RS_instructPPT_Cgdgugfhgfgfcer5_final.pdf

© 2008 Cisco Systems, Inc. All rights reserved. Cisco Confidential
Presentation_ID 1
Chapter 5: Inter-VLAN
Routing
Routing & Switching

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Chapter 5
5.1 Inter-VLAN Routing Configuration
5.2 Troubleshooting Inter-VLAN Routing
5.3 Layer 3 Switching
5.4 Summary

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Chapter 5: Objectives
 Describe the three primary options for enabling inter-VLAN routing.
 Configure legacy inter-VLAN routing.
 Configure router-on-a-stick inter-VLAN routing.
 Troubleshoot common inter-VLAN configuration issues.
 Troubleshoot common IP addressing issues in an inter-VLAN-routed
environment.
 Configure inter-VLAN routing using Layer 3 switching.
 Troubleshoot inter-VLAN routing in a Layer 3-switched environment.

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5.1 Inter-VLAN Routing
Configuration

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Inter-VLAN Routing Operation
What is Inter-VLAN routing?
 Layer 2 switches cannot forward traffic between VLANs without the
assistance of a router.
 Inter-VLAN routing is a process for forwarding network traffic from
one VLAN to another, using a router.

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Legacy Inter-VLAN Routing
In the past:
 Actual routers were used to route between VLANs.
 Each VLAN was connected to a different physical router interface.
 Packets would arrive on the router through one through interface,
be routed and leave through another.
 Because the router interfaces were connected to VLANs and had
IP addresses from that specific VLAN, routing between VLANs was
achieved.
 Large networks with large number of VLANs required many router
interfaces.

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Router-on-a-Stick Inter-VLAN Routing
 The router-on-a-stick approach uses a different path to route
between VLANs.
 One of the router’s physical interfaces is configured as a 802.1Q
trunk port so it can understand VLAN tags.
 Logical subinterfaces are created; one subinterface per VLAN.
 Each subinterface is configured with an IP address from the VLAN it
represents.
 VLAN members (hosts) are configured to use the subinterface
address as a default gateway.
 Only one of the router’s physical interface is used.

Presentation_ID 8
Multilayer Switch Inter-VLAN Routing
 Multilayer switches can perform Layer 2 and Layer 3 functions,
replacing the need for dedicated routers.
 Multilayer switches support dynamic routing and inter-VLAN routing.
 The multilayer switch must have IP routing enabled.
 A switch virtual interface (SVI) exists for VLAN 1 by default. On a
multilayer switch, a logical (layer 3) interface can be configured for
any VLAN.
 The switch understands network-layer PDUs; therefore, can route
between its SVIs, just as a router routes between its interfaces.
 With a multilayer switch, traffic is routed internal to the switch device.
 This routing process is a suitable and scalable solution.

Presentation_ID 9
Configure Legacy Inter-VLAN Routing
Preparation
 Legacy inter-VLAN routing requires routers to have multiple
physical interfaces.
 Each one of the router’s physical interfaces is connected to a
unique VLAN.
 Each interface is also configured with an IP address for the subnet
associated with the particular VLAN.
 Network devices use the router as a gateway to access the
devices connected to the other VLANs.

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Preparation (cont.)

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Switch Configuration

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Router Interface Configuration

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Configure Router-on-a-Stick
Preparation
 An alternative to legacy inter-VLAN routing is to use VLAN trunking
and subinterfaces.
 VLAN trunking allows a single physical router interface to route
traffic for multiple VLANs.
 The physical interface of the router must be connected to a trunk
link on the adjacent switch.
 On the router, subinterfaces are created for each unique VLAN.
 Each subinterface is assigned an IP address specific to its subnet
or VLAN and is also configured to tag frames for that VLAN.

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Switch Configuration

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Router Subinterface Configuration

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Verifying Subinterfaces

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Verifying Subinterfaces (cont.)

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 Access to devices on remote VLANs can be tested using
the ping command.
 The ping command sends an ICMP echo request to the
destination address.
 When a host receives an ICMP echo request, it responds with an
ICMP echo reply.
 Tracert is a useful utility for confirming the routed path taken
between two devices.
Verifying Routing

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6.2 Troubleshoot Inter-
VLAN Routing

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 When using the legacy routing model, ensure that the switch ports
connect to the router interfaces and are configured with the correct
VLANs.
 Use the switchport access vlan [appropriate vlan#]
command to correct any erroneous VLAN port assignment.
 Ensure that the router is connected to the correct switch port.
 When using router-on-a-stick, ensure that the switch port
connected to the router is configured as a trunk link.
 Use the switchport mode trunk command to make the
switch port a trunk.
Inter-VLAN Configuration Issues
Switch Port Issues

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Verify Switch Configuration

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 With router-on-a-stick configurations, a common problem is
assigning the wrong VLAN ID to the subinterface.
 The show interface command can help detect this problem.
 If this is the case, use the encapsulation dot1q <vlan_id>
interface command to fix the problem.
Verify Router Configuration

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Verify Router Configuration (cont.)

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 When using legacy inter-VLAN routing, ensure that the router has
the correct IP address and mask on the interfaces connecting to
the switch.
 Ensure that the network devices are configured with the correct IP
address and mask.
 In the router, use the ip address command to fix any erroneous
IP assignments.
 In the PCs, refer to the installed operating system documentation
to properly change IP information.
IP Addressing Issues
Errors with IP Address and Subnet Masks

Presentation_ID 25
 Use the show ip interface command to verify if the correct IP
address is configured in the router.
 Use the show running-config when troubleshooting router-
related problems.
 When troubleshooting addressing issues, ensure that the
subinterface is configured with the correct address for that VLAN.
 Subinterface IDs are often configured to match the VLAN number,
which makes it easier to manage inter-VLAN configuration, but this
is not a requirement.
IP Addressing Issues
Verifying IP Address and Subnet Mask
Configuration Issues

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5.3 Layer 3 Switching

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 Layer 3 switches usually have packet-switching throughputs in the
millions of packets per second (pps).
 All Catalyst multilayer switches support the following types of Layer 3
interfaces:
• Routed port
• Switch virtual interface (SVI)
 High-performance switches, such as the Catalyst 6500 and Catalyst
4500, are able to perform most of the router’s functions.
 Several models of Catalyst switches require enhanced software for
specific routing protocol features.
Layer 3 Switching Operation and Configuration
Introduction to Layer 3 Switching

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 Today’s routing has become faster and cheaper and can be
performed at hardware speed.
 Routing can be transferred to core and distribution devices with
little to no impact on network performance.
 Many users are in separate VLANs, and each VLAN is usually a
separate subnet. This implies that each distribution switch must
have IP addresses matching each access switch VLAN.
 Layer 3 (routed) ports are normally implemented between the
distribution and the core layer. This model is less dependent on
spanning tree, because there are no loops in the Layer 2 portion of
the topology.
Inter-VLAN Routing with Switch Virtual Interfaces

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 By default, an SVI is created for the default VLAN (VLAN 1). This
allows for remote switch administration.
 Any additional SVIs must be created by the administrator.
 SVIs are created the first time the VLAN interface configuration
mode is entered for a particular VLAN SVI.
 Enter the interface vlan 10 command to create an SVI
named VLAN 10.
 The VLAN number used corresponds to the VLAN tag associated
with data frames on an 802.1Q encapsulated trunk.
 When the SVI is created, ensure that the specific VLAN is present in
the VLAN database.
Inter-VLAN Routing with SVIs (Cont.)

Presentation_ID 30
 SVIs advantages include:
• Much faster than router-on-a-stick, because everything is
hardware-switched and routed.
• No need for external links from the switch to the router for
routing.
• Not limited to one link. Layer 2 EtherChannels can be used
between the switches to get more bandwidth.
• Latency is much lower, because it does not need to leave the
switch.
Inter-VLAN Routing with SVIs (Cont.)

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 A routed port is a physical port that acts similarly to an interface on a
router.
 Routed ports are not associated with any VLANs.
 Layer 2 protocols, such as STP, do not function on a routed interface.
 Routed ports on a Cisco IOS switch do not support subinterfaces.
 To configure routed ports, use the no switchport interface
configuration mode command.
 Note: Routed ports are not supported on Catalyst 2960 Series
switches.
Inter-VLAN Routing with Routed Ports

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 The Cisco Switch Database Manager (SDM) provides multiple
templates for the Cisco Catalyst 2960 switch.
 The SDM lanbase-routing template can be enabled to allow the
switch to route between VLANs and to support static routing.
 Use the show sdm prefer command to verify which template is
in use.
 The SDM template can be changed in global configuration mode
with the sdm prefer command.
Configuring Static Routes on a Catalyst 2960

Presentation_ID 33
To troubleshoot Layer 3 switching issues, verify the following for
accuracy:
 VLANs
• VLANs must be defined across all the switches.
• VLANs must be enabled on the trunk ports.
• Ports must be in the right VLANs.
 SVIs
• SVIs must have the correct IP address or subnet mask.
• SVIs must be up.
• SVIs must match with the VLAN number.
Troubleshooting Layer 3 Switching
Layer 3 Switch Configuration Issues

Presentation_ID 34
To troubleshoot Layer 3 switching issues, verify the following for
accuracy:
 Routing
• Routing must be enabled.
• Each interface or network should be added to the routing
protocol.
 Hosts
• Hosts must have the correct IP address or subnet mask.
• Hosts must have a default gateway associated with an SVI or
routed port.
Troubleshooting Layer 3 Switching
Layer 3 Switching Configuration Issues (Cont.)

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Chapter 5: Summary
This chapter described and explained the following concepts:
 Inter-VLAN routing, the process of routing traffic between different
VLANs, using either a dedicated router or a multilayer switch
 Legacy, router-on-a-stick, and multilayer switch inter-VLAN routing
 Layer 3 switching, SVIs, and routed ports
 Troubleshooting inter-VLAN routing with a router or a Layer 3
switch
 Common errors involving VLAN, trunk, Layer 3 interface, and IP
address configurations

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Chapter 5: Summary

Presentation_ID 37

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