Faculty of Electronics and Telecommunications
INSTITUTE OF TELECOMMUNICATIONS
RAE411 Telecommunications Software
Tianhua Chen
2022/10/21
1
Basic Concepts of Graphs and Related Terms
▪ Graph is a non linear data structure; A map is a well-known example of a graph. In a map various
connections are made between the cities. The cities are connected via roads, railway lines and
aerial network. We can assume that the graph is the interconnection of cities by roads. Euler used
graph theory to solve Seven Bridges of Königsberg problem. Is there a possible way to traverse
every bridge exactly once – Euler Tour
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A graph contains a set of points known as nodes (or vertices) and set of links known as edges (or Arcs) which
connects the vertices.
Graph Terminology
1. Vertex : An individual data element of a graph is called as Vertex. Vertex is also known as node. In above
example graph, A, B, C, D & E are known as vertices.
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2. Edge : An edge is a connecting link between two vertices. Edge is also known as Arc. An edge is represented as
(starting Vertex, ending Vertex).
In above graph, the link between vertices A and B is represented as (A,B).
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▪ Edges are three types:
▪ 1. Undirected Edge - An undirected edge is a bidirectional edge. If there is an undirected edge
between vertices A and B then edge (A , B) is equal to edge (B , A).
▪ 2. Directed Edge - A directed edge is a unidirectional edge. If there is a directed edge between
vertices A and B then edge (A , B) is not equal to edge (B , A).
▪ 3. Weighted Edge - A weighted edge is an edge with cost on it. Edges can be weighted; for example,
"distance," "travel time," and "fare" between two stops in a transit network can all be used as
weights.
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Types of Graphs
1. Undirected Graph
A graph with only undirected edges is said to be undirected graph.
2. Directed Graph
A graph with only directed edges is said to be directed graph.
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3. Complete Graph
A graph in which any V node is adjacent to all other nodes present in the graph is known as a complete graph. An
undirected graph contains the edges that are equal to edges = n(n-1)/2 where n is the number of vertices present
in the graph. The following figure shows a complete graph.
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4. Regular Graph
Regular graph is the graph in which nodes are adjacent to each other, i.e., each node is accessible from any
other node.
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5. Cycle Graph
A graph having cycle is called cycle graph. In this case the first and last nodes are the same. A closed simple
path is a cycle.
6. Acyclic Graph
A graph without cycle is called acyclic graphs.
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7. Weighted Graph
A graph is said to be weighted if there are some non negative value assigned to each edges of the graph.
The value is equal to the length between two vertices. Weighted graph is also called a network.
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Outgoing Edge
A directed edge is said to be outgoing edge on its origin vertex.
Incoming Edge
A directed edge is said to be incoming edge on its destination vertex.
Degree
Total number of edges connected to a vertex is said to be degree of that vertex.
Indegree
Total number of incoming edges connected to a vertex is said to be indegree of that vertex.
Outdegree
Total number of outgoing edges connected to a vertex is said to be outdegree of that vertex.
Parallel edges or Multiple edges
If there are two undirected edges to have the same end vertices, and for two directed edges to have the same
origin and the same destination. Such edges are called parallel edges or multiple edges.
Self-loop
An edge (undirected or directed) is a self-loop if its two endpoints coincide.
Simple Graph
A graph is said to be simple if there are no parallel and self-loop edges.
Adjacent nodes
When there is an edge from one node to another then these nodes are called adjacent nodes.
Incidence
In an undirected graph the edge between v1 and v2 is incident on node v1 and v2.
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Walk
A walk is defined as a finite alternating sequence of vertices and edges, beginning and ending with
vertices, such that each edge is incident with the vertices preceding and following it.
Closed walk
A walk which is to begin and end at the same vertex is called close walk. Otherwise it is an open walk.
If e1,e2,e3,and e4 be the edges of pair of vertices (v1,v2),(v2,v4),(v4,v3) and (v3,v1) respectively ,then v1
e1 v2 e2 v4 e3 v3 e4 v1 be its closed walk or circuit.
Path
A open walk in which no vertex appears more than once is called a path.
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If e1 and e2 be the two edges between the pair of vertices (v1,v3) and (v1,v2) respectively, then v3 e1 v1 e2
v2 be its path.
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Length of a path
The number of edges in a path is called the length of that path. In the following, the length of the path is 3.
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Circuit
A closed walk in which no vertex (except the initial and the final vertex) appears more than once is called a
circuit.
A circuit having three vertices and three edges.
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Routing and Forwarding
Forwarding versus Routing
– Forwarding:
– to select an output port based on destination address and routing table
– Routing:
– process by which routing table is built
▪ Routing is not the same as Forwarding
▪ Routing is the building of maps
– Each routing protocol usually has its own routing database
– Routing protocols populate the forwarding table
▪ Forwarding is passing the packet to the next hop device
– Forwarding table contains the best path to the next hop for each prefix
– There is only ONE forwarding table
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Routing
Forwarding versus Routing
– Forwarding:
– to select an output port based on destination address and routing table
– Routing:
– process by which routing table is built
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Routing
Forwarding versus Routing
– Forwarding:
– to select an output port based on destination address and routing table
– Routing:
– process by which routing table is built
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Routing
• Forwarding table VS Routing table
• Forwarding table
• Used when a packet is being forwarded and so must contain enough information to
accomplish the forwarding function
• A row in the forwarding table contains the mapping from a network number to an outgoing
interface and some MAC information, such as Ethernet Address of the next hop.
• Routing table
• Built by the routing algorithm as a precursor to build the forwarding table
• Generally contains mapping from network numbers to next hops
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Routing
Example rows from (a) routing and (b) forwarding tables
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Routing
Network as a Graph
The basic problem of routing is to find the lowest-cost path between any two nodes
Where the cost of a path equals the sum of the costs of all the edges that make up the path
Undirected Graph
Cycle Graph
Sub Graph
Connected Graph
Routers are used as vertices, and network connections between routers are used as edge weights, which
can include factors such as the speed of network connections, network load levels, and priority by time
periods. As an abstraction, we combine all influencing factors into a single weight.
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Routing
• For a simple network, we can calculate all shortest paths and load them into some nonvolatile
storage on each node.
• Such a static approach has several shortcomings
• It does not deal with node or link failures
• It does not consider the addition of new nodes or links
• It implies that edge costs cannot change
• What is the solution?
• Need a distributed and dynamic protocol
• Two main classes of protocols
• Distance Vector
• Link State
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Distance Vector
▪ Each node constructs a one dimensional array (a vector) containing the “distances” (costs) to all
other nodes and distributes that vector to its immediate neighbors
▪ Starting assumption is that each node knows the cost of the link to each of its directly connected
neighbors
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Distance Vector
Initial distances stored at each node (global view)
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Distance Vector
Initial routing table at node A
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Distance Vector
Final routing table at node A
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Distance Vector
A to G:
A, B, C, D, G (cost: 4)
A, C, D, G (cost: 3)
A, F, G (cost: 2)
Final distances stored at each node (global view)
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Distance Vector
▪ The distance vector routing algorithm is sometimes called as Bellman-Ford algorithm
▪ Every T seconds each router sends its table to its neighbor each router then updates its table
based on the new information
▪ Problems include fast response to good new and slow response to bad news. Also too many
messages to update
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Distance Vector
• When a node detects a 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 2-hop 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
▪
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Distance Vector
▪ Slightly different circumstances can prevent the network from stabilizing
– Suppose the link from A to E goes down
– In the next round of updates, A advertises a distance of infinity to E, but B and C advertise a
distance of 2 to E
– Depending on the exact timing of events, the following might happen
▪ Node B, upon hearing that E can be reached in 2 hops from C, concludes that it can reach
E in 3 hops and advertises this to A
▪ Node A concludes that it can reach E in 4 hops and advertises this to C
▪ Node C concludes that it can reach E in 5 hops; and so on.
▪ This cycle stops only when the distances reach some number that is large enough to be
considered infinite
Count-to-infinity problem
–
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Count-to-infinity Problem
▪ Use some relatively small number as an approximation of infinity
▪ For example, the maximum number of hops to get across a certain network is never going to be
more than 16
▪ One technique to improve the time to stabilize routing is called split horizon
– When a node sends a routing update to its neighbors, it does not send those routes it learned
from each neighbor back to that neighbor
– For example, if B has the route (E, 2, A) in its table, then it knows it must have learned this
route from A, and so whenever B sends a routing update to A, it does not include the route (E,
2) in that update
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Count-to-infinity Problem
▪ In a stronger version of split horizon, called split horizon with poison reverse
– B actually sends that back route to A, but it puts negative information in the route to ensure
that A will not eventually use B to get to E
– For example, B sends the route (E, ∞) to A
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Example
F to C:
FABC = 6+3+4=13
FEBC = 2+1+4=7
FEDC = 2+1+9=12
FAEBC = 6+1+1+4 = 12
FAEDC = 6+1+1+9=17
FEABC = 2+1+3+4=10
FABEDC = 6+3+1+1+9=20
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How to find the distance vector router table for all nodes?
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Routing Information Protocol (RIP)
Example Network running RIP
Rather than advertising the cost of reaching other routers,
the routers advertise the cost of reaching networks.
Router C would advertise to router A the fact it can reach
1, network 2 and 3 cost 0
2, network 5 and 6 cost 1
3, network 4 at cost 2
RIPv2 Packet Format
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Link State Routing
Strategy: Send to all nodes (not just neighbors) information about directly connected links (not entire
routing table).
▪ Link State Packet (LSP)
– id of the node that created the LSP
– cost of link to each directly connected neighbor
– sequence number (SEQNO)
– time-to-live (TTL) for this packet
▪ Reliable Flooding
– store most recent LSP from each node
– forward LSP to all nodes but one that sent it
– generate new LSP periodically; increment SEQNO
– start SEQNO at 0 when reboot
– decrement TTL of each stored LSP; discard when TTL=0
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Link State
Reliable Flooding
Flooding of link-state packets. (a) LSP arrives at node X; (b) X floods LSP to A and C; (c) A and C
flood LSP to B (but not X); (d) flooding is complete
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Dijkstra’s Shortest-Path Algorithm
▪ Iterative algorithm
– After k iterations, know least-cost path to k nodes
▪ S: nodes whose least-cost path definitively known
– Initially, S = {u} where u is the source node
– Add one node to S in each iteration
▪ D(v): current cost of path from source to node v
– Initially, D(v) = c(u,v) for all nodes v adjacent to u
– … and D(v) = ∞ for all other nodes v
– Continually update D(v) as shorter paths are learned
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Dijsktra’s Algorithm
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1 Initialization:
S: Least cost path known
2 S = {u}
3 for all nodes v
D(v): Known shortest cost from
4
if (v is adjacent to u)
source to v
5
D(v) = c(u,v)
6
else D(v) = ∞
C(w,v): Known cost from w to v
7
8 Loop: Do
9 find w not in S with the smallest D(w)
10 add w to S
11 update D(v) for all v adjacent to w and not in S:
12
D(v) = min{D(v), D(w) + c(w,v)}
13 until all nodes in S
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1
1
2
5
4
3
Dijkstra’s Algorithm Example
3
2
4
1
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2
Dijkstra’s Algorithm Example
Loop: Do
4
1 D(w)
3 not in S 1with the smallest
find w
add w to S
2
5
forall v adj to1 w && not in S:
1
2
5
3
4
3
1
D(v) = min{ D(v), D(w) + c(w,v) }
1
4 in S
until all nodes
3
2
4
3
2
3
2
1
4
1
4
1
1
2
2
5
5
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4
3
4
3
1
1
36
1
4
1
1
1
2
2
5
5
4
3
4
3
Dijkstra’s Algorithm Example
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2
3
2
4
1
1
3
2
3
2
1
4
1
4
1
1
2
2
5
5
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4
3
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3
1
1
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u
3
2
v
w
1
4
1
2
x
s
5
4
3
y
t
1
• Shortest-path tree from u
Shortest-Path Tree
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z
(u,v)
(u,w)
(u,w)
(u,v)
(u,v)
(u,w)
(u,w)
link
• Forwarding table at u
v
w
x
y
z
s
t
38
u
39
3
2
v
w
1
Fig. 1
1
4
2
x
Fig. 3
5
s
4
3
y
t
1
z
Fig. 5
Fig. 2
Fig. 4
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Shortest Path Routing
▪ In practice, each switch computes its routing table directly from the LSP’s it has collected using a
realization of Dijkstra’s algorithm called the forward search algorithm
▪ Specifically each switch maintains two lists, known as Tentative and Confirmed
▪ Each of these lists contains a set of entries of the form (Destination, Cost, NextHop)
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Shortest Path Routing
▪ The algorithm
– Initialize the Confirmed list with an entry for myself; this entry has a cost of 0
– For the node just added to the Confirmed list in the previous step, call it node Next, select its
LSP
– For each neighbor (Neighbor) of Next, calculate the cost (Cost) to reach this Neighbor as the
sum of the cost from myself to Next and from Next to Neighbor
▪ If Neighbor is currently on neither the Confirmed nor the Tentative list, then add
(Neighbor, Cost, Nexthop) to the Tentative list, where Nexthop is the direction I go to
reach Next
▪ If Neighbor is currently on the Tentative list, and the Cost is less than the currently listed
cost for the Neighbor, then replace the current entry with (Neighbor, Cost, Nexthop) where
Nexthop is the direction I go to reach Next
– If the Tentative list is empty, stop. Otherwise, pick the entry from the Tentative list with the
lowest cost, move it to the Confirmed list, and return to Step 2.
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Shortest Path Routing
42
Open Shortest Path First (OSPF)
OSPF Header Format
OSPF Link State Advertisement
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Distance Vector vs. Link State Routing
Node A: to reach F go to B
Node B: to reach F go to D
Node D: to reach F go to E
Node E: go directly to F
A
B
C
▪ With distance vector routing, each node has information only about the next hop:
▪
▪
▪
▪
▪
E
Distance vector routing makes
poor routing decisions if
directions are not completely
correct
(e.g., because a node is down).
D
▪
If parts of the directions incorrect, the routing may be incorrect until the routing algorithms has reconverged.
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44
F
Distance Vector vs. Link State Routing
C
A
B
E
C
F
A
C
B
C
A
E
F
D
C
B
E
B
A
D
E
F
D
D
In link state routing, each node has a complete map of the topology
B
F
▪
A
E
▪
D
If a node fails, each
node can calculate
the new route
A
E
C
▪
B
Difficulty: All nodes need to
have a consistent view of the
network
A
D
45
B
E
C
F
D
F
F
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Link State Routing: Properties
▪ Each node requires complete topology information
▪ Link state information must be flooded to all nodes
▪ Guaranteed to converge
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46
Link State Routing: Basic principles
1. Each router establishes a relationship (“adjacency”) with its neighbors
2. Each router generates link state advertisements (LSAs) which are distributed to all
routers
LSA = (link id, state of the link, cost, neighbors of the link)
3. Each router maintains a database of all received LSAs (topological database or link
state database), which describes the network as a graph with weighted edges
4. Each router uses its link state database to run a shortest path algorithm (Dijikstra’s
algorithm) to produce the shortest path to each network
47
47
Received
LSAs
LSAs are flooded
to other interfaces
Link State
Database
Dijkstra’s
Algorithm
Operation of a Link State Routing protocol
48
IP Routing
Table
48
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49
OSPF
▪ OSPF = Open Shortest Path First
▪ The OSPF routing protocol is the most important link state routing protocol on the Internet
▪ The complexity of OSPF is significant
▪ History:
– 1989: RFC 1131 OSPF Version 1
– 1991: RFC 1247 OSPF Version 2
– 1994: RFC 1583 OSPF Version 2 (revised)
– 1997: RFC 2178 OSPF Version 2 (revised)
– 1998: RFC 2328 OSPF Version 2 (current version)
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50
Provides authentication of routing messages
Enables load balancing by allowing traffic to be split evenly across routes with equal cost
Type-of-Service routing allows to setup different routes dependent on the TOS field
Supports subnetting
Supports multicasting
Allows hierarchical routing
Features of OSPF
▪
▪
▪
▪
▪
▪
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Example Network
Router IDs are
selected
independent of
interface addresses
Link costs are called Metric
Metric is in the range [0 , 216]
Metric can be asymmetric
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Link State Advertisement (LSA)
Link ID = 10.1.1.1, Metric = 4
Link ID = 10.1.2.1, Metric = 3
Link ID = 10.10.10.1, Metric = 0
▪ The LSA of router 10.10.10.1 is as
follows:
▪ Link State ID:
10.10.10.1 = can be Router ID
▪ Advertising Router:
10.10.10.1 = Router ID
▪ Number of links:
3 = 2 links plus router itself
▪ Description of Link 1:
▪ Description of Link 2:
▪ Description of Link 3:
Each router sends its LSA to all routers in the network
(using a method called reliable flooding)
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4
/2
Network and Link State Database
10.10.10.1
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2.0
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LS Type
10.1.10.1
Link StateID
10.1.10.2
10.1.10.1
Adv. Router
4
/2
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.5
.3
0xe39a
0x6b53
0x219e
0x9b47
Checksum
10.1.5.0/24
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.2
Router-LSA
10.1.10.3
10.1.10.4
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10.10.10.6
0x8000003a
0x80000003
0x80000007
0x80000006
LS SeqNo
18
20
1712
1618
0
LS Age
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0x80000038
1680
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Router-LSA
10.1.10.4
10.1.10.5
.1
Router-LSA
10.1.10.5
10.1.10.6
Each router has a
database which
contains the LSAs
from all other routers
Router-LSA
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Link State Database
▪ The collection of all LSAs is called the link-state database
▪ Each router has and identical link-state database
Useful for debugging: Each router has a complete description of the network
–
▪ If neighboring routers discover each other for the first time, they will exchange their link-state
databases
▪ The link-state databases are synchronized using reliable flooding
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55
OSPF Packet Format
IP header
OSPF packets are not
carried as UDP payload!
OSPF has its own IP
protocol number: 89
OSPF Message
Header
TTL: set to 1 (in most cases)
OSPF Message
LSA
Header
LSA
LSA
Data
LSA
... ...
Body of OSPF Message
Message Type
Specific Data
Destination IP: neighbor’s IP address or 224.0.0.5
(ALLSPFRouters) or 224.0.0.6 (AllDRouters)
56
LSA
56
OSPF Packet Format
2: current version
is OSPF V2
OSPF Message
Header
Message types:
1: Hello (tests reachability)
2: Database description
3: Link Status request
4: Link state update
5: Link state acknowledgement
Standard IP checksum taken
over entire packet
version
message length
Body of OSPF Message
type
authentication type
Area ID
source router IP address
checksum
authentication
32 bits
authentication
Authentication passwd = 1: 64 cleartext password
Authentication passwd = 2: 0x0000 (16 bits)
KeyID (8 bits)
Length of MD5 checksum (8 bits)
Nondecreasing sequence number (32 bits)
ID of the Area
from which the
packet originated
0: no authentication
1: Cleartext
password
2: MD5 checksum
(added to end
packet)
Prevents replay
attacks
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57
OSPF LSA Format
LSA
LSA
Header
LSA
Data
LSA
Header
Link 1
Link 2
Link Age
Link Type
Link State ID
advertising router
Metric
Metric
length
link sequence number
Link ID
Link Data
#TOS metrics
Link Data
Link ID
#TOS metrics
checksum
Link Type
Link Type
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Discovery of Neighbors
10.1.10.2
Scenario:
Router 10.1.10.2 restarts
▪ Routers multicasts OSPF Hello packets on all OSPF-enabled interfaces.
▪ If two routers share a link, they can become neighbors, and establish an adjacency
10.1.10.1
OSPF Hello
OSPF Hello: I heard 10.1.10.2
▪ After becoming a neighbor, routers exchange their link state databases
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59
Acknowledges
receipt of
description
Sends database
description.
(description only
contains LSA
headers)
10.1.10.1
OSPF Hello
OSPF Hello: I heard 10.1.10.2
Database Description: Sequence = X+1
Database Description: Sequence = X+1, 1 LSA header=
Router-LSA,
10.1.10.2, 0x80000005
Database Description: Sequence = X, 5 LSA headers =
Router-LSA, 10.1.10.1, 0x80000006
Router-LSA,
10.1.10.2, 0x80000007
Router-LSA,
10.1.10.3, 0x80000003
Router-LSA,
10.1.10.4, 0x8000003a
Router-LSA,
10.1.10.5, 0x80000038
Router-LSA,
10.1.10.6, 0x80000005
Database Description: Sequence = X
Database
description of
10.1.10.2
Sends empty
database
description
Scenario:
Router 10.1.10.2 restarts
10.1.10.2
After neighbors are discovered the nodes exchange their databases
Discovery of
adjacency
Neighbor discovery and
database synchronization
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60
10.1.10.1 sends
requested LSAs
Regular LSA exchanges
61
10.1.10.1
Link State Request packets, LSAs =
Router-LSA,
10.1.10.1,
Router-LSA,
10.1.10.2,
Router-LSA,
10.1.10.3,
Router-LSA,
10.1.10.4,
Router-LSA,
10.1.10.5,
Router-LSA,
10.1.10.6,
Link State Update Packet, LSAs =
Router-LSA, 10.1.10.1, 0x80000006
Router-LSA, 10.1.10.2, 0x80000007
Router-LSA, 10.1.10.3, 0x80000003
Router-LSA, 10.1.10.4, 0x8000003a
Router-LSA, 10.1.10.5, 0x80000038
Router-LSA, 10.1.10.6, 0x80000005
Link State Update Packet, LSA =
Router-LSA,
10.1.1.6, 0x80000006
10.1.10.2
10.1.10.2 explicitly
requests each LSA
from 10.1.10.1
10.1.10.2 has more
recent value for
10.0.1.6 and sends it
to 10.1.10.1
(with higher sequence
number)
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10.10.10.1
Update
database
ACK
LSA
10.10.10.2
Update
database
10.10.10.2
LSA
LSA
10.10.10.5
Update
database
10.10.10.4
LSA-Updates are distributed to all other routers via Reliable Flooding
Example: Flooding of LSA from 10.10.10.1
Routing Data Distribution
•
•
62
LSA
ACK
Update
database
10.10.10.6
Update
database
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Dissemination of LSA-Update
▪ A router sends and refloods LSA-Updates, whenever the topology or link cost changes. (If a
received LSA does not contain new information, the router will not flood the packet)
▪ Exception: Infrequently (every 30 minutes), a router will flood LSAs even if there are no new
changes.
▪ Acknowledgements of LSA-updates:
▪ explicit ACK, or
implicit via reception of an LSA-Update
▪
63
OSPF Design Service Provider Networks
64
OSPF Areas and Rules
•
•
•
•
Backbone area (0)
must exist
All other areas
must have
connection to
backbone
Backbone must be
contiguous
Do not partition
area (0)
Area 2
Backbone
Router
Internet
Area 4
Area
Border
Router
Area 0
Ruteador
Interno
Area 3
Area 1
Autonomous
System (AS)
Border Router
65
Autonomous Systems
▪
▪
An autonomous system is a region of the Internet that is administered by a single entity.
Examples of autonomous regions are:
▪ UVA’s campus network
▪ MCI’s backbone network
Regional Internet Service Provider
▪
▪
Routing is done differently within an autonomous system (intradomain routing) and between
autonomous system (interdomain routing).
66
Ethernet
Ethernet
Router
Router
Autonomous
System 1
Router
Ethernet
Ethernet
Autonomous
System 2
Router
Router
Ethernet
Autonomous Systems (AS)
Ethernet
Router
67
67
OSPF Design
▪ Figure out your addressing first – OSPF and addressing go together
– The objective is to maintain a small link-state DB
– Create address hierarchy to match the network topology
– Separate blocks for infrastructure, customer interfaces, customers, etc.
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OSPF Design
▪ Examine the physical topology
– Is it meshed or hub-and-spoke (star)
▪ Try to use as Stubby an area as possible
– It reduces overhead and LSA counts
▪ Push the creation of a backbone
– Reduces mesh and promotes hierarchy
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OSPF Design
▪ One SPF per area, flooding done per area
– Try not to overload the ABRs (Area border router)
▪ Different types of areas do different flooding
– Normal areas
– Stub areas
– Totally stubby (stub no-summary)
– Not so stubby areas (NSSA)
70
OSPF Design
▪ Redundancy
– Dual links out of each area – using metrics (cost) for traffic
engineering
– Too much redundancy …
▪ Dual links to backbone in stub areas must be the same –
otherwise sub-optimal routing will result
▪ Too much redundancy in the backbone area without good
summarization will affect convergence in the area 0
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OSPF for ISPs
▪ OSPF features you should consider:
– OSPF logging neighbor changes
– OSPF reference cost
– OSPF router ID command
– OSPF Process Clear/Restart
72
OSPF Best Common Practices – Adding Networks
73
OSPF – Network Aggregation
•
BCP – Individual OSPF network
statement for each infrastructure link
– Have separate IP address blocks
for infrastructure and customer
links
– Use IP unnumbered interfaces or
BGP to carry /30 to customers
– OSPF should only carry
infrastructure routes in an ISP’s
network
OC12c
ISP Backbone
OC48
Customer Connections
OC12c
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OSPF – Adding Networks
▪ Redistribute connected subnet
– Works for all connected interfaces on the router but sends networks as external types-2s –
which are not summarized
▪ router ospf 100
redistribute connected subnets
▪
▪ Not recommended
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OSPF – Adding Networks
▪ Specific network statements
– Each interface requires an OSPF network statement. Interfaces that
should not bet broadcasting Hello packets need a passive-interface
statement
▪ router ospf 100
▪
network 192.168.1.1 0.0.0.3 area 51
▪
network 192.168.1.5 0.0.0.3 area 51
passive interface Serial 1/0
▪
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OSPF – Adding Networks
▪ Network statements - wildcard mask
– Every interface covered by a wildcard mask used in the OSPF network
statement. Interfaces that should not be broadcasting Hello packets
need a passive-interface statement or default passive-interface should
be used
▪ router ospf 100
▪
network 192.168.1.0 0.0.0.255 area 51
▪
default passive-interface default
no passive interface POS 4/0
▪
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OSPF – Adding Networks
▪ The key theme when selecting which method to use is to keep the links-state DB as small as
possible
– Increases stability
– Reduces the amount of information in the LSAs
– Speeds up convergence time
78
OSPF – Useful Features
79
OSPF Logging Neighbor Changes
▪ The router will generate a log message whenever an OSPF neighbor changes state
▪ Syntax:
▪ [no] ospf log-adjacency-changes
▪ A typical log message:
▪ %OSPF-5-ADJCHG: Process 1, Nbr 223.127.255.223 on Ethernet0 from
LOADING to FULL, Loading Done
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Number of State Changes
▪ The number of state transitions is available via SNMP (ospfNbrEvents) and the
CLI:
– show ip ospf neighbor [type number] [neighbor-id] [detail]
▪ Detail—(Optional) Displays all neighbors given in detail (list all neighbors).
When specified, neighbor state transition counters are displayed per
interface or neighbor ID
81
State Changes (Cont.)
▪ To reset OSPF related statistics, use the clear ip ospf counters EXEC command.
– clear ip ospf counters [neighbor [<type number>] [neighbor-id]]
82
OSPF Cost: Reference Bandwidth
▪ Bandwidth used in metric calculation
– Cost = 10^8/BW
– Not useful for BW > 100 Mbps but can be changed
▪ Syntax:
– ospf auto-cost reference-bandwidth <reference-bandwidth>
▪ Default reference bandwidth is still 100Mbps for backward compatibility
83
OSPF Router ID
▪ If the loopback interface exists and has an IP address, that is used as the router ID in
routing protocols - stability!
▪ If the loopback interface does not exist, or has no IP address, the router ID is the highest
IP address configured – danger!
▪ Subcommand to manually set the OSPF router ID :
– router-id <ip address>
84
OSPF Clear/Restart
▪ clear ip ospf [pid] redistribution
–This command can clear redistribution based on OSPF routing process ID. If no PID
is given, it assumes all OSPF processes
▪ clear ip ospf [pid] counters
–This command clear counters based on OPSF routing process ID. If no PID is given,
it assumes all OSPF processes
▪ clear ip ospf [pid] process
–This command will restart the specified OSPF process. If no PID is given, it assumes
all OSPF processes. It attempts to keep the old router-id, except in cases where a new
router-id was configured, or an old user configured router-id was removed. It requires
user confirmation because it will cause network churn.
85
OSPF Command Summary
86
Redistributing Routes into OSPF
– ROUTER OSPF <pid#x>
– REDISTRIBUTE {protocol} <as#y>
–
<metric>
–
<metric-type (1 or 2)
–
<tag>
–
<subnets>
–
87
NETWORK <n.n.n.n> <mask> AREA <area-id>
AREA <area-id> STUB {no-summary}
AREA <area-id> AUTHENTICATION
AREA <area-id> DEFAULT_COST <cost>
AREA <area-id> VIRTUAL-LINK <router-id>...
AREA <area-id> RANGE <address mask>
OSPF Router Sub-Commands
▪
▪
▪
▪
▪
▪
88
IP OSPF COST <cost>
IP OSPF PRIORITY <8-bit-number>
IP OSPF HELLO-INTERVAL <number-of-seconds>
IP OSPF DEAD-INTERVAL <number-of-seconds>
IP OSPF AUTHENTICATION-KEY <8-bytes-of-password>
Interface Sub-Commands
▪
▪
▪
▪
▪
89
Internet Control Message Protocol (ICMP)
Destination host unreachable due to link /node failure
Reassembly process failed
Time To Live had reached 0 (so datagrams don't cycle forever)
IP header checksum failed
▪ Defines a collection of error messages that are sent back to the source host whenever a router or
host is unable to process an IP datagram successfully
–
–
–
–
▪ ICMP-Redirect
– From router to a source host
– With a better route information
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91