Ip Routing Overview; Routing Protocols - Alcatel Omni Switch/Router User Manual

IP Routing Overview

IP Routing Overview
In switching, traffic may be transmitted from one media type to another within the same
broadcast domain (or group/
happens at layer 3, the network layer. In routing, traffic may be transmitted across groups/
, and broadcast or multicast traffic is prevented from being transmitted across those
VLANs
domains (unless some other mechanism is set up on the switch, such as
multicast routing).
In IP routing, the switch builds routing tables to keep track of optimal destinations for traffic
it receives that is destined for remote networks. The switch also sends and receives routing
messages, or advertisements, to/from other routers in the network. When the switch receives
a packet to be routed, it strips off the
It looks up the source/destination address in the routing table, and then adds the appropriate
address to the packet.
MAC
Calculating routing tables and stripping/adding
switch software unless a Hardware Routing Engine (HRE) or HRE-X is installed. The HRE or
HRE-X significantly improves routing performance. See Chapter 1, "Omni Switch/Router Chas-
sis and Power Supplies," " for information about the HRE-X. On the Omni S/R, IP routing has
a fastpath mechanism with some additional statistics display on the IP Statistics and Errors
screen available through the
12).
IP is associated with several layer 3 and layer 4 protocols. Some of these protocols are built
into the base code loaded into the switch. Others are included as part of Advanced Routing
software. Some protocols are specifically used for routing; others are used by any host or end
station that has an IP address. A brief overview of supported IP protocols is included here.

Routing Protocols

When IP routing is enabled, the switch uses routing protocols to build routing tables that
keep track of stations in the network and to decide the best path for forwarding data. These
routing protocols include:
• Routing Information Protocol (
exchange information in an autonomous system.
"least-cost path" method.
called RouteD.
which is part of Alcatel's optional Advanced Routing software.
RouteD or GateD, allows the switch to learn routing information from other, neighboring
routers.
RIP
• Open Shortest Path First (
function similar to
a datagram.
Routing software.
• Border Gateway Protocol (
between autonomous systems.
Advanced Routing software.
Page 25-2
). Switching happens at layer 2, the physical layer; routing
VLAN
MAC
command (see Viewing IP Statistics and Errors on page 25-
)—An interior gateway protocol that defines how routers
RIP
services are performed by a program operating in the switch
RIP
and services are also available from a program called GateD,
RIP RIP II
)—An interior gateway protocol that provides a routing
OSPF
but which uses different techniques to determine the best route for
RIP
services are provided by GateD, part of Alcatel's optional Advanced
OSPF
)—An exterior gateway protocol that provides for routing
BGP
is not part of the base code but is included in the
BGP
header and examines the IP header of the packet.
headers to packets is performed by
MAC
makes routing decisions using a
RIP
RIP
forwarding or IP
UDP
, whether performed by