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Chapter 6
InterVLAN Routing
© 1999, Cisco Systems, Inc.
6-1
Objectives
Upon completion of this chapter, you will
be able to perform the following tasks:
• Identify the network devices required to effect
interVLAN routing
• Configure a default gateway to ensure network
reachability
• Configure a route processor to facilitate
interVLAN routing
© 1999, Cisco Systems, Inc.
www.cisco.com
BCMSN—6-2
InterVLAN Routing
In this chapter, we discuss the following
topics:
• InterVLAN routing issues
• Distribution layer topology
• Configuring interVLAN routing
© 1999, Cisco Systems, Inc.
www.cisco.com
BCMSN—6-3
InterVLAN Routing
In this section we discuss the following
topics:
• InterVLAN Routing Issues
–Isolated Collision Domains
–Finding the Route
–Supporting Multiple VLAN traffic
• Distribution Layer Topology
• Configuring InterVLAN Routing
© 1999, Cisco Systems, Inc.
www.cisco.com
BCMSN—6-4
Problem: Isolated Broadcast
Domains
172.16.20.4
VLAN20
VLAN10
VLAN30
• Because of their nature, VLANs inhibit communication
between VLANs
© 1999, Cisco Systems, Inc.
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BCMSN—6-5
Solution: Routing Between
VLANs
172.16.20.4
VLAN20
VLAN10
VLAN30
• Communications between VLANs require a routing
processor
© 1999, Cisco Systems, Inc.
www.cisco.com
BCMSN—6-6
Problem: Finding the Route
I need to send this
packet to 172.16.20.4. That
address is not on my
local segment.
VLAN20
VLAN10
172.16.10.3
172.16.20.4
Network
172.16.20.0
Network
172.16.10.0
• InterVLAN communications introduce the problem of
where end-user stations send nonlocal packets
© 1999, Cisco Systems, Inc.
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BCMSN—6-7
Solution: Defining a Default
Gateway
I will send
the packet to
my default router.
VLAN20
VLAN10
172.16.10.3
172.16.20.4
Network
172.16.10.0
Network
172.16.20.0
I know where
network
172.16.20.0 is!
• End-user stations send nonlocal packets to a default
router
© 1999, Cisco Systems, Inc.
www.cisco.com
BCMSN—6-8
Problem: Supporting Multiple
VLAN Traffic
I need information
from File Server A.
I need information
from File Server A.
I need information
from File Server A.
?
???
VLAN10
VLAN20
VLAN30
I have three
distinct streams of
traffic destined for
the same place!
File Server A
172.16.3.127
• Multiple VLANs interfacing with a single route processor
require multiple connections or VLAN trunking
© 1999, Cisco Systems, Inc.
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BCMSN—6-9
Solution: Multiple Links
VLAN10 VLAN20 VLAN30
VLAN60
• The router can support a separate interface for each VLAN
© 1999, Cisco Systems, Inc.
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BCMSN—6-10
Solution: Inter-Switch Link
VLAN10 VLAN20 VLAN30
VLAN60
Eth 3/0.1
3/0.2
3/0.3
Eth 3/1.1
3/1.2
3/1.3
• The router can support a single ISL link for multiple VLANs
© 1999, Cisco Systems, Inc.
www.cisco.com
BCMSN—6-11
InterVLAN Routing
In this section we discuss the following
topics:
• InterVLAN Routing Fundamentals
• Distribution Switch Topology
–External Route Processors
–Internal Route Processors
• Configuring InterVLAN Routing
© 1999, Cisco Systems, Inc.
www.cisco.com
BCMSN—6-12
Distribution Layer Route
Processors
Distribution Layer
• The distribution-layer device is a combination of a
high-end switch and a route processor
© 1999, Cisco Systems, Inc.
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BCMSN—6-13
External Route Processor
VLAN41
Network 172.16.41.3
VLAN41
Network 172.16.41.4
Switch A
Switch B
Switch C
VLAN42
Network 172.16.42.5
• An external Cisco high-end router and a Catalyst 5000
switch with an NFFC or NFFCII
• Connected by multiple Ethernet connections or an ISL link
© 1999, Cisco Systems, Inc.
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BCMSN—6-14
Internal Route Processors
VLAN41
Network 172.16.41.3
VLAN41
Network 172.16.41.4
VLAN42
Network 172.16.42.5
• Multilayer switches integrate Layer 2 and Layer 3
functionality in a single box
© 1999, Cisco Systems, Inc.
www.cisco.com
BCMSN—6-15
Internal Route Processors (cont.)
RSFC
• RSFC is a daughter card
on the Supervisor Engine
IIG and IIIG
RSM
• RSM can reside in slots 2 through 12 of a Catalyst 5000 switch
© 1999, Cisco Systems, Inc.
www.cisco.com
BCMSN—6-16
InterVLAN Routing
In this section we discuss the following topics:
• InterVLAN Routing Fundamentals
• Distribution Layer Topology
• Configuring InterVLAN Routing
– Locating and accessing the route
processor
– Configuring an interface
– Defining a default gateway
– Testing the link
© 1999, Cisco Systems, Inc.
www.cisco.com
BCMSN—6-17
Locating the Route Processor
Switch (enable) show module
Mod Module-Name
--- ------------1
2
3
Ports
----0
24
1
Module-Type
--------------------Supervisor III
10/100BaseTX Ethernet
Route Switch
Model
Serial-Num
--------- --------WS-X5530 010821493
WS-X5225R 012145458
WS-X5302 006825295
Status
------ok
ok
ok
• Specifying a particular module number displays information on
that module
• Not specifying a module number displays information on all
modules installed in the system
© 1999, Cisco Systems, Inc.
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BCMSN—6-18
Accessing the Route
Processor
Switch (enable) session 3
• Eliminates the need to connect a terminal
directly to the RSM console port
© 1999, Cisco Systems, Inc.
www.cisco.com
BCMSN—6-19
Identifying the Route Processor
Router(config)#hostname RSM143
Router(config)exit
RSM143#
• The hostname uniquely identifies each route
processor within the network
© 1999, Cisco Systems, Inc.
www.cisco.com
BCMSN—6-20
Enabling an IP Routing Protocol
RSM141(config)#ip routing
RSM141(config)#router igrp 1
RSM141(config-router)#network 172.16.0.0
172.16.10.0
172.16.30.0
Network
172.16.0.0
• Routing protocols determine optimal paths
through the network and transport information
across these paths
© 1999, Cisco Systems, Inc.
www.cisco.com
BCMSN—6-21
Configuring an VLAN Interface on
an Internal Route Processor
RTR144(config)#interface vlan41
RTR144(config-if)#ip address 172.16.10.3 255.255.255.0
RTR144(config-if)#exit
RTR144(config)#interface vlan42
RTR144(config-if)#ip address 172.16.20.3 255.255.255.0
RTR144(config-if)#exit
• The internal route processor automatically encapsulates
packets using ISL
• Initial configuration requires a no shutdown command
© 1999, Cisco Systems, Inc.
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BCMSN—6-22
Configuring an VLAN Interface on
an External Route Processor
Slot
Port
Subinterface Number
RSM144(config)#interface fastethernet 0/1.1
RSM144(config-if)#encapsulation isl 10
RSM144(config-if)#ip address 172.16.10.3 255.255.255.0
RSM144(config-if)#exit
RSM144(config)#interface fastethernet 0/1.2
RSM144(config-if)#encapsulation isl 20
RSM144(config-if)#ip address 172.16.20.3 255.255.255.0
RSM144(config-if)#exit
Encapsulation Type and
VLAN Number
Interface FA 0/1
Subinterface 0/1.1
Interface FA 0/1
Subinterface 0/1.2
• Subinterfaces allow for routing multiple data streams through a
single physical interface
• Initial configuration requires a no shutdown command
© 1999, Cisco Systems, Inc.
www.cisco.com
BCMSN—6-23
Defining a Default Gateway
ASW31#config t
Enter configuration commands, one per line. End with CNTL/Z
ASW31(config)#ip default-gateway 172.16.1.163
Default Gateway
172.16.1.163
VLAN40
VLAN30
172.16.1.163
Default Gateway
172.16.1.163
ASW41#config t
Enter configuration commands, one per line. End with CNTL/Z
ASW41(config)#ip default-gateway 172.16.1.163
• Defining a default gateway facilitates interVLAN
communications
© 1999, Cisco Systems, Inc.
www.cisco.com
BCMSN—6-24
Testing the Link
PC41#ping 172.16.10.3
Sending 5, 100-byte ICMP Echos to 172.16.10.3,
time out is 2 seconds:
!!!!!
Success rate is 100 percent (5/5),
round-trip min/avg/max 0/0/0/ ms
• The ping command tests connectivity to remote hosts
© 1999, Cisco Systems, Inc.
www.cisco.com
BCMSN—6-25
Laboratory Exercise:
Visual Objective
Switch Block X
© 1999, Cisco Systems, Inc.
VLAN x1
VLAN x2
VLAN x3
VLAN x4
www.cisco.com
BCMSN—6-26
Summary
• InterVlan routing is a requirement to enable
communication between devices in separate
VLANs.
• Most devices are configured with the IP address
of a default router to which all non-local network
packets are sent.
• The Inter-Switch Link (ISL) protocol is used to
facilitate multiple VLAN traffic over a single link.
• The distribution layer routing processor can be
an internal or external router/switch topology.
© 1999, Cisco Systems, Inc.
www.cisco.com
BCMSN—6-28
Review
• List at least two problems that can impede
communications between VLANs, and
identify a solution for each problem.
• Identify at least two Cisco platform solutions
for an internal route processor topology at
the distribution layer.
• Compare and contrast the steps used to
configure an interface on an RSM and an ISL
link on an external router.
© 1999, Cisco Systems, Inc.
www.cisco.com
BCMSN—6-29