- Suppose node A sends
its update first:
The distance vector message is processed by nodes B, C, E and F...
Example: Node F:
- Initially:
d(F,B) via A = infinite - Now:
From distance message: d(A,B) = 1 (A --- B) Node F knows: d(F,A) = 1 (F --- A) Therefore: d(F,B) = 2 (F --- via A --- B)
- Note:
- Originally: d(F,B) = ∞
- Now: d(F,B) = 2 !!!
Therefore:
- Schedule: (F, B, 2) for transmission (to all neighbors)
Result: after incorporating all changes
- Initially:
- Suppose node B
transmits distance vector updates
next:
The distance vector message is processed by nodes A and C...
Example: at Node A (very similar to the previous example)
- Initially:
d(A,C) via B
= infinite
- A can get to B with cost = 1
- From (B, C, 1)
(B can
get to C
with cost 1),
A
can get to C
via A with
cost 1 + 1 = 2
- The cost
d(A,C)
changed
Schedule: (A, C, 2) for transmission to all neighbors
Result after incorporating all changes:
A strange phenomenon at Node A:
Question:
- How does a
packet travel:
A -----> via B -----> Fwith cost 3 ???
Answer: (just follow the routing table in node A and B)
Conclusion:
- The distance table can contain "loop back" routes
Note:
- Due to this "feature"
that the Distance Vector Algorithm
will have convergence problems
in some network topologies !!!
We will study this problem in more detail later
Dictactical note:
- It's tedious to
go through the entire example.
- You can go through it yourself....
- Initially:
d(A,C) via B
= infinite
- Suppose node C transmits updates next:
The distance vector message is processed by nodes A, B and D...
- Suppose node D transmits updates next:
The distance vector message is processed by nodes C and G...
- Suppose node G transmits updates next:
Which will cause the following updates in tables of nodes D and F:
NOTE:
- Only now. nodes
D has
discovered a route to F
(and vice versa)
- You can see that
the Distance Vector algoritm
will converge
very slowly
to a final solution
The final solution will the same the one obtained by the Dijkstra's algorithm...
- Only now. nodes
D has
discovered a route to F
(and vice versa)
- Suppose node F transmits updates next:
Which will cause the following updates in tables of nodes A and G:
