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IP Routing: OSPF
Network Protocols and Standards
Autumn 2004-2005
Nov 11, 2004
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Issues
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Design of OSPF
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Separating hosts and routers
Broadcast networks (Ethernet, FDDI, …)
Non-broadcast networks (ATM, X.25, …)
Splitting very large networks into areas
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Separating Hosts and Routers
Instead of link-state records for hosts:
R-H1
R-H2
R-H3
R
H1
H2
H3
Use simplification based on subnet model
- One link between the router and the subnet:
“link to stub network”
- Identified by its subnet number
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Stub Networks
RT
RT
N

N
Graph Representation
Multiaccess/broadcast stub network
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Single router attached to the network
N is network IP address and associated mask
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Point to Point Networks
RT1
RT2
RT1
RT2
Graph Representation

Unnumbered point-to-point network
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Interfaces to point-to-point network not assigned
IP addresses
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Point to Point Networks
RT1
Ia
Ib
RT2
RT1
RT2
Ia
Ib
Graph Representation

Numbered point-to-point network
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Ia and Ib are interface IP addresses
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Stub Networks
RT
RT
Host

Host
Graph Representation
Host directly attached to a router
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Host route
Mask is 0xFFFF FFFF (255.255.255.255)
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Broadcast Networks

Such networks characterized by:
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Full connectivity
Broadcast capability
Issues (when N routers coexist on a broadcast
network):
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# Adjacencies: N(N-1)/2
Each router would advertise:
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N-1 links to other routers
One link to the subnetwork
Solution: reduce number of adjacencies to N
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One router is designated through election
Election of the designated router is through “Hello Protocol”
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Broadcast Networks

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Reducing the number of link state records
using designated router
Database will include two links per router
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One link from router to virtual node
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Called router link type 2 (transit network)
Advertised by the router itself
Appropriate metric
Link from virtual node to router
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Nov 11, 2004
Called network link
Advertised by designated router
Network links have a null metric
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Broadcast Networks
RT2
RT1
RT2
N
RT1
RT3
RT3
RT4
Multiaccess/broadcast transit network
- 3 or more routers attached to the network
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RT4
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Broadcast Networks

Simplifying flooding
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A router sends a link state advertisement to the
designated router only using 224.0.0.6 “all
designated routers” multicast address
If advertisement is new, designated router floods
the link state on all its interfaces (including the
network on which it is received!) using 224.0.0.5
“all OSPF routers” multicast address
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Broadcast Networks


Designated router forms adjacencies with all routers
in the broadcast network
For reliability, there is one backup designated router

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Backup designated router also forms adjacencies with all
routers in the broadcast network
Listens to 224.0.0.6 but remains silent in terms of flooding,
etc.
Failure of designated router is discovered by means of Hello
protocol
Smoother transition occurs since the backup router has
already formed all the adjacencies
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Non-broadcast Networks

IP over X.25 networks
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Popular in Europe in 1980’s
IP over ATM

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Static configurations
Avoid N(N-1)/2 overhead
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Use designated router
On-demand circuits
Permanent circuits are for links between routers
and the “designated router”
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Link State DB records

Five types of link state records
1.
2.
3.
4.
5.
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Router link
Network link
Summary link (IP network)
Summary link (to a border router)
External link
Types 3 and 4 are used when OSPF
areas are used
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OSPF Router ID

Each OSPF router is assigned an OSPF
router ID
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32 bit number uniquely identifying the
router within the OSPF domain
When the router interfaces have IP
addresses assigned to them, then the
OSPF router ID is one of the router’s IP
address
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LSA Header
OSPF HEADER
TYPE=4
ADVERTISEMENT 1
LS AGE
OPTIONS
LS TYPE
LINK STATE ID
ADVERTISING ROUTER
LS SEQUENCE NUMBER
…
LS CHECKSUM
LENGTH
Advertisement
Header
#ADVERTISEMENTS
4 BYTES
ADVERTISEMENT DATA (LINKS)
ADVERTISEMENT N
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LSA Header Fields
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Link state Type
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Link state ID
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Identifies one particular advertisement
Checksum
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Chosen by the advertising router
Generally an IP address
Sequence Number
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Type of LS record (1, 2, 3, 4, or 5)
Protects header as well as content
Length

Total length of the record (including the 20-byte header)
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LSA Header Fields
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Advertising router
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Age
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The OSPF ID of the sender
16-bit unsigned integer indicating the time in seconds since
the link state record was first advertised
Options:

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E T
RFC 1583
Definition
Type of Service
External Links
E: used in Hello protocol
T: Set when router supports nonzero TOS

Nov 11, 2004
Removed from the latest version of the standard (RFC2328)
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Multiple Areas
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Hierarchical routing
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Decreased routing overhead

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Size of link state DB
Duration of route computation
Volume of messages exchanged
Split the network into set of independent parts by a
backbone
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Each area operates like an independent network
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Database includes only the state of the area’s links
Flooding stops at the boundaries
Routers compute routes within the area
Cost of routing proportional to the size of the area
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Multiple Areas
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How to glue the network together?
Some routers belong to several areas
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Typically to the backbone and to one lower-level
area
At least one area border router in each area
Area border routers

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Maintain several link state databases (one for each
area to which they belong)
Emit special link state records (summaries) to
signal reachability of networks in each area
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Stub Areas

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Areas where there is only one exit point, or the exit
point is not a function of the external destination
Stub area does not need to know the topology of the
rest of the AS
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All external traffic goes to the exit point
Obviously, no AS boundary router can be internal to
the stub areas
R
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OSPF Packet Formats
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OSPF directly over IP, using protocol number 89
OSPF does not explicitly support fragmentation, but
protocol messages can generally be split
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This should be used rather than IP fragmentation
OSPF packets are sent with an IP TOS of 0
OSPF packets are sent with IP precedence set to
Internetwork control
All OSPF packets use the same OSPF header
OSPF Multicast addresses (sent with TTL=1)
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224.0.0.5: All OSPF routers
224.0.0.6: OSPF designated and backup routers
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The Common OSPF Header
VERSION #
TYPE
PACKET LENGTH
ROUTER ID
AREA ID
CHECKSUM
AUTYPE
AUTHENTICATION
AUTHENTICATION
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OSPF Header Fields
Version #: set to 2 (current version)
Type: The OSPF packet type
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Hello
Database description
Link state request
Link state update
Link state acknowledgement
1.
2.
3.
4.
5.
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Packet length: Number of bytes in the header including the
header
Router ID: The IP selected for identifying the router
Area ID: The value 0 is reserved for backbone area.
Commonly, an IP address is used to identify the areas
Checksum: Computed on the whole OSPF packet, excluding
the 8-octet authentication field
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OSPF Header Fields
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AUTYPE: Identifies the authentication algorithm. Only three
values are identified in the standard itself:
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0: No authentication
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1: Simple authentication
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Exchanges not authenticated
Authentication field ignored; can be set to anything
“Clear password” type of authentication; all packets must contain the
right value, pre-configured for that area
Used to prevent unconfigured routers from joining in
2: Cryptographic authentication
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Secret key is used to generate a digest of the packet
Digest is added at the end of the packet; size not included in the
header
64-bit field is restructured to contain digest size, key ID, and sequence
number (to protect against replay attacks)
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