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Similar to Ccna 3 week 2 (20)
Ccna 3 week 2
- 2. Copyright © 2005 University of BoltonCopyright © 2005 University of Bolton
Distance Vector vs Link State
Distance Vector
– Copies Routing Table to
neighbours
– Updates frequently
– Distance metrics e.g. hop count
– Views network from prospective
of neighbours
– Slow to converge
– Loops a problem
– Easy to configure
– Consumes bandwidth
Link State
– Uses shortest path
– Updates event triggered
– Sends link-state packets to all
– Common view of network
– Fast convergence
– Less vulnerable to loops
– Harder to Configure
– Requires more memory and
CPU power
- 3. Copyright © 2005 University of BoltonCopyright © 2005 University of Bolton
Link State Features
Router learn network info from ALL routers in a net
Routers multicast frequent hello packets
containing details of directly connected networks
Link-State Advertisments keep track of other
routers
Information used by router to calculate Shortest
Path to each network
- 4. Copyright © 2005 University of BoltonCopyright © 2005 University of Bolton
Example Topology
- 5. Copyright © 2005 University of BoltonCopyright © 2005 University of Bolton
Maintaining Routing Information
When a failure occurs, LSAs are multicast
– Routers forward multicasts on all ports except receiver
Each Link-State router updates its topology database with
new network state and recalculates
A link = an interface
Link-State info includes: IP Addr & subnet, net type,
connected routers
Best routes calculated from topology database and placed
in routing table
- 6. Copyright © 2005 University of BoltonCopyright © 2005 University of Bolton
Network View
- 7. Copyright © 2005 University of BoltonCopyright © 2005 University of Bolton
Advantages & Disadvantages
Advantages
– Fast convergence: changes
reported at once
– Robust against loops
– Routers all know topology
– Link-state packets
sequenced and aged
– Careful design minimises
link-state table size
Disadvantages
– Significant RAM and CPU
– Network design follows strict
rules
– Network admin must be
knowledgable (you guys!)
– Initial flooding impedes
performance
- 8. Copyright © 2005 University of BoltonCopyright © 2005 University of Bolton
Comparison
After initial flood, routing table only changes if net
conditions change
– The partial update only contains information about links that have
changed.
– Reduces network load, faster propagation of changes
Link-state protocols support CIDR and VLSM.
In fact, link-state protocols generally outperform distance
vector protocols on any size network.
– Not always implemented due to hardware requirements and
admin expertise
- 10. Copyright © 2005 University of BoltonCopyright © 2005 University of Bolton
Introduction
Open non-proprietary standard protocol for TCP/IP
Widely used in industry
Preferable to RIP v1 or v2 because metrics are
better than simple hop count
Uses a hierarchical approach, with different areas
defined within a network
- 11. Copyright © 2005 University of BoltonCopyright © 2005 University of Bolton
OSPF Terminology
Link: an interface on a routerLink-state: The status of the link between two routersTopological Database: The status of all routers on the
network
Area: each router within an area has the same link-state
information
Cost: assigned to each link, based on bandwidthRouting Table: generated when routing algorithm runs on
topological database
Adjacency Database: List of routers communicating directlyDesignated Router (DR): Elected to represent all routers
on the network
- 12. Copyright © 2005 University of BoltonCopyright © 2005 University of Bolton
OSPF vs Distance Vector
Speed of convergence
Support for Variable Length Subnet Mask (VLSM)
Network size
Path selection
Grouping of members
- 13. Copyright © 2005 University of BoltonCopyright © 2005 University of Bolton
SPF Algorithm
- 14. Copyright © 2005 University of BoltonCopyright © 2005 University of Bolton
SPF Algorithm
- 15. Copyright © 2005 University of BoltonCopyright © 2005 University of Bolton
OSPF Network Types
OSPF interfaces automatically recognize 3 types
of networks:
– Broadcast multi-access, such as Ethernet
– Point-to-point networks
– Nonbroadcast multi-access (NBMA), e.g. Frame Relay
Not desirable for every router to be adjacent to
every other in a broadcast network – overhead
Pick Designated Router (and Backup)
- 16. Copyright © 2005 University of BoltonCopyright © 2005 University of Bolton
Use of Designated Router
Routers send their updates to the designated
router (Multicast to 224.0.0.6)
Designated Router updates all routers on network
using 224.0.0.5
As Designated Router is single point of failure,
Backup is also established
- 17. Copyright © 2005 University of BoltonCopyright © 2005 University of Bolton
Hello Packets
Small layer 3 packets
Sent every 10 seconds to multicast 224.0.0.5
Used to ensure neighbours still running
Protocol used in election of DR
- 18. Copyright © 2005 University of BoltonCopyright © 2005 University of Bolton
Protocol Steps
Use hello packets to announce selves
Develop adjacency relationships with neighbours
Use LSA and LSU packets to learn about topology
Calculate routing tables from topology
If changes occur, recalculate
- 19. Copyright © 2005 University of BoltonCopyright © 2005 University of Bolton
Configure
R1(config)# router ospf <proc_id>
R1(config-router)# network <addr> <wildcard>
area <area>
- 20. Copyright © 2005 University of BoltonCopyright © 2005 University of Bolton
Need to add loopback address
No active interfaces means router drops out of
OSPF
Logical loopback keeps process available
If present, loopback used as router ID
R1(config)# interface loopback 1
R1(config-if)# ip address <addr> <subnet>
- 21. Copyright © 2005 University of BoltonCopyright © 2005 University of Bolton
Link costs
OSPF uses 108
/bandwidth as link cost
Set bandwidth of interface if needed:
R1(config-if)# bandwidth 56
May be necessary if mixed network uses other
calculations
R1(config-if)# ip ospf cost <number>
- 22. Copyright © 2005 University of BoltonCopyright © 2005 University of Bolton
Authentication
Can require common password for updates
May be encrypted using MD5
See lab for details
- 23. Copyright © 2005 University of BoltonCopyright © 2005 University of Bolton
Timers
OSPF routers must use the same timer values
Router(config-if)#ip ospf hello-intervalseconds
Router(config-if)#ip ospf dead-intervalseconds
- 24. Copyright © 2005 University of BoltonCopyright © 2005 University of Bolton
Default Route
On the router connected to default connection:
Router(config)#ip route 0.0.0.0 0.0.0.0 [interface |
next-hop address ]
Must then instruct router to propagate default route
Router(config-router)#default-information
originate