Routing Protocols and CIDR

1. Routing Protocols and CIDR BSAD 141 Dave Novak Sources: Network+ Guide to Networks, Dean 2013

2. Outline • Routing • Static and Dynamic routing • Routing Protocols • EGPs • IGPs • CIDR

3. Internet Routing (review) • On a small internetwork routers job is simply to forward packets destined for remote network to that network • Separate local traffic from remote traffic • On larger, more complex internetworks, the router must select the “best” or most efficient route from source to destination • Often measured by fewest hops

4. Routing Tables • All TCP/IP devices have some type of routing table • A table to determine where to send packets • MAC address mapped to IP address • Systems store local address mappings and can usually transmit local packets directly to the receiving system • Systems typically use a default address (the IP address of a router) for non local transmission

5. Routing Tables • You can go to the command prompt on your computer and type in “netstat –r”

6. Internet Routing • Routers populate their routing tables with destination IP address and best route info • Two broad categories of routing • 1) Static – routes that do not change • Network admin creates table manually • 2) Dynamic – system changes routing table information over time • Router uses routing protocols to exchange information with routers around it to learn optimal routes to different destinations

7. Static Routing • Commonly used on many personal devices • Routes are fixed and do not change dynamically • Think of hard coding a specific value versus using variables than can change values • Useful on small internetworks • Doesn’t scale • No information available for any networks the device is not directly attached to

8. Dynamic Routing • Indirectly collects information on networks the router is not directly attached to through communication with other routers • Routing information is continually updated based on changing conditions • Used on most routers • Reduce management workload • Required on Internet or large internetwork • Scaleable

9. Routing in the Internet • If routing propagation software allowed one router to exchange information directly with all other routers there would be scaling problems

10. Routing in the Internet • Scalability is addressed using a two-level hierarchy • Networks are divided into groups • Within groups, routers exchange information using routing propagation software • One (or a couple) member of each group summarize information from within the group and pass that information to other groups

11. Autonomous System Concept • Routing groups are created based on AS concept • One central authority in charge of a contiguous set of routers and networks • Can be made for economic, technical, and or administrative reasons • University • Corporation • ISP

12. Routing Protocols • Divided into two categories • 1) Routers within AS use Interior Gateway Protocol (IGP) to exchange routing information between them • Several different IGPs available • Each AS chooses its own IGP

13. Routing Protocols • 2) Router designated to communicate with other AS’s use Exterior Gateway Protocol (EGP) to exchange routing information with a designated router in another AS • EGP summarizes information from the AS before passing that information to another AS

14. Routing Protocols

15. Optimal Routes • No universal agreement about which path is optimal • In dynamic environments like the Internet, what is optimal may change - frequently • Different applications have different needs • Interactive login – path with least delay • Large graphics – path with max throughput • Real time audio – path with min variance in delay • Routing metric

16. Routing Metrics - examples • Hop count • Hop corresponds to an intermediate network (router) • Number of intermediate destinations between point of origin and final destination • Administrative cost • Assigned manually to control which path can be used • Maybe administration doesn’t want traffic to traverse a certain route as a 1st choice

17. Routing in EGP • Border Gateway Protocol (BGP) is most popular EGP routing protocol used to pass information between different AS

18. Routing in EGP • BGP possesses following properties: • Routing among autonomous systems • Routes are given as paths of AS • Provision for policies • Allows sender and receiver to enforce policies • Facilities for transit routing • Distinguish between AS that will pass information on and those that won’t • Reliable transport • Uses TCP

19. Routing in IGP • Interior Gateway Protocol (IGP): different protocols are commonly used to pass information within a particular AS • 1) Routing Information Protocol (RIPv2) • 2) Open Shortest Path First (OSPF) • 3) Enhanced Interior Gateway Routing Protocol (EIGRP)

20. Open Shortest Path First (OSPF) • Most widely used IGP in enterprise networks • Adjacent routers periodically probe each other • Broadcast link-status message • Compute shortest path • Can subdivide AS into logical areas • The AS imposes a hierarchy within the AS • Scales to handle more routers • Limits broadcast to specific area

21. Open Shortest Path First (OSPF) • Uses link-state routing • Measures properties of links (like bandwidth) • Able to update routing tables more quickly • Load balancing by splitting traffic between routes with equal metrics • Less network traffic

22. Routing algorithm

23. Classless Inter-domain Routing (CIDR) • A more flexible way to reference and allocate the limited address space used in standard IPv4 • Also referred to as supernetting • Combining two or more subnetworks with a common CIDR prefix for routing purposes • A hierarchical allocation of address space that allows large ISPs to control segments of address space

24. Classless Inter-domain Routing (CIDR) • CIDR is an alternative to traditional subnetting • Review: • Subnetting allows for logical partitioning of class-based IP addresses into separate groups • Requires the use of a subnet mask

25. Classless Inter-domain Routing (CIDR) • Subnetting extends the network address by using a subnet mask to create additional organizational hierarchies within each IPv4 class

26. Classless Inter-domain Routing (CIDR) • Assume a standard class C IPv4 address space • 24 bits in the prefix (the network address) • 8 bits in the suffix (the hosts on that network) • Subnetting allows 1, 2, 3, or 4 bits from the suffix to be “moved” to the prefix • Example: “moving” 1 bit from suffix to prefix creates two separate logical networks with 128 hosts / subnet

27. Classless Inter-domain Routing (CIDR) • Example: “moving” 2 bits from suffix to prefix creates four separate logical networks with 64 hosts / subnet • Example: “moving” 3 bits from suffix to prefix creates eight separate logical networks with 32 hosts / subnet

28. Classless Inter-domain Routing (CIDR) • CIDR organizes IP addresses into logical networks in a manner that is independent of the value of the addresses • Can “recombine” or group separate subnets for routing purposes

29. Classless Inter-domain Routing (CIDR) • This effectively aggregates the routes in routing table entries from individual smaller networks • GREATLY reduces routing table entries • Requires the use of routing protocols that support CIDR including: EIGRP, RIP-v2, OSPF, and BGP

30. Classless Inter-domain Routing (CIDR) “A company that operates 150 accounting services in each of 50 districts has a router in each office connected with a frame relay link to its corporate headquarters. Without supernetting, the routing table on any given router might have to account for 150 routers in each of the 50 districts, or 7500 different networks. However, if a hierarchical addressing system is implemented with supernetting, then each district has a centralized site as interconnection point. Each route is summarized before being advertised to other districts. Each router now only recognizes its own subnet and the other 49 summarized routes.” (Source: example is DIRECTLY from http://en.wikipedia.org/wiki/Supernet)

32. CIDR Notation example Source: CIDR conversion table, University of Wisconsin: https://kb.wisc.edu/ns/page.php?id=3493

33. CIDR Notation • xxx.xxx.xxx.xxx/n (n is # of (leftmost) ‘1’ bits in the mask • IPv4 Class C address example • 192.60.128.0/22 = 11111111.11111111.11111100.00000000

34. CIDR Notation • 192.60.128.0/23 = 11111111.11111111.11111110.00000000

35. Classless Inter-domain Routing (CIDR) • CIDR aggregation REQUIRES network segments to contiguous or numerically adjacent (cannot aggregate 192.168.20.0 and 192.168.23.0 unless 192.168.21.0 and 192.168.22.0 are also included in 192.168.20.20/22

36. Summary • Routing • Static and Dynamic routing • Routing Protocols • EGPs • IGPs • CIDR