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Cisco ME 3400 EthernetAccess Switch SoftwareConfiguration Guide
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Chapter32 Configuring IP Unicast Routing
Understanding IP Routing
Understanding 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 th e
VLAN, referred to as inter-VLAN routing. You configure one or more routers to route traffic to the
appropriate destination VLAN.
Figure 32-1 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.
Figure32-1 Routing Topology Example
When Host A in VLAN10 needs to communicate with Host B in VLAN 10, it sends a packet addressed
to that host. SwitchA forwards the packet directly to Host B, without sending it to the router.
When Host A sends a packet to HostC in VLAN 20, Switch A forwards the packet to the router, which
receives the traffic on the VLAN10 interface. The router checks the routing table, finds the correct
outgoing interface, and forwards the packet on the VLAN20 interface to Switch B. Switch B receives
the packet and forwards it to HostC.

Types of Routing

Routers and Layer 3 switches can route packets in three different ways:
•By using default routing
•By using preprogrammed static routes for the traffic
•By dynamically calculating routes by using a routing protocol
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 port s through a single path into and out of a
network. Static routing is secure and uses little bandwidth, but does not automatically respond to chang es
in the network, such as link failures, and therefore, might result in un reachable desti nations. As netw orks
grow, static routing becomes a labor-intensive liability.
Dynamic routing protocols are used by routers to dynamically calc ula te the best rout e f or for warding
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 ne ighb ors. Dist an ce-vec tor pr otoc ols u se on e
or a series of metrics for calculating the best routes. Th ese prot ocol s are ea sy t o co nfigure and use .
92996
A
BC
Host
Host
Host
Switch A Switch B
VLAN 10 VLAN 20
IEEE 802.1Q
trunks