1. COS 461: Computer Networks Spring 2014
Routing Recitation #5
COS 461: Computer Networks
Spring 2014

2. Link State (Djikstra’s)
dest
link b
(a,?) 6 e d c
(a,?) 4 22 7 d
(a,?) b c 7 10 18 e
(a,?) a f 4 f
(a,?)

3. Link State (Djikstra’s)
dest
link b
(a,b) 6 e d c
(a,?) 4 22 7 d
(a,?) b c 7 10 18 e
(a,?) a f 4 f
(a,?)

4. Link State (Djikstra’s)
dest
link b
(a,b) 6 e d c
(a,f) 4 22 7 d
(a,?) b c 7 10 18 e
(a,?) a f 4 f
(a,?)

5. Link State (Djikstra’s)
dest
link b
(a,b) 6 e d c
(a,f) 4 22 7 d
(a,f) b c 7 10 18 e
(a,?) a f 4 f
(a,?)

6. Link State (Djikstra’s)
dest
link b
(a,b) 6 e d c
(a,f) 4 22 7 d
(a,f) b c 7 10 18 e
(a,f) a f 4 f
(a,?)

7. Link State (Djikstra’s)
dest
link b
(a,b) 6 e d c
(a,f) 4 22 7 d
(a,f) b c 7 10 18 e
(a,f) a f 4 f
(a,f)

8. Which routing protocol requires the least amount of state on the router?
link state
distance vector
path vector

9. Which routing protocol requires the least amount of state on the router?
link state
distance vector
path vector
D_x(y) = cost of least-cost path from x to y
Update distances based on neighbors
Path vector: send the entire path for each destination d

10. Which of the following, if true, ensures packets from a to e always traverse c?
B. Y > X + 1
C. Y > X
D. A or B D

11. Which of the following, if true, ensures packets from a to e always traverse c?
B. Y > X + 1
C. Y > X
D. A or B D

12. Which of the following, if true, ensures packets from b to e always traverse d?
B. Y > X + 1
C. Y > X
D. A or B B

13. Which of the following, if true, ensures packets from b to e always traverse d?
B. Y > X + 1
C. Y > X
D. A or B B

14. Distance Vector Routing Initial Routing table at E18 C 21 11 12 5 A D E 15 17
Initial Routing table at E
Destination
Cost
Next Hop A B C D

15. Distance Vector Routing Initial Routing table at E18 C 21 11 12 5 A D E 15 17
Initial Routing table at E
Destination
Cost
Next Hop A
Inf
--- B C D

16. Distance Vector Routing Initial Routing table at E18 C 21 11 12 5 A D E 15 17
Initial Routing table at E
Destination
Cost
Next Hop A
Inf
--- B C D

17. Distance Vector Routing Initial Routing table at E18 C 21 11 12 5 A D E 15 17
Initial Routing table at E
Destination
Cost
Next Hop A
Inf
--- B C 11 D

18. Distance Vector Routing Initial Routing table at E18 C 21 11 12 5 A D E 15 17
Initial Routing table at E
Destination
Cost
Next Hop A
Inf
--- B C 11 D 17

19. Distance Vector Routing Routing table at E after one iteration18 C 21 11 12 5 A D E 15 17
Routing table at E after one iteration
Destination
Cost
Next Hop A
Inf
--- B C 11 D 17

20. Distance Vector Routing Routing table at E after one iteration18 C 21 12 5 11 A D E 15 17
Routing table at E after one iteration
Destination
Cost
Next Hop A 32 D B
Inf
--- C 11 17

21. Distance Vector Routing Routing table at E after one iteration18 C 21 12 5 11 A D E 15 17
Routing table at E after one iteration
Destination
Cost
Next Hop A 32 D B 29 C 11 17

22. Distance Vector Routing Routing table at E after one iteration18 C 21 12 5 11 A D E 15 17
Routing table at E after one iteration
Destination
Cost
Next Hop A 32 D B 29 C 11 17

23. Distance Vector Routing Routing table at E after one iteration18 C 21 12 5 11 A D E 15 17
Routing table at E after one iteration
Destination
Cost
Next Hop A 32 D B 29 C 11 16

24. Distance Vector Routing Routing table at E after two iterations18 C 21 12 5 11 A D E 15 17
Routing table at E after two iterations
Destination
Cost
Next Hop A 32 D B 29 C 11 16

25. Distance Vector Routing Routing table at E after two iterations18 C 21 12 5 11 A D E 15 17
Routing table at E after two iterations
Destination
Cost
Next Hop A 31 C B 29 11 D 16

26. Distance Vector Routing Routing table at E after two iterations18 C 21 12 5 11 A D E 15 17
Routing table at E after two iterations
Destination
Cost
Next Hop A 31 C B 28 11 D 16

27. Distance Vector Routing Routing table at E after two iterations18 C 21 12 5 11 A D E 15 17
Routing table at E after two iterations
Destination
Cost
Next Hop A 31 C B 28 11 D 16

28. Distance Vector Routing Routing table at E after two iterations18 C 21 12 5 11 A D E 15 17
Routing table at E after two iterations
Destination
Cost
Next Hop A 31 C B 28 11 D 16

29. Distance Vector RoutingB 18 C X 12 5 11 A D E 15 17
For what value of x does the routing table at E not change anymore after two iterations?
For all X >= 2
For all X >= 3
For all X <= 4
For all X <= 3

30. Distance Vector RoutingB 18 C X 5 11 12 A D E 15 17
For what value of x does the routing table at E not change anymore after two iterations?
If x =2, the routing table will change: E->A is through C, distance 30 instead of 31 through D (2nd hop)
For all X >= 2
For all X >= 3
For all X <= 4
For all X <= 3

31. Border Gateway Protocol (BGP)
Assuming standard BGP routing policies (Gao-Rexford model), will packets take the drawn path?
True or False

32. Border Gateway Protocol (BGP)
Assuming standard BGP routing policies (Gao-Rexford model), will packets take the drawn path?
True or False
Why?

33. Border Gateway Protocol (BGP)
Assuming standard BGP routing policies (Gao-Rexford model), will packets take the drawn path?
True or False
Why?
Don’t export routes learned from a peer or provider to another peer or provider

34. Border Gateway Protocol (BGP)
Assuming standard BGP routing policies (Gao-Rexford model), will packets take the drawn path?
True or False

35. Border Gateway Protocol (BGP)
Assuming standard BGP routing policies (Gao-Rexford model), will packets take the drawn path?
True or False
Why?

36. AS X would have to pay AS U whereas it could go through AS Y for free
Border Gateway
Protocol (BGP)
Assuming standard BGP routing policies (Gao-Rexford model), will packets take the drawn path?
True or False
Why?
AS X would have to pay AS U whereas it could go through AS Y for free

37. Border Gateway Protocol (BGP)
Assuming standard BGP routing policies (Gao-Rexford model), will packets take the drawn path?
True or False

38. Border Gateway Protocol (BGP)
Assuming standard BGP routing policies (Gao-Rexford model), will packets take the drawn path?
True or False
Why?

39. Border Gateway Protocol (BGP)
Assuming standard BGP routing policies (Gao-Rexford model), will packets take the drawn path?
True or False
Why?
Y and Z would prefer the peering link over the provider link through W to save money.

40. Border Gateway Protocol (BGP)
Assuming standard BGP routing policies (Gao-Rexford model), will packets take the drawn path?
True or False

41. Border Gateway Protocol (BGP)
Assuming standard BGP routing policies (Gao-Rexford model), will packets take the drawn path?
True or False
Why?

42. Border Gateway Protocol (BGP)
Assuming standard BGP routing policies (Gao-Rexford model), will packets take the drawn path?
True or False
Why?
AS Z would not advertise a route to its provider, AS W, to its peer, AS Y, and vice versa

43. Border Gateway Protocol (BGP)
Assuming standard BGP routing policies (Gao-Rexford model), will packets take the drawn path?
True or False

44. Border Gateway Protocol (BGP)
Assuming standard BGP routing policies (Gao-Rexford model), will packets take the drawn path?
True or False
Why?

45. Border Gateway Protocol (BGP)
Assuming standard BGP routing policies (Gao-Rexford model), will packets take the drawn path?
True or False
Why?
AS X would not advertise a peer link to its provider, AS U (it would pay for being a middle man)

46. Routing Loops b d 2 1 a 1 c e 1 2
If the link (c – e) fails, and the nodes run a link-state routing protocol, can a temporary forwarding loop occur? If so, which node pairs may see their traffic loop?

47. Routing Loops b d 2 1 a 1 c e 1 2
Nodes a, c, d, and e could potentially see their outgoing packets loop. a  e c <-> e d  c

48. Routing Loops b d 2 1 a 1 c e 1 2
Suppose network operator Olivia decides to bring down the link c – e for maintenance. Olivia figures she can issue a series of link weight changes in the network to shift traffic away from c – e such that no temporary forwarding loops occur. She’s right; what series of changes to c – e’s weight would achieve this?

49. Routing Loops b d 2 1 a 1 c e 1 2
Change c – e weight to 4. a to e moves to a – b. c to e remains on c – e. c to d? d to c moves to d – b. e to c remains on c – e.

50. Routing Loops b d 2 1 a 1 c e 1 2
Change c – e weight to 6. c to e moves to c – a. e to c moves to e – d. No traffic left on c – e. Olivia can bring down the link.

51. b->a->d h->e->d f->e->d c->b->e->d
BGP Routing
Peer Peer
Provider -> Customer
1. Which of the following paths to d are valid?
b->a->d
h->e->d
f->e->d
c->b->e->d

52. b->a->d h->e->d f->e->d c->b->e->d
BGP Routing
Peer Peer
Provider -> Customer
1. Which of the following paths to d are valid?
Not c because don’t export paths through peers to other peers
b->a->d
h->e->d
f->e->d
c->b->e->d

53. e->b->c->f->i e->d->g->h->i
BGP Routing
Peer Peer
Provider -> Customer
2. Which path does e take to reach i ?
e->h->i
e->f->i
e->b->c->f->i
e->d->g->h->i

54. e->b->c->f->i e->d->g->h->i
BGP Routing
Peer Peer
Provider -> Customer
2. Which path does e take to reach i ?
H would be paying to be the middle man, E would be paying B and f would be paying C, and then G would be paying to be the middle man
e->h->i
e->f->i
e->b->c->f->i
e->d->g->h->i

55. e->b->c->f->i e->d->g->h->i
BGP Routing
Peer Peer
Provider -> Customer
3. If the link e-f is removed then which path does e take to reach i ?
e->h->i
e->f->i
e->b->c->f->i
e->d->g->h->i

56. e->b->c->f->i e->d->g->h->i
BGP Routing
Peer Peer
Provider -> Customer
3. If the link e-f is removed then which path does e take to reach i ?
Not a because h would be paying to be the middle man
e->h->i
e->f->i
e->b->c->f->i
e->d->g->h->i

57. e->h e->f f->i d->g c->f
BGP Routing
Peer Peer
Provider -> Customer
4. Suppose AS b provides a dump of all BGP routes they learn for every destination and we use them to reconstruct the AS-level topology, which of the following business relations will be missing ?
e->h
e->f
f->i
d->g
c->f

58. e->h e->f f->i d->g c->f
BGP Routing
Peer Peer
Provider -> Customer
4. Suppose AS b provides a dump of all BGP routes they learn for every destination and we use them to reconstruct the AS-level topology, which of the following business relations will be missing ?
e->h
e->f
f->i
d->g
c->f

59. a and h a and c a, b and h a, b and c h
BGP Routing
Peer Peer
Provider -> Customer
5. What is the minimum set of ASes that must provide “dumps” of every AS path they learn for every edge in the graph to be visible in at least one dump?
a and h
a and c
a, b and h
a, b and c h

60. a and h a and c a, b and h a, b and c h
BGP Routing
Peer Peer
Provider -> Customer
5. What is the minimum set of ASes that must provide “dumps” of every AS path they learn for every edge in the graph to be visible in at least one dump?
a and h
a and c
a, b and h
a, b and c h

61. BGP Routing
6. BGP supports flexible routing policies. Internet Service Providers (ISPs) often have a “prefer customer” policy where they prefer to route through a customer, even if a shorter route exists through a peer or provider. Why? How is this policy realized in BGP?

62. BGP Routing
6. BGP supports flexible routing policies. Internet Service Providers (ISPs) often have a “prefer customer” policy where they prefer to route through a customer, even if a shorter route exists through a peer or provider. Why? How is this policy realized in BGP?
Directing traffic through a customer generates revenue, whereas sending through a peer or provider is (at best) revenue neutral and may, in fact, cost money.
The policy is realized in BGP by having an import policy that assigns a higher local-preference value to routes learned from customer ASes.