Information Technology Reference
In-Depth Information
connected directly to a router's Ethernet interface, a
crossover
cable is used.
Similar to LAN interfaces, each WAN interface has its own IP
address
and
subnet
mask, making it a
member of a specific
network
. Remember, MAC addresses are used only on
Ethernet
interfaces and
are not on WAN interfaces.
Routers and the Network Layer
Key to understanding the role of a router in the network is to understand that a router is a Layer
3
device responsible for forwarding packets. However, a router also operates at Layers 1 and 2.
The main purpose of a router is to connect multiple networks and forward packets destined for its own
networks or other networks. A router is considered a Layer 3 device because its primary forwarding
decision is based on the information in the Layer
3
IP
packet
, specifically the destination IP
address
.
This is known as
routing
.
When a router receives a packet, it examines the
destination
IP address. If the packet does not belong
to any of the router's directly connected networks, the router must
forward
this packet to another
router
or drop the packet.
When forwarding a packet, the router will encapsulate the Layer
3
IP packet into the data portion of a
Layer
2
data-link
frame
appropriate for the exit interface. The Layer
2 frame
will then be encoded into
the Layer
1
physical signals used to represent these bits over the physical link.
Label the External Components of a Router Exercise
Choose the correct label description for each number shown in Figure 1-1.
Figure 1-1
Rear View of an 1841 Cisco Router
Figure 1-1 Label Description:
7
Alternative management port that can support remote access through a modem
3
Single-slot USB port
1
4-port Cisco EtherSwitch 10BASE-T/100BASE-TX autosensing high-speed WAN interface
card
6
FastEthernet port 0/0
8
High-speed WAN interface card with two serial interfaces
2
Compact flash module
5
Management port used for local access to the device; must be used for initial configuration
4
FastEthernet port 0/1
