A Router is a Layer 3 device designed to connect different networks together (e.g., connecting a LAN to the Internet or connecting different VLANs). While switches use MAC addresses, routers use IP Addresses to make forwarding decisions.
The Routing Table (RIB)
The router maintains a Routing Table (Routing Information Base) which contains the best paths to destination networks.
- Directly Connected: Networks physically attached to the router interfaces.
- Static Routes: Manually configured paths.
- Dynamic Routes: Learned via protocols like OSPF, EIGRP, or BGP.
Packet Forwarding Process
When a router receives a packet, it performs the following steps:
- De-encapsulation: It removes the Layer 2 header (Ethernet frame) to inspect the Layer 3 packet.
- Destination Lookup: It checks the Destination IP address against its Routing Table.
- Longest Match Rule: If multiple routes match, the router selects the one with the longest subnet mask (most specific match).
- Encapsulation: It encapsulates the packet in a new Layer 2 frame appropriate for the outgoing interface (e.g., new Source/Dest MAC addresses).
- Forwarding: The frame is sent out the exit interface.
Router# show ip route Codes: L - local, C - connected, S - static, R - RIP, M - mobile, B - BGP D - EIGRP, EX - EIGRP external, O - OSPF, IA - OSPF inter area Gateway of last resort is not set 192.168.10.0/24 is variably subnetted, 2 subnets, 2 masks C 192.168.10.0/24 is directly connected, GigabitEthernet0/0 L 192.168.10.1/32 is directly connected, GigabitEthernet0/0 S 10.0.0.0/8 [1/0] via 192.168.20.2
Key Difference: Switches connect devices in the same network. Routers connect different networks.