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Routing/Routed Protocols

Routing/Routed Protocols. Part I. Routed Protocol Definition:. Routed Protocol – used to transmit user data (packets) through an internetwork. Routed protocols are assigned to an interface and determine the method of packet delivery. Examples: IP, IPX, AppleTalk, DECNet, Banyan Vines.

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Routing/Routed Protocols

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  1. Routing/Routed Protocols Part I

  2. Routed Protocol Definition: • Routed Protocol – used to transmit user data (packets) through an internetwork. Routed protocols are assigned to an interface and determine the method of packet delivery. • Examples: IP, IPX, AppleTalk, DECNet, Banyan Vines

  3. Routing Protocol Definition: • Routing protocol – any protocol that defines algorithms to be used for updating routing tables between routers. Basically, a routing protocol determines the path of a packet through an internetwork. • Examples: RIP, RIPv2, IGRP, EIGRP, OSPF, IS-IS, BGP

  4. Remember: • A Routed Protocol – defines logical addressing. Most notable example on the test – IP • A Routing Protocol – fills the routing table with routing information. Examples on the test – RIP, IGRP, EIGRP, OSPF, IS-IS

  5. CCNA Exam Tips -- Routing • Since IP routing is basically what Cisco routers do, this protocol is the backdrop for the whole CCNA exam. See prior chapter notes on “IP Addressing” for more info. • Next, some routing protocols that are strongly rumored to be on the CCNA 801 will be outlined. • But first, some generalities about routing protocols…

  6. Main Goals of Routing Protocols • To fill the routing table with current best, loop-free routes • To notice when routes in the table are no longer valid and remove them from the routing table • To add new routes or replace lost routes • The time for finding a working route is called convergence.

  7. Two Categories of Routing Protocols • Exterior Routing Protocols – used for use between different organizations such as ISPs or ISPs and their customers. • Ex: Border Gateway Protocol (BGP) • Interior Routing Protocols – used to distribute routing information inside a single organization. • Ex: RIP, IGRP, EIGRP, OSPF, IS-IS

  8. Border Gateway Protocol (BGP) • The most popular exterior routing protocol & the only one on the CCNA 801 exam • ISPs use BGP to exchange routing info between themselves and other ISPs and customers. • BGP advertises only routing info to specifically defined peers using TCP. • BGP does not use a metric like internal routing protocols

  9. Terminology of Interior Routing Protocols This is not as painful as it sounds. There are only 6 basic concepts.

  10. TYPE of routing protocol • Each interior routing protocol can be characterized based on the underlying logic used by the routing protocol. • The underlying logic is referred to as the TYPE of routing protocol. • The three types are: • Distance vector • Link-state • Hybrid

  11. Full/partial Update • Full routing updates – entire routing tables are sent regularly • Partial routing updates – only a subset of the routing table is sent, typically just information about changed routes. • Partial routing updates require less overhead than full routing updates.

  12. Convergence • Convergence refers to the time required for routers to react to changes in the network.

  13. Metric • The metric refers to the numeric value that describes how good a particular route is. • The lower the value, the better the route.

  14. Support for VLSM • Variable-length subnet masking (VLSM) means that, in a single Class A, B, or C network, multiple subnet masks can be used. • The advantage of VLSM is that it enables you to vary the size of each subnet, based on the needs of that subnet. • Some routing protocols support VLSM, and some do not

  15. Classless or Classful • Classless routing protocols transmit the subnet mask along with each route in the routing updates sent by that protocol. • Classful routing protocols do not transmit mask information. • Only classless routing protocols support VLSM. To say that a routing protocol is classless is to say that it supports VLSM.

  16. Now we apply those terms to some interior routing protocols. .

  17. Distance Vector Protocols: RIP and IGRP • Distance vector protocols advertise routing information by sending messages, called routing updates, out the interfaces on a router. • These updates contain a series of entries, with each entry representing a subnet and a metric. • Failure to receive updates from a neighbor in a timely manner results in the removal of the routes previously learned from that neighbor.

  18. Distance Vector Protocols: RIP and IGRP • Routers send periodic full updates and expect to receive periodic updates from neighboring routers. • When possible, routers use broadcasts or multicasts to send routing updates. This way, all neighbors on a LAN can receive the same routing information in a single update. • If a router learns multiple routes to the same subnet, the router chooses the best route based on the metric.

  19. Routing Information Protocol (RIP) • Been around 15+ years for use with IP networks. • Easier to use than some newer routing protocols, but severely limited in comparison.

  20. Basic RIP Summary • Based on distance vector logic • Uses hop count for the metric • Hop count = number of routers between two points • Sends periodic full routing updates every 30 seconds • Converges slowly, often taking 3 to 5 minutes • Does not support VLSM, also making it a classful routing protocol

  21. Interior Gateway Routing Protocol (IGRP) • IGRP is a Cisco-proprietary IP routing protocol created to provide a better distance vector protocol. • The most obvious difference between RIP-1and IGRP is the metric. • IGRP advertises up to five parameters that describe the metric for each route, although, by default only two are used – bandwidth and delay.

  22. IGRP • Other three possible parameters used to describe IGRP metric can include: reliability, load, and MTU (maximum transmission unit). • IGRP calculates the metric based on a mathematical formula that “you do not really need to know for the exam.” (Wendell Odom, CCNA INTRO, p.415)

  23. Distance Vector Protocols

  24. Link-State Protocols: OSPF and Integrated IS-IS • The goal of link-state protocols is to fill the routing tables with the current best routes. • Link-state advertises a large amount of topological info about the network • Discovers neighbor routers before exchanging routing information. • A router running a link-state protocol uses more memory and more processing cycles than do distance vector protocols.

  25. Link-State Protocols: OSPF and Integrated IS-IS • To figure out the current best routes, a router processes the link-state topology database using an algorithm called the Dijkstra Shortest Path First (SPF) algorithm. • This info helps link-state protocols avoid loops & converge quickly. • Quick convergence – often less than 10 seconds.

  26. Open Shortest Path First (OSPF) • OSPF is the most popular link-state IP routing protocol today. • Because OSPF does not send full updates on a regular short interval (like RIP), the overall number of bytes sent for routing information is typically smaller.

  27. Open Shortest Path First (OSPF) • OSPF uses a concept called cost for the metric. Each link is considered to have a cost; a route’s cost is the sum of the cost for each link. • By default, Cisco derives the cost value for a link from the bandwidth. • OSPF supports VLSM.

  28. Integrated IS-IS • OSI defines a network layer protocol called the Connectionless Network Protocol (CLNP). It also defines a routing protocol – a routing protocol used to advertise CLNP routes, called Intermediate System-to-Intermediate System (IS-IS). IS-IS advertises CLNP routes between “intermediate systems,” which is what OSI calls routers.

  29. Integrated IS-IS • Integrated IS-IS has the capability to advertise IP routes as well as CLNP routes. • “…most installations could care less about CLNP.” (Wendell Odom, CCNA INTRO, p.419) • Supports VLSM

  30. Balanced Hybrid Protocols: Enhanced IGRP • EIGRP uses features similar to link-state protocols, and others similar to distance vector protocols, and yet others unlike either of the two. • The internal workings of EIGRP depend on an algorithm called the Diffusing Update Algorithm (DUAL). • Requires less processing than the Dijkstra SPF algorithm.

  31. EIGRP Summary • A balanced hybrid protocol • Converges in less than 3 seconds • Discovers neighbors (via Hello packets) before sending them information. • Requires little design effort • Supports VLSM • Cisco proprietary • Metric based on bandwidth & delay, scaled by multiples of 256.

  32. That’s IT Next week, May 3, to be discussed: -- Routing Protocol Configuration Commands -- Routing Protocol Logic -- VLSM

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