Dynamic Routing Protocols II
OSPF
Relates to Lab 4. This module covers link state
routing and the Open Shortest Path First (OSPF)
routing protocol.
1
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
F
• If parts of the directions incorrect, the routing may be incorrect until the
routing algorithms has re-converged.
2
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
B
C
D
E
A
F
B
C
D
E
C
D
E
B
C
D
E
A
A
B
B
F
C
A
D
F
F
E
B
C
D
E
F
F
3
Link State Routing: Properties
• Each node requires complete topology information
• Link state information must be flooded to all nodes
• Guaranteed to converge
4
Link State Routing: Basic princples
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
5
Operation of a Link State Routing protocol
Received
LSAs
Link State
Database
Dijkstra’s
Algorithm
IP Routing
Table
LSAs are flooded
to other interfaces
6
Dijkstra’s Shortest Path Algorithm for a Graph
Input: Graph (N,E) with
N the set of nodes and E the set of edges
dvw
link cost (dvw = infinity if (v,w)  E, dvv = 0)
s
source node.
Output: Dn
cost of the least-cost path from node s to node n
M = {s};
for each n  M
Dn = dsn;
while (M  all nodes) do
Find w  M for which Dw = min{Dj ; j  M};
Add w to M;
for each n  M
Dn = minw [ Dn, Dw + dwn ];
Update route;
enddo
7
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
1991: RFC 1247
1994: RFC 1583
1997: RFC 2178
1998: RFC 2328
OSPF Version 1
OSPF Version 2
OSPF Version 2 (revised)
OSPF Version 2 (revised)
OSPF Version 2 (current version)
8
Features of OSPF
• 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
9
Example Network
10.10.10.2
.2
10.1.1.0 / 24
.4
2 .0
/2
4
.3
•Link costs are called Metric
/2
.5
5
.5
.5
10.1.5.0/24
10.10.10.2
.0
.1 .
.3
1
.8
3
.6
.1
2
.6
.4
10.1.3.0 / 24
10
3
1
10.1.7.0 / 24
.2
.3
• Metric is in the range [0 ,
.4
10.1.4.0 / 24
.1
Router IDs are
selected
independent of
interface addresses
2
4
.2
10.10.10.6
10
4
.1
10.10.10.4
10.1.6.0 / 24
10.10.10.1
10.10.10.5
216]
• Metric can be asymmetric
10
Link State Advertisement (LSA)
10.10.10.1
4
.1
.2
.2
10.1.1.0 / 24
10.1.4.0 / 24
3
2
.1 .
2 .0
/2
10.1.3.0 / 24
.2
10
4
• The LSA of router 10.10.10.1 is as
follows:
.1
• Link State ID:
10.10.10.1 = Router ID
• Advertising Router:
10.10.10.1 = Router ID
• Number of links:
3 = 2 links plus router itself
10.10.10.2
.3
.3
.3
• Description of Link 1:
Link ID = 10.1.1.1, Metric = 4
• Description of Link 2:
Link ID = 10.1.2.1, Metric = 3
• Description of Link 3:
Link ID = 10.10.10.1, Metric = 0
10.1.5.0/24
10.10.10.3
Each router sends its LSA to all routers in the network
(using a method called reliable flooding)
11
Network and Link State Database
10.10.10.2
.2
.2
2 .0
/2
4
.3
4
.5
/2
.3
.6
.0
.1 .
.4
.8
10
.2
.3
.6
10.1.7.0 / 24
10.1.6.0 / 24
10.1.4.0 / 24
.1
Each router has a
database which
contains the LSAs
from all other routers
.4
10.1.3.0 / 24
10.1.1.0 / 24
.4
10.10.10.6
.1
.1
10.10.10.4
10
10.10.10.1
.5
.5
10.1.5.0/24
10.10.10.2
10.10.10.5
LS Type
Link StateID
Adv. Router
Checksum
LS SeqNo
LS Age
Router-LSA
10.1.10.1
10.1.10.1
0x9b47
0x80000006
0
Router-LSA
10.1.10.2
10.1.10.2
0x219e
0x80000007
1618
Router-LSA
10.1.10.3
10.1.10.3
0x6b53
0x80000003
1712
Router-LSA
10.1.10.4
10.1.10.4
0xe39a
0x8000003a
20
Router-LSA
10.1.10.5
10.1.10.5
0xd2a6
0x80000038
18
Router-LSA
10.1.10.6
10.1.10.6
0x05c3
0x80000005
1680
12
Link State Database
• The collection of all LSAs is called the link-state database
• Each router has an 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
13
OSPF Packet Format
OSPF Message
IP header
OSPF packets are not
carried as UDP payload!
OSPF has its own IP
protocol number: 89
OSPF Message
Header
Body of OSPF Message
Message Type
Specific Data
LSA
LSA
... ...
LSA
TTL: set to 1 (in most cases)
LSA
Header
LSA
Data
Destination IP: neighbor’s IP address or 224.0.0.5
(ALLSPFRouters) or 224.0.0.6 (AllDRouters)
14
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
15
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
16
Discovery of Neighbors
• 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
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
17
Neighbor discovery and
database synchronization
10.1.10.1
Discovery of
adjacency
Scenario:
Router 10.1.10.2 restarts
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
Sends empty
database
description
Database
description of
10.1.10.2
Database Description: Sequence = X+1
18
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
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)
Link State Update Packet, LSA =
Router-LSA,
10.1.1.6, 0x80000006
19
Routing Data Distribution
• LSA-Updates are distributed to all other routers via Reliable
Flooding
• Example: Flooding of LSA from 10.10.10.1
10.10.10.1
10.10.10.2
LSA
ACK
10.10.10.4
LSA
Update
database
Update
database
10.10.10.2
Update
database
LSA
10.10.10.6
LSA
ACK
Update
database
Update
database
10.10.10.5
20
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?
21
Dynamic Routing Protocols III
More OSPF
Relates to Lab 4. This module covers additional details on
the Open Shortest Path First (OSPF) routing protocol.
Functional Requirements of OSPF
• Fast convergence and low consumption of network resources
• A descriptive routing metric
– Configurable
– Value ranges between 1 and 65,535
– No restriction on network diameters (RIP has a limit of 15)
• Equal-cost multipath
– A way to do load balancing
Functional Requirements of OSPF
• Routing Hierarchy
– Support large routing domains
• Separate internal and external routes
• Support of flexible subnetting schemes
– Route to arbitrary [address,mask] combinations using
variable length subnet masks (VLSMs)
• Security
• Type of Service Routing
OSPF Basics
(The Essence)
• Distributed, replicated database model
– Describes complete routing topology
• Link State Advertisements (LSAs, sometimes called Link
State Announcements)
– Carry local piece of routing topology
• Distribution of LSAs using reliable flooding
• Link state database
– Identical for all the routers
OSPF Packet Format
OSPF Message
IP header
OSPF packets are not
carried as UDP payload!
OSPF has its own IP
protocol number: 89
OSPF Message
Header
Body of OSPF Message
Message Type
Specific Data
LSA
LSA
TTL: set to 1 (in most cases)
LSA
Header
Destination IP: neighbor’s IP address or 224.0.0.5
(ALLSPFRouters) or 224.0.0.6 (AllDRouters)
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
LSAs (1)
• Identifying LSAs
– LS type field
– Link State ID field
• Mostly carries addressing information
• E.g. IP address of externally reachable network
– Advertising Router field
• Originating router’s OSPF router ID
LSAs (2)
• Identifying LSA instances
– Needed to update self-originated LSAs
– LS Sequence Number field
• 32 bit values
• Monotonically increasing until some max value
• 600 years to roll over!
• LSA checksum and LS Age guard against potential
problems
LSAs (3)
• Verifying LSA contents
– LS Checksum field
• Computed by the originating router and left unchanged
thereafter
• LS age field not included in checksum
• Removing LSAs from databases
– LS Age field
• Ranges from 0 to 30 min.
• Max Age LSAs used to delete outdated LSAs
LSAs (4)
• Other LSA Header fields
– Options field
• Sometimes used to give special treatment during
flooding or routing calculations
– Length field
• Includes LSA header and contents
• Ranges from 20-65535 bytes
Link State Database
•
•
•
•
•
Collection of all OSPF LSAs
Databases exchanged between neighbors
Synchronization thru reliable flooding
Gives the complete routing topology
Each OSPF router has identical link-state database
Reliable Flooding
• Robustness
– Updates flooded over all the links , so failure of any link
does not affect database synchronization
– LSAs refreshed every 30 minutes
– LSA checksum field detects corruption
– Flooding loops avoided by LS Age field
– MinLSInterval limits rate of LSA origination
– Receivers can refuse to accept LSA updates if they
received an update less than a second ago
Routing Calculations
•
•
•
•
•
Link costs configurable by administrator
Smaller values for more preferred links
Must make sense to add link costs
Different costs for each link direction possible
Dijkstra’s shortest path algorithm
– Incrementally calculates tree of shortest paths
– Each link in the network examined once
– Computes multiple shortest paths (equal-cost multipath)
IP Multicast to Send/Receive Changes
• Multi-Access networks
– All routers must accept packets sent to the AllSPFRouters
(224.0.0.5) address
– All Designated Router (DR) and Backup Designated Router
(BDR) must accept packets sent to the AllDRouters (224.0.0.6)
address
• Hello packets are sent to the AllSPFRouters address
(Unicast for point-to-point and virtual links)
Hierarchical Routing
• Technique used to build large networks
• Minimizes consumption of network resources:
– Router memory
– Router computing resources
– Link bandwidth
• Flat Routing: linear increase in routing table size
• Hierarchical Routing: size increases logarithmically
An Example of Hierarchical Routing (1)
10.0.3
10.3.0.0/16
10.3.1
10.0.0.0/8
10.3.2
10.1.3
10.2.3
10.1.0.0/16
10.2.0.0/16
10.1.1
10.1.2
10.2.1
10.2.2
An Example of Hierarchical Routing (2)
•
•
•
•
Consider a router in 10.1.1
Assume 16 entries in each of the first level partitions
With flat routing, 9*16 = 144 entries/router
With 3 level hierarchy, the router has 16 entries
within 10.1.1.0/24 + entries for 10.1.2.0/24,
10.1.3.0/24,10.2.0.0/16 and 10.3.0.0/16 for a total of
20 entries.
• Significant reduction in routing table size
• But might lead to suboptimal routing
OSPF Areas
• Two-level hierarchical routing scheme
through the use of areas
• Areas identified by 32-bit id
• Each area has its own link state database
which is a collection of network-LSAs and
router-LSAs
• Area’s topology hidden from all other areas
• Interconnection of areas through area border
routers (ABRs)
• ABR leaks IP addressing information to other
areas through summary LSAs
OSPF Areas
• Reduction in link state databases of an area
• Reduction in amount of flooding traffic needed for
synchronization
• Reduction in the cost of the shortest path
calculations
• Increased robustness
Area Organization
• All the areas are connected to area 0.0.0.0 also
called the Backbone Area
• Need not have a direct physical connection though
– Virtual links provide logical link to backbone
– Summary LSAs tunneled across non backbone areas
• Exchange of routing information between areas
using Distance Vector Protocol
– Absence of redundant paths between areas
– Not subject to convergence problems
OSPF Areas
• Group of nodes/networks
• Per area topology DB
– Invisible outside the area
– Reduces routing traffic
• Backbone Area is contiguous
Area 0
Area 2
Area 3
Backbone Area
– All others areas must connect to
the backbone
• Virtual Links
Area 4
Area 1
Router Classification
IR
Area 3
Area 2
ABR/BR
Area 0
ASBR
To another AS
Area 1
IR/BR
• Internal Router (IR)
• Area Border Router
(ABR)
• Backbone Router (BR)
• Autonomous System
Border Router (ASBR)
OSPF Route Types
Area 2
Area 0
ABR
Area 3
Intra-Area Route
– All routes within an area
ASBR
To Another AS
Inter-Area Route
– Routes announced from area to
another by an ABR
External Route
– Routes imported into OSPF from
another protocol or Static routes
Inter-Area Route Summarization
• Prefix or all subnets
• Prefix or all networks
R2
FDDI
With
Summarization
Dual Ring
Network Next Hop
Summarization
Area 0
R1 (ABR)
1
R1
Area 1
Network Next Hop
Without
Backbone
1.A
R1
1.B
R1
1.C
R1
1.A
1.B
1.C
External Routes
• Redistributed into OSPF
• Flooded without changes throughout the AS
• OSPF supports two type of external metrics
– Type 1
– Type 2 (Default)
RIP
IGRP
OSPF
Redistribute
EIGRP
BGP
etc.
Topology/Links-State DB
• A router has a separate DB for each area it
belongs to
• All routers within an area have an identical DB
• SPF calculation is done separately for each area
• LSA flooding is limited to the particular area
Protocol Functionality
• Bringing up adjacencies
• LSA Types
• Area Classification
The Hello Protocol
• Responsible to establish and maintain neighbor
relationships
• Elects designated router in multi-access
networks
Hello
FDDI
Dual Ring
Hello
Hello
Designated Router (DR)
One per multi-access network
Generates network links advertisements
Assists in DB synchronization
Designated
Backup
Router
Designated
Router
Designated Router by Priority
• Configured priority (per interface)
• Otherwise determined by the highest router ID
131.108.3.2
131.108.3.3
DR
R1 Router ID = 144.254.3.5
144.254.3.5
R2 Router ID = 131.108.3.3
Neighbor States
• 2-way
– The router sees itself in other Hello packets
– DR is selected from neighbors in state 2-way or greater
2-way
DR
BDR
Neighbor States
• Full
– Routers are fully adjacent
– DB is synchronized
– Relationship to the DR and
BDR
Full
DR
BDR
When to Become Adjacent
•
•
•
•
•
•
Underlying network is point-to-point
Underlying network type is virtual link
The router itself is the DR
The router itself is the BDR
The neighboring router is the DR
The neighboring router is the BDR
LSAs Propagate Along Adjacencies
DR
BDR
• LSAs acknowledged along adjacencies
Different Types of LSAs
• Five LSA types
– Type 1 :
– Type 2 :
– Type 3 y 4:
– Type 5 y 7:
Router LSA
Network LSA
Summary LSA
External LSA
Router LSA (Type 1)
• Describes the state and cost of the router’s link to
the area
• All the router’s links in an area must be described
in a single LSA
• Flooded throughout the particular area and not
beyond
• Router indicates whether it is an ASBR, ABR, or
the end point of a virtual link
Network LSA (Type 2)
• Generated for every transit broadcast or NBMA
network
• Describes all the routers attached to the
network
• Only the DR originates this type of LSA
• Flooded throughout the area and not beyond
Summary LSA (Type 3 y 4)
• Describes a destination outside the area but
still within the AS
• Flooded throughout a single area
• Originated by an ABR
• Only intra-area routes are advertised into
the backbone (Area 0)
• Type 4 is the information about the ASBR
External LSA (Type 5)
• Defines routes to destinations outside the AS
• Default route is also sent as external
• Two Types of external LSA:
• E1: Considers the total cost of to the external destination
• E2: Considers only the cost of the outgoing interface to the
external destination
Issues not covered
• OSPF Network Types
– Broadcast subnets
– NBMA Subnets
•
•
•
•
OSPF Extensions
Multicast Routing using OSPF (MOSPF)
OSPF Management
and a whole lot of others!
Further Reading
• John T. Moy, OSPF - An Anatomy of an Internet
Routing Protocol
• Christian Huitema, Routing in the Internet
• RFC 2178