Advanced Computer Networks: Lecture 21. This lecture will cover the following: distance vector routing example; distance vector routing table; count to infinity problem; split horizon; reliable flooding; route calculation: Dijkstra’s shortest path algorithm;...
CS716 Advanced Computer Networks By Dr. Amir Qayyum Lecture No. 21 Distance Vector Routing Example Information in routing table of each node: Iteration 1 At Distance to reach node node A B C D E F G A 0 1 1 x 1 1 x B 1 0 1 x x x x C 1 1 0 1 x x x D x x 1 0 x x 1 E 1 x x x 0 x x F 1 x x x x 0 1 G x x x 1 x 1 0 B C A D E F G Distance Vector Routing Example Information in routing table of each node: Iteration 2 At Distance to reach node node A B C D E F G A 0 1 1 2 1 1 2 B 1 0 1 2 2 2 x C 1 1 0 1 2 2 2 D 2 2 1 0 x 2 1 E 1 2 2 x 0 2 x F 1 2 2 2 2 0 1 G 2 x 2 1 x 1 0 B C A D E F G Distance Vector Routing Example Information in routing table of each node: Iteration 3 At Distance to reach node node A B C D E F G A 0 1 1 2 1 1 2 B 1 0 1 2 2 2 3 C 1 1 0 1 2 2 2 D 2 2 1 0 3 2 1 E 1 2 2 3 0 2 3 F 1 2 2 2 2 0 1 G 2 3 2 1 3 1 0 B C A D E F G Distance Vector Routing Table B C A D E F G Destination Cost NextHop A A C C D C E A F A G A Distance Vector Routing: Link Failure • F detects that link to G has failed • F sets distance to G to infinity and sends update to A • A sets distance to G to infinity since it uses F to reach G • A receives periodic update from C with 2hop path to G • A sets distance to G to 3 and sends update to F • F decides it can reach G in 4 hops via A B C A D E F G Count to Infinity Problem • Link from A to E fails • A advertises distance of infinity to E, but B and C advertise a distance of 2 to E ! • B decides it can reach E in 3 hops; advertises this to all • A decides it can read E in 4 hops; advertises this to all A • C decides that it can reach E in 5 hops… • We are counting to infinity … F B C D E G Split Horizon C : 2 : B AA C : 2 : B DD Loop of > 2 nodes fails split horizon !!! BB C : 1 : C C : ∞ : CC Reliable Flooding X A C B D X A C B (a) X A C B (c) D (b) D X A C B D (d) 10 • Route Calculation: Dijkstra’s Shortest Path Algorithm Let – – – – – N denotes set of nodes in the graph l (i, j) denotes nonnegative cost (weight) for edge (i, j) s denotes this node M denotes the set of nodes incorporated so far C(n) denotes cost of the path from s to node n M = {s} for each n in N - {s} C(n) = l(s, n) // calculate cost to each node while (M != N) M = M union {w} such that C(w) is the minimum for all w in (N - M) for each n in (N - M) C(n) = MIN(C(n), C (w) + l(w, n )) 11 Link State Algorithm Initialize confirmed with entry for self (cost = 0) For newly added node (next), select its LSP For each neighbor of next, calculate cost to reach neighbor as the sum of cost from self to next and from next to neighbor If neighbor is currently in neither confirmed nor tentative, add to tentative, where nexthop is the direction to reach next If neighbor is currently in tentative and cost is less than current cost for neighbor , then replace current entry with , where nexthop is the direction to reach next If tentative is empty, stop. Otherwise pick entry from tentative with the lowest cost, move it to confirmed and return to step 2 12 Route Calculation At node D Confirmed list Tentative list (D,0,) (D,0,) (C,2,C), (B,11,B) (D,0,), (C,2,C) (B,11,B) (D,0,), (C,2,C) (B,5,C), (A,12,C) 5 (D,0,), (C,2,C), (B,5,C) (A,12,C) (D,0,), (C,2,C), (B,5,C) (A,10,C) A (D,0,), (C,2,C), (B,5,C), (A,10,C) B 10 11 D C ... {s} for each n in N - {s} C(n) = l(s, n) // calculate cost to each node while (M != N) M = M union {w} such that C(w) is the minimum for all w in (N - M) for each n in (N - M) C(n) = MIN(C(n),.. .Lecture? ?No.? ?21 Distance Vector Routing Example Information in routing table of each node: Iteration 1