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TCOM 515
Lecture 3
Objectives
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OSPF Packets
OSPF Operation
OSPF Neighbors & Adjacencies
Designated Routers and Backups
Areas
Link State Advertisements LSAs
OSPF Background
OSPF version 1 - only used in experimental form - RFC 1131
OSPF version 2 - RFCs 1247,1583,2178, 2328 (most current) since
1991
John Moy is the expert on OSPF. He wrote most of the RFCs and
the definitive OSPF book, OSPF Anatomy of an Internet Routing
Protocol.
OSPF stands for Open Shortest Path First and is the IETF’s
recommended IGP.
Advantages of OSPF include: fast reconvergence, support for large
networks, classless routing, VSLM, equal cost load balancing,
and route tagging.
Most Large networks use OSPF or IS-IS as their IGP.
Existing working group for OSPF - show draft & website
OSPF Terms
Interface or Link - connection from local router to attached network
Link State - status of link between two routers, relationship to neighbor
router
Cost - value assigned to link, based on media speed
Autonomous system - group of router exchanging routing info using same
routing protocol
Area - set of routers and networks that have the same area designation,
each router in an area has same info
Neighbors - two routers that share a common network, discover and
exchange routing info across it
Hello - Protocol used to establish and maintain neighbor relationships
LSA - Link-State Advertisements, includes interfaces, associated cost and
network information
Neighbor Database - list of all neighbors with established two-way
communications
Link-State Database - listing of link-state entries from all other routers in
area, same database for each router in an area, generated from
LSAs received
OSPF Operation
1.
OSPF enabled router sends hello packets out all enable interfaces
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2.
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Routers with shared link may become neighbors via negotiation
Some neighbors form adjacencies based on network and hello type.
Router send Link State Advertisements (LSA) over its adjacencies.
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LSA include interface link, state and cost information.
4.
Router receives other LSAs and records it in its Link State Database. Then it
sends the LSA out its enabled interfaces.
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6.
LSAs flood the OSPF area and each router has same Database.
Router uses Shortest Path First Algorithm (SPF) to build a SPF tree with a
node for each router/destination.
7.
Router uses the SPF tree to build its routing table.
*LSAs are only sent out every 30 minutes or if a topology change happens.
OSPF Neighbors
OSPF routers build a neighbor table based on its exchange of hello
packets out its OSPF enabled interfaces
To become neighbors, the two routers must share a link, both interfaces
must be OSPF enable.
The routers will negotiate via the use of the hello packets on the
agreement of certain parameters: Area ID, Authentication, Network
Mask.
Neighbors are identified in OSPF by their Router ID
The Router ID is:
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The numerically highest loopback interface IP address on the router.
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If there are no loopbacks, the numerically highest physical interface
IP address of the router.
Loopbacks are preferred because of the stability of the interface ( no
flapping) and the network administrator’s ability to change and
assign the loopbacks.
OSPF Metric & Paths
OSPF Metric is cost based on the sum of the costs of all outgoing interfaces to a
destination.
Recorded as a 16 bit number between 1 and 65,535. This metric is applied on all
router links. The lower the cost the better.
The cost metric - 10^8/ bandwidth of a link so:
FastEthernet = 100Mbps => cost of 1
Ethernet = 10Mbps => cost of 10
T1 = 1.544 Mbps => cost of 64
Path Types
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Intra-area - within one of the router’s attached areas
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Inter-area - destination in another area but same OSPF domain
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Type 1 External - to destination outside of OSPF domain with metric
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Type 2 External - to destination outside of OSPF domain without metric
OSPF Packet Header
Version number - OSPF version number, most recent version 2
Type - OSPF packet type following the header
1 - Hello - Establishes and maintains neighbor relationships.
2 - Database description - contents of the topological database upon adjacency formation
3 - Link State request - Requests pieces of the topological database from neighbor routers
4 - Link State Update—Responds to a link state request packet
5 - Link State Acknowledgment—Acknowledges link state update packets.
Packet length - Entire packet length, including the OSPF header, in bytes.
Router ID - Identifies the originating router
Area ID - Identifies the area from where the packet originated.
Checksum - IP checksum for entire packet including header
Authentication type - mode of authentication
1 - Null, none
2 - Simple, clear text
3 - MD5 encrypted
Authentication - information needed to authentication at other end, password, Key, etc
Data—Contains encapsulated upper-layer information.
OSPF Packet Types
Hello Packet & Protocol
• Used to discover neighbors
• Used as keepalives for existing neighbors
• Elect Designated Routers and Backup Designated routers
• 10 second interval by default
Database Descriptions
• Used to build adjacencies by matching LSAs in the Database
Link State Request
• Used to request new or more recent LSAs from neighbors
Link State Update
• Used to flood LSAs and responds to Link State Requests
Links State Acknowledgement
• Used to ACK LSAs to make the process reliable
Hello Packet
Additional fields after OSPF packet header:
Network mask 4 bytes- subnet mask of interface of originating packet
Hello Interval 2 bytes - time in seconds between Hello packets
on interface
Options 1 byte - checks compatible capabilities
Router Priority 1 byte - for DR and BDR election
Router Dead Interval 4 bytes - time in seconds to wait for hello
packet before considered dead neighbor
Designated Router 4 bytes - IP address of interface of the DR
on the network
Backup Designated Router 4 bytes- IP address of interface of the
BDR on the network
Neighbor 8 bytes - multiple instances of this field are possible,
neighbors on the network from which originating router has received
hellos
Link State Request Packet
Additional fields after OSPF packet header:
Link State Type 4 bytes - LS type number
1- Router LSA
2 - Network LSA
3 - Network Summary LSA
4 - ASBR Summary LSA
5 - AS External LSA
6 - Group Membership LSA
7 - NSSA External LSA
8 - External Attributes LSA
9, 10 ,11 - Opaque LSAs
Link State ID 4 bytes - Type dependant on LSA header
Advertising Router 4 bytes - Router ID of LSA originating
router
Link State Update &
Acknowledgement Packets
Update additional fields to OSPF packet header:
Number of LSAs 8 bytes - number of LSAs to follow
LSA - full LSA, multiple LSAs can be included as
Specified in the number field
Acknowledgement additional fields to OSPF packet header:
LSA headers - headers of all LSAs to be acknowledged,
Each header is 5 bytes long.
LSA Types
1 - Router - produced by each router w/ info on interfaces, state and cost
for each area it belongs to, flooded within area
2 - Network - produced by each DR listing all connected routers on
multiaccess networks flooded to within the area the network belongs to
3 - Network Summary - produced by ABRs to let one area know about
another
4 - ASBR Summary - produced by ABRs to advertise ASBRs outside of
the area
5 - AS External - originated by ASBRs to advertise external AS routes
6 - Group Membership
7 - NSAA External - allows ASBR in NSA to advertise external AS routes
8 - External Attributes
9 - Opaque (link local)
10 - Opaque (area local)
11 Opaque (AS)
OSPF Areas
•Area - logical grouping of OSFP routes and links that
effectively divide an OSPF domain into sub-domains.
•A router in an area knows nothing detailed of topology
outside of its own area.
•Expressed in an Area ID of 32 bits, most often as
decimal, but can be in dotted decimal like an IP
address.
•Area 0 must exist in all OSPF implementations and
should be the backbone area of the network.
•The use of areas allows administrators to cluster
groups of routers together to reduce the CPU load and
memory needed for running OSPF on every router.
OSPF Area Traffic
Intra-area traffic - includes any traffic that originates
and is delivered within the same area, not leaving it
Inter-area traffic - includes any traffic that originates
on a router in one area and is delivered by a router in
another areas, transiting between at least 2 areas.
External traffic - includes any traffic that originates
from within one OSPF domain and transits to a router
in another Autonomous System, requires a EGP
OSPF Area Types
Backbone area - only one per OSPF domain, should be area 0,
receives LSA types 1 - 5
Non-backbone, Non-stub area - also receives LSA types 1-5
Stub area - receives LSA types 1-4, instead of receiving ASBR
summary Type 4s with real info, default routes are inserted by
ABRs to reduce routes, load and memory.
Requirements for a Stub Area:
•All routers must have identical Link State Databases.
•There can be no virtual links.
•No router in a stub area can be a ASBR.
•There can be more than 1 ABR but there is no preference.
OSPF Area Types 2
Totally Stubby area - Only LSA types 1 and 2 are allowed. A
special default only route version of LSA is sent by the ABRs. This
reduces the routes, CPU load and memory usage even more.
Not-So-Stubby area - LSA types 1 - 4 are allowed, in addition two
type 7 which allows external routes to be advertised into the area.
These special Type 7 LSA are the only difference from normal
stubby areas.
OSPF Router Types
Internal Router - all interfaces belong to the same area
Area Border Router (ABR) - router has interfaces in more than
one area and acts a a gateway for traffic between the areas, “interarea traffic.” An ABR must have one interface in the Backbone
area. It has a separate Link State database for each area.
Backbone Router - router has at least one interface in the
backbone area. Every ABR is a Backbone router, but not every
backbone router is an ABR.
Autonomous Systems Boundary Router (ASBR) - a router that
acts as a gateway for traffic external to the OSPF domain using
routes learned through another routing protocol.
Designated Routers
Multi-access networks need DRs and BDRs:
•The existence of a large number of unneccessary LSAs created on
the network.
•The flooding of these LSAs by all the routers would disrupt the
operation of the network.
DR - Designated Router
•Elected base on router priority
•Forms an adjacency with each router and receives all LSAs
•Sends LSAs to every router
•Forms adjacency with BDR
•Exists on an Interface
BDR - Backup Designated router
•Forms an adjacency with each router in case DR disappears
•Forms adjacency with DR
•Takes over for DR in case of failure
DR Election
1. Each router makes a list including
• All eligible routers
• All routers declaring themselves to be the DR
• All routers declaring themselves to the be BDR
2. Makes a subset list of all routers no claiming to be DR as
possible BDRs
3. From that subset, neighbor with highest priority is selected as
BDR
4. If no router in subset claims to be BDR, than any neighbor with
highest priority is elected BDR.
5. In case of tie with priority, the neighbor with the highest Router
ID wins the tie.
6. If there is no DR, the BDR becomes the DR and a new BDR is
elected through the same process.
**If a new router comes online with a higher priority, it will not knock
the BDR or the DR out of place, only the failure of either one
will result in a new election.
OSPF SPF Example
Make a SPF tree for this example from Router 1 perspective.
Then build your routing table.
OSPF SPF Example Cont
Link Speed
Cost
Links
100Mbps
1
A, C, F
10Mbps
10
B, D, G, H, J
T1-1.544Mbps
64
E, I, K
OSPF SPF Example Cont
This is the routing table from Router 1.
Destination
Cost
Next Hop
2
1
2
3
2
2
4
12
2
5
2
2
6
12
2
7
11
2
OSPF SPF Example 2
Links C and F having failed.
Make a new SPF tree for Router 1 and build its routing table.
OSPF SPF Example 2 Cont
This is the SPF tree.
OSPF SPF Example 2 Cont
This is the routing table from Router 1.
Destination
Cost
Next Hop
2
1
2
3
10
3
4
20
3
5
31
2
6
21
2
7
11
2
Summary
•OSPF Areas
•OSPF Packet Types
•OSPF Header
•OSPF DRs and BDRs
•OSPF Neighbors
•OSPF SPF
•OSPF LSAs