Routers

Cisco IOS will not allow two active interfaces on the same router to belong to the same network.

LAN Interfaces

• Ethernet interface also has a Layer 2 MAC address and participates in the Ethernet LAN in the same way as any other hosts on that LAN. For example, a router Ethernet interface participates in the ARP process for that LAN.
• When two routers are connected directly through the Ethernet interfaces, or when a PC NIC is connected directly to a router Ethernet interface, a crossover cable is used.

WAN Interfaces

• The Layer 2 encapsulation can be of different types, such as PPP, Frame Relay, and HDLC (High-Level Data Link Control).
• Similar to LAN interfaces, each WAN interface has its own IP address and subnet mask, which identifies it as a member of a specific network.

Note: MAC addresses are used on LAN interfaces, such as Ethernet, and are not used on WAN interfaces. However, WAN interfaces use their own Layer 2 addresses depending on the technology.

Router Interfaces

The router in the figure has four interfaces. Each interface has a Layer 3 IP address and subnet mask that configures it for a different network. The Ethernet interfaces also have Layer 2 Ethernet MAC addresses.

The WAN interfaces are using different Layer 2 encapsulations. Serial 0/0/0 is using HDLC and Serial 0/0/1 is using PPP. Both of these serial point-to-point protocols use a broadcast address for the Layer 2 destination address when encapsulating the IP packet into a data link frame.

The main purpose of a router is to connect multiple networks and forward packets destined either 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 process is known as routing.

When each router receives a packet, it searches its routing table to find the best match between the destination IP address of the packet and one of the network addresses in the routing table.

Once a match is found, the packet is encapsulated in the layer 2 data link frame for that outgoing interface. The type of data link encapsulation depends on the type of interface, such as Ethernet or HDLC.

The router participates in Layer 1 and Layer 2 processes as well.

After a router has examined the destination IP address of a packet and consulted its routing table to make its forwarding decision, it can forward that packet out the appropriate interface toward its destination.

The router encapsulates the Layer 3 IP packet into the data portion of a Layer 2 data link frame appropriate for the exit interface. The type of frame can be an Ethernet, HDLC, PPP, or some other Layer 2 encapsulation - whatever encapsulation is used on that particular interface.

The Layer 2 frame is encoded into the Layer 1 physical signals that are used to represent bits over the physical link.

Each router in the path from source to destination performs this same process of decapsulation, searching the routing table, and then re-encapsulation.

---