Because routers use Layer 3 addresses, which often have structure to the address, routers can use techniques, such as address summarization, in building networks that maintain performance and responsiveness as they grow in size. By imposing a hierarchical structure on a network, routers can efficiently use redundant paths and determine optimal routes in a constantly changing network environment.
This section describes router functions that are vital in a switched LAN design:
Broadcast and multicast control
If your user applications require broadcast and/or multicast support, such as videoconferencing, IPTV, or streaming data, such as a stock ticker, you should manage the broadcasts and multicasts that can cause network congestion. Routers are best suited to control these broadcasts and multicasts in your network by performing the following functions:
Caching the addresses of remote hosts? When hosts send a broadcast packet determining the address of a remote host that the router already knows about, the router responds on behalf of the remote host and filters the packet from leaving the local network by dropping the broadcast packet.
Caching advertised network services? When a router learns of new network services, the router caches the necessary information and does not forward the broadcasts related to the new service. When a client of that network service sends a broadcast locating that service, the router responds on behalf of the new service and filters the broadcast from the rest of the network by dropping the broadcast packet, sparing other network hosts from having to respond. For example, Novell Internetwork Packet Exchange (IPX) clients use broadcasts to find local services; and in a network without a router, every server responds to every client broadcast by multicasting its list of services. Routers manage these Novell broadcasts by collecting services not local to the switch and sending out periodic updates describing the services offered on the entire network.
Providing special protocols? Special multicast protocols, such as the Internet Group Multicast Protocol (IGMP) and Protocol Independent Multicast (PIM). These new multicast protocols enable multicasting applications to "negotiate" with routers, switches, and workstations to determine which devices belong to a multicast group. This negotiation helps limit the range and impact of the multicast stream on the network.
A good network design contains a mix of appropriately scaled switching and routing implementations. Given the effects of broadcast radiation on CPU performance, well-managed switched LAN designs must include routers for broadcast and multicast management to keep your network from being saturated and crippled with unnecessary traffic.
In addition to preventing broadcasts from radiating throughout the network, spreading uncontrolled, routers are also responsible for providing services to each LAN segment. The following list identifies some examples of these services provided in a network environment:
Address Resolution Protocol (ARP)? Rather than a workstation's ARP request flooding the network forcing every host to respond, the router can respond to the ARP request on behalf of the owner.
Dynamic Host Configuration Protocol (DHCP)? With this protocol, an IP address is automatically assigned to a workstation on a TCP/IP network. DHCP saves you from having to manually assign permanent IP addresses to each workstation, a daunting task if you have hundreds of workstations to support. Routers can provide DHCP services to the network, preventing the DHCP broadcast from wandering the network waiting for a DHCP server to respond to the request.
Simple Network Management Protocol (SNMP)? This protocol is a widely used for network monitoring and management. Network monitoring and management information is passed from SNMP agents to the workstation console used to oversee the network. Routers can act as SNMP agents for devices connected to their LAN or WAN interfaces, avoiding the addition of costly network probes on each network segment.
In a flat network, a single router would be bombarded by myriad requests needing responses, in turn taxing the router processor. Therefore, you need to consider the number of routers that can provide reliable services to a given subset of VLANs and that some type of hierarchical design needs to be considered for your network.
Routers are used to connect networks of different media types, such as Ethernet and Token Ring, translate the OSI Layer 3 network addresses, and fragment packets as necessary. Routers perform these same functions in switched LAN designs. Most switching is done within like media, such as Ethernet and Token Ring switches, with some capability of connecting to another media type, as discussed in the section "Media Dependence" earlier in this chapter. If a requirement for a switched network design is to provide high-speed connectivity between unlike media, however, routers will be required in your network design.