The anatomy of a network router’s routing table
A route table is essentially a database used by a router to determine the optimal path for forwarding data packets. It consists of a collection of entries, each representing a destination network or a specific host. These entries contain essential information that helps the router make informed routing decisions.
Key Components of a Route Table Entry:
- Destination Network or Host: This field specifies the destination network or host address for which the entry is applicable. It can be an IP address, a subnet, or a range of addresses.
- Next-Hop: The next-hop field identifies the next router or gateway to which the data packet should be forwarded on its journey. It can be an IP address or an interface through which the router should transmit the packet.
- Interface: This field designates the network interface through which the data packet should exit the router to reach its destination. It typically includes Ethernet interfaces, serial interfaces, or virtual interfaces like loopback.
- Metric or Cost: The metric represents the cost or distance to the specified destination. Lower values are preferred, as they signify a more direct and efficient path.
- Route Type: Routers maintain various route types, including connected routes, static routes, and dynamic routes (learned through routing protocols like OSPF or BGP). The route type helps the router prioritize entries based on their source and reliability.
How a Route Table Works
- Packet Arrival: When a data packet arrives at the router, it examines the destination IP address in the packet’s header to determine where the packet should be sent.
- Route Lookup: The router consults its route table to find the most specific entry that matches the destination IP address. It searches for the entry with the longest matching prefix (subnet mask) and uses this information to identify the appropriate route.
- Next-Hop Determination: The router identifies the next-hop address or interface specified in the selected route table entry. This is the router or gateway to which the packet should be forwarded.
- Forwarding Decision: The router makes a forwarding decision based on the next-hop information. If the next-hop is on a directly connected network, the packet is sent directly to the destination. If not, the router forwards the packet to the next-hop router, and the process is repeated until the packet reaches its final destination.
Dynamic Route Table Updates
Route tables are not static; they evolve dynamically based on network changes and routing protocols. When a router learns about new routes or network changes, it updates its route table accordingly. This ensures that the router always has the most up-to-date information for efficient routing.
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