Information About IP Routing In some network environments, VLANs are associated with individual networks or subnetworks. In an IP network, each subnetwork is mapped to an individual VLAN. Configuring VLANs helps control the size of the broadcast domain and keeps local traffic local. However, network devices in different VLANs cannot communicate with one another without a Layer 3 device (router) to route traffic between the VLAN, referred to as inter-VLAN routing. You configure one or more routers to route traffic to the appropriate destination VLAN. Figure 1: Routing Topology Example This figure shows a basic routing topology. Switch A is in VLAN 10, and Switch B is in VLAN 20.
The router has an interface in each VLAN. When Host A in VLAN 10 needs to communicate with Host B in VLAN 10, it sends a packet that is addressed to that host. Switch A forwards the packet directly to Host B, without sending it to the router. When Host A sends a packet to Host C in VLAN 20, Switch A forwards the packet to the router, which receives the traffic on the VLAN 10 interface. The router checks the routing table, finds the correct outgoing interface, and forwards the packet on the VLAN 20 interface to Switch B. Switch B receives the packet and forwards it to Host C. Types of Routing Routers and Layer 3 switches can route packets in these ways:
• By using default routing • By using preprogrammed static routes for the traffic Default routing refers to sending traffic with a destination unknown to the router to a default outlet or destination. Static unicast routing forwards packets from predetermined ports through a single path into and out of a network. Static routing is secure and uses little bandwidth, but does not automatically respond to changes in the network, such as link failures, and therefore, might result in unreachable destinations. As networks grow, static routing becomes a labor-intensive liability. Dynamic routing protocols are used by routers to dynamically calculate the best route for forwarding traffic.
There are two types of dynamic routing protocols: • Routers using distance-vector protocols maintain routing tables with distance values of networked resources, and periodically pass these tables to their neighbors. Distance-vector protocols use one or a series of metrics for calculating the best routes. These protocols are easy to configure and use. • Routers using link-state protocols maintain a complex database of network topology, based on the exchange of link-state advertisements (LSAs) between routers. LSAs are triggered by an event in the network, which speeds up the convergence time or time that is required to respond to these changes.
Configuring IP Unicast Routing Information About IP Routing Configuring IP Unicast Routing 2