Read the Advertisement

B Advertisement

Neighbor B shares the distances it currently knows.

highlighted = computed this step

B shares its distances

B does not send A a full path here; it advertises the cost it currently knows for each destination.

dB(D)=4d_B(D)=4
B advertised vectorRouter A recomputes a distance-vector update from B's advertised table.B advertised vector - A via B, route A to B to D, cost 5destcurrentB advertises1 + advdecisionupdatednext hopA012keep self0selfB101tie1BC523improve3BD99945improve5B

B says C is close

B advertises C at cost 2. A has not decided yet whether that helps.

dB(C)=2d_B(C)=2
B advertised vectorRouter A recomputes a distance-vector update from B's advertised table.B advertised vector - A via B, route A to B to D, cost 5destcurrentB advertises1 + advdecisionupdatednext hopA012keep self0selfB101tie1BC523improve3BD99945improve5B

B says D is reachable

B advertises D at cost 4, which gives A its first finite-looking offer for D.

dB(D)=4d_B(D)=4
B advertised vectorRouter A recomputes a distance-vector update from B's advertised table.B advertised vector - A via B, route A to B to D, cost 5destcurrentB advertises1 + advdecisionupdatednext hopA012keep self0selfB101tie1BC523improve3BD99945improve5B

Summary

The next chapter adds A's link cost to each advertised distance and then compares the candidate against A's current table.

1+dB(dest)1+d_B(\text{dest})
B advertised vectorRouter A recomputes a distance-vector update from B's advertised table.B advertised vector - A via B, route A to B to D, cost 5destcurrentB advertises1 + advdecisionupdatednext hopA012keep self0selfB101tie1BC523improve3BD99945improve5B