Overview of Layer 3 Switching and Software Features

• 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.

• 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

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.

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.

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.

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.

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.)

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

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.

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.

• 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.

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.

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.

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.