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This chapter gives an overview of Layer 3 switching. It describes the Catalyst 2948G-L3 switch router interface types, and shows how the Catalyst 2948G-L3 switch router fits 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:
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:
Compared to other 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.
The Catalyst 2948G-L3 is a fixed configuration, multiprotocol 10/100/1000 Ethernet switch router based on Layer 3 software.
Table 1-1 lists the interfaces supported on the Catalyst 2948G-L3 switch router.
| Interface Types | No. of Ports |
|---|---|
10/100 Mbps Fast Ethernet---UTP | 48 |
1 Gbps Gigabit Ethernet uplink Note Second Gigabit Ethernet port does notsupport full line rate switching. | 2 |
Figure 1-1 shows how the Catalyst 2948G-L3 is used in a small campus backbone.
This section lists the switching software features on the Catalyst 2948G-L3 switch router.
This section briefly describes the key features supported in Layer 3 switching software.
| 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.
See "Configuring Networking Protocols."
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 different quality of service is provided to each class.
Packet scheduling and discarding policies are determined by the class to which the packets belong. 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."
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), Fast EtherChannel, and load sharing across equal cost Layer 3 paths and spanning trees (for Layer 2 based networks).
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.
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:
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.
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).
See "Configuring EtherChannel."
You can bundle up to two gigabit Ethernet connections as one logical link, which can provide up to 4-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. Each channel group has its own IP address.When a packet is queued 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, the traffic load is shared across all links within the bundled ports.
See "Configuring EtherChannel."
Specifically, local or unroutable traffic is bridged among the bridged interfaces in the same bridge group, while routable traffic is routed 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:
See the "Configuring IRB" section.
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. A maximum of 16 spanning-tree instances are supported, one for each bridge group.
To configure the Spanning-Tree Protocol, see the Cisco IOS Command Reference publication.
Layer 3 switching software supports up to 244 VLAN subinterfaces per system and up to 32 VLAN subinterfaces per physical port. Because routing will take place, each VLAN is assumed to terminate at the switch router. Since this might not necessarily be the case, integrated routing and bridging (IRB) is also supported. To configure IRB, see the "About Integrated Routing and Bridging" section.
To configure VLANs, you first define a subinterface at the interface, and map a VLAN to the subinterface.
See the "About Virtual LANs" section.
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 also route and bridge between IEEE 802.1Q and ISL stations.
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.
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 or the 802.1Q trunking protocol, and can route and bridge 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, and see the "About Encapsulation over EtherChannel" section.
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."
Posted: Fri Apr 14 09:51:58 PDT 2000
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