CMPT 471 Networking II

1. CMPT 471Networking II OSPF

2. Dynamic Routing • In very simple small networks static routing may be adequate. • As networks grow and become more complex, including multiple or redundant paths between hosts and routers static routing becomes difficult to manage • Redundant routes are only useful if they can be used immediately on failure of the original route • In a large network the reporting of failure and response by a network manager cannot be accomplished on a short enough time scale for the redundant routes to be useful

3. Dynamic Routing • A routing algorithm to update the routing tables to respond to changes and failures in the network becomes a necessary component of the system • Addition of a routing algorithm does not change the forwarding algorithm used to deliver packets, segments …

4. Shortcomings of RIP • Simple metric (hop count) not useful when links have different capacities (it takes longer to transmit over a lower capacity link) • Allows one route between each pair of stations, even if multiple routes with the same metric value exists (no load balancing) • Slow convergence, count to infinity problem, mitigated by using a small maximum hop count but still an issue. Looping can be caused during convergence • No authentication in RIP1, malicious user can hijack traffic by providing low cost routes. Authentication available in RIP2 • Useful for small simple networks only, limited by maximum hop count, size of routing data proportional to the size of the network, and poor control of looping

5. Approaches – Link-state • When router initialized and at intervals thereafter, it determines link cost on each interface (cost to each directly connected node) • Advertises set of link costs to all other nodes in topology • Each node constructs routing table containing minimum cost paths to all attached nodes ( costs and first hop to each router) using the data received from all other nodes advertisements. • Open shortest path first (OSPF) protocol uses link-state routing. (a common IRP) • Second generation routing algorithm for ARPANET

6. link state methods • The count to infinity problem is solved since information is sent to all routers not just nearest neighbors. This gives rapid convergence • The size of the routing data packet is no longer proportional to the size of the network, it is determined by the number of nearest neighbors. • Each node calculates its own optimal paths, reducing probability of loops caused by old information (old due to longer propagation time across network)

7. Open Shortest Path First: OSPF • IGP of Internet, supports classless addressing, subnetting, authentication, and load balancing (use of multiple equal cost paths to distribute load) • (RFC 2328) Uses a Link State Routing Algorithm. Each router • keeps a database of information based on local costs and received update packets from other routers in the AS. • Each router builds a directed graph showing topology and path costs

8. Open Shortest Path First: OSPF • Uses a Link State Routing Algorithm (RFC 2328) Each router • Knows whether data originates within or outside the AS. • Can choose correct gateway to send packets through to the outside world • Uses the database and Dykstra’s algorithm to determine least cost paths for the whole AS using itself as the source node, preventing looping • Advertises its locally determined routing table periodically

9. Directed Graph Network model • Models flow between routers • Vertices or nodes are routers and networks • Types of Network • Transit: data not originating in network can pass through the network, more than one router is attached to the network • Stub: data not originating in network can enter only. One router is attached to network

10. Directed Graph Network model • Types of Network • Transit and Stub • Edges, associated costs at output of routers • Connect two routers with a pair of edges • Connect router to transit network with pair of edges • Connect router to stub network with single edge

11. 5 R2 R1 3 Simplest OSPF graphs: 1 • AS must contain at least two routers. Simplest case is two routers connected by a point to point connection • The two routers are separated by a transit network, with three or more routers connect to the network it is known as a multi access network 0 5 R2 R1 0 3 0 R3 6

12. Simplest OSPF graphs: 2 • Stub network, single router connects network to the AS. Does not mean no data is coming from the stub network, simply that no data is coming through the network from the other side • Flow costs can be asymmetric. • Flow costs for the path to the next router are associated with the router as the packet leaves the router • No additional cost is added for propagation from the intermediate network to the next router, all cost is associated with the path from the originating router to the transit 5 R1

13. From notes of Lou Hafer, after RFC1131: Sample AS

14. RFC 1131: Sample AS: Directed Graph

15. RFC 2328: Sample AS: Directed Graph

16. Neighbor Routers • Any pair of routers attached to the same single network segment (single broadcast address) can become neighbors • To become neighbors they must agree that they are neighbors • The pair of routers negotiates this agreement using and exchange of Hello packets (more later) to assure a two way link is established

17. Adjacent routers • Routers establish an adjacency if they will be exchanging LSAs.(link state announcement packets that carry routing information between routers) • A router on a particular physical segment will not necessarily be adjacent to all other routers on that segment • A router with multiple interfaces may simultaneously be adjacent to routers on more than one network segment

18. Designated Routers (1) • Each multi access network will have a designated router (and a backup designated router in case the designated router fails). • The designated router will be chosen by the exchange of hello packets (more later) • Consider all possible connections between the routers connected to the network (n[n-1]/2 connections) and the n connections to the network. This gives a total of n2 connections. Each of these connections would generate one record in the routing database for the overall network

19. n(n-1)/2 links Designated Routers

20. Designated Routers (2) • When a designated router is elected, one of its responsibilities is to advertize the routes for the local network segment to the other routers on the same local network segment • Adjacencies will be established from the designated router to all other routers on the network segment • we need only advertise these n-1 connections between routers on the local network segment to other routers outside of the local segment

21. Designated Routers D (n-1) links will become adjacencies n(n-1)/2 links

22. Designated Routers (3) • When a designated router is elected, another of its responsibility is to advertize the routes for the local network segment to other designated routers for other segments. • Adjacencies will be established from the designated router to all other routers on other network segments accessible through the other interfaces attached to the designated router. • we need only advertise one connection from the designated router to each network’s designated router in the OSPF routing database.

23. Designated Routers D (n-1) adjacencies n(n-1)/2 links D Local segment

24. Designated Routers • Only the designated router for each network will advertises routes to routers connected to the network. These routes are advertised by sending a network link state advertisement to all routers adjacent to the designated router. This advertisement contains a list of all routers connected to the network, the cost for the path from each of these routers to the network is 0. • All routers attached to the network will advertise a list of routes for their neighbor routers. Routes to neighbor routers

25. Operation • A database corresponding to the directed graph is kept by each router. • Dijkstra’s algorithm (shortest path first SPF) and database information are used, to find the next hop on the least cost path from this router (the source) to all other routers. • Only next hop information is used to forward packets

26. Operation • Link state messages (called Link state advertisements, LSAs) from other routers in the AS are used to build/update the database • A LSA includes information on path costs to nearest neighbor routers only (router R6 would have path costs to router R3 R5 and R10 in its LSA) • LSAs are flooded to all adjacent routers throughout the AS • LSAs are timestamped, the routers database is only updated when the message is newer than the last update for that node

27. Designated Routers: Example • Example for network 3 with designated router R2 • As designated router for N3 R2 has entries • R1 0 • R2 0 • R3 0 • R4 0 • As neighbor to other routers R2 has entries • N2 3 • N3 1 • Routers R1 R3 and R4 do not duplicate advertisements for N3.

28. RFC 1131: Sample AS: SPF tree

29. SPF Routing table for router 6 RFC1131: routing table for router 6

30. Dividing an AS into Areas • Many networks are large and complex it is often useful to divide them into areas and deal with each smaller area separately • OSPF includes mechanisms for dealing with ASs partitioned into areas. • When an AS is divided into areas the areas are chosen so they can be connected by a backbone of routers • Any router which is part of an area, but also communicates with other areas, is also a part of the backbone area.

31. Dividing an AS into Areas • The backbone is a special area. The information passing between all other areas travels through the backbone routers (dark ovals in diagram) • Routers in each area run their own copy of the routing process and have their own topological link-state data base. Routers in one area have no detailed knowledge of routing in other areas. • An area includes all routers with a connection to any contained network

32. Internal routers 1,2,5,6,8,9,12 Area Border routers 3,4,7,10,11 1b Includes Router 4 2b 23b Includes Routers 7 and 11 From notes of Lou Hafer, after RFC1131: AS divided into areas

33. Types of routers in an AS • Internal Routers: all connected networks belong to the same area or with only backbone interfaces • Area Border Routers: not internal, run one copy of routing algorithm for backbone, and one copy for each attached area • Backbone Routers: has at least one interface to the backbone. Can be an internal router if all interfaces are with the backbone. Otherwise it is an area border router • AS boundary routers: Part of the AS but also communicates with routers outside the AS using and EGP. Can be routers of an of the above types

34. Inter area routing • Routing a packet between two areas through the backbone • The overall algorithm find the lowest cost sum of • The lowest cost path from the source to the AS border router in the source area • The lowest cost path through the AS backbone from the AS border router for the source area the AS border router for the destination area • The lowest cost path from the AS border router for the destination area to the destination

35. From notes of Lou Hafer, after RFC1131: AS area 1

36. From notes of Lou Hafer, after RFC1131: AS backbone