Chapter 35 Configuring Ip Unicast Routing; Understanding Ip Routing; Types Of Routing - Cisco ME 3400 Software Configuration Manual

Understanding IP Routing

support routing and forces any routing to be done through software. This overloads the CPU and severely
degrades routing performance. For more information on the SDM templates, see
"Configuring SDM Templates"
release.
Understanding IP Routing
In an IP network, each subnetwork is mapped to an individual VLAN. 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 35-1
router has an interface in each VLAN.
Figure 35-1
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:
•
•
•
Cisco ME 3400 Ethernet Access Switch Software Configuration Guide
35-2
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
IEEE 802.1Q
By using default routing—sending traffic with a destination unknown to the router to a default outlet
or destination.
By using preprogrammed static routes for the traffic
Static unicast routing forwards packets from predetermined ports through a single path into and out
of a network. Static routing does not automatically respond to changes in the network and therefore,
might result in unreachable destinations.
By dynamically calculating routes by using a routing protocol
Dynamic routing protocols are used by routers to dynamically calculate the best route for forwarding
traffic. Routing protocols supported by the switch are Routing Information Protocol (RIP), Border
Gateway Protocol (BGP), Open Shortest Path First (OSPF) protocol, Enhanced IGRP (EIGRP),
System-to-Intermediate System (IS-IS), and Bidirectional Forwarding Detection (BFD).
or see the sdm prefer command in the command reference for this
VLAN 20
Switch B
trunks
Chapter 35 Configuring IP Unicast Routing
Chapter 6,
C
Host
OL-9639-07