Upload
martin-bratina
View
241
Download
0
Tags:
Embed Size (px)
DESCRIPTION
OSPF networking protocol basics
Citation preview
Agenda
• What is OSPF?• Why is OSPF needed?• How OSPF works?• LAB
2 © 2012 AT&T Intellectual Property. All rights reserved. AT&T and the AT&T logo are trademarks of AT&T Intellectual Property.
What is OSPF?
• Open Shortest Path First• Internal routing protocol (IGP)• Open Standard. RFC 2328• Link State Protocol• Designed for IP networks
© 2012 AT&T Intellectual Property. All rights reserved. AT&T and the AT&T logo are trademarks of AT&T Intellectual Property.3
Why is OSPF needed?
• Open standard • Link State Protocol• Scalability• Fast convergence• Supports authentication/security• Supports VLSM (Variable Length Subnet Masks)• Support IPv6 (OSPFv3)• Support load balancing• Consumes low bandwidth• Uses a hierarchical structure
© 2012 AT&T Intellectual Property. All rights reserved. AT&T and the AT&T logo are trademarks of AT&T Intellectual Property.4
© 2012 AT&T Intellectual Property. All rights reserved. AT&T and the AT&T logo are trademarks of AT&T Intellectual Property.5
How OSPF works?
© 2012 AT&T Intellectual Property. All rights reserved. AT&T and the AT&T logo are trademarks of AT&T Intellectual Property.6
How OSPF works?
• Uses a Link State Logic. • Neighbor discovery• Topology database exchange• Route computation
• Divides routers into areas for route optimization and segmentation
© 2012 AT&T Intellectual Property. All rights reserved. AT&T and the AT&T logo are trademarks of AT&T Intellectual Property.7
Neighbor Discovery
• Neighbor discovery• Each router discover its neighbors routers sending hello messages• Routers from adjacencies with its neighbors
• Neighbor adjacency requirements• Interfaces in the same subnet• Different router ID between neighbors• Routers in the same area• Same authentication parameters
© 2012 AT&T Intellectual Property. All rights reserved. AT&T and the AT&T logo are trademarks of AT&T Intellectual Property.8
Topology Database Exchange
• Topology Database Exchange• Each router generates its own network topology information (LSA
Message)• Each router flood its LSA message to its neighbors• Each router stores a LSA copy on its internal OSPF topology database
(LSDB)• Each router has the same LDSB
• Link State Advertisement (LSA) Message information• Router ID of the LSA• List of the router interfaces, IP address, subnet and mask• List of routers reachable (neighbors) on each interface
Link State Advertisements(LSAs)
LS Age
Options LS Type
Link State ID
Advertising Router
LS Sequence Number
LS Checksum
Length
LSA Header
0 16
© 2012 AT&T Intellectual Property. All rights reserved. AT&T and the AT&T logo are trademarks of AT&T Intellectual Property.9
© 2012 AT&T Intellectual Property. All rights reserved. AT&T and the AT&T logo are trademarks of AT&T Intellectual Property.10
Route Computation
• Route Computation• Each router analyzes its topology database and executes Shortest Path
First (SPF) algorithm• Router choses the shortest (best) route to each destination from its
perspective (SPF Tree)• Router generates entries in the routing table for those selected
destinations
© 2012 AT&T Intellectual Property. All rights reserved. AT&T and the AT&T logo are trademarks of AT&T Intellectual Property.11
SPF Tree
BEFORE run SPF AFTER run SPF
Communication between OSPF Routers
• OSPF packets encapsulated in IP packets• IP protocol # 89• Standard 24 byte header• OSPF packet type field• OSPF router ID of sender• Packet checksum• Authentication fields• OSPF Area ID• Uses Multicast IP address 224.0.0.5 for all OSPF routers• DR router use Multicast IP address 224.0.0.6
© 2012 AT&T Intellectual Property. All rights reserved. AT&T and the AT&T logo are trademarks of AT&T Intellectual Property.12
The Hello Protocol
• Maintains neighbor relationships• Elects Designated Router (DR) in multi-access networks• The Hello Packet• Hello packets sent out every 10 seconds• RouterDead Interval (default 40 seconds)• Network Mask• List of neighbors• Ensures that link is bidirectional
© 2012 AT&T Intellectual Property. All rights reserved. AT&T and the AT&T logo are trademarks of AT&T Intellectual Property.13
© 2012 AT&T Intellectual Property. All rights reserved. AT&T and the AT&T logo are trademarks of AT&T Intellectual Property.14
The Hello Protocol
Hello Hello
Hello Hello
© 2012 AT&T Intellectual Property. All rights reserved. AT&T and the AT&T logo are trademarks of AT&T Intellectual Property.15
Designated Router (DR)
• One per multi access network• Generates Network Link Advertisements (LSA type 2)• All routers form adjacency to it• Assist in DB synchronization• Elected by priority. It tie, it is elected by the highest router ID
Adjacencies WITHOUT DR Adjacencies WITH DR
DR
Database Synchronization
• Crucial to ensure correct and loop free routing• Must be done before 2 neighbors start
communication• Whenever new LSAs are introduced • uses reliable flooding• Each router sends LSA headers (small LSA’s) to its
neighbor when connection comes up• Requests only those LSAs which are recent
© 2012 AT&T Intellectual Property. All rights reserved. AT&T and the AT&T logo are trademarks of AT&T Intellectual Property.16
Database Exchange
• Neighboring routers first exchange hellos• A database description packet establishes the
sequence number• The other router sends LSA headers• Sequence number incremented for every pair of
database description packets • implicit acknowledgement for the previous pair• After examining LSA headers explicit request sent
for complete LSA
© 2012 AT&T Intellectual Property. All rights reserved. AT&T and the AT&T logo are trademarks of AT&T Intellectual Property.17
Reliable Flooding
• Starts when a router wants to update self-originated LSAs
• Neighbor installs more recent LSAs into its database
• Floods out on all interfaces except the one on which it arrived
• Reliability-retransmissions until acks received
© 2012 AT&T Intellectual Property. All rights reserved. AT&T and the AT&T logo are trademarks of AT&T Intellectual Property.18
Reliable Flooding (cont..)
10.1.1.1 10.1.1.2 10.1.1.4 10.1.1.6
10.1.1.3 10.1.1.5
Time T1
uu
u
© 2012 AT&T Intellectual Property. All rights reserved. AT&T and the AT&T logo are trademarks of AT&T Intellectual Property.19
Reliable Flooding (cont..)
10.1.1.1 10.1.1.2 10.1.1.4 10.1.1.6
10.1.1.3 10.1.1.5
Time T2u
uu
u
u
© 2012 AT&T Intellectual Property. All rights reserved. AT&T and the AT&T logo are trademarks of AT&T Intellectual Property.20
Reliable Flooding (cont..)
10.1.1.1 10.1.1.2 10.1.1.4 10.1.1.6
10.1.1.3 10.1.1.5
Time T3
u
u
© 2012 AT&T Intellectual Property. All rights reserved. AT&T and the AT&T logo are trademarks of AT&T Intellectual Property.21
Reliable Flooding (cont..)
10.1.1.1 10.1.1.2 10.1.1.4 10.1.1.6
10.1.1.3 10.1.1.5
Time T3+
ack ack ack ack
ack
© 2012 AT&T Intellectual Property. All rights reserved. AT&T and the AT&T logo are trademarks of AT&T Intellectual Property.22
Routing Calculations
• Link costs configurable by administrator• Smaller values for more preferred links• 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)
© 2012 AT&T Intellectual Property. All rights reserved. AT&T and the AT&T logo are trademarks of AT&T Intellectual Property.24
LAB
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• All Areas connected through Backbone Area (Area 0)• Interconnection of areas through area border routers (ABRs) • ABR leaks IP addressing information to other areas through
summary LSAs
Sample Area Configuration
AREA 0Backbone
AREA 1AREA 35
Internet
ASBRABR
ABR
OSPF Areas (Cont…)
• 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• Routing protection• Hidden prefixes
Incorporating external routing information
• Special routers called AS boundary routers at the edge of OSPF domain
• ASBRs originate AS-External LSAs• only routes for which the choice of an ASBR makes
sense are imported• otherwise default routes are used
Interaction with areas
• Inter Area communication• LSA type 1. Router LSA. All routers in an area• LSA type 2. Network LSA. DR generates it.
• Intra Area communication• LSA type 3. Summary ABR LSA. • LSA type 5. Summary ASBR LSA.
OSPF Area Types
• Restrict the amount of external routing information within an area
• Used when resources especially router memory is very limited
• Many types of restricted areas• Stub Areas• Others: not covered on this training
OSPF Area Types (cont…)
• Stub Areas• Don’t support ASBRs and hence no AS-External-LSAs• Routing to external destinations based on default routes
originated by the area’s border routers• Summary LSAs also made optional• Must lie on the edge of OSPF routing domain• Inter-area routing may also be based on default routes• Improved scaling• But not preferred due to the possibility of suboptimal
routes
© 2012 AT&T Intellectual Property. All rights reserved. AT&T and the AT&T logo are trademarks of AT&T Intellectual Property.32
LAB
OSPF configuration
Normal area configuration
R1#configure terminal
R1(config)#router ospf 1
R1(config-router)# router-id 150.1.1.1
R1(config-router)# network 10.1.1.0 0.0.0.255 area 0
Stub area configuration
R1#configure terminal
R1(config)#router ospf 1
R1(config-router)# router-id 150.1.1.1
R1(config-router)# network 10.1.1.0 0.0.0.255 area 1
R1(config-router)# area 1 stub
OSPF verification
• R1# show ip ospf neighbors• R1# show ip ospf interfaces• R1# show ip ospf database• R1# show ip route• R1# show ip interfaces brief
Issues not covered
• OSPF Area Types• NSSA• Totally stubby• Totally NSSA• OSPF Network Types• Broadcast subnets• NBMA Subnets• OSPF in the WAN• OSPF Management• Virtual Links• OSPF and IPv6• Many others
Thank You!