Computer Networks · M2 · read

Routing, forwarding, distance and next hop

Routing: build the shortest path one step at a time

Unit 4 · shortest paths and routing tables

Definition. Routing discovers or computes paths through a network; forwarding uses a table to send an individual packet toward the selected next hop.

Simple meaning. Routing table banata hai; forwarding us table ko use karta hai.

Understand it step by step

  1. A route contains a destination prefix and an outgoing interface or next hop. A metric measures the desirability/cost of that route.
  2. A path is a sequence of permitted edges. Its weighted cost is the sum of the edge weights, not merely the number of vertices drawn.
  3. A shortest-path algorithm maintains a tentative distance from the source to each vertex. Start the source at 0 and all others at ∞.
  4. A predecessor records the vertex just before a destination on the chosen path. Trace predecessors backward, then reverse the list to obtain the path in forward order.
  5. A next hop is the first router after the source on that path. It is different from a final destination’s immediate predecessor.

Practice question. Differentiate routing and forwarding, with an example. [3 practice marks]

Answer

Routing determines suitable network paths and produces routing/forwarding information. Forwarding applies that information to a particular packet. If A has learned that network X is reached through B, computing that choice is routing and sending a packet to B is forwarding.

Yaad rakho: Predecessor = pichhla node; next hop = source ke baad pehla node.