Introduction to Routing & Routing Protocol 1

Agenda – - Router Operations – - Static Route – - Default Route – - Dynamic Route – - Class of Dynamic Routing Protocol – Administrative Distance – - Best Route Selection – - Distance Vector Protocol – - Link State Routing Protocol – - RIP V1 & V2 – - RIP Configuration – - Structured approach for Troubleshooting 2

Router Operations :- Routing is the process by which items get from one location to another. In networking, a router is the device used to route traffic. Routers can forward packets over static routes or dynamic routes, based on the router configuration. A router needs to do the following: – Know the destination address. – Identify the sources from which the router can learn. – Discover possible routes to the intended destination. – Select the best route. – Maintain and verify routing information.

 Routers must learn destinations that are not directly connected. Router Operations (Cont.)

Static Routes :- Static routers use a route that a network administrator enters into the router manually to reach the next hop. Configure unidirectional static routes to and from a stub network to allow communications to occur.

Static Route Configuration – Defines a path to an IP destination network or subnet or host – Address = IP address of the next hop router – Interface = outbound interface of the local router RouterX(config)# ip route network [mask] {address | interface}[distance] [permanent]

Static Route Example  This is a unidirectional route. You must have a route configured in the opposite direction. RouterA(config)# ip route RouterA(config)#ip route s0/0/0 or

Verifying the Static Route Configuration RouterA# show ip route Codes: C - connected, S - static, I - IGRP, R - RIP, M - mobile, B - BGP D - EIGRP, EX - EIGRP external, O - OSPF, IA - OSPF inter area E1 - OSPF external type 1, E2 - OSPF external type 2, E - EGP i - IS-IS, L1 - IS-IS level-1, L2 - IS-IS level-2, * - candidate default U - per-user static route Gateway of last resort is to network /8 is subnetted, 1 subnets C is directly connected, Fastethernet 0/0 S* /0 is directly connected, Serial0/0/0

Configuring a Default Route The ip default-network command establishes a default route in networks using dynamic routing protocols. – Router(config-router)#ip default-network network-number Creating an ip route to /0 is another way to configure a default route. – Router(config)#ip route [next-hop-ip-address | exit- interface]

Default Routes  This route allows the stub network to reach all known networks beyond Router A.

Configuring Default Route By default, routers learn paths to destinations in three different ways: Static routes: manually defines the static routes as next hop to destination. Default routes: manually defines default routes as path when there is no known route to destination. Dynamic routes: router learns the path by receiving periodic updates from other routers. The ip default-network command is usually configured on the routers that connect to a router with a static default route. HongKong1(config)#ip route

Static vs. Dynamic Routes Static Route – Uses a route that a network administrator enters into the router manually Dynamic Route – Uses a route that a network routing protocol adjusts automatically for topology or traffic changes

What Is a Dynamic Routing Protocol?  Routing protocols are used between routers to determine paths to remote networks and maintain those networks in the routing tables.  After the path is determined, a router can route a routed protocol to the learned networks.

 An autonomous system is a collection of networks within a common administrative domain.  Interior gateway protocols operate within an autonomous system.  Exterior gateway protocols connect different autonomous systems. Autonomous Systems: Interior and Exterior Routing Protocols

Classes of Routing Protocols

Classful Routing Protocol – Classful routing protocols do not include the subnet mask with the route advertisement. – Within the same network, consistency of the subnet masks is assumed. – Summary routes are exchanged between foreign networks. – These are examples of classful routing protocols: RIPv1 IGRP

Classless Routing Protocol – Classless routing protocols include the subnet mask with the route advertisement. – Classless routing protocols support a variable- length subnet mask (VLSM). – Summary routes can be manually controlled within the network. – These are examples of classless routing protocols: RIPv2 EIGRP OSPF IS-IS

Administrative Distance :- is used to rate the trustworthiness of the routing protocol. - Administrative distance is the feature that routers use in order to select the best path when there are multiple routes to the same destination from different routing protocols. - Administrative distance defines the reliability of a routing protocol. - Each routing protocol is prioritized in order of most to least reliable (believable) with the help of an administrative distance value.Lower the AD vlaue higher is the reliability of that routing protocol. 18

19 Default Distance Value Table Route SourceDefault Distance Values Connected interface0 Static route1 Enhanced Interior Gateway Routing Protocol (EIGRP) summary route 5 External Border Gateway Protocol (BGP) 20 Internal EIGRP90 IGRP100 OSPF110 Intermediate System-to- Intermediate System (IS-IS) 115 Routing Information Protocol (RIP) 120 Exterior Gateway Protocol (EGP)140 On Demand Routing (ODR)160 External EIGRP170 Internal BGP200 Unknown*255

Administrative Distance: Ranking Routing Sources Routers choose the routing source with the best administrative distance:  OSPF has an administrative distance of 110.  EIGRP has an administrative distance of 90.

Administrative Distance: Ranking Routes

Selecting the Best Route Using Metrics Routing protocols use metrics to determine the best route to a destination if there are multiple route to that destination from same Routing Protocol.

Determining the Route Next Hop Destination/next hop associations tell a router that a particular destination can be reached optimally by sending the packet to a particular router.

Distance Vector Routing Protocols Routers pass periodic copies of their routing table to neighboring routers and accumulate distance vectors.

Sources of Information and Discovering Routes Routers discover the best path to destinations from each neighbor.

Maintaining Routing Information Updates proceed step by step from router to router.

Inconsistent Routing Entries: Counting to Infinity and Routing Loops Each node maintains the distance from itself to each possible destination network.

Counting to Infinity Slow convergence produces inconsistent routing.

Counting to Infinity (Cont.) Router C concludes that the best path to network is through router B.

Counting to Infinity (Cont.) Router A updates its table to reflect the new but erroneous hop count.

Counting to Infinity (Cont.) The hop count for network counts to infinity.

Solution to Counting to Infinity: Defining a Maximum A limit is set on the number of hops to prevent infinite loops.

Routing Loops Packets for network bounce (loop) between routers B and C.

Solution to Routing Loops: Split Horizon It is never useful to send information about a route back in the direction from which the original information came.

Solution to Routing Loops: Route Poisoning and Poison Reverse Routers advertise the distance of routes that have gone down to infinity.

Solution to Routing Loops: Route Poisoning and Poison Reverse (Cont.) Poison reverse overrides split horizon.

Solution to Routing Loops: Hold-Down Timers The router keeps an entry for the “possibly down” state in the network, allowing time for other routers to recompute for this topology change.

Triggered Updates The router sends updates when a change in its routing table occurs.

Eliminating Routing Loops

Eliminating Routing Loops (Cont.)

Link-State Routing Protocols After an initial flood of LSAs, link-state routers pass small, event-triggered link-state updates to all other routers.

OSPF Hierarchical Routing  Consists of areas and autonomous systems  Minimizes routing update traffic

Link-State Routing Protocol Algorithms

Benefits and Drawbacks of Link-State Routing – Benefits of link-state routing: Fast convergence: – Changes are reported immediately by the affected source Robustness against routing loops: – Routers know the topology – Link-state packets are sequenced and acknowledged Hierarchical network design enables optimization of resources. – Drawbacks of link-state routing: Significant demands for resources: – Memory (three tables: adjacency, topology, forwarding) – CPU (Dijkstra’s algorithm can be intensive, especially when there are many instabilities) Requires very strict network design Configuration can be complex when tuning various parameters and when design is complex

RIP Overview – is one of the oldest distance-vector routing protocols – Use Hop-count as routing metric selects the path. – prevents routing loops by implementing a limit on the number of hops allowed in a path from the source to a destination. – The maximum number of hops allowed for RIP is 15.Hop 16 is consider unreachable. – So this max hop count limit the size of network that RIP can support. – RIP implements the split horizon, route poisoning and hold-down mechanisms to prevent incorrect routing information from being propagated – Routes update every 30 seconds, Routing Information Protocol send complete routing table every 30 sec. to all its neighboring devices. – This cause a lot of bandwidth utilize on these update flooding. Even If there is no change in routing information even then these updates are flooded on the network. – As the network size grow there would be massive traffic burst every 30 sec. – This cause high latency & slow convergence of Routing Table. – AD value is 120, which is least preferred among Dynamic Routing Protocol.

Split Horizon :- split-horizon route advertisement is a method of preventing routing loops in distance-vector routing protocols by prohibiting a router from advertising a route back onto the interface from which it was learned. Thus when a device that participates in such route advertisements receives an update from an interface, it (the device) does not forward updates through the same interface. By doing so, routing loops are prevented. Route Poisoning :- Distance-vector routing protocols in computer networks use route poisoning to indicate to other routers that a route is no longer reachable and should not be considered from their routing tables. RIP, use a maximum hop count to determine how many routers the traffic must go through to reach the destination. Each route has a hop count number assigned to it which is incremented as the routing information is passed from router to router. A route is considered unreachable if the hop count exceeds the maximum allowed. Max limit is 15 hop, 16 is consider unreachable.RIP Hold Time :- A router will wait for a particular amount of time before considering a route unreachable, That time is called Hold time for which router wait before flush the route, for RIP Holdtimer is 180 second.

RIPv1 has the following limitations :- – It does not send subnet mask information in its updates. – It sends updates as broadcasts on – It does not support authentication. – It is not able to support VLSM or classless interdomain routing (CIDR).

RIPv2 Features

RIPv1 and RIPv2 Comparison RIPv1RIPv2 Routing protocolClassfulClassless Supports variable-length subnet mask?NoYes Sends the subnet mask along with the routing update?NoYes Addressing typeBroadcast Multicast Routing updates over Defined in …RFC 1058 RFCs 1721, 1722, and 2453 Supports manual route summarization?NoYes Authentication support?NoYes

IP Routing Configuration Tasks – Router configuration –Select routing protocols –Specify networks or interfaces

RIP Configuration – Starts the RIP routing process RouterX(config)# router rip RouterX(config-router)# network network-number  Selects participating attached networks  Requires a major classful network number  Enables RIP version 2 RouterX(config-router)# version 2

RIP Configuration Example

Verifying the RIP Configuration Routing Protocol is "rip" Sending updates every 30 seconds, next due in 6 seconds Invalid after 180 seconds, hold down 180, flushed after 240 Invalid after 180 seconds, hold down 180, flushed after 240 Outgoing update filter list for all interfaces is not set Incoming update filter list for all interfaces is not set Redistributing: rip send version 2, receive version 2 Default version control: send version 2, receive version 2 Interface Send Recv Triggered RIP Key-chain FastEthernet0/0 2 2 Serial0/0/2 2 2 Automatic network summarization is in effect Maximum path: 4 Routing for Networks: Routing Information Sources: Gateway Distance Last Update :00:25 Distance: (default is 120) RouterA#

Displaying the IP Routing Table

Troubleshooting RIPv2 The debug ip rip Command

Summary – Routing is the process by which items get from one location to another. – Dynamic routing protocols determine how updates are conveyed, what knowledge is conveyed, when to convey knowledge, and how to locate recipients of the updates. – A routing protocol that has a lower administrative value is more trustworthy than a protocol that has a higher administrative value. – There are three classes of routing protocols: distance vector, link-state, and balanced hybrid. – The ip classless command can be used to prevent a router from dropping a packet that is destined for an unknown subnetwork of a directly attached network if a default route is configured.

Summary (Cont.) – RIP is a distance vector routing protocol that uses hop count as the matrix for route selection and broadcasts updates every 30 seconds. – RIPv1 uses classful routing protocol; RIPv2 uses classless routing protocol. RIPv2 supports VLSM, manual route summarization, and authentication; RIPv1 does not support these activities. – To enable a dynamic routing protocol, first a routing protocol is selected, then IP network numbers are assigned without values being specified (except OSPF). – The router command starts the routing process. The network command allows the routing process to determine which interfaces will participate in sending and receiving the routing updates.

Summary (Cont.) – The router RIP command selects RIP as the routing protocol. The network command identifies a participating attached network. – The show ip command displays information about routing protocols and the routing table. – The debug ip rip command displays information on RIP routing transactions.

Structured Approach to Troubleshooting

Typical Layer 1 Errors Broken cables Disconnected cables Cables connected to the wrong ports Intermittent cable connection Wrong cables used for the task at hand Transceiver problems DCE cable problems DTE cable problems Devices turned off

Typical Layer 2 Errors Improperly configured serial interfaces Improperly configured Ethernet interfaces Improper encapsulation set Improper clock rate settings on serial interfaces Network interface card (NIC) problems

Typical Layer 3 Errors Routing protocol not enabled Wrong routing protocol enabled Incorrect IP addresses Incorrect subnet masks

Layer 3 Troubleshooting Using Ping

Layer 7 Troubleshooting Using Telnet

Troubleshooting Layer 1 Using show interfaces Command The show interfaces serial command

Troubleshooting Using show cdp neighbors Command

Troubleshooting Using show cdp neighbors detail Command

Troubleshooting Using traceroute Command

Troubleshooting Routing Issues The show ip route Command

Troubleshooting Routing Issues The show ip protocols Command

Troubleshooting Using show controllers serial Command The show controllers serial Command

Introduction to debug Debug syntax