1. Introduction to IP Routing
Graduate Program in Computer Science
Aristotle University of Thessaloniki
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2. IP as Routing Protocol
IP is a connectionless, unreliable, best-effort delivery protocol.
IP accepts whatever data is passed down to it from the upper layers and forwards the data in the form of IP Packets.
All the nodes are identified using an IP address.
Packets are delivered from the source to the destination using IP address

3. IP Routing Source Destination Router Routing Table
Application
Application
Transport
Router
Transport
Network
Network
Network
Link
Link
Link
Routing Table
Destination IP address
IP address of a next-hop router
Flags
Network interface specification

4. IP Routing How does a device know where to send a packet?
All devices need to know what IP addresses are on directly attached networks
If the destination is on a local network, send it directly there

5. IP Routing (cont) If the destination address isn’t local
Most non-router devices just send everything to a single local router
Routers need to know which network corresponds to each possible IP address

6. Router
A router is a device that determines the next network point to which a packet should be forwarded toward its destination
Allow different networks to communicate with each other
A router creates and maintains a table of the available routes and their conditions, and uses this information to determine the best route for a given packet.
A packet will travel through a number of network points with routers before arriving at its destination.
There can be multiple routes defined. The route with a lower weight/metric will be tried first.

7. IP Routing

8. Packet Propagation

9. IP Routing Protocols Static Routing Dynamic Routing
IGP (Interior Gateway Protocol): Route data within an Autonomous System
RIP (Routing Information Protocol)
RIP-2 (RIP Version 2)
OSPF (Open Shortest Path First)
IGRP (Interior Gateway Routing Protocol)
EIGRP (Enhanced Interior Gateway Routing Protocol)
IS-IS (Intermediate System to Intermediate System)
EGP (Exterior Gateway Protocol): Route data between Autonomous Systems
BGP (Border Gateway Protocol)

10. Terminology Autonomous System (AS)
Each AS is a group of networks & routers administered by a single authority using a common routing protocol.
Interior Gateway Protocol (IGP)
Routers within single AS communicate using one of several dynamic routing protocols, known generically as an IGP.
Exterior Gateway Protocols (EGP)
Communication between routers belonging to different AS requires additional protocol, so-called EGP.

11. Terminology Split Horizon
Router B sends info to Router A about router’s C networks
Router A now knows about router’s C networks
Router A will NOT tell B about router’s C networks
Thus, loops are prevented
Split Horizon
Split horizon could be seen as redundant, since without it router B would never prefer routing traffic through A, because the networks of C would be in its routing table 3 hops away vs. 1 hop away by sending them directly to C. But if the link between B and C went down, then B would send traffic destined to C’s networks through A, creating a loop…
Routes that came from an interface are NOT advertised to the same interface!

12. Split Horizon with Poison Reverse
Terminology
Router B sends info to Router A about router’s C networks
Router A now knows about router’s C networks
Router A will tell B that router’s C networks are unreachable through A
Split Horizon with Poison Reverse
Routes that came from an interface are advertised as unreachable to the same interface!

13. Classful Routing Protocols
Classful routing protocols do not send subnet mask information in routing updates.
The first routing protocols, such as RIP
When network addresses were allocated based on classes.
Class A, B, or C.
Routing protocol did not need to include the subnet mask in the routing update.
Network mask determined based on value of first octet of the network address.

14. Classful & Classless Classful (does not support CIDR and VLSM)
Classless (supports CIDR and VLSM)
Interior Routing Protocols or Interior Gateway Protocols (IGP)
Distance Vector
RIPv1 – Simple, Classful, limited metrics (hop count)
RIPv2 – Simple, Classless, limited metrics (hop count)
Cisco Proprietary
IGRP – Simple, Classful, better metric (BW, delay, reliab., load)
EIGRP – Simple, Classless, same metric, DUAL (backup routes)
Link State
OSPF – Perceived complex, classless, Cisco metric BW, IETF
IS-IS - Perceived complex, classless, metric “default”, ISO
CIDR, ακρωνύμιο των λέξεων "Classless Inter Domain Routing"

15. Introduction to Distance Vector Routing Protocols
Configuring and maintaining static routes for a large network would be overwhelming.
What happens when that link goes down at 3:00 a.m.?

16. Introduction to Distance Vector Routing Protocols
RIP: Routing Information Protocol originally specified in RFC 1058.
Metric: Hop count
Hop count greater than 15 means network is unreachable.
Routing updates: Broadcast/multicast every 30 seconds
IGRP: Interior Gateway Routing Protocol - Cisco proprietary
Composite metric: Bandwidth, delay, reliability and load
Routing updates: Broadcast every 90 seconds
IGRP is the predecessor of EIGRP and is now obsolete
EIGRP: Enhanced IGRP – Cisco proprietary
It can perform unequal-cost load balancing.
It uses Diffusing Update Algorithm (DUAL) to calculate the shortest path.
No periodic updates, only when a change in topology.
IGRP and EIGRP: Cisco never submitted RFCs to IETF for these protocols.

17. Meaning of Distance Vector
Routes are advertised as vectors of distance and direction.
Distance is defined in terms of a metric
Such as hop count,
Direction is simply the:
Next hop router or
exit interface.
Routing protocol
Does not know the topology of an internetwork.
Only knows the routing information received from its neighbors.
Distance Vector routing protocol does not have the knowledge of the entire path to a destination network.

18. Meaning of Distance Vector
R1 knows that:
Distance: to /24 is 1 hop
Direction: out interface S0/0/0 toward R2
Remember: R1 does not have a topology map, it only knows distance and direction!

19. Operation of Distance Vector Routing Protocols
Periodic updates
Some distance vector routing protocols periodically broadcast the entire routing table to each of its neighbors. (RIP and IGRP)
30 seconds for RIP
90 seconds for IGRP
Inefficient: updates consume bandwidth and router CPU resources
Periodic updates always sent, even no changes for weeks, months,…

20. Operation of Distance Vector Routing Protocols
Neighbor of R1
R3 is unaware of R1 and learns R1’s networks by R2, R4
Neighbor of R1
Neighbors are:
routers that share a link
use the same routing protocol.
Router is only aware of:
Network addresses of its own interfaces
Network addresses that its neighbors know.
It has no broader knowledge of the network topology.

21. Link-State Protocol Operation
Link-state routing protocol can create a “complete view,” or topology, of the network.
Like having a complete map of the network topology
Link-state protocols are associated with Shortest Path First (SPF) calculations.
A link-state router uses the link-state information to:
Create a topology map
Select the best path to all destination networks in the topology.

22. Link-State Protocol Operation
Link-state protocols work best in situations where
The network design is hierarchical, usually occurring in large networks.
The administrators have a good knowledge of the implemented link-state routing protocol.
Fast convergence of the network is crucial. Link-state protocols are triggered on events and do not rely on periodic updates – 30/90 seconds network downtime is UNACCEPTABLE!

23. Link-State Protocol Characteristics
With link-state routing protocols, each router has the full picture of the network topology, and can independently make a decision based on an accurate picture of the network topology.
To do so, each link-state router keeps a record of:
Its immediate neighbor routers.
All the other routers in the network, or in its area of the network, and their attached networks.
The best paths to each destination.

24. Link-State Protocol Advantages
Respond quickly to network changes.
Send triggered updates when a network change occurs.
Send periodic updates (link-state refresh), at long intervals, such as every 30 minutes.
Uses LSAs to confirm topology information before the information ages out of the link-state database.

25. OSPF Router Tables / Databases
OSPF maintains three databases which are used to create three tables.
Database
Table
Description
Adjacency Database
Neighbor Table
List of all neighbor routers to which a router has established bidirectional communication.
This table is unique for each router.
Can be viewed using the show ip ospf neighbor command.
Link-state Database
Topology Table
List of information about all other routers in the network.
The database shows the network topology.
All routers within an area have identical link-state databases.
Can be viewed using the show ip ospf database command.
Forwarding Database
Routing Table
List of routes generated when an algorithm is run on the link-state database.
Each router’s routing table is unique and contains information on how and where to send packets to other routers.
Can be viewed using the show ip route command.

26. Link-State Advertisements (LSAs)
When a change occurs in the network topology, the router experiencing the change creates a link-state advertisement (LSA) concerning that link.
LSAs are also called link-state protocol data units (PDUs).
The LSA is multicasted to all neighboring devices using either or and confirmed!
Routers receiving the LSA immediately forward it to all neighboring routers.
Link-state information must be synchronized between routers. To accomplish this, LSAs have the following characteristics:
LSAs are reliable; there is a method for acknowledging their delivery.
LSAs are flooded throughout the area (or throughout the domain if there is only one area).
LSAs have a sequence number and a set lifetime, so each router recognizes that it has the most current version of the LSA.
LSAs are periodically refreshed to confirm topology information before they age out of the LSDB.

27. Link-State Database (LSDB)
Routers receiving add the LSA to their link-state database (LSDB).
The LSDB is used to calculate the best paths through the network.
OSPF best route calculation is based on Edsger Dijkstra's shortest path first (SPF) algorithm.

28. SPF Routing Algorithm
The SPF algorithm accumulates costs along each path, from source to destination.
The accumulated costs is then used by the router to build a topology table.

29. SPF Tree and Routing Table
The topology table is essentially an SPF tree which contains a listing of all OSPF networks and the costs to reach them.
The resulting best routes are then considered to be added to the routing table.

30. OSPF Areas
To minimize processing and memory requirements, OSPF can divide the routing topology into a two-layer hierarchy called areas.
Characteristics of OSPF areas include:
Minimizes routing table entries.
Localizes impact of a topology change within an area.
Detailed LSA flooding stops at the area boundary.
Requires a hierarchical network design.
OSPF allows for the creation of multiple areas so that the network administrator can:
Reduce the size of routing tables
Isolate topology changes as much as possible to the area in which they occur
Allow only summary LSA updates to cross area boundaries
Reap all the benefits of using a hierarchical addressing scheme.

31. OSPF Two-Layer Hierarchy
Backbone Area
Referred to as Area 0
Also known as the Transit Area
Regular (Standard) Areas
Also known as non-backbone areas
All regular areas must connect to the backbone area
Standard areas can be further defined as stub areas, totally stubby areas, and Not-so- stubby areas (NSSAs)
You should be aware that the backbone area is also referred to as a transit area. This distinction is a minor one, but is enough to cause confusion for students. Another type of transit area is one that is configured with virtual links. Recall that virtual links are a temporary solution when an organization has two backbones that are physically and logically disconnected. A virtual link between the two backbones is also called a transit area.
All other areas are known as regular areas (standard areas, stub areas, totally stubby areas, and NSSAs). These are covered later in the presentation.
If link between A – B goes down, it is of no general interest
Cisco recommends:
An area should have no more than 50 routers.
A router should not be in more than 3 areas.

32. Purpose of a Metric Routing protocol metrics: RIP: Hop count
IGRP and EIGRP: Bandwidth, delay, reliability and load
OSPF (Cisco’s version): Bandwidth
IS-IS: Four values (Cisco uses “default”) – Covered in CCNP
BGP: Attributes – Covered in CCNP

33. Metric Parameters R1 to reach the 172.16.1.0/24 network.
56 Kbps
R1 to reach the /24 network.
RIP: Fewest number of hops via R2.
OSPF: Path with the lowest cost through R3.
This results in faster packet delivery.

34. Configuring RIP Example
/24
/24
64 kbps .1 .2 .2 R1 R2 .1
Fa0/0
Fa0/0
S0/0/1 R3
S0/0/1
R1(config)# router rip
R1(config-router)# version 2
R1(config-router)# network
R1(config-router)# no auto-summary
R1(config-router)# exit
R2(config)# router rip
R2(config-router)# version 2
R2(config-router)# network
R2(config-router)# no auto-summary
R2(config-router)# exit
Although the two examples shown are a commonly used combination of a network statement and a wildcard mask, others could also work. For instance, a range of subnets could be specified.
Notice that the process-ids do not need to match.

35. Verifying RIP Command Description
show ip protocols
Displays networks router is advertising, route sources & administrative distance
show ip route
Displays the routing table

36. OSPF Metric Calculation
Bandwidth
High
Low
Lower Cost
Higher Cost
The OSPF metric calculation is based on cost.
Cost is an indication of the overhead required to send packets across a certain interface.
The cost of an interface is inversely proportional to the bandwidth of that interface.
A higher bandwidth is attributed a lower cost.
A lower bandwidth is attributed a higher cost.

37. OSPF Cost Formula Cost = 100,000,000 / Bandwidth (bps) For example:
10BaseT = 100,000,000 / 10,000,000 = 10
T1 = 100,000,000 / 1,544,000 = 64
For interfaces faster than 100 Mbps, the cost reference can be altered using the auto-cost reference-bandwidth command which is covered later in this presentation.

38. Configuring Single-Area OSPF Example
OSPF Area 0
/24
/24
64 kbps .1 .2 .2 R2 .1 R1
Fa0/0
Fa0/0
S0/0/1 R3
S0/0/1
R1(config)# interface Fa0/0
R1(config-if)# ip address
R1(config-if)# no shut
R1(config-if)# exit
R1(config)#
R2(config)# interface Fa0/0
R2(config-if)# ip address
R2(config-if)# no shut
R2(config-if)# interface S0/0/1
R2(config-if)# ip address
R2(config-if)# bandwidth 64
R2(config-if)# exit
R2(config)#
Although the two examples shown are a commonly used combination of a network statement and a wildcard mask, others could also work. For instance, a range of subnets could be specified.
Notice that the process-ids do not need to match.
R3(config)# interface S0/0/1
R3(config-if)# ip address
R3(config-if)# bandwidth 64
R3(config-if)# no shut
R3(config-if)# exit
R3(config)#

39. Configuring Single-Area OSPF Example
OSPF Area 0
/24
/24
64 kbps .1 .2 .2 R2 .1 R1
Fa0/0
Fa0/0
S0/0/1 R3
S0/0/1
R1(config)# router ospf 1
R1(config-router)# network area 0
R1(config-router)#
R2(config)# router ospf 50
R2(config-router)# network area 0
R2(config-router)# network area 0
R2(config-router)#
Although the two examples shown are a commonly used combination of a network statement and a wildcard mask, others could also work. For instance, a range of subnets could be specified.
Notice that the process-ids do not need to match.
R3(config)# router ospf 100
R3(config-router)# network area 0
R3(config-router)#

40. Configuring Multi-Area OSPF Example
OSPF Area 0
OSPF Area 1
/24
/24
64 kbps .1 .2 .2 R1 R2 .1
Fa0/0
S0/0/1 R3
Fa0/0
S0/0/1
R1(config)# router ospf 1
R1(config-router)# network area 0
R1(config-router)#
R2(config)# router ospf 50
R2(config-router)# network area 1
R2(config-router)# network area 0
R2(config-router)#
Notice that the /24 network is now advertised in area 1.
R3(config)# router ospf 100
R3(config-router)# network area 1
R3(config-router)#

41. Alternate Multi-Area OSPF Configuration
OSPF Area 0
OSPF Area 1
/24
/24
64 kbps .1 .2 .2 .1 R1 R2
S0/0/1 R3
Fa0/0
Fa0/0
S0/0/1
R1(config)# router ospf 1
R1(config-router)# network area 0
R1(config-router)#
R2(config)# interface S0/0/1
R2(config-if)# ip ospf 50 area 1
R2(config-if)# exit
R2(config)#
R2(config)# router ospf 50
R2(config-router)# network area 0
R2(config-router)#
To demonstrate an alternative, interface S0/0/1 of R2 is configured to be in area 1.
R3(config)# router ospf 100
R3(config-router)# network area 1
R3(config-router)#

42. Verifying OSPF Command Description
show ip protocols
Displays OSPF process ID, router ID, networks router is advertising & administrative distance
show ip ospf neighbors
Displays OSPF neighbor relationships.
show ip route
Displays the routing table.
show ip ospf interface
Displays hello interval and dead interval
show ip ospf
Displays OSPF process ID, router ID, OSPF area information & the last time SPF algorithm calculated

43. OSPF vs… OSPF requires more resources from router Fast convergence
LSNDI RMRA
4 Feb 2008
OSPF vs…
OSPF requires more resources from router
Fast convergence
Less overhead – good for large networks
Supports VLSM
Complex to configure for advanced needs!

44. …vs EIGRP EIGRP – Cisco proprietary routing protocol
LSNDI RMRA
4 Feb 2008
…vs EIGRP
EIGRP – Cisco proprietary routing protocol
Uses partial updates and neighbour discovery
Like OSPF but easier to configure
Good for large multiprotocol networks that use Cisco routers

45. Setting up EIGRP Needs Autonomous System Number
LSNDI RMRA
4 Feb 2008
Setting up EIGRP
Needs Autonomous System Number
Router(config)#router eigrp 123
Router(config-router)#net
Router(config-router)#net
Router(config-router)#exit
Uses Wildcard Mask

46. Verifying EIGRP Router# show ip protocols
LSNDI RMRA
4 Feb 2008
Verifying EIGRP
Router# show ip protocols
This shows the routing protocol in use and other useful information too
Router# sh ip eigrp ?
interfaces IP-EIGRP interfaces
neighbors IP-EIGRP neighbors
topology IP-EIGRP Topology Table
traffic IP-EIGRP Traffic Statistics

47. LSNDI RMRA
4 Feb 2008
Εργασία
Στο zip αρχείο διάλεξη 10 - εργασία.zip υπάρχει ένα pkt αρχείο με μια δικτυακή τοπολογία που της λείπει το routing, με αποτέλεσμα οι υπολογιστές των διαφόρων υποδικτύων να μην μπορούν να επικοινωνήσουν μεταξύ τους.
Αποκαταστήστε την δρομολόγηση μεταξύ των υποδικτύων σύμφωνα με τις οδηγίες των διαφανειών που επίσης βρίσκονται εντός του zip αρχείου.
Παραδοτέο: ένα pdf με όλα τα βήματά σας καθώς και τα αποτελέσματα των βημάτων.