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Understanding IP Routing
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 34-1
router has an interface in each VLAN.
Figure 34-1
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 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 three different ways:
•
•
•
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. 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.
The switch supports only the Routing Information Protocol (RIP), which uses a single distance metric
(cost) to determine the best path. It also supports default routing and static routing.
Cisco Catalyst Blade Switch 3020 for HP Software Configuration Guide
34-2
shows a basic routing topology. Switch A is in VLAN 10, and Switch B is in VLAN 20. The
Routing Topology Example
VLAN 10
Switch A
A
Host
B
Host
ISL Trunks
By using default routing
By using preprogrammed static routes for the traffic
By dynamically calculating routes by using a routing protocol
Chapter 34
VLAN 20
Switch B
C
Host
Configuring IP Unicast Routing
OL-8915-03
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