CSCD 433/533 Advanced Networks Spring PDF Free Download

CSCD 433/533 Advanced Networks Spring 2016 Lecture 13 Router Algorithms and Design Chapter 5 1

Topics Router Algorithms Routing in General Hierarchical routing Interior Gateway Protocols OSPF mention of RIP Brief Comparison Properties of OSPF OSPF Areas 2

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 Properties More 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 12

Aggregated Network vs. Reality AS Autonomous Systems BGP for links between regions The AS graph may look like this Other routing protocols within regions Reality may be closer to this

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 7 Entries 17 Entries Hierarchical Routing

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 21

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 22

Org Chart for ICAAN and IANA ICAAN

AS Based Routing in Internet Two Types of Internet Routing 1. 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 24

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.

27 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

28 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

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 29

Choosing the Best Path From RouterA - After running Djikstra's Algorithm Shortest Path First Best routes identified 14.0.0.0/8 2 11.0.0.0/8 15 B 2 15.0.0.0/8 10.0.0.0/8 A 5 13.0.0.0/8 12.0.0.0/8 2 C 16.0.0.0/8 D 2 17.0.0.0/8 2 10 18.0.0.0/8 E 20.0.0.0/8 2 2 19.0.0.0/8 30

Results Put into Routing Table RouterA s Routing Table 10.0.0.0/8 connected e0 11.0.0.0/8 connected s0 12.0.0.0/8 connected s1 13.0.0.0/8 connected s2 14.0.0.0/8 17 s0 15.0.0.0/8 17 s1 16.0.0.0/8 4 s1 10.0.0.0/8 17.0.0.0/8 4 s1 e0 18.0.0.0/8 14 s1 19.0.0.0/8 6 s1 20.0.0.0/8 16 s1 11.0.0.0/8 s0 A s2 s1 15 5 13.0.0.0/8 2 14.0.0.0/8 12.0.0.0/8 B C D 2 16.0.0.0/8 2 2 17.0.0.0/8 2 19.0.0.0/8 2 10 15.0.0.0/8 18.0.0.0/8 E 20.0.0.0/8 2

OSPF Hierarchy

33 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

Areas Make OSPF Scalable Area: 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 37

38 Traffic Types Three types of traffic: Intra-area Traffic Inter-area Traffic External AS External Traffic

Router Types Backbone Router Autonomous system Boundary Router (ASBR) Area Border Router (ABR) External AS Internal Router Note: ABR are also Backbone Routers. 39

Network Hierarchy Transit Area (Backbone Area or Area 0) Nonbackbone Area Q: What is the backbone used for? 40

Network Hierarchy Backbone Area Area 0 Area 19 Area 12 Area 13 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 41

LSA types 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 42

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.

Router LSA (Type1) Produced by every router in an area. Now, LSA type1 is produced by 192.168.30.10. Identified by the router ID of the originating router. RID:190.168.30.10 Link1 Link2 Area0 Floods within its area only, does not cross ABR. Area1 Area2 Includes list of directly attached links. 45

LS Type Advertisement 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. 46

Network LSA (Type2) Network (type 2) LSA for each transit broadcast or NBMA network in an area. Originated by the DR of broadcast network. Includes subnet mask of link. RID:190.168.30.20 Link:190.168.30.18/29 Area0 Floods within its area only, does not cross ABR. RID:190.168.30.10 Area1 Area2 Describes the network and list of attached routers. 47

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. 48

49 Network Summary LSA( Type3) Also advertises the intra-area and inter-area routes into the backbone LSA type3 is originated by the ABR of originating area. Link:172.16.121.0/2 4 Area0 Also advertises the intra-area and interarea routes into the backbone. Link:192.168.13.16/ 28 Tells the internal Routers what destinations the ABR can reach The destination is network which ABR can reach Area1 Through a single area, and belonging to the AS. Area2

LS Type Advertisement 5 AS external link advertisements. Originated by ASBR. They describe routes to destinations external to the AS. Flooded all over except stub areas. 51

53 Types of Areas Backbone Area Stub Area Totally Stubby Area NSSAs (Not-so-stubby Area) Not Backbone Not Stubby Area (Regular)

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. 56

Summary Advantages: Fast Converge. Loop-free Hierarchical Management. Authentication Supported. Suitable for Large-scale Network. Disadvantages: Complicated Configuration. Equal priority Load balance. 57

This week, Reminder, no Lab due to Takehome Midterm Takehome Midterm on Wednesday 58

Active BGP Entries 5/2/16 18 Source: http://www.cidr-report.org/

Org Chart for ICAAN and IANA ICAAN 23

A 30

OSPF Hierarchy 32 32

Optimization of OSPF Area Why? An identical LSDB only in its area. Detailed LSAs Area 0 stopped at the area bondary. Area 1 Area 2 No detailed knowledge of the topology outside of area 1. The necessary databases require more memory. The complex algorithm requires more CPU time. So, fewer LSAs, The flooding less impact of LSAs on adversely the CPU, affects less demand for system resources. available bandwidth, particularly in unstable internetworks. 36 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 area Most flooding limited to the area. Smaller link state databases, fewer LSAs, less impact on the CPU. 36

Traffic Types Three types of traffic: Intra-area Traffic Inter-area Traffic External AS External Traffic 38

Router Types Backbone Router Autonomous system Boundary Router (ASBR) Area Border Router (ABR) External AS Internal Router Note: ABR are also Backbone Routers. 39 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. 39

Network Hierarchy Transit Area (Backbone Area or Area 0) Nonbackbone Area Q: What is the backbone used for? 40 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. 40

Network Summary LSA( Type3) Also advertises the intra-area and inter-area routes into the backbone LSA type3 is originated by the ABR of originating area. Link:172.16.121.0/2 4 Area0 Also advertises the intra-area and interarea routes into the backbone. Link:192.168.13.16/ 28 Tells the internal Routers what destinations the ABR can reach The destination is network which ABR can reach Area1 Through a single area, and belonging to the AS. Area2 49 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. 49

ASBR Summary LSA (Type 4) RIP ASBR(192.168.30.12) LSA type4 is originated by the ABR of originating area. Area0 The destination they advertise is an ASBR router. Area1 Area2 Type 4 LSA is identical to Type3 LSAs, except that the destination they advertise is an ASBR router. 50 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. 50

LS Type Advertisement 5 AS external link advertisements. Originated by ASBR. They describe routes to destinations external to the AS. Flooded all over except stub areas. 51

External LSAs (Type5) 10.83.10.0/24 172.20.57.254 LSA type 5 is originated by the ASBR of originating area. ASBR(192.168.30.12 ) Area0 The Destination external to the OSPF AS. Area1 Type 5 LSAs, the only type flooded throughout the entire AS. Area2 52 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. 52

Types of Areas Backbone Area Stub Area Totally Stubby Area NSSAs (Not-so-stubby Area) Not Backbone Not Stubby Area (Regular) 53

Stub Areas and Not So Stubby Areas Stub Area is A stub area is an area into which AS External LSAs are not flooded. Not so stubby Area is Allows the injection of external routes in a limited fashion into the stub area. 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 54 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 54

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. Stub areas must not have virtual links going through them. 55 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. 55