Distance vector routing: worked example

Network from the figure (link costs): A–B = 5, A–C = 2, A–D = 3, B–C = 4, B–E = 3, C–E = 4.

Principle

In distance vector routing each router keeps a table of the best-known distance to every destination and the next hop. Periodically it sends its vector (distance to all nodes) only to its direct neighbours. Each router then applies the Bellman-Ford equation:

Dₓ(y) = min over neighbours v of { c(x, v) + Dᵥ(y) }

The process repeats until no table changes (convergence).

Step 0: initial vectors (only direct neighbours known, ∞ = unknown)

Order: [A, B, C, D, E]

Router	A	B	C	D	E
A	0	5	2	3	∞
B	5	0	4	∞	3
C	2	4	0	∞	4
D	3	∞	∞	0	∞
E	∞	3	4	∞	0

Step 1: after the first exchange (using neighbours' initial vectors)

Example calculations:

• A → E: via B = 5 + 3 = 8; via C = 2 + 4 = 6; via D = 3 + ∞. Minimum = 6 (via C).
• B → D: via A = 5 + 3 = 8; via C = 4 + ∞; via E = 3 + ∞. Minimum = 8 (via A).
• C → D: via A = 2 + 3 = 5.
• D → B: via A = 3 + 5 = 8; D → C: via A = 3 + 2 = 5.
• E → A: via B = 3 + 5 = 8; via C = 4 + 2 = 6.

Router	A	B	C	D	E
A	0	5	2	3	6
B	5	0	4	8	3
C	2	4	0	5	4
D	3	8	5	0	∞
E	6	3	4	∞	0

Step 2: after the second exchange

• D → E: via A = 3 + 6 = 9.
• E → D: via B = 3 + 8 = 11; via C = 4 + 5 = 9.

All other entries are unchanged.

Step 3: third exchange gives no change, so the tables have converged

Final routing tables (destination: cost via next hop)

Router	To A	To B	To C	To D	To E
A	0	5 (B)	2 (C)	3 (D)	6 (C)
B	5 (A)	0	4 (C)	8 (A)	3 (E)
C	2 (A)	4 (B)	0	5 (A)	4 (E)
D	3 (A)	8 (A)	5 (A)	0	9 (A)
E	6 (C)	3 (B)	4 (C)	9 (C)	0

Example: the best A→E path is A → C → E with cost 2 + 4 = 6. The best D→E path is D → A → C → E with cost 3 + 2 + 4 = 9.

Remarks

• Each router knows only its neighbours and their vectors, not the whole topology.
• Count-to-infinity: if a link fails, wrong information may circulate slowly; it is reduced with split horizon, poison reverse, and triggered updates.
• The protocol used in practice for this method is RIP.