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OSPF
CSC/ECE 573, Sections 001
Fall, 2012
Introduction
1-1
OSPF: Open (= non-proprietary) SPF
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RFC 2328 (OSPF version 2)
 Recommended interior (intra-domain) routing
protocol for TCP/IP
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Goals
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link state routing using Dijkstra's algorithm
converge faster than RIP
exchange less information than RIP
Runs directly over IP (not UDP or TCP);
protocol number 89
OSPF “advanced” features (not in RIP)
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Network Layer
security: all OSPF messages authenticated (to prevent
malicious intrusion)
multiple same-cost paths allowed (only one path in RIP)
For each link, multiple cost metrics for different TOS
(e.g., satellite link cost set “low” for best effort; high for
real time)
integrated uni- and multicast support:
– Multicast OSPF (MOSPF) uses same topology data
base as OSPF
hierarchical OSPF in large domains.
4-3
OSPF (cont’d)
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Optimization metrics
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hop-count
– delay
– throughput, etc.
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When several equal-cost routes exist, can send
traffic along each of them
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for load-balancing
Reliability
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flooding, with neighbor acknowledgments
reoriginate LSAs at 30 minute intervals
all LSAs are checksummed
can withdraw state using LSA with maximum age
OSPF (cont’d)
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Subnetting
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designed to work with variable-length subnets and
CIDR
Security
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simple cleartext password
MD5 message digest, based on shared secret key
Area Routing
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Routing domain is split into areas
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Area = generalization of a subnet
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“backbone” and attached areas
areas do not overlap
2-level hierarchy; each area must attach directly to backbone
its topology and details are not visible outside the area
each area runs a copy of the link-state protocol
smaller link-state databases
Routers at boundaries intercommunicate
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summarize (aggregate) routing prefixes and advertise fewer
routes
Areas in an Autonomous System
OSPF Areas (cont’d)
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Exchange of summary LSA’s across the
backbone: Distance Vector!
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But using flooding as method of distribution
Virtual links to handle connections of border
routers to the backbone area
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Result: border routers do not have to be physically
directly connected to each other
“tunnels” through non-border routers  virtual link
OSPF Classes of Routers
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Internal routers: wholly within an area
Area border routers: connect two or more areas
Backbone routers: on the backbone
AS boundary routers: talk to routers in other AS
The classes are allowed to overlap
OSPF Classes of Routers (cont'd)
Connections And Networks
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OSPF supports three types of connections
and networks
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Point-to-point lines between exactly two routers
Multiaccess networks with broadcasting (e.g.,
most LANs)
Multiaccess networks without broadcasting (e.g.,
most WANs)
OSPF abstracts the collection of actual
networks, routers, and lines into a directed
graph
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(hosts do not play a role in OSPF)
LAN Abstraction
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N routers connected to a broadcast LAN
 Model as N*(N-1)/2 point-to-point connections 
expensive!
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Instead, create one LSA for the LAN, containing links to
all routers
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size of link-state database (N routers, O(N2) links)
# of messages exchanged (O(N2) for N routers in LAN)
N+1 nodes, N links, N messages on LAN
A “Designated Router” is responsible for originating the
LSA representing the LAN
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there may also be a “backup designated router”
LAN Abstraction Example
OSPF Network Representation
Example
OSPF Network Representation
Example (cont'd)
Message Types
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Hello
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Link state update
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at boot time, used to discover who the neighbors are;
also used to periodically test reachability
flooded to the whole network; provides the sender's
cost to its neighbors
Link state ACK
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ACKs link state update  reliable flooding
Message Types (cont’d)
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Database description
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when a line between routers is brought up, gives
sequence # of all link state entries a sender has
Link state request
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after exchanging database description messages,
sender requests link state information for links for
which the other router has more recent info
LSA Types: Router LSA
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Lists router's interfaces, their cost, the
network/router they connect to, the range of IP
addresses directly accessible via the interfaces
LSA Types: Network LSA
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Performs the LAN abstraction: lists all routers
connected to the network
LSA Types: Summary Link
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Link to border router, advertises IP
addresses between areas
External Link LSA
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Link to another site, imports routing
information from other AS