ROUTING PROTOCOLS
PART II
ET4187/ET5187 Advanced Telecommunication Network
2
Open Shortest Path First (OSPF)
3





RFC 2328
OSPF is a link state protocol
OSPF provides a number of features not found in
distance vector protocols.
Support for these features has made OSPF a
widely-deployed routing protocol in large
networking environments.
In fact, RFC 1812 – Requirements for IPv4 Routers,
lists OSPF as the only required dynamic routing
protocol.
Link State Routing

Based on Dijkstra’ s Shortest-Path-First algorithm.

Each router starts by knowing:



Each router advertises to the entire network (flooding):



Prefixes of its attached networks.
Links to its neighbors.
Prefixes of its directly connected networks.
Active links to its neighbors.
Each router learns:

A complete topology of the network (routers, links).

Each router computes shortest path to each destination.

In a stable situation, all routers have the same graph, and compute
the same paths.
Link state routing: graphical illustration
Global view:
b
3
a
a’s view:
3
a
d
b
6
b
d’s view:
c
c
1
a
c’s view:
2
c
6
a
b’s view: 3
1
2
d
c
b
1
c
2
d
6
Collecting all views yield a global & complete view of the network!
Operation of a Link State Routing protocol
Received
LSAs
Link State
Database
Dijkstra’s
Algorithm
LSAs are flooded
to other interfaces
IP Routing
Table
Link State Routing: Properties

Each node requires complete topology information

Link state information must be flooded to all nodes

Guaranteed to converge
Distance Vector vs. Link State Routing

With distance vector routing, each node has information only about
the next hop:






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
Distance vector routing makes
poor routing decisions if
directions are not completely
correct
(e.g., because a node is down).
A
B
C
D
E
If parts of the directions incorrect, the routing may be incorrect until the
routing algorithms has re-converged.
F
Distance Vector vs. Link State Routing

In link state routing, each node has a complete map of
the topology
A


If a node fails, each
node can calculate
the new route
B
C
D
E
A
F
A
Difficulty: All nodes need to
have a consistent view of the
network
A
B
C
D
E
B
C
D
E
A
F
B
C
D
E
B
C
D
E
A
F
B
C
D
E
F
F
A
F
B
C
D
E
F
Distance Vector vs. Link State Routing
Link State
•
•
•
•
•
•
Vectoring
Topology information is flooded
within the routing domain
Best end-to-end paths are
computed locally at each
router.
Best end-to-end paths
determine next-hops.
•
Based on minimizing some
notion of distance
Works only if policy is shared
and uniform
Examples: OSPF, IS-IS
•
•
•
•
•
Each router knows little about
network topology
Only best next-hops are chosen
by each router for each
destination network.
Best end-to-end paths result
from composition of all nexthop choices
Does not require any notion of
distance
Does not require uniform
policies at all routers
Examples: RIP, BGP
Dynamic Routing Protocols
Open Shortest Path First
OSPF terminology
12

OSPF areas
 OSPF
networks are divided into a collection of areas.
 An area consists of a logical grouping of networks and
routers.
 The area can coincide with geographic or
administrative boundaries.
 Each area is assigned a 32-bit area ID.
13

Subdividing the network provides the following benefits:

Within an area, every router maintains an identical topology
database describing the routing devices and links within the area.




Areas limit the potentially explosive growth in the number of link
state updates.


These routers have no knowledge of topologies outside the area.
They are only aware of routes to these external destinations.
This reduces the size of the topology database maintained by each
router.
Most LSAs are distributed only within an area.
Areas reduce the CPU processing required to maintain the
topology database.

The SPF algorithm is limited to managing changes within the area.
14

Backbone area and area 0
All OSPF networks contain at least one area, this area is
known as area 0 or the backbone area
 Additional areas can be created based on network
topology or other design requirements.
 In networks containing multiple areas, the backbone
physically connects to all other areas.
 OSPF expects all areas to announce routing information
directly into the backbone.
 The backbone then announces this information into other
areas

15
16

Intra-area, area border, and AS boundary routers
 Intra-area routers



Area border routers (ABR)






This class of router is logically connected to two or more areas.
One area must be the backbone area.
An ABR is used to interconnect areas.
They maintain a separate topology database for each attached area.
ABRs also execute separate instances of the SPF algorithm for each area.
AS boundary routers (ASBR)




This class of router is logically located entirely within an OSPF area.
Intra-area routers maintain a topology database for their local area.
It is located at the periphery of an OSPF internetwork.
It functions as a gateway exchanging reachability between the OSPF network and
other routing environments
ASBRs are responsible for announcing AS external link advertisements through the AS.
Each router is assigned a 32-bit router ID (RID).
 The RID uniquely identifies the device.
 One popular implementation assigns the RID from the lowest-numbered IP
address configured on the router.
17

Physical network types


Point-to-point: Point-to-point networks directly link two routers.
Multi-access: Multi-access networks support the attachment of more than
two routers.

Broadcast networks: have the capability of simultaneously directing a packet
to all attached routers.



Non-broadcast networks: do not have broadcasting capabilities.



This capability uses an address that is recognized by all devices.
Ethernet and token-ring LANs are examples of OSPF broadcast multi-access
networks.
Each packet must be specifically addressed to every router in the network.
X.25 and frame relay networks are examples of OSPF non-broadcast multiaccess networks.
Point-to-multipoint: Point-to-multipoint networks are a special case of
multi-access, non-broadcast networks.


In a point-to-multipoint network, a device is not required to have a direct
connection to every other device.
This is known as a partially meshed environment.
18

Neighbor routers and adjacencies


Routers that share a common network segment establish a neighbor
relationship on the segment.
Routers must agree on the following information to become neighbors:





Area ID: The routers must belong to the same OSPF area.
Authentication: If authentication is defined, the routers must specify the same
password.
Hello and dead intervals: The routers must specify the same timer intervals
used in the Hello protocol.
Stub area flag: The routers must agree that the area is configured as a stub
area.
After two routers have become neighbors, an adjacency relationship can
be formed between the devices.


Neighboring routers are considered adjacent when they have synchronized
their topology databases.
This occurs through the exchange of link state information.
19

Designated and backup designated router
The exchange of link state information between neighbors
can create significant quantities of network traffic.
 To reduce the total bandwidth required to synchronize
databases and advertise link state information, a router
does not necessarily develop adjacencies with every
neighboring device:

Multi-access networks: Adjacencies are formed between an
individual router and the (backup) designated router.
 Point-to-point networks: An adjacency is formed between both
devices.

20

Designated and backup designated router (cont.)


Each multi-access network elects a designated router (DR) and backup
designated router (BDR).
The DR performs two key functions on the network segment:



The BDR forms the same adjacencies as the designated router.


It forms adjacencies with all routers on the multi-access network. This causes
the DR to become the focal point for forwarding LSAs.
It generates network link advertisements listing each router connected to the
multi-access network.
It assumes DR functionality when the DR fails.
Each router is assigned an 8-bit priority, indicating its ability to be
selected as the DR or BDR.


A router priority of zero indicates that the router is not eligible to be
selected.
The priority is configured on each interface in the router.
21


The relationship between neighbors.
No adjacencies are formed between routers that are
not selected to be the DR or BDR.
22

Link state database
 The
link state database is also called the topology
database.
 It contains the set of link state advertisements
describing the OSPF network and any external
connections.
 Each router within the area maintains an identical copy
of the link state database.
23

Link state advertisements and flooding

LSAs are exchanged between adjacent OSPF routers.

This is done to synchronize the link state database on each device.
When a router generates or modifies an LSA, it must
communicate this change throughout the network.
 The router starts this process by forwarding the LSA to each
adjacent device.
 Upon receipt of the LSA, these neighbors store the
information in their link state database and communicate the
LSA to their neighbors.
 This store and forward activity continues until all devices
receive the update.
 This process is called reliable flooding.

24

Two steps taken to ensure this flooding doesn’t
overloading the network with excessive quantities of
LSA traffic:

Each router stores the LSA for a period of time
If, during that time, a new copy of the LSA arrives, the router
replaces the stored version.
 However, if the new copy is outdated, it is discarded.


To ensure reliability, each link state advertisement must be
acknowledged.
Multiple acknowledgements can be grouped together into a single
acknowledgement packet.
 If an acknowledgement is not received, the original link state
update packet is retransmitted.

25

Five types of information contained in link state
advertisements:

Router LSAs




It describes the state of the router's interfaces (links) within the area.
Generated by every OSPF router.
The advertisements are flooded throughout the area.
Network LSAs



It lists the routers connected to a multi-access network.
Generated by the DR on a multi-access segment.
The advertisements are flooded throughout the area.
26

Summary LSAs (Type-3 and Type-4)


It generated by an ABR.
Two types of summary link advertisements:






Type-3 summary LSAs describe routes to destinations in other areas
within the OSPF network (inter-area destinations).
Type-4 summary LSAs describe routes to ASBRs.
Summary LSAs are used to exchange reachability information between
areas.
Normally, information is announced into the backbone area.
The backbone then injects this information into other areas.
AS external LSAs



It describes routes to destinations external to the OSPF network.
They are generated by an ASBR.
The advertisements are flooded throughout all areas in the OSPF
network.
27
OSPF link state advertisements
Example Network
10.1.1.2
.1
4
.2
.2
3
2
• Metric is in the range [0 , 216]
• Metric can be asymmetric
3
.6
1
.5
.3
5
.6
10.1.7.0 / 24
.4
.3
.3
1
.4
.2
.5
.5
10.1.5.0/24
10.1.2.3
• Link costs are called Metric
.4
10.1.4.0 / 24
10.1.1.0 / 24
Router IDs can be
selected
independent of
interface addresses,
but usually chosen to
be the smallest
interface address
2
10.1.3.0 / 24
.1
10.1.7.6
10.1.4.4
10.1.6.0 / 24
10.1.1.1
10.1.5.5
Link State Advertisement (LSA)
10.1.1.1
10.1.1.2
10.1.1.0 / 24
3
2
.2
.2
.4
.4
.4
.3
.5
.3
.3
.5
10.1.5.5

The LSA of router 10.1.1.1 is as follows:

Link State ID:
10.1.1.1

Advertising Router:
Number of links:
10.1.1.1 = Router ID
3 = 2 links plus router itself
Description of Link 1:
Description of Link 2:
Description of Link 3:
Link ID = 10.1.1.2, Metric = 4
Link ID = 10.1.2.2, Metric = 3
Link ID = 10.1.1.1, Metric = 0




.6
.5
10.1.5.0/24
10.1.2.3
.6
10.1.7.0 / 24
10.1.4.0 / 24
10.1.6.0 / 24
.1
.2
10.1.3.0 / 24
4
.1
10.1.7.6
10.1.4.4
= Router ID
Network and Link State Database
10.1.1.1
10.1.1.0 / 24
Each router has a
database which
contains the LSAs
from all other routers
.2
.2
.4
10.1.4.0 / 24
.4
.4
.3
.5
.3
.3
.6
10.1.7.0 / 24
10.1.6.0 / 24
.1
.2
10.1.7.6
10.1.4.4
10.1.3.0 / 24
.1
10.1.1.2
.6
.5
.5
10.1.5.0/24
10.1.5.5
10.1.2.3
LS Type
Link StateID
Adv. Router
Checksum
LS SeqNo
LS Age
Router-LSA
10.1.1.1
10.1.1.1
0x9b47
0x80000006
0
Router-LSA
10.1.1.2
10.1.1.2
0x219e
0x80000007
1618
Router-LSA
10.1.2.3
10.1.2.3
0x6b53
0x80000003
1712
Router-LSA
10.1.4.4
10.1.4.4
0xe39a
0x8000003a
20
Router-LSA
10.1.5.5
10.1.5.5
0xd2a6
0x80000038
18
Router-LSA
10.1.7.6
10.1.7.6
0x05c3
0x80000005
1680
OSPF Packet Format
OSPF Message
IP header
OSPF Message
Header
OSPF packets are not
carried as UDP payload!
OSPF has its own IP
protocol number: 89
TTL: set to 1 (in most cases)
Body of OSPF Message
Message Type
Specific Data
LSA
LSA
Header
Destination IP: neighbor’s IP address or 224.0.0.5
(ALLSPFRouters) or 224.0.0.6 (AllDRouters)
LSA
LSA
Data
... ...
LSA
OSPF Packet Format
OSPF Message
Header
2: current version
is OSPF V2
version
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
Authentication passwd = 1:
Authentication passwd = 2:
Body of OSPF Message
type
message length
source router IP address
ID of the Area
from which the
packet originated
Area ID
checksum
authentication type
authentication
authentication
32 bits
64 cleartext password
0x0000 (16 bits)
KeyID (8 bits)
Length of MD5 checksum (8 bits)
Nondecreasing sequence number (32 bits)
0: no authentication
1: Cleartext
password
2: MD5 checksum
(added to end
packet)
Prevents replay
attacks
OSPF LSA Format
LSA
Link Age
LSA
Header
LSA
Header
LSA
Data
Link Type
Link State ID
advertising router
link sequence number
checksum
length
Link ID
Link 1
Link Data
Link Type #TOS metrics
Metric
Link ID
Link 2
Link Data
Link Type #TOS metrics
Metric
Discovery of Neighbors


Routers multicasts OSPF Hello packets on all OSPFenabled interfaces.
If two routers share a link, they can become neighbors, and
establish an adjacency
10.1.10.1
10.1.10.2
Scenario:
Router 10.1.10.2 restarts
OSPF Hello
OSPF Hello: I heard 10.1.10.2

After becoming a neighbor, routers exchange their link state
databases
Neighbor discovery and database synchronization
Scenario:
Router 10.1.10.2
restarts
Discovery of
adjacency
10.1.10.1
10.1.10.2
OSPF Hello
OSPF Hello: I heard 10.1.10.2
After neighbors are discovered the nodes exchange their databases
Database Description: Sequence = X
Sends database
description.
(description only
contains LSA
headers)
Acknowledges
receipt of
description
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+1, 1 LSA header=
Router-LSA,
10.1.10.2, 0x80000005
Database Description: Sequence = X+1
Sends empty
database
description
Database
description of
10.1.10.2
Regular LSA exchanges
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,
10.1.10.1 sends
requested LSAs
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
10.1.10.2
10.1.10.2 explicitly
requests each LSA
from 10.1.10.1
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 not new changes.
Acknowledgements of LSA-updates:



explicit ACK, or
implicit via reception of an LSA-Update
Question: If a new node comes up, it could build the
database from regular LSA-Updates (rather than
exchange of database description). What role do the
database description packets play?