1. Computer Networks with Internet Technology William Stallings
Interior Routing Protocols
Computer Networks with Internet Technology William Stallings
Chapter 11
Interior Routing Protocols
Chapter 15

2. Autonomous Systems (AS)
Interior Routing Protocols
Autonomous Systems (AS)
Group of routers exchanging information via common routing protocol
Set of routers and networks managed by single organization
Connected
Except in time of failure
Chapter 15

3. Interior Routing Protocol (IRP)
Interior Routing Protocols
Interior Routing Protocol (IRP)
Passes routing information between routers within AS
Does not need to be implemented outside AS
Allows IRP to be tailored
May be different algorithms and routing information in different connected AS
Need minimum information from other connected AS
At least one router in each AS must talk
Use Exterior Routing Protocol (ERP)
Chapter 15

4. Exterior Routing Protocol (ERP)
Interior Routing Protocols
Exterior Routing Protocol (ERP)
Pass less information than IRP
Router in first system determines route to target AS
Routers in target AS then co-operate to deliver datagram
ERP does not deal with details within target AS
Chapter 15

5. Figure 11.4 Application of Exterior and Interior Routing Protocols
Chapter 15

6. Approaches to Routing – Distance-vector
Interior Routing Protocols
Approaches to Routing – Distance-vector
Each node (router or host) exchange information with neighboring nodes
Neighbors are both directly connected to same network
First generation routing algorithm for ARPANET
Node maintains vector of link costs for each directly attached network and distance and next-hop vectors for each destination
Used by Routing Information Protocol (RIP)
Requires transmission of lots of information by each router
Distance vector to all neighbors
Contains estimated path cost to all networks in configuration
Changes take long time to propagate
Chapter 15

7. Approaches to Routing – Link-state
Interior Routing Protocols
Approaches to Routing – Link-state
Designed to overcome drawbacks of distance-vector
When router initialized, it determines link cost on each interface
Advertises set of link costs to all other routers in topology
Not just neighboring routers
From then on, monitor link costs
If significant change, router advertises new set of link costs
Each router can construct topology of entire configuration
Can calculate shortest path to each destination network
Router constructs routing table, listing first hop to each destination
Router does not use distributed routing algorithm
Use any routing algorithm to determine shortest paths
In practice, Dijkstra's algorithm
Open shortest path first (OSPF) protocol uses link-state routing.
Also second generation routing algorithm for ARPANET
Chapter 15

8. Exterior Router Protocols – Path-vector
Interior Routing Protocols
Exterior Router Protocols – Path-vector
Provide information about which networks can be reached by a given router and ASs crossed to get there
Does not include distance or cost estimate
Each block of information lists all ASs visited on this route
Enables router to perform policy routing
e.g. avoid path to avoid transiting particular AS
e.g. link speed, capacity, tendency to become congested, and overall quality of operation, security
e.g. minimizing number of transit ASs
Chapter 15

9. Interior Routing Protocols
11.2 Least Cost Algorithms
Least-cost criterion
If minimize number of hops, link value 1
Link value may be inversely proportional to capacity, proportional to current load, or some combination
May differ in different two directions
E.g. if cost equaled length of queue
Cost of path between two nodes as sum of costs of links traversed
For each pair of nodes, find least cost path 
Two common algorithms
Dijkstra's algorithm
Bellman-Ford algorithm
Chapter 15

10. 11.3 Distance Vector Routing: RIP
Interior Routing Protocols
11.3 Distance Vector Routing: RIP
Each node exchange information with neighbors
Directly connected by same network
Each node maintains three vectors
Link cost
Distance vector
Next hop vector
Every 30 seconds, exchange distance vector with neighbors
Use this to update distance and next hop vector
Chapter 15

11. Figure 11.1 A Configuration of Routers and Networks
Interior Routing Protocols
Figure 11.1 A Configuration of Routers and Networks
(changed to 1)
(changed to 1)
Chapter 15

12. Figure 11.8 Distance Vector Algorithm Applied to Figure 11.1
Interior Routing Protocols
Figure Distance Vector Algorithm Applied to Figure 11.1
Chapter 15

13. RIP Details – Incremental Update
Interior Routing Protocols
RIP Details – Incremental Update
Updates do not arrive from neighbors within small time window
RIP packets use UDP
Tables updated after receipt of individual distance vector
Add any new destination network
Replace existing routes with small delay ones
If update from router R, update all routes using R as next hop
Chapter 15

14. RIP Details – Topology Change
Interior Routing Protocols
RIP Details – Topology Change
If no updates received from a router within 180 seconds, mark route invalid
Invalid timer: 180 sec
Assumes router crash or network connection unstable
Set distance value to infinity
Actually 16
Chapter 15

15. Counting to Infinity Problem (1)
Interior Routing Protocols
Counting to Infinity Problem (1)
Slow convergence may cause:
All links are assumed cost 1
B has distance to network 5 as 2, next hop D
A & C have distance 3
and next hop B
Chapter 15

16. Counting to Infinity Problem (2)
Interior Routing Protocols
Counting to Infinity Problem (2)
Suppose router D fails:
B determines network 5 no longer reachable via D
Sets distance to 4 based on report from A or C
At next update, B tells A and C this
A and C receive this and increment their network 5 distance to 5
4 from B plus 1 to reach B
B receives distance count 5 and assumes
network 5 is 6 away
Repeat until reach infinity (16)
Takes 8 to 16 minutes to resolve ×
Chapter 15

17. Interior Routing Protocols
Split Horizon
Counting to infinity problem caused by misunderstanding between B and A, and B and C
Each thinks it can reach network 5 via the other
Split Horizon rule says do not send information about a route back in the direction it came from
Router sending information is nearer destination than you
That is, A should not tell B “the distance to network 5”.
Erroneous route now eliminated within time out period (180 seconds)
Chapter 15

18. Interior Routing Protocols
Poisoned Reverse
Send updates with hop count of 16 to neighbors for route learned from those neighbors
If two routers have routes pointing at each other advertising reverse route with metric 16 breaks loop immediately
B tells A and C “distance to network 5 is 16”
Chapter 15

19. Figure 11.9 RIP Packet Format (v1)
Interior Routing Protocols
Figure RIP Packet Format (v1)
Command:
1: request, 2: response
Address Family identifier:
IP, IPX, …
Over UDP
Multicast:
Chapter 15

20. Interior Routing Protocols
RIP v2
Route Tag: 0 or AS#
Chapter 15

21. Interior Routing Protocols
Chapter 15

22. RIP Packet Format Notes
Interior Routing Protocols
RIP Packet Format Notes
Command: 1=request 2=reply
Updates are replies whether asked for or not
Initializing node broadcasts request
Requests are replied to immediately
Version: 1 or 2
Address family: 2 for IP
IP address: non-zero network portion, zero host portion
Identifies particular network
Metric
Path distance from this router to network
Typically 1, so metric is hop count
Chapter 15

23. Interior Routing Protocols
RIP Limitations
Destinations with metric more than 15 are unreachable
If larger metric allowed, convergence becomes lengthy
Simple metric leads to sub-optimal routing tables
Packets sent over slower links
Accept RIP updates from any device
Misconfigured device can disrupt entire configuration
Chapter 15

24. 11.4 Link-State Protocol: OSPF
Interior Routing Protocols
11.4 Link-State Protocol: OSPF
RIP limited in large internets
Open Shortest Path First (OSPF)
OSPF preferred interior routing protocol for TCP/IP based internets
Link state routing used
Directly over IP
Chapter 15

25. Interior Routing Protocols
Link State Routing
When initialized, router determines link cost on each interface
Router advertises these costs to all other routers in topology
Router monitors its costs
When changes occurs, costs are re-advertised
Each router constructs topology and calculates shortest path to each destination network
Not distributed version of routing algorithm
Can use any algorithm
Dijkstra
Chapter 15

26. Interior Routing Protocols
Flooding
Packet sent by source router to every neighbor
Incoming packet resent to all outgoing links except source link
Duplicate packets already transmitted are discarded
Prevent incessant retransmission
All possible routes tried so packet will get through if route exists
Highly robust
At least one packet follows minimum delay route
Reach all routers quickly
All nodes connected to source are visited
All routers get information to build routing table
High traffic load
Chapter 15

27. Figure 11.10 Flooding Example
Interior Routing Protocols
Figure Flooding Example
Chapter 15

28. Interior Routing Protocols
OSPF Overview
Router maintains descriptions of state of local links
Transmits updated state information to all routers it knows about
Router receiving update must acknowledge
Lots of traffic generated
Each router maintains database
Directed graph
Chapter 15

29. Interior Routing Protocols
Router Database Graph
Vertices
Router
Network
Transit
Stub
Edges
Connecting two routers
Connecting router to network
Built using link state information from other routers
Chapter 15

30. Figure 11.11 Sample Autonomous System
Interior Routing Protocols
Figure Sample Autonomous System
Chapter 15

31. Figure 11.12 Directed Graph of Autonomous System of Figure 19.7
Interior Routing Protocols
Figure Directed Graph of Autonomous System of Figure 19.7
Chapter 15

32. Interior Routing Protocols
Link Costs
Cost of each hop in each direction is called routing metric
OSPF provides flexible metric scheme based on type of service (TOS)
Normal (TOS 0)
Minimize monetary cost (TOS 2)
Maximize reliability (TOS 4)
Maximize throughput (TOS 8)
Minimize delay (TOS 16)
Each router generates 5 spanning trees (and 5 routing tables)
Chapter 15

33. Figure 11.13 The SPF Tree for Router R6
Interior Routing Protocols
Figure The SPF Tree for Router R6
Chapter 15

34. Interior Routing Protocols
Areas
Make large internets more manageable
Configure as backbone and multiple areas
Area – Collection of contiguous networks and hosts plus routers connected to any included network
Backbone – contiguous collection of networks not contained in any area, their attached routers and routers belonging to multiple areas
Chapter 15

35. Interior Routing Protocols
Operation of Areas
Each area runs a separate copy of the link state algorithm
Topological database and graph of just that area
Link state information broadcast to other routers in area
Reduces traffic
Intra-area routing relies solely on local link state information
Chapter 15

36. Interior Routing Protocols
Inter-Area Routing
Path consists of three legs
Within source area
Intra-area
Through backbone
Has properties of an area
Uses link state routing algorithm for inter-area routing
Within destination area
Chapter 15

37. Figure 11.14 OSPF Packet Header
Interior Routing Protocols
Figure OSPF Packet Header
* directly over IP
Chapter 15

38. Interior Routing Protocols
Packet Format Notes
Version number: 2 is current
Type: one of 5, see next slide
Packet length: in octets including header
Router id: this packet’s source, 32 bit
Area id: Area to which source router belongs
Authentication type: null, simple password or encryption
Authentication data: used by authentication procedure
Chapter 15

39. Interior Routing Protocols
OSPF Packet Types
Hello: used in neighbor discovery
Database description: Defines set of link state information present in each router’s database
Link state request
Link state update
Link state acknowledgement
Chapter 15