Alcatel Carrier Internetworking Solutions Switch/Router ユーザーズマニュアル

Page 25-2

In switching, traffic may be transmitted from one media type to another within the same 
broadcast domain (or group/

VLAN

). Switching happens at layer 2, the physical layer; routing 

happens at layer 3, the network layer. In routing, traffic may be transmitted across groups/

VLANs

, and broadcast or multicast traffic is prevented from being transmitted across those 

domains (unless some other mechanism is set up on the switch, such as 

UDP

 forwarding or IP 

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 

MAC

 header and examines the IP header of the packet. 

It looks up the source/destination address in the routing table, and then adds the appropriate 

MAC

 address to the packet.

Calculating routing tables and stripping/adding 

MAC

 headers to packets is performed by 

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 

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.

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 (

RIP

)—An interior gateway protocol that defines how routers 

exchange information in an autonomous system. 

RIP

 makes routing decisions using a 

“least-cost path” method. 

RIP

 services are performed by a program operating in the switch 

called RouteD. 

RIP

 and 

RIP

 

II

 services are also available from a program called GateD, 

which is part of Alcatel’s optional Advanced Routing software. 

RIP

, whether performed by 

RouteD or GateD, allows the switch to learn routing information from other, neighboring 

RIP

 routers.

• Open Shortest Path First (

OSPF

)—An interior gateway protocol that provides a routing 

function similar to 

RIP

 but which uses different techniques to determine the best route for 

a datagram. 

OSPF

 services are provided by GateD, part of Alcatel’s optional Advanced 

Routing software.

• Border Gateway Protocol (

BGP

)—An exterior gateway protocol that provides for routing 

between autonomous systems. 

BGP

 is not part of the base code but is included in the 

Advanced Routing software.