output of the show ip
route command. For hosts on a local area network, this
gateway is set to whatever machine has a direct connection to
the outside world, and it is the Default Gateway listed in the
workstation TCP/IP settings. If the default route is being
configured for a router which itself is functioning as the
gateway to the public Internet, then the default route will
point to the gateway machine at an Internet service provider
(ISP) site. Default routes are implemented using the ip
route command. Router(Config)#ip route 0.0.0.0 0.0.0.0
192.168.1.1 In this example, 192.168.1.1 is the gateway.
Inter-VLAN connectivity can be achieved through either logical
or physical connectivity. Logical connectivity involves a
single connection, or trunk, from the switch to the router.
That trunk can support multiple VLANs. This topology is called
a router on a stick because there is a single connection to the
router. However, there are multiple logical connections between
the router and the switch. Physical connectivity involves a
separate physical connection for each VLAN. This means a
separate physical interface for each VLAN. Early VLAN designs
relied on external routers connected to VLAN-capable switches.
In this approach, traditional routers are connected via one or
more links to a switched network. The router-on-a-stick designs
employ a single trunk link that connects the router to the rest
of the campus network. Inter-VLAN traffic must cross the Layer
2 backbone to reach the router where it can move between VLANs.
Traffic then travels back to the desired end station using
normal Layer 2 forwarding. This out-to-the-router-and-back flow
is characteristic of router-on-a-stick designs. Interactive
Media Activity Drag and Drop: Inter-VLAN Routing Issues and
Solutions When the student has completed this activity, the
student will learn about some of the problems when using VLAN.
They will also learn some of the solutions.
Content
9.3 Inter-VLAN Routing Overview 9.3.4
Physical and logical interfaces In a traditional situation, a
network with four VLANs would require four physical connections
between the switch and the external router. As technologies
such as Inter-Switch Link (ISL) became more common, network
designers began to use trunk links to connect routers to
switches. Although any trunking technology such as ISL, 802.1Q,
802.10, or LAN emulation (LANE) can be used, Ethernet-based
approaches such as ISL and 802.1Q are most common. The Cisco
Proprietary protocol ISL as well as the IEEE multivendor
standard 802.1q are used to trunk VLANs over Fast Ethernet
links. The solid line in the example refers to the single
physical link between the Catalyst Switch and the router. This
is the physical interface that connects the router to the
switch. As the number of VLANs increases on a network, the
physical approach of having one router interface per VLAN
quickly becomes unscalable. Networks with many VLANs must use
VLAN trunking to assign multiple VLANs to a single router
interface. The dashed lines in the example refer to the
multiple logical links running over this physical link using
subinterfaces. The router can support many logical interfaces
on individual physical links. For example, the Fast Ethernet
interface FastEthernet 0/0 might support three virtual
interfaces numbered FastEthernet 1/0.1, 1/0.2 and 1/0.3. The
primary advantage of using a trunk link is a reduction in the
number of router and switch ports used. Not only can this save
money, it can also reduce configuration complexity.
Consequently, the trunk-connected router approach can scale to
a much larger number of VLANs than a one-link-per-VLAN design.
Content 9.3 Inter-VLAN Routing
Overview 9.3.5 Dividing physical interfaces into
subinterfaces A subinterface is a logical interface within a
physical interface, such as the Fast Ethernet interface on a
router. Multiple subinterfaces can exist on a single physical
interface. Each subinterface supports one VLAN, and is assigned
one IP address. In order for multiple devices on the same VLAN
to communicate, the IP addresses of all meshed subinterfaces
must be on the same network or subnetwork. For example, if
subinterface 2 has an IP address of 192.168.1.1 then
192.168.1.2, 192.168.1.3, and 192.1.1.4 are the IP addresses of
devices attached to subinterface 2. In order to route between
VLANs with subinterfaces, a subinterface must be created for
each VLAN. The next section discusses the commands necessary to
create subinterfaces and apply a trunking protocol and an IP
address to each subinterface.
Content
9.3 Inter-VLAN Routing Overview
9.3.6 Configuring inter-VLAN routing This section
demonstrates the commands necessary to configure inter-VLAN
routing between a router and a switch. Before any of these
commands are implemented, each router and switch should be
checked to see which VLAN encapsulations they support. Catalyst
2950 switches have supported 802.1q trunking since the release
of Cisco IOS release 12.0(5.2)WC(1), but they do not support
Inter-Switch Link (ISL) trunking. In order for inter-VLAN
routing to work properly, all of the routers and switches
involved must support the same encapsulation. On a router, an
interface can be logically divided into multiple, virtual
subinterfaces. Subinterfaces provide a flexible solution for
routing multiple data streams through a single physical
interface. To define subinterfaces on a physical interface,
perform the following tasks: - Identify the interface.
- Define the VLAN encapsulation.
- Assign an IP
address to the interface.
To identify the
interface, use the interface command in global
configuration mode. Router(config)#interface
fastethernet port-number. subinterface-number The
port-number identifies the physical interface, and the