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CSCD 433/533Advanced NetworksSpring 2016

Lecture 13

Router Algorithms and DesignChapter 5

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Topics

• Router Algorithms• Routing in General• Hierarchical routing• Interior Gateway Protocols

• OSPF mention of RIP• Brief Comparison• Properties of OSPF• OSPF Areas

Routing in General

• Recall• Networks appear as graphs• Abstraction where nodes are routers, switches

or hosts• Edges are links between nodes

• Physical or virtual• Assign weight to them• Network links are bi-directional

• Useful for modeling networks

Purpose of Routing

Recall• Routers interconnects two or more networks • Two major problems in delivering packets in networks

• What are they? How to build forwarding tables in all network nodes How to do forwarding (efficiently) could say optimal

Routing Algorithms

Ideally, how should routing work ?

• Think long-term,

• Think globally,

• Think shared networks

Routing Algorithms• Properties

• Robust• Stable• Fair • Efficient

Robust• Should run for years without system-wide failure • Expected - hardware and software failures• Hosts, routers and lines will fail repeatedly, network

topology change frequently• Ideally, good routing algorithm should be able to cope with

changes without rebooting network

Routing Algorithms• Properties

• Stable• Important goal for routing algorithm• Routing algorithm should converge quickly to set of paths

and stay there• Communication may be disrupted until routing algorithm

reaches equilibrium

• Fair• Means traffic sources get a fair share of resources• Should be equitable distribution across bandwidth

Routing Algorithms

• Properties• Efficient

• What are we trying to optimize ?• Packet delay• Number of hops or minimum distance

packets must travel• Ether way tends to improve delay and

improve throughput!!!

Routing PropertiesMore Questions

• How well do existing protocols achieve• Robustness, Stability, Fairness and Efficiency?

And• How can a network be organized to improve upon

these properties ?

• Want routing to happen effortlessly• No-one should notice it at all• Alice and Bob can carry on endless

conversations without interruption ...

Routing Protocols Design

• Design decisions that affect routing performance include• How often to send information to neighbors?

• How much information to send?

• Can we create hierarchy to reduce number of routers? And, number of router messages sent

Tiered Routing

Tiered Routing in Internet• Network Gets Bigger

• Router table grows proportionally• Router memory consumed by ever increasing tables• More CPU time needed to scan

• Hierarchy• Network divided up into regions• Routers know only whats in their region

• Why is this good?• Each router knows all details about routing in own region• Does not need to know about internal structure of other regions

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Aggregated Network vs. Reality

The AS graphmay look like this

Reality may be closer to this…

BGP for links between regions

Other routing protocols within regions

AS Autonomous Systems

Tiered Routing in Internet

• Different networks Interconnected• Natural to regard each one as a separate region• Frees routers in one network from having to know

topology of other networks

• Just Increasing from Single to Two level tables• Reduced entries from 17 down to 7• Gains in space of table

• Design Question - Is there an optimal number of routing levels?

Hierarchical Routing

Hierarchical Routing

17 Entries

7 Entries

Tiered Routing in Internet

• Answer to Question• What is the optimal number of levels in routing ?• Hypothesized back in 1979 • Optimal number of levels for N routers

• Is, ln N for N router subnet • Requires a total of e ln N entries per level

• Began with Farouk Kamoun and Leonard Kleinrock paper, 1977

http://citeseerx.ist.psu.edu/viewdoc/download?doi=10.1.1.6.4852&rep=rep1&type=pdf

Internet Organized into AS's

• Someone observed ...• Use Principle of Hierarchy to organize Internet and

thus reduce routing tables in size

• Flat routing linearly increases routing table size• Hierarchy, table size increases logarithmically• Created Autonomous Systems as organization

structure for routing (AS's)

• Mentioned in CSCD330

Active BGP Entries

Source: http://www.cidr-report.org/

Hierarchy vs. Fully Connected

What is the trade-off between fully connecting routers vs. more hierarchy?

Hierarchy - More complexity• Designate special purpose routers Protocols more complicated Will need backups for potential failures

Fully Connected Router tables need more space More routers to run router algorithms Convergence take more time

AS Numbers (ASNs)

ASNs are 16 bit values

• EWU: 3935• MIT: 3• Northwestern University: 103• UC San Diego: 7377• AT&T: 7018, 6341, 5074, … • UUNET: 701, 702, 284, 12199, …• Sprint: 1239, 1240, 6211, 6242, … https://www.ultratools.com/tools/asnInfo

ASNs represent units of routing policy

Internet Organized into AS’s• Internet organized into a series of

Administrative Systems • Each controlled by a single administrative entity• Distinct regions of administrative control

• Hierarchy of Autonomous Systems• Large, tier-1 provider – create network backbone• Medium-sized regional provider with smaller

backbone• Small network run by a single company or university

• Like EWU

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Autonomous System Defined

• A collection of routers under same technical and administrative domain• Each AS, has globally unique number assigned to them from a centralized authority (ARIN)

• The American Registry for Internet Numbers (ARIN)• Responsible for tracking and assigning these numbers http://www.arin.net/index.shtml

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Org Chart for ICAAN and IANA

ICAAN

AS Based Routing in Internet

Two Types of Internet Routing1. Interdomain routing between ASes

• Routing policies based on Business relationships• No common metrics, and limited cooperation• BGP: policy-based, path-vector routing protocol

2. Intradomain routing within an AS• Shortest-path routing based on Link metrics• Routers all managed by single institution• OSPF and IS-IS: link-state routing protocol• RIP and EIGRP: distance-vector routing protocol

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OSPF and little bit of RIP

Link State Algorithm Terms

• Link Interface on a router• LS - Link state Description of interface and of its

relationship to its neighboring routers, including:– IP address/mask of the interface, – The type of network it is connected to– The routers connected to that network– The metric (cost) of that link

• LSA - Link State Advertisements• LSDB - The collection of all the link-states would

form a link-state database.

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Brief Review of RIP• Original Interior AS protocol

– Works well in small systems– Maximum hop count is 15– Only uses hop count as link weight– Sends entire database every 30 seconds– Only sends to its neighbors– Suffers from problems

• Count to Infinity Problem• Slow Convergence

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Review of OSPF• Broadcasts link-state advertisements (LSAs) to all

other routers, when change in link status• Also, broadcasts state

• Once every 30 minutes, even if link state not changed!!!

• OSPF cost advertised in LSAs (Link State Advertisements)

• Can configure cost• Delay, data rate, monetary cost, or

other factors– Cisco’s OSPF metric based on bandwidth

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OSPF Operation• OSPF

• Over time, OSPF routers gather received LSAs into LSDB• Synchronize LSDBs between all neighboring routers• Every router has same LSDB• From LSDB, entries for router’s routing table are calculated

using Djikstra's SPF Algorithm

• Looked at an example in CSCD330

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From RouterA - After running Djikstra's Algorithm – Shortest Path First – Best routes identified

A C

D

2

5

B

E

15 2

102

2

2

22

11.0.0.0/8

14.0.0.0/8

12.0.0.0/8

13.0.0.0/8

17.0.0.0/8

16.0.0.0/8

15.0.0.0/8

18.0.0.0/8

20.0.0.0/8

19.0.0.0/8

10.0.0.0/8

Choosing the Best Path

Results Put into Routing Table

RouterA’s Routing Table

10.0.0.0/8 connected e0

11.0.0.0/8 connected s0



14.0.0.0/8 17 s0

15.0.0.0/8 17 s1

16.0.0.0/8 4 s1

17.0.0.0/8 4 s1

18.0.0.0/8 14 s1

19.0.0.0/8 6 s1

20.0.0.0/8 16 s1

A C

D

2

5

B

E

15 2

102

2

2

22

11.0.0.0/8

14.0.0.0/8

12.0.0.0/8

13.0.0.0/8

17.0.0.0/8

16.0.0.0/8

15.0.0.0/8

18.0.0.0/8

20.0.0.0/8

19.0.0.0/8

10.0.0.0/8

s0

s1

s2

e0

OSPF Hierarchy

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OSPF Adds Hierarchy

• OSPF Areas• Were defined to limit reachability of routers in large

networks• Areas puts boundaries on explosion of link-state

updates• Flooding and calculation of Dijkstra's algorithm on a

router is limited to changes within an area

Multiple OSPF Areas, WHY ?

• Three issues can overwhelm an OSPF router in heavily populated OSPF network

1.High demand for router processing and memory resources

2.Large routing tables, and 3.Large topology tables

• OSPF allows large areas to be separated into smaller, more manageable areas

More Terms for OSPF

Internal router Routers that have all their interfaces within the same area are

called internal routers Internal routers in the same area have identical link-state

databases and run a single copy of the routing algorithm. Area Border Router (ABR)

Router that has an interface to a specific area and also the Backbone area, Area 0

Autonomous System Border Router (ASBR) These routers can import non-OSPF network information to the OSPF network, and vice versa this is referred to as redistribution

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Optimization of OSPFAreaWhy?

So, fewer LSAs, less impact on the CPU, less demand for system resources.

Area 1 Area 2

Area 0

An identical LSDB only in its area.

No detailed knowledge of the topology outside of area 1. The necessary databases require more

memory.The complex algorithm requires more CPU time.The flooding of LSAs adversely affects available bandwidth, particularly in unstable internetworks.

Detailed LSAs stopped at the area bondary.

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• AreaArea: Collection of OSPF routers.– Every OSPF router must belong to at least one area– Every OSPF network must have an Area 0 (backbone

area)– All other Areas should “touch” Area 0– Routers in same area have same link-state database– Creates a tree-like structure with Area 0 as root

Areas Make OSPF Scalable

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External AS

Traffic Types

External Traffic

Three types of traffic:

Intra-area Traffic

Inter-area Traffic

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Router Types Backbone Router

Area Border Router

(ABR)

Internal Router

Autonomous system

Boundary Router

(ASBR)

External AS

Note: ABR are also Backbone Routers.

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Network Hierarchy

Transit Area (Backbone Area or Area 0)

Nonbackbone Area

Q: What is the backbone used for?

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Network HierarchyBackbone Area

Area 0

Area 13

Area 12

Area 19

This prevents routing loops because is now hierarchy within areas Routers are not all equal and there is limited passage of LSA's by area

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Type Code Description Produced by

1 Router LSA Each router

2 Network LSA DR

3 Network Summary LSA ABR

4 ASBR Summary LSA ABR

5 AS External LSA ASBR

6 Group Membership LSA

7 NSSA External LSA ASBR

8 External Attributes LSA for BGP

9 Opaque LSA (link-local scope) not been deployed

10 Opaque LSA (area-local scope) not been deployed

11 Opaque LSA (AS scope) not been deployed

LSA types

LS Type Advertisements

1. Router Link advertisements. Generated by each router for each area it belongs to. They describe the states of the router's link to the area. These are only flooded within a particular area.

2. Network Link advertisements. Generated by Designated Routers. They describe the set of routers attached to a particular network. Flooded in the area that contains the network

3 or 4. Summary Link advertisements. Generated by Area Border routers. They describe inter-area (between areas) routes.

• Type 3 describes routes to networks, also used for aggregating routes• Type 4 describes routes to ASBR.

5. AS external link advertisements.Originated by ASBR. They describe routes to destinations external to the AS. Flooded all over except stub areas.

LS Type Advertisement 1

Router Link advertisements.

Generated by each router for each area it belongs to. They describe the states of the router's link to the area. These

are only flooded within a particular area.

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Area2Area1

Area0

Router LSA (Type1)

Produced by every router in an area.

Now, LSA type1 is produced by 192.168.30.10.

Link1

Link2

Identified by the router ID of the originating router.

Floods within its area only, does not cross ABR.

Includes list of directly attached links.

RID:190.168.30.10

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Network Link advertisements. Generated by Designated Routers. They describe the set of routers attached to a particular

network. Flooded in the area that contains the network.

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Area2Area1

Area0

Network LSA (Type2)Network (type 2) LSA for each transit broadcast or NBMA network in an area.

Originated by the DR of broadcast network.


Describes the network and list of attached routers.

Includes subnet mask of link.

RID:190.168.30.20

Link:190.168.30.18/29

RID:190.168.30.10

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LS Link Advertisement 3 or 4

Summary Link advertisements. Generated by Area Border routers. They describe inter-area (between areas) routes.

• Type 3 describes routes to networks, also used for aggregating routes

• Type 4 describes routes to ASBR.

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Network Summary LSA( Type3)

Area2Area1

Area0

LSA type3 is originated by the ABR of originating area.

Tells the internal Routers what destinations the ABR can reach

The destination is network which ABR can reach

Also advertises the intra-area and inter-area routes into the backbone

Through a single area, and belonging to the AS.

Link:172.16.121.0/24

Link:192.168.13.16/28

Also advertises the intra-area and inter-area routes into the backbone.

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ASBR Summary LSA (Type 4)

Area2Area1

Area0


The destination they advertise is an ASBR router.

Type 4 LSA is identical to Type3 LSAs, except that

the destination they advertise is an ASBR router.

ASBR(192.168.30.12)

RIP

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AS external link advertisements. Originated by ASBR. They describe routes to destinations external to the AS.

Flooded all over except stub areas.

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Area2Area1

Area0

LSA type 5 is originated by the ASBR of originating area.

ASBR(192.168.30.12)

External LSAs (Type5)

The Destination external to the OSPF AS.

Type 5 LSAs, the only type flooded throughout the entire AS.

10.83.10.0/24 172.20.57.254

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Types of Areas

Backbone Area

Stub Area

Totally Stubby Area

NSSAs (Not-so-stubby Area)

Not Backbone Not Stubby Area

(Regular)

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Stub Areas and Not So Stubby Areas

Why?In many cases, these External LSAs may make up a large percentage of the LSAs in the databases of every router

And, not every router needs to know about all the external destinations

Stub Area is A stub area is an area into which AS External LSAsare not flooded.

Not so stubby Area isAllows the injection of external routes in a limited fashion into the stub area.

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Stub areas must not have virtual links going through them.

Stub Area Rules

LSAs type 4&5 are blocked.

LSAs type 3&4&5 are blocked.

Stub areas cannot have an ASBR, and they should have one ABR.

There can be two or more ABRs, but because of default route, suboptimal routing paths to external autonomous systems can occur.

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LSA Types Allowed Per Area Type

Area Type 1&2 3&4 5 7

Backbone (area 0) Yes Yes Yes No

Non-backbone, non-stub Yes Yes Yes No

Stub Yes Yes No No

Totally stubby Yes No* No No

Not-so-stubby Yes Yes No Yes

Except for a single type 3 LSA per ABR, advertising the default route.

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Summary

Disadvantages: Complicated Configuration.

Equal priority Load balance.

Advantages:Fast Converge.

Loop-free

Hierarchical Management.

Authentication Supported.

Suitable for Large-scale

Network.

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• This week, Reminder, no Lab due to Takehome Midterm• Takehome Midterm on Wednesday

10

Routing Protocols Design

• Design decisions that affect routing performance include• How often to send information to neighbors?

• How much information to send?

• Can we create hierarchy to reduce number of routers? And, number of router messages sent

5/2/16 18

Active BGP Entries

Source: http://www.cidr-report.org/

19

Hierarchy vs. Fully Connected

What is the trade-off between fully connecting routers vs. more hierarchy?

Hierarchy - More complexity• Designate special purpose routers Protocols more complicated Will need backups for potential failures

Fully Connected Router tables need more space More routers to run router algorithms Convergence take more time

23

Org Chart for ICAAN and IANA

ICAAN

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32

OSPF Hierarchy

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More Terms for OSPF

Internal router Routers that have all their interfaces within the same area are

called internal routers Internal routers in the same area have identical link-state

databases and run a single copy of the routing algorithm. Area Border Router (ABR)

Router that has an interface to a specific area and also the Backbone area, Area 0

Autonomous System Border Router (ASBR) These routers can import non-OSPF network information to the OSPF network, and vice versa this is referred to as redistribution

36

36

Optimization of OSPFAreaWhy?

So, fewer LSAs, less impact on the CPU, less demand for system resources.

Area 1 Area 2

Area 0

An identical LSDB only in its area.

No detailed knowledge of the topology outside of area 1. The necessary databases require more

memory.The complex algorithm requires more CPU time.The flooding of LSAs adversely affects available bandwidth, particularly in unstable internetworks.

Detailed LSAs stopped at the area bondary.

Dividing an internetwork into areas is a response to three concerns commonly expressed about link state protocols.

The necessary databases require more memory than a distance vector protocol requires.

The complex algorithm requires more CPU time than a distance vector protocol requires.

The flooding of link state packets adversely affects available bandwidth, particularly in unstable internetworks.

No detailed knowledge of the topology outside of their area. An identical link state database only in its areaMost flooding limited to the area.Smaller link state databases , fewer LSAs, less impact on the CPU.

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External AS

Traffic Types

External Traffic

Three types of traffic:

Intra-area Traffic

Inter-area Traffic

39

39

Router Types Backbone Router

Area Border Router

(ABR)

Internal Router

Autonomous system

Boundary Router

(ASBR)

External AS

Note: ABR are also Backbone Routers.

Internal Routers are routers whose interfaces all belong to the same area. These routers have a single link state database.

Area Border Routers (ABRs) connect one or more areas to the backbone and act as a gateway for inter-area traffic. An ABR always has at least one interface that belongs to the backbone, and must maintain a separate link state database for each of its connected areas. For this reason, ABRs often have more memory and perhaps more powerful processors than internal routers. An ABR will summarize the topological information of its attached areas into the backbone, which will then propagate the summary information to the other areas.

Backbone Routers are routers with at least one interface attached to the backbone. Although this requirement means that ABRs are also Backbone Routers, Figure 9.21 shows that not all Backbone Routers are ABRs. An Internal Router whose interfaces all belong to area 0 is also a Backbone Router.

Autonomous System Boundary Routers (ASBRs) are gateways for external traffic, injecting routes into the OSPF domain that were learned (redistributed) from some other protocol, such as the BGP and EIGRP processes shown in Figure 9.21. An ASBR can be located anywhere within the OSPF autonomous system; it may be an Internal, Backbone , or ABR.

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40

Network Hierarchy

Transit Area (Backbone Area or Area 0)

Nonbackbone Area

Q: What is the backbone used for?

It is best to avoid the need for them by ensuring that areas, particularly backbone areas, are designed with redundant links to prevent partitioning. When two or more internetworks are merged, sufficient planning should take place beforehand so that no area is left without a direct link to the backbone.

41

Network HierarchyBackbone Area

Area 0

Area 13

Area 12

Area 19

This prevents routing loops because is now hierarchy within areas Routers are not all equal and there is limited passage of LSA's by area

42

Type Code Description Produced by

1 Router LSA Each router

2 Network LSA DR

3 Network Summary LSA ABR

4 ASBR Summary LSA ABR

5 AS External LSA ASBR

6 Group Membership LSA

7 NSSA External LSA ASBR

8 External Attributes LSA for BGP

9 Opaque LSA (link-local scope) not been deployed

10 Opaque LSA (area-local scope) not been deployed

11 Opaque LSA (AS scope) not been deployed

LSA types

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LS Type Advertisements

1. Router Link advertisements. Generated by each router for each area it belongs to. They describe the states of the router's link to the area. These are only flooded within a particular area.

2. Network Link advertisements. Generated by Designated Routers. They describe the set of routers attached to a particular network. Flooded in the area that contains the network

3 or 4. Summary Link advertisements. Generated by Area Border routers. They describe inter-area (between areas) routes.

• Type 3 describes routes to networks, also used for aggregating routes• Type 4 describes routes to ASBR.

5. AS external link advertisements.Originated by ASBR. They describe routes to destinations external to the AS. Flooded all over except stub areas.

44


Router Link advertisements.

Generated by each router for each area it belongs to. They describe the states of the router's link to the area. These

are only flooded within a particular area.

45

Area2Area1

Area0

Router LSA (Type1)

Produced by every router in an area.

Now, LSA type1 is produced by 192.168.30.10.

Link1

Link2

Identified by the router ID of the originating router.


Includes list of directly attached links.

RID:190.168.30.10

46


Network Link advertisements. Generated by Designated Routers. They describe the set of routers attached to a particular

network. Flooded in the area that contains the network.

47

Area2Area1

Area0

Network LSA (Type2)Network (type 2) LSA for each transit broadcast or NBMA network in an area.

Originated by the DR of broadcast network.


Describes the network and list of attached routers.

Includes subnet mask of link.

RID:190.168.30.20

Link:190.168.30.18/29

RID:190.168.30.10

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LS Link Advertisement 3 or 4

Summary Link advertisements. Generated by Area Border routers. They describe inter-area (between areas) routes.

• Type 3 describes routes to networks, also used for aggregating routes

• Type 4 describes routes to ASBR.

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49

Network Summary LSA( Type3)

Area2Area1

Area0


Tells the internal Routers what destinations the ABR can reach

The destination is network which ABR can reach

Also advertises the intra-area and inter-area routes into the backbone

Through a single area, and belonging to the AS.

Link:172.16.121.0/24

Link:192.168.13.16/28

Also advertises the intra-area and inter-area routes into the backbone.

When another router receives a Network Summary LSA from an ABR, it does not run the SPF algorithm. Rather, it simply adds the cost of the route to the ABR and the cost included in the LSA. A route to the advertised destination, via the ABR, is entered into the route table along with the calculated cost. This behavior—depending on an intermediate router instead of determining the full route to the destination—is distance vector behavior. So, while OSPF is a link state protocol within an area, it uses a distance vector algorithm to find inter-area routes.[13]

[13] This distance vector behavior is the reason for requiring a backbone area and requiring that all inter-area traffic pass through the backbone. By forming the areas into what is essentially a hub-and-spoke topology, the route loops to which distance vector protocols are prone are avoided.

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50

ASBR Summary LSA (Type 4)

Area2Area1

Area0


The destination they advertise is an ASBR router.

Type 4 LSA is identical to Type3 LSAs, except that

the destination they advertise is an ASBR router.

ASBR(192.168.30.12)

RIP



51


AS external link advertisements. Originated by ASBR. They describe routes to destinations external to the AS.

Flooded all over except stub areas.

52

52

Area2Area1

Area0

LSA type 5 is originated by the ASBR of originating area.

ASBR(192.168.30.12)

External LSAs (Type5)

The Destination external to the OSPF AS.

Type 5 LSAs, the only type flooded throughout the entire AS.

10.83.10.0/24 172.20.57.254



53

Types of Areas

Backbone Area

Stub Area

Totally Stubby Area

NSSAs (Not-so-stubby Area)

Not Backbone Not Stubby Area

(Regular)

54

54

Stub Areas and Not So Stubby Areas

Why?In many cases, these External LSAs may make up a large percentage of the LSAs in the databases of every router

And, not every router needs to know about all the external destinations

Stub Area is A stub area is an area into which AS External LSAsare not flooded.

Not so stubby Area isAllows the injection of external routes in a limited fashion into the stub area.

The performance of routers within a stub area can be improved, and memory conserved, by the reduced size of their databases. Of course, the improvement will be more marked in internetworks with a large number of type 5 LSAs. There are, however, four restrictions on stub areas

55

55

Stub areas must not have virtual links going through them.

Stub Area Rules

LSAs type 4&5 are blocked.

LSAs type 3&4&5 are blocked.

Stub areas cannot have an ASBR, and they should have one ABR.

There can be two or more ABRs, but because of default route, suboptimal routing paths to external autonomous systems can occur.

Totally stubby areas use a default route to reach not only destinations external to the autonomous system but also all destinations external to the area. The ABR of a totally stubby area will block not only AS External LSAs but also all Summary LSAs—with the exception of a single type 3 LSA to advertise the default route.

56

LSA Types Allowed Per Area Type

Area Type 1&2 3&4 5 7

Backbone (area 0) Yes Yes Yes No

Non-backbone, non-stub Yes Yes Yes No

Stub Yes Yes No No

Totally stubby Yes No* No No

Not-so-stubby Yes Yes No Yes

Except for a single type 3 LSA per ABR, advertising the default route.

57

Summary

Disadvantages: Complicated Configuration.

Equal priority Load balance.

Advantages:Fast Converge.

Loop-free

Hierarchical Management.

Authentication Supported.

Suitable for Large-scale

Network.

cscd 433/533 advanced networks - ewupenguin.ewu.edu/.../cscd433-lecture13-2016-routing-ospf.pdf ·...

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