Routing Protocols and CIDR BSAD 146 Dave Novak Sources: Network+ Guide to Networks, Dean 2013

Outline Routing Static and Dynamic routing Routing Protocols EGPs IGPs CIDR

Last time we discussed IPv4 address space review Subnetting conceptual review Subnetting example

Internet Routing (review) On a small internetwork the job of a router is simply to forward packets destined for remote network off of the home network Separate local traffic from remote traffic On larger, more complex internetworks, routers consider different performance metrics and select the “best” or most efficient route from source to destination Often measured by fewest hops (based on graph theory)

Internet Routing (review) Routers contain information about the devices and traffic conditions (congestion, etc.) on the network they are attached to Routers contain at least some information about other routers that they can directly reach The amount of information routers share with other routers depends on many things including configuration, security protocols or methods, and routing protocols that are used

Routing Tables (review) 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

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

Internet Routing Routers populate their routing tables with destination IP addresses and best route info Two broad categories of routing 1) Static – routes that do not change Tables created for fixed routes or reference to only one or two routers, which are always used 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

Static Routing Commonly used on many personal devices At least part of route is fixed and does not change 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 router is not directly attached to

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 Scalable

Routing in the Internet If routing propagation software required routers to exchange information directly with all other routers there would be scaling problems

Routing in the Internet Scalability is addressed using a two-level hierarchy Networks and routers are partitioned 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

Autonomous System (AS) 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

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

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

Routing Protocols

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

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 1 st choice

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

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

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)

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

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

Routing algorithm

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

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

Subnetting IPv4 (review) Subnetting extends the network address by using a subnet mask to create additional organizational hierarchies within each IPv4 class

Subnetting IPv4 (review) 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

Subnetting IPv4 (review) 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

Classless Inter-domain Routing (CIDR) CIDR allows IP addresses to be organized into logical networks in a manner that is relatively independent of the value of the IP addresses Allows flexibility in defining logical networks as well as in creating “routing groups” of addresess Can “recombine” or group separate subnets for routing purposes

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

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

CIDR Notation example Source: CIDR conversion table, University of Wisconsin:

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

CIDR Notation /23 =

Classless Inter-domain Routing (CIDR) CIDR aggregation REQUIRES network segments to contiguous or numerically adjacent (cannot aggregate and unless and are also included in /22

Summary Routing Static and Dynamic routing Routing Protocols EGPs IGPs CIDR