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CCNP Practical

Studies: Switching

By Justin Menga

Authors

Safari Books Online

Imprints

Inter-VLAN Routing Architectures

Within a LAN topology, inter-VLAN routing is used to route packets between different VLANs. Three common

inter-VLAN routing architectures are used in modern LAN networks today:

Router-on-a-stick

Router-on-a-stick using trunks

Layer 3 switching

This section examines each of these in detail, outlining any restrictions or issues associated with each.

Table of Contents

About the Author

About the Technical

Reviewers

Introduction

Chapter 1. Switching

Connectivity

Chapter 2. VLAN

Operations

Chapter 3. Trunking and

Bandwidth Aggregation

Chapter 4. Spanning Tree

Chapter 5. Inter-VLAN

Routing

Inter-VLAN Routing

Architectures

Multilayer LAN Design

Scenario 5-1:

Conﬁguring Basic IP

Routing

Scenario 5-2:

Conﬁguring Layer 3

Switching

Scenario Prerequisites

Summary

Chapter 6. Layer 3

Switching

Chapter 7. Multicast

Routing and Switching

Chapter 8. Traffic Filtering

and Security

Chapter 9. Quality of

Service

Chapter 10. Maintenance,

Monitoring, and

Troubleshooting

Chapter 11.

Comprehensive Switching

Self-Study Lab

Appendix A.

Comprehensive Switching

Self-Study Lab Part I

Solution

Appendix B.

Comprehensive Switching

Self-Study Lab Part II

Solution

Router-on-a-Stick

The

router-on–a-stick

architecture is the most basic method of inter-VLAN routing. In this architecture, a router is

simply connected to each VLAN and forwards inter-VLAN traffic between the appropriate VLANs.

Figure 5-1

shows this

architecture.

Figure 5-1

Router-on–a-Stick

As you can see in

Figure 5-1,

the router has a physical Ethernet interface dedicated for each VLAN. If IP hosts on VLAN

100 need to communicate with hosts of VLAN 200, IP packets with the appropriate source and destination IP addresses

are sent to the router, which looks up the destination IP address and forwards to the appropriate host on the

destination VLAN. The router-on-a-stick architecture is simple to understand because the Layer 2 functions (provided

by a switch) and Layer 3 functions (provided by a router) are physically separated.

The major issue with this architecture is performance. Because routers are software-based, they cannot route packets

as fast as switches (hardware-based) can switch frames. Even if you are using high-performance routers, the physical

interface connecting each VLAN to the router is a bottleneck because it can transmit packets only at 10 Mbps, 100

Mbps, or 1 Gbps depending on the interface type. This restriction means that the router becomes a performance

bottleneck when routing between high-speed VLANs.

Another issue with this architecture is the number of routers and physical interfaces required to support multiple

VLANs. A dedicated Ethernet interface is required per VLAN. Routers are low-density devices, meaning that there is a

high cost per port and multiple routing devices might be required to support multiple VLANs, increasing the complexity

of the network.

Finally, all inter-VLAN traffic must travel via the router. In

Figure 5-1,

even though the PCs in VLAN 100 and VLAN 200

are connected to the same switch, all inter-VLAN traffic between the PCs must be sent through the router, which is

inefficient.

Router-on-a-Stick Using Trunks

As discussed in the last section, the router-on-a-stick architecture has physical limitations based upon a dedicated

physical interface being required for each VLAN. This limitation can be removed by using trunk interfaces, where

multiple VLANs are supported on a single physical interface by using tagging technologies such as 802.1Q or ISL.

Rather than using physical interfaces to attach the router to each VLAN, virtual or logical interfaces are used to attach

the router to each VLAN.

Figure 5-2

shows this architecture.

Figure 5-2

Router-on-a-Stick Using Trunks

Figure 5-2,

virtual interfaces (rather than physical interfaces) are used to connect the router to each VLAN. A single

physical trunk interface transports tagged VLAN traffic to the router, with the tag determining to which virtual interface

a frame should be forwarded for routing. Apart from the differences between using physical interfaces per VLAN as

opposed to virtual interfaces per VLAN, this architecture is essentially identical to the traditional router-on-a-stick

architecture and suffers the same performance limitations, because the routing engine is still software-based and the

trunk interface is limited to 10 Mbps, 100 Mbps, or 1 Gbps.

Routing Using Layer 3 Switches

The architectures discussed thus far represent the traditional inter-VLAN routing architectures. The major issue with

these architectures is performance—if gigabit speed routing is required between VLANs, extremely high performance

and costly routers are required. A new form of inter-VLAN routing on the LAN has emerged in recent years called

Layer

3 switching.

With a Layer 3 switch, the traditionally separated Layer 2 and Layer 3 functions are combined into a single

1z3

23.10.2013, 23:11

CCNP Practical Studies: Switching | Inter-VLAN Routing Arc...

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device, eliminating the bottleneck associated with the cable between a router and switch by replacing the cable with a

high-speed backplane connection. Layer 3 switches also typically perform routing in specially designed hardware

circuitry rather than software, using specialized hardware that can perform routing functions at high speed. This means

that the performance of Layer 3 switches is much higher than traditional router-on-a-stick architectures. For example,

if you use a Cisco 3640 series router in the router-on-a-stick architecture, you can achieve routing speeds of up to

40,000 packets per second. If you compare this with a Cisco Catalyst 3550-24-EMI Layer 3 switch, which is actually

cheaper than a Cisco 3640 router, you can route packets at up to 6.6 million packets per second. This is obviously

quite a difference and highlights the limitations of using router-on-a-stick architectures for inter-VLAN routing on the

LAN. Of course, the Cisco 3640 router still has a place in the network; it supports a wide variety of diverse media,

including serial and ATM connections for WAN connectivity; also supports advanced features such as ﬁrewalling,

encryption, and so on—all of which are not supported on Cisco Catalyst switches.

The Layer 3 switch uses application-speciﬁc integrated circuits (ASICs), which are hardware chips that can route traffic

at very high speeds. These ASICs are installed on the switching engine of a Layer 3 switch, which traditionally switches

frames at Layer 2. The ASICs allow the switching engine to also switch frames that contain packets sent between

different VLANs. Each ASIC is programmed with the information required to route traffic from one VLAN to another,

without having to pass the traffic through the CPU of the routing engine. This information includes the egress port,

egress VLAN, and new destination MAC address that should be written for the frame that is sent. Some form of route

cache is normally used to store such information, with the ASIC searching the cache for routing information for the

destination IP address of packets as they are received. How this information is programmed into the route cache

depends on the Layer 3 switch architecture used; however, the end result is essentially the same.

In addition to the high-speed routing feature, these ASICs also can apply security access control list (ACL) ﬁltering and

Layer 3 quality of service (QoS) classiﬁcation, all at wire-speed, meaning these useful features can be turned on without

affecting performance.

NOTE

The internal mechanics of Layer 3 switching are covered in more detail in Chapter 6, "Layer 3 Switching."

When examining the architecture of a Layer 3 switch, it is important to understand that several different approaches to

Layer 3 switching implemented by Cisco exist:

Router-on-a-stick—

Some chassis-based Catalyst switches (e.g., the Catalyst 4000 and 5000) support routing

modules, which are effectively routers on a blade. Apart from having a high-speed connection to the switch

backplane, the routing module is essentially a router-on-a-stick, with all routed traffic requiring processing through

the routing module. This architecture is not really Layer 3 switching at all because the switch hardware has no

special ASICs for Layer 3 switching; instead, it is a high-speed, router-on-a-stick architecture.

Multilayer switching (MLS)—

In this architecture, hardware-based ASICs on the switching component of the Layer 3

switch refer to a cache that is populated with the required information to route a packet received on one VLAN to

another VLAN, without having to pass the packet through the routing engine. With MLS, the Layer 3 switching cache

is populated after the ﬁrst packet of a particular ﬂow (connection) is received and the route processor is queried for

routing information.

Cisco Express Forwarding (CEF)—

This architecture is identical to MLS in terms of the hardware-based ASICs

referring to a Layer 3 cache for information as to how to route packets between VLANs without involving the router

processor. CEF differs from MLS in terms of the way the Layer 3 cache is populated. CEF pre-populates the caches

with full routing information, which means the route processor never needs to be queried for the initial routing

information that is required in a MLS architecture.

In this chapter, you learn how to conﬁgure the Catalyst 4000 using the Layer 3 routing module in a router-on-a-stick

architecture. You also learn how to conﬁgure Layer 3 switching on the Catalyst 3550, which is based upon a CEF

architecture. In Chapter 6, you learn about MLS and CEF on the Catalyst 6000/6500 family of switches.

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