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Chapter 38 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 the
VLAN, referred to as inter-VLAN routing. You configure one or more routers to route traffic to the
appropriate destination VLAN.
Figure 38-1 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.
Figure 38-1 Routing Topology Example
When Host A in VLAN 10 needs to communicate with Host B in VLAN 10, it sends a packet 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 surveys 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.
This section contains information on these routing topics:
• Types of Routing, page 38-3
• IP Routing and Switch Stacks, page 38-4
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
• By dynamically calculating routes by using a routing protocol
6. BGP = Border Gateway Protocol.
7. VRF = Virtual Private Networks routing/forwarding.
8. CE = customer edge.
9. CEF = Cisco Express Forwarding.
10. dCEF = distributed CEF.
11. PBR = policy-based routing.
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A
B
C
Host
Host
Host
Switch A Switch B
VLAN 10 VLAN 20
ISL Trunks
