Routing protocols Dynamic routing Lecture 4 Dynamic

Routing protocols: Dynamic routing Lecture 4

Dynamic routing protocols A routing protocol is a set of processes, algorithms, and messages that are used to exchange routing information and populate the routing table with the best paths. The purpose of a routing protocol includes: Discovery of remote networks Maintaining up-to-date routing information Choosing the best path to destination networks Ability to find a new best path if the current path is no longer available. 2

Dynamic Routing Protocol Operation The purpose of all routing protocols: to learn about remote networks and to quickly adapt whenever there is a change in the topology. The methods used by a routing protocol depend upon the algorithm it uses and the operational characteristics of that protocol. In general, the operations of a dynamic routing protocol can be described as follows: The router sends and receives routing messages on its interfaces. The router shares routing messages and routing information with other routers that are using the same routing protocol. Routers exchange information to learn about remote networks. When a router detects a topology change the routing protocol can advertise this change to other routers. 3

Dynamic vs. Static 4

Autonomous system (AS) Autonomous system (routing domain, автономна система, домен маршрутизації) - is a collection of routers under a common administration. Internet is based on the autonomous system concept, there are two types of routing protocols required: Interior Gateway Protocols (IGP, внутрішні шлюзові протоколи) are used for routing inside an autonomous system. Exterior Gateway Protocols (EGP, зовнішні шлюзові протоколи) are used for routing between autonomous systems. 5

6

Interior Gateway Protocols (IGPs) classification Distance vector routing protocols (дистанційновекторні протоколи маршрутизації) : RIP (Routing Information Protocol) IGRP (Interior Gateway Routing Protocol) EIGRP (Enhanced IGRP) Link-state routing protocols (протоколи стану каналу): OSPF (Open Shortest Path First) IS-IS (Intermediate System-Intermediate System) 7

Convergence (збіжність) is when all routers have complete and accurate information about the network. Convergence time is the time it takes routers to share information, calculate best paths, and update their routing tables. A network is not completely operable until it has converged. Convergence properties include the speed of propagation of routing information and the calculation of optimal paths. The faster the convergence, the better the routing protocol. Generally, RIP and IGRP are slow to converge, whereas EIGRP and OSPF are faster to converge. 8

Metric A metric (метрика) is a value used by routing protocols to determine which path is most preferable when there are multiple paths to the same remote network. The lower the metric, the better the route. The parameters used to calculate metric: Hop count (к-ть проміжних вузлів або стрибків) - the number of routers a packet must traverse. Bandwidth (пропускна здатність) – the summarized bandwidth along the route. Load (завантаженість) - the traffic utilization of a certain link. Delay (затримка) - the time a packet takes to traverse a path. Reliability (надійність) - the probability of a link failure. Cost (вартість) - A value determined either by the IOS or by the network administrator to indicate preference for a route. 9

RIP Metric Hop count - Best path is chosen by the route with the lowest hop count. The metric associated with a certain route can be best viewed using the show ip route command. The metric value is the second value in the brackets for a routing table entry: R 172. 16. 3. 0/24 [120/2] via 172. 16. 4. 1, 00: 26, Serial 0/0/1 10

IGRP, EIGRP Metric K 1(bandwidth)=1 K 2(load)=0 K 3 (delay)=1 K 4, K 5 (reliability)=0 11

OSPF Metric The OSPF metric is called cost. A cost is associated with the output side of each router interface. The lower the cost, the more likely the interface is to be used to forward data traffic. 12

The cost of an OSPF route is the accumulated value from one router to the destination network. 13

Administrative distance (AD, адміністративна відстань) - defines the preference of a routing source - is used to select the best path when it learns about the same destination network from two or more different routing sources. - is an integer value from 0 to 255. The lower the value the more preferred the route source. An administrative distance of 0 is the most preferred. An administrative distance of 255 means the router will not believe the source of that route and it will not be installed in the routing table. AD of route sources: 0 – directly connected 1 - static 90 – EIGRP 100 – IGRP 110 – OSPF 120 - RIP 14

Administrative distance R 2# show ip route (**ouput omitted** ) 15

Distance Vector Routing Protocols Routes are advertised as vectors of distance and direction. Distance is defined in terms of a metric such as hop count and direction is simply the next-hop router or exit interface. Bellman-Ford algorithm is used for the best path route determination. Periodic Updates are sent at regular intervals (30 seconds for RIP and 90 seconds for IGRP). Broadcast Updates are sent to 255. Entire routing tables are sent to each of its neighbors. Updates consume bandwidth and CPU resources to process the updates. Slow to converge. Routing loops. Do not have an actual map of the network topology (routing by rumors). Typical protocols: RIP, IGRP 16

Network discovery If router’s interfaces are configured correctly, the router will initially discover its own directly connected networks. 17

Routing information exchange If a routing protocol is configured, the routers begin exchanging routing updates. Initially, these updates only include information about their directly connected networks. Upon receiving an update, the router checks it for new information. 18

Discovering new networks Any routes that are not currently in its routing table are added. At this point the routers have knowledge about their own directly connected networks and about the connected networks of their nearest neighbors 19

Next update Continuing the journey toward convergence, the routers exchange the next round of periodic updates. Each router again checks the updates for new information. 20

Convergence Finally, routers got information about all existing networks. The routes to each network with the lowest metric, installed into the routing table, are used to sent packet to the destination needed. 21

RIP configuration To enable a dynamic routing protocol, enter the global configuration mode and use the router command: R(config)# router rip The prompt changes from a global configuration prompt to the router configuration mode for RIP : R(config-router)# To enable RIP routing for a network, use the command for each directly connected network: Router(config-router)# network IP-network-address 22

R 2(config)#router rip R 2(config-router)#network 192. 168. 2. 0 R 2(config-router)#network 192. 168. 3. 0 R 1(config)#router rip R 1(config-router)#network 192. 168. 1. 0 R 2(config-router)# network 192. 168. 4. 0 R 1(config-router)#network 192. 168. 2. 0 R 3(config)#router rip R 3(config-router)#network 192. 168. 4. 0 R 3(config-router)#network 192. 168. 5. 0 23

24

Routing table examination: show ip route R 192. 168. 5. 0/24 [120/2] via 192. 168. 2. 2, 00: 23, Serial 0/0/0 25

Routing protocol verification: show ip protocols Routing protocol configured Timers Interfaces used in updating Advertised local networks Neighbors, sending updates Administrative distance 26

Routing protocols: distance vector vs. link state Distance vector protolols (RIP, IGRP) Link-state protocols (OSPF, IS-IS) - Copy routing tables to neighbours - Update frequently - Full copies of the routing table are sent - The route information is known from the point of view of the neighbours - Slow to converge - Susceptible to routing loops - Easy to configure and administer - Do not support subnetting - Consumes a lot of bandwidth - Collect routes from ALL the routers in the system - Triggered updates are sent only when some changes happen - Updates contain only changes - Fast to converge - Not susceptible to routing loops - Hard to configure. Require a knowledgeable administrator. - Support subnetting with varying mask length - Need more memory and processing power 27

Questions ? ? ? 28