routing algo n

UNICAST ROUTING PROTOCOLS
 A routing table can be either static or dynamic.
 A static table is one with manual entries.
 A dynamic table is one that is updated automatically
when there is a change somewhere in the Internet.
 A routing protocol is a combination of rules and
procedures that lets routers in the Internet inform each
other of changes.

Bellman Ford Algorithm or Ford Fulkerson Algorithm
Distance vector routing

Distance vector routing tables

Initialization of tables in distance vector routing
In distance vector routing, each node shares its routing table with its immediate
neighbors periodically (30s) and when there is a change.

Updating in distance vector routing

 A problem with distance-vector routing is that
any decrease in cost (good news) propagates
quickly, but any increase in cost (bad news) will
propagate slowly.
The problem is referred to as count to infinity. It
sometimes takes several updates before the cost
for a broken link is recorded as infinity by all
routers.
Problem with distance vector routing

LINK STATE ROUTING
 Link state routing has a different philosophy from that of
distance vector routing.
 In link state routing, if each node in the domain has the
entire topology of the domain—the list of nodes and links,
how they are connected including the type, cost (metric),
and the condition of the links (up or down)—the node can
use the Dijkstra algorithm to build a routing table.

Building Routing Tables
 Creation of the states of the links by each
node, called the link state packets (LSP)
 Dissemination of LSPs to every other
routers, called flooding (efficiently)
 Formation of a shortest path tree for each
node
 Calculation of a routing table based on the
shortest path tree

Creation of LSP
• LSP data: E.g. the node ID, the list of links, a
sequence number, and age.
• LSP Generation
– When there is a change in the topology of
the domain
– On a periodic basis
• There is no actual need for this type of LSP,
normally 60 minutes or 2 hours

Example of formation of shortest path tree

Forming shortest path three for router A in a graph

PATH VECTOR ROUTING
 Distance vector and link state routing are both interior
routing protocols.
 They can be used inside an autonomous system. Both of
these routing protocols become intractable when the
domain of operation becomes large.
 Distance vector routing is subject to instability if there is
more than a few hops in the domain of operation.
 Link state routing needs a huge amount of resources to
calculate routing tables. It also creates heavy traffic
because of flooding.
 There is a need for a third routing protocol which we call
path vector routing.

Initial routing tables in path vector routing

Stabilized tables for three autonomous systems

Internal and external BGP sessions
BGP supports classless addressing and CIDR.

Unicasting
In unicasting, the router forwards the received packet through only one of its
interfaces.

Multicasting
In multicasting, the router may forward the received packet through several of
its interfaces.

1. Consider a network with five nodes, N1 to N5, as shown below.
The network uses a Distance Vector Routing protocol. Once the routes have stabilized, the
distance vectors at different nodes are as following. N1: (0, 1, 7, 8, 4)
N2: (1, 0, 6, 7, 3) N3: (7, 6, 0, 2, 6) N4: (8, 7, 2, 0, 4) N5: (4, 3, 6, 4, 0)
Each distance vector is the distance of the best known path at the instance to nodes, N1 to
N5, where the distance to itself is 0. Also, all links are symmetric and the cost is identical in
both directions. In each round, all nodes exchange their distance vectors with their
respective neighbors. Then all nodes update their distance vectors. In between two rounds,
any change in cost of a link will cause the two incident nodes to change only that entry in
their distance vectors. The cost of link N2-N3 reduces to 2(in both directions). After the next
round of updates, what will be the new distance vector at node, N3.
(A) (3, 2, 0, 2, 5)
(B) (3, 2, 0, 2, 6)
(C) (7, 2, 0, 2, 5)
(D) (7, 2, 0, 2, 6)
Exercise

1. Consider a network with five nodes, N1 to N5, as shown below.
The network uses a Distance Vector Routing protocol. Once the routes have stabilized, the
distance vectors at different nodes are as following. N1: (0, 1, 7, 8, 4)
N2: (1, 0, 6, 7, 3) N3: (7, 6, 0, 2, 6) N4: (8, 7, 2, 0, 4) N5: (4, 3, 6, 4, 0)
Each distance vector is the distance of the best known path at the instance to nodes, N1 to
N5, where the distance to itself is 0. Also, all links are symmetric and the cost is identical in
both directions. In each round, all nodes exchange their distance vectors with their
respective neighbors. Then all nodes update their distance vectors. In between two rounds,
any change in cost of a link will cause the two incident nodes to change only that entry in
their distance vectors. 52. The cost of link N2-N3 reduces to 2(in both directions). After the
next round of updates, what will be the new distance vector at node, N3.
(A) (3, 2, 0, 2, 5)
(B) (3, 2, 0, 2, 6)
(C) (7, 2, 0, 2, 5)
(D) (7, 2, 0, 2, 6)
Exercise
N3: (7, 2, 0, 2, 6)
N3 knows (1,0,2,7,3) from N2 and (8,7,2,0,4) from N4.
N3: (3,2,0,2,5)

2. Consider the same data as given in previous question.
After the update in the previous question, the link N1-N2 goes down. N2 will
reflect this change immediately in its distance vector as cost, infinite. After the
NEXT ROUND of update, what will be cost to N1 in the distance vector of N3?
(A) 3
(B) 9
(C) 10
(D) Infinite
Exercise

2. Consider the same data as given in previous question.
After the update in the previous question, the link N1-N2 goes down. N2 will
reflect this change immediately in its distance vector as cost, infinite. After the
NEXT ROUND of update, what will be cost to N1 in the distance vector of N3?
(A) 3
(B) 9
(C) 10
(D) Infinite
Exercise
N1: (0,infinity,3,8,4)
N2: (infinity,0,2,4,3)
N3: (3,2,0,2,5)
N4: (8,4,2,0,4)
N5: (4,3,5,4,0)
After the next round of exchange, N3 knows
the distance vector (infinity,0,2,4,3) from N2
and (8,4,2,0,4) from N4.
So N3 will update its distance vector to
(10,2,0,2,5).

3. Consider a network with 6 routers R1 to R6 connected with links having weights as
shown in the following diagram
All the routers use the distance vector based routing algorithm to update their routing
tables. Each router starts with its routing table initialized to contain an entry for each
neighbor with the weight of the respective connecting link. After all the routing tables
stabilize, how many links in the network will never be used for carrying any data?
(A) 4
(B) 3
(C) 2
(D) 1
Exercise

Sol: We can check one by one all shortest distances. When we check for all shortest
distances for Ri we don’t need to check its distances to R0 to Ri-1 because the network
graph is undirected.
Following will be distance vectors of all nodes.
Shortest Distances from R1 to R2, R3, R4, R5 and R6
R1 (5, 3, 12, 12, 16)
Links used: R1-R3, R3-R2, R2-R4, R3-R5, R5-R6
Shortest Distances from R2 to R3, R4, R5 and R6
R2 (2, 7, 8, 12)
Links used: R2-R3, R2-R4, R4-R5, R5-R6
Shortest Distances from R3 to R4, R5 and R6
R3 (9, 9, 13)
Shortest Distances from R4 to R5 and R6
R4 (1, 5)
Links used: R4-R5, R5-R6
Shortest Distance from R5 to R6
R5 (4)
Links Used: R5-R6
If we mark, all the used links one by one, we can see that following links are never used.
R1-R2
R4-R6

3. Consider a network with 6 routers R1 to R6 connected with links having weights as
shown in the following diagram
All the routers use the distance vector based routing algorithm to update their routing
tables. Each router starts with its routing table initialized to contain an entry for each
neighbour with the weight of the respective connecting link. After all the routing
tables stabilize, how many links in the network will never be used for carrying any
data?
(A) 4
(B) 3
(C) 2
(D) 1
Exercise

4. Suppose the weights of all unused links in the previous question are
changed to 2 and the distance vector algorithm is used again until all
routing tables stabilize. How many links will now remain unused?
(A) 0
(B) 1
(C) 2
(D) 3
Exercise

Sol: The distance vectors of all nodes
R1 (2, 3, 9, 10, 11)
Links used: R1-R2, R1-R3, R2-R4, R4-R5, R4-R6
R2 (2, 7, 8, 9)
R3 (9, 9, 11)
R4 (1, 2)
Links used: R4-R5, R4-R6
R5 (3)
Links Used: R5-R4, R4-R6
If we mark, all the used links one by one, we can see that all links are used except
the following link.
R5-R6

6. Consider the following three statements about link state and distance vector
routing protocols, for a large network with 500 network nodes and 4000 links.
[S1] The computational overhead in link state protocols is higher than in distance
vector protocols.
[S2] A distance vector protocol (with split horizon) avoids persistent routing
loops, but not a link state protocol.
[S3] After a topology change, a link state protocol will converge faster than a
distance vector protocol.
Which one of the following is correct about S1, S2, and S3 ?
(A) S1, S2, and S3 are all true.
(B) S1, S2, and S3 are all false.
(C) S1 and S2 are true, but S3 is false
(D) S1 and S3 are true, but S2 is false
Exercise

routing algo n

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