Catalyst 2948G-L3 and Catalyst 4908G-L3 Software Feature and Configuration Guide IOS Software, Release 12.0(7)W5(15d)
Overview of Layer 3 Switching and Software Features
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Overview of Layer 3 Switching and Software Features

Table Of Contents

Overview of Layer 3 Switching and Software Features

About Layer 3 Switching

Layer 3 Switch Router Interface Types

Network Configuration Example

Layer 3 Switching Software Features

About Key Features

Distributed Hardware Forwarding

Cisco IOS Routing Protocols

QoS-Based Forwarding

Network Class Redundancy

Remote Monitoring

Load Balancing

Cisco Discovery Protocol

Cisco Express Forwarding

Hot Standby Router Protocol

Fast EtherChannel

Gigabit EtherChannel

Integrated Routing and Bridging

Spanning Tree Protocol

Virtual LANs

IEEE 802.1Q VLAN Encapsulation

Inter-Switch Link VLAN Encapsulation

Switching Database Manager


Overview of Layer 3 Switching and Software Features


This chapter gives an overview of Layer 3 switching. It describes both the Catalyst 2948G-L3 and the Catalyst 4908G-L3 switch router interface types, and shows how these Layer 3 switch routers fit into the network. Also included is a list of Layer 3 switching software features with brief descriptions of selected features. This chapter includes the following sections:

About Layer 3 Switching

Layer 3 Switch Router Interface Types

Network Configuration Example

Layer 3 Switching Software Features

About Key Features

About Layer 3 Switching

Layer 3 switching refers to a class of high-performance routers optimized for the campus LAN or intranet, providing both wirespeed Ethernet routing and switching services.

A Layer 3 switch router performs the following three major functions:

Packet switching

Route processing

Intelligent network services

Compared to conventional software based routers, Layer 3 switch routers process more packets faster by using application-specific integrated circuit (ASIC) hardware instead of microprocessor-based engines.
Layer 3 switch routers also improve network performance with two software functions—route processing and intelligent network services.

Layer 3 Switch Router Interface Types

Both the Catalyst 2948G-L3 and the Catalyst 4908G-L3 are fixed configuration switch routers based on Layer 3 switching software. The Catalyst 2948G-L3 is a multiprotocol 10/100/1000 Ethernet switch router and the Catalyst 4908G-L3 is a multiprotocol Gigabit Ethernet switch router.

Table 1-1 lists the interfaces supported in the Layer 3 switch routers.

Table 1-1 Interfaces Supported in Layer 3 Switch Routers

Layer 3 Switch Routers
Interface Types
No. of Ports

Catalyst 2948G-L3

10/100 Mbps Fast Ethernet—UTP

48

1 Gbps Gigabit Ethernet

2

Catalyst 4908G-L3

1 Gbps Gigabit Ethernet

8


Network Configuration Example

Figure 1-1 shows how the Layer 3 switch routers are used in a small campus backbone.

Figure 1-1 The Layer 3 Switch Routers in a Small Campus Backbone

Layer 3 Switching Software Features

This section lists the switching software features on the Catalyst 2948G-L3 and the Catalyst 4908G-L3 switch routers.

Layer 1 features

10/100BaseTX half duplex and full duplex (Catalyst 2948G-L3)

1000BaseSX, LX, and long haul (-LX/LH, -ZX) full duplex

Layer 2 bridging features

Layer 2 transparent bridging

Layer 2 Media Access Control (MAC) learning, aging, and switching by hardware

Spanning Tree Protocol (IEEE 802.1D) per bridge group

A maximum of 16 active bridge groups

Up to 4K MAC addresses

Integrated routing and bridging (IRB)

Layer 2 entries, IP routing, IP multicast routing, and Novell IPX routing share the 24K content addressable memory (CAM) on the Catalyst 2948G-L3 and the 32K CAM on the Catalyst 4908G-L3

Virtual LAN (VLAN) features

Inter-Switch Link (ISL)-based VLAN trunking

IEEE 802.1Q-based VLAN trunking

Layer 3 routing, switching, and forwarding

IP, IPX, and IP multicast routing and switching between Ethernet ports

Constrained multicast flooding (CMF)

QoS-based forwarding based on IP precedence

Load balancing among equal cost paths based on source and destination IP and IPX addresses

Layer 2 entries, IP routing, IP multicast routing, and Novell IPX routing share the 24K content addressable memory (CAM) on the Catalyst 2948G-L3 and the 32K CAM on the Catalyst 4908G-L3

Up to 18K IP routes

Up to 20K IP host entries

Up to 20K IPX routes

Up to 20K IPX host entries

Up to 128 IP multicast groups

Supported routing protocols

Routing Information Protocol (RIP and RIP II)

Interior Gateway Routing Protocol (IGRP)

Enhanced Interior Gateway Routing Protocol (EIGRP)

Open Shortest Path First (OSPF)

Internet Packet Exchange (IPX), RIP, and EIGRP

Protocol Independent Multicast (PIM)—sparse and dense modes

Secondary addressing

Static routes

Classless interdomain routing (CIDR)

Fast EtherChannel (FEC) features (Catalyst 2948G-L3)

Bundling of up to four Fast Ethernet ports

Load sharing based on source and destination IP and IPX addresses of unicast packets

Load sharing for bridge traffic based on MAC address

Inter-Switch Link (ISL) on the Fast EtherChannel

Integrated routing and bridging (IRB) on the Fast EtherChannel

IEEE 802.1Q trunking on the Fast EtherChannel

Up to 16 active FEC port channels


Note The Catalyst 4908G-L3 switch router does not have Fast Ethernet interfaces, which can be assigned to an EtherChannel.


Gigabit EtherChannel (GEC) features

Bundling the two Gigabit Ethernet ports on the Catalyst 2948G-L3 and up to four Gigabit Ethernet ports on the Catalyst 4908G-L3

Load sharing based on source and destination IP or IPX addresses of unicast packets

Load sharing for bridge traffic based on MAC address

Inter-Switch Link (ISL) on the Gigabit EtherChannel

Integrated routing and bridging (IRB) on the Gigabit EtherChannel

IEEE 802.1Q trunking on the Gigabit EtherChannel

Up to one active GEC port channel in the Catalyst 2948G-L3 and up to four active GEC port channels in the Catalyst 4908G-L3

Additional Protocols and Features

Bootstrap Protocol (BOOTP)

Cisco Discovery Protocol (CDP) support on Ethernet ports

Cisco Group Management Protocol (CGMP) server support

Dynamic Host Configuration Protocol (DHCP) relay

Hot Standby Routing Protocol (HSRP) over 10/100 Ethernet, Gigabit Ethernet, FEC, GEC, and BVI (Bridge Group Virtual Interface)

Internet Control Message Protocol (ICMP)

Internet Group Management Protocol (IGMP)

Internet Packet Exchange Service Advertisement Protocol (IPX SAP) and SAP filtering

Integrated routing and bridging (IRB) routing mode support

Simple Network Management Protocol (SNMP)

About Key Features

This section briefly describes the key features supported in Layer 3 switching software.

Distributed Hardware Forwarding

Layer 3 switching software employs a distributed architecture in which the control path and data path are relatively independent. The control path code, such as routing protocols, runs on the processor, whereas the data packets are switched by the Ethernet interfaces and the switching fabric.

A microcoded application-specific integrated circuit (ASIC) handles all packet switching for the interfaces. The following are the main functions of the control layer between the routing protocol and the firmware datapath microcode:

Managing the internal data and control circuits for the packet forwarding and control functions

Extracting the other routing and packet forwarding related control information from the Layer 2 and Layer 3 bridging and routing protocols and the configuration data, and then conveying the information to the interfaces to control the datapath

Collecting the datapath information, such as traffic statistics, from the interfaces to the processor

Handling certain data packets sent from the Ethernet interfaces to the processor

Cisco IOS Routing Protocols

Layer 3 switching software provides a comprehensive suite of routing protocols based on Cisco IOS software. The following networking protocols and routing protocols are supported on the Layer 3 switch routers.

Networking Protocols
Supported Routing Protocols

IP

RIP, RIP-2, OSPF, IGRP, EIGRP, PIM

IPX

IPX RIP, EIGRP


Many of the Cisco IOS routing protocol features, such as route redistribution and load balancing over equal cost paths (for OSPF and EIGRP) are supported. Configuration of these routing protocols is identical to the configuration methods currently employed on all Cisco routers.


Note Layer 2 entries, IP routing, IP multicast routing, and Novell IPX routing share the 24K content addressable memory (CAM) on the Catalyst 2948G-L3 and the 32K CAM on the Catalyst 4908G-L3.


See "Configuring Networking Protocols."

QoS-Based Forwarding

Quality of service (QoS) includes technologies such as Resource Reservation Protocol (RSVP) and weighted round-robin (WRR), which help control bandwidth, network delay, jitter, and packet loss in networks that become congested. The QoS identifier provides specific treatment to traffic in different classes, so that each class receives different quality of service.

The class to which the packets belong determine packet scheduling and discarding policies. For example, the overall service given to packets in the premium class will be better than that given to the standard class; the premium class is expected to experience lower loss rate or delay.

The switch router has QoS-based forwarding for IP traffic only. The implementation of QoS forwarding is based on local administrative policy and IP precedence. The mapping between the IP precedence field and the QoS field determines the delay priority of the packet.

See "Configuring Quality of Service."

Network Class Redundancy

The redundancy of Cisco IOS software provides key network features, such as Hot Standby Router Protocol (HSRP), routing protocol convergence with Routing Information Protocol (RIP), Open Shortest Path First (OSPF), Enhanced Interior Gateway Routing Protocol (EIGRP), EtherChannel, and load sharing across equal cost Layer 3 paths and spanning trees (for Layer 2 based networks).

Remote Monitoring

Layer 3 switching software supports the first four remote monitoring (RMON) groups.

RMON is a network management protocol for gathering network information and monitoring traffic data within remote LAN segments from a central location. RMON allows you to monitor all nodes and their interaction on a LAN segment. RMON, used in conjunction with the SNMP agent in the router, allows you to view both the traffic that flows through the router and segment traffic not necessarily destined for the router. Layer 3 switching software combines RMON alarms and events with existing MIBs so you can choose where monitoring will occur.

Refer to the Cisco IOS Configuration Fundamentals Configuration Guide.

Load Balancing

A router that employs load balancing can distribute traffic over all its network ports that are the same distance from the destination address. Load balancing increases the utilization of network segments, thus increasing effective network bandwidth.

Layer 3 switching software uses source + destination-based load balancing, an enhanced version of the Cisco IOS software per-destination load balancing. Essentially, this method takes certain bits from the source and destination IP and IPX addresses and maps them into a path.

Using this method has the following two benefits:

The traffic is distributed more effectively.

There is almost no impact on the datapath performance.

Layer 3 switching software supports load balancing on two equal cost paths using the source and destination IP and IPX address. Per-packet load balancing is not supported.

Cisco Discovery Protocol

Cisco Discovery Protocol (CDP) is a device-discovery protocol that is both media and protocol independent. CDP is available on all Cisco products, including routers, switches, bridges, and access servers. Using CDP, a device can advertise its existence to other devices and receive information about other devices on the same LAN. CDP enables Cisco products to exchange information with each other regarding their MAC addresses, IP addresses, and outgoing interfaces. CDP runs over the data link layer only, thereby allowing two systems that support different network-layer protocols to learn about each other. Each device configured for CDP sends periodic messages to a multicast address. Each device advertises at least one address at which it can receive Simple Network Management Protocol (SNMP) messages.

Cisco Express Forwarding

Layer 3 switching software features Cisco Express Forwarding (CEF). CEF is advanced Layer 3 IP switching technology. CEF optimizes network performance and scalability for networks with large and dynamic traffic patterns, such as the Internet, on networks characterized by intensive Web-based applications, or interactive sessions. Although you can use CEF in any part of a network, it is designed for high-performance, highly resilient Layer 3 IP backbone switching.

CEF manages route distribution and forwarding by distributing routing information from the central processor to the individual Ethernet interfaces. This technology, used within the Internet, provides scalability in large campus core networks. CEF provides Layer 3 forwarding based on a topology map of the entire network, resulting in high-speed routing table lookups and forwarding.

One of the key benefits of CEF in Layer 3 switching is its routing convergence. Since the forwarding information base (FIB) is distributed to all interfaces, whenever a route goes away or is added, the FIB updates that information and provides it to the interfaces. Thus, central processor interrupts are minimized. The interfaces receive the new topology very quickly and reconverge around a failed link based on the routing protocol being used.


Caution Cisco strongly recommends that you do not issue any CEF configuration commands. The CEF default settings should not be altered, and doing so may adversely affect the performance of your system.

Hot Standby Router Protocol

The Hot Standby Router Protocol (HSRP) provides high network availability by routing IP traffic from hosts on Ethernet networks without relying on the availability of any single router. This feature is particularly useful for hosts that do not support a router discovery protocol and do not have the functionality to switch to a new router when their selected router reloads or loses power.

Devices that are running the HSRP detect a failure by sending and receiving multicast User Datagram Protocol (UDP) "hello" packets. When HSRP detects that the designated active router has failed, the selected backup router assumes control of the HSRP group's MAC and IP addresses. (You can also select a new standby router at that time.)

The chosen MAC address and IP addresses are unique and do not conflict with any others on the same network segment. The MAC address is selected from a pool of Cisco MAC addresses. Configure the last byte of the MAC address by configuring the HSRP group number. You also configure the unique virtual IP address. The IP address must be specified on a single router within the same group. When the HSRP is running, it selects an active router and instructs its device layer to listen on an additional (dummy) MAC address.

Layer 3 switching software supports HSRP over 10/100 Ethernet, Gigabit Ethernet, FEC, GEC, and BVI (Bridge Group Virtual Interface).

Fast EtherChannel

Fast EtherChannel (FEC) establishes a high-bandwidth connection between two Layer 3 switch devices. You can use up to four Fast Ethernet connections as one Layer 3 forwarding path, which can provide up to 800 Mbps full duplex aggregate capacity. If link detection determines a failure of any one link, the packets are switched on the remaining active links in the FEC. No dependencies are placed on which ports to configure in the channel.

Fast EtherChannel uses a source-destination IP and IPX address load-balancing scheme for up to four ports in a channel group. Each channel group has its own IP and IPX address.When you queue a packet to exit out of the port channel interface, the last two bits of the IP and IPX source and destination address determine which interface in the channel the packet takes.

See "Configuring EtherChannel."

Gigabit EtherChannel

Gigabit EtherChannel (GEC) allows grouping of Gigabit Ethernet ports into a single multigigabit logical EtherChannel link. GEC establishes a high-bandwidth connection between two Catalyst switch routers.

You can bundle up to two Gigabit Ethernet connections on the Catalyst 2948G-L3 as one logical link, which can provide up to 4-Gb full-duplex aggregate capacity. On the Catalyst 4908G-L3, you can bundle up to four Gigabit Ethernet connections, which provide up to 8-Gb full-duplex aggregate capacity. If a failure of any one link is detected, the packets are switched on the remaining active link in the Gigabit EtherChannel.

Gigabit EtherChannel uses a source-destination IP address load-balancing scheme for up to two ports in a channel group on the Catalyst 2948G-L3 and up to four ports in a channel group on the Catalyst 4908G-L3. Each channel group has its own IP address.When you queue a packet to exit out of the port channel interface, the last two bits of the IP source and destination address determine which interface in the channel the packet takes.

As with all EtherChannel technologies, all links share the traffic load within the bundled ports.

See "Configuring EtherChannel."

Integrated Routing and Bridging

Integrated routing and bridging (IRB) allows you to route a given protocol between routed interfaces and various bridge groups or between bridge groups within a single router. Multiple ports in the switch router can reside in one bridge group with one IP address and be routed to other switch router interfaces with different IP addresses.

Specifically, you bridge local or unroutable traffic among the bridged interfaces in the same bridge group, while you route routable traffic to other routed interfaces or bridge groups.

Layer 3 switching software supports IRB for IP and IPX only.

Here are some examples of when to use IRB:

When you want to interconnect a bridged network with a routed network, the IRB feature enables the switch router to act as a true brouter.

For example, when you are migrating a bridged network to a routed network, or when the remote site does not have routing capabilities, you can use the switch router to interconnect the bridged and routed networks.

When you want to conserve IP or IPX addresses by connecting network segments with bridges and assigning each bridge group one network address.

When you want to break one big segment into several small segments to improve the performance of the end stations.


Note Layer 2 entries, IP routing, IP multicast routing, and Novell IPX routing share the 24K content addressable memory (CAM) on the Catalyst 2948G-L3 and the 32K CAM on the Catalyst 4908G-L3.


See the "Configuring IRB" section.

Spanning Tree Protocol

The Spanning Tree Protocol is a bridge protocol that enables a learning bridge to dynamically work around loops in a network topology by creating a spanning tree. Bridges exchange Bridge Protocol Data Unit (BPDU) messages with other bridges to detect loops, and then remove the loops by shutting down selected bridge interfaces.

The Spanning Tree Protocol is a standardized technique for maintaining a network of multiple bridges or switches. When the topology changes, the Spanning Tree Protocol transparently reconfigures bridges and switches to avoid the creation of loops by placing ports in a forwarding or blocking state. Each bridge group has a separate instance of the Spanning Tree Protocol.

The Spanning Tree Protocol parameters are set for each bridge group. For each Spanning Tree instance, you configure a set of global options with a set of port parameters. The port parameter list contains only ports that are members of a given bridge group. The Layer 3 switch routers support a maximum of 16 bridge groups, which run their own instance of spanning tree.

To configure the Spanning Tree Protocol, see the Cisco IOS Command Reference publication.

Virtual LANs

A virtual LAN (VLAN) configures switches and routers according to logical rather than physical topologies. Using VLANs, a network administrator can combine any collection of LAN segments within an internetwork into an autonomous user group, which appears as a single LAN. VLANs logically segment the network into different broadcast domains so that packets are switched only between ports within the VLAN. Typically, a VLAN corresponds to a particular subnet, although not necessarily.

To configure VLANs, you first define a subinterface at the interface, and map a VLAN to the subinterface. Layer 3 switching software supports up to 244 VLAN subinterfaces per system and up to 32 VLAN subinterfaces per physical port.

See the "About Virtual LANs" section.

IEEE 802.1Q VLAN Encapsulation

The IEEE 802.1Q standard provides a method for secure bridging of data across a shared backbone. Layer 3 switching software supports IEEE 802.1Q VLAN encapsulation over all media including Fast Ethernet, Gigabit Ethernet, Fast EtherChannel, and GigaChannel. The Layer 3 switch router can route between IEEE 802.1Q and ISL trunks.

IEEE 802.1Q encapsulation uses an internal, or one level, packet tagging scheme to multiplex VLANs across a single physical link, while maintaining strict adherence to the individual VLAN domains. IEEE 802.1Q can have access ports, or untagged ports where frames are assigned to VLANs based on a port VLAN identifier (PVID), or native VLAN for the port. It can also have trunked ports where some frames can be tagged and others untagged. IEEE 802.1Q uses Per VLAN Spanning Tree Plus (PVST+), mapping multiple spanning trees to the spanning tree of pure IEEE 802.1Q switches.

See the "Configuring 802.1Q VLAN Encapsulation" section.

Inter-Switch Link VLAN Encapsulation

Layer 3 switching software also supports Inter-Switch Link (ISL) encapsulation over all media, including Fast Ethernet, Gigabit Ethernet, Fast EtherChannel, and GigaChannel. The Layer 3 switch router can be deployed in environments with the ISL trunking protocol, and can route between ISL and 802.1Q stations.

ISL encapsulation uses an external, or two level, packet tagging scheme to multiplex VLANs across a single physical link, while maintaining strict adherence to the individual VLAN domains. With ISL, all packets must be tagged on a physical link.

ISL uses one spanning tree per VLAN (PVST) over ISL trunks.

See the "Configuring ISL VLAN Encapsulation" section.

Switching Database Manager

Layer 3 switching software features the switching database manager (SDM). SDM resides on the central processor and its primary function is to maintain the Layer 3 switching database in ternary content addressable memory (TCAM). SDM maintains the address entries contained in TCAM in an appropriate order. SDM manages TCAM space by partitioning protocol-specific switching information into multiple regions.

The key benefit of SDM in Layer 3 switching is its ability to configure the size of the protocol regions in TCAM. SDM enables exact-match and longest-match address searches, which result in high-speed forwarding.

See "Configuring Switching Database Manager."