Routing Basics The term routing is used for taking a packet from one device and sending it through the network to another device on a different network. To be able to route packets, a router must know, at a minimum, the following: Destination address Neighbor routers from which it can learn about remote networks Possible routes to all remote networks The best route to each remote network How to maintain and verify routing information
Configuring StaticIP Routing Static routing has the following benefits: 1. There is no overhead on the router CPU, which means you could possibly buy a cheaper router than if you were using dynamic routing. 2. There is no bandwidth usage between routers, which means you could possibly save money on WAN links. 3. It adds security, because the administrator can choose to allow routing access to certain networks only. Static routing has the following disadvantages: 1. The administrator must really understand the internetwork and how each router is connected in order to configure routes correctly. 2. If a network is added to the internetwork, the administrator has to add a route to it on all routers—by hand. 3. It’s not feasible in large networks because maintaining it would be a full-time job in itself.
Static routing Static Routes built by using IP route command
Default Routing Default routes are used to route packets with destinations that do not match any of the other routes in the routing table. Routers are typically configured with a default route for Internet-bound traffic, since it is often impractical and unnecessary to maintain routes to all networks in the Internet. A default route is actually a special static route that uses this format: ip route [next-hop-address]
Verifying static route configuration it is important to verify that they are present in the routing table and that routing is working as expected. The command show run is used to view the active configuration in RAM to verify that the static route was entered correctly. Router#sh run The show ip route command is used to make sure that the static route is present in the routing table. Router#sh ip route
Troubleshooting static route If any problem in connection between networks follow this steps: 1. verifying static routes (by using sh ip route) to make sure the route found in routing table. 2. If the problem exists, run a traceroute command.
Dynamic Routing Overview Routing protocols Routed protocol Autonomous systems Purpose of a routing protocol and autonomous systems Identifying the classes of routing protocols
Routing protocols It is the communication used between routers. It allows one router to share information with other routers regarding the networks. The information a router gets from another router, using a routing protocol, is used to build and maintain a routing table. Examples of routing protocols are: RIP, IGRP, EIGRP and OSPF
Routed protocol It is used to direct user traffic. It provides enough information in its network layer address to allow a packet to be forwarded from one host to another based on the addressing scheme. Examples of routed protocols are: IP and IPX.
Autonomous systems An autonomous system (AS) is a collection of networks under a common administration sharing a common routing strategy. To the outside world, an AS is viewed as a single entity. The AS may be run by one or more operators while presenting a consistent view of routing to the external world.
Purpose of a routing protocol and autonomous systems The goal of a routing protocol is to build and maintain the routing table. Routers use routing protocols to manage information received from other routers. Autonomous systems (AS) provide the division of the global internetwork into smaller and more manageable networks. Each AS has its own set of rules and policies and an AS number that will uniquely distinguish it from other autonomous systems throughout the world.
The Types of routing protocols Most routing algorithms can be classified into one of two categories: 1. Distance Vector The distance vector routing approach determines the direction (vector) and distance to any link in the internetwork. As an example: RIP and IGRP. 2. Link State The link-state approach, also called shortest path first, recreates the exact topology of the entire internetwork. As an example: OSPF. 3. Hybrid Mixed between distance vector and link state As an example: EIGRP.
Distance Vector Routing Distance vector routing updates Distance vector routing loop issues Defining a maximum count Elimination routing loops through split horizon Route poisoning Preventing routing loops with holddown timers
Distance vector routing updates Routing table updates occur periodically or when the topology in a distance vector protocol network changes. Distance vector algorithms call for each router to send its entire routing table to each of its adjacent neighbors. The administrative distance (AD) is used to rate the trustworthiness of routing information received on a router from a neighbor router. An administrative distance is an integer from 0 to 255, where 0 is the most trusted and 255 means no traffic will be passed via this route. If a router receives two updates listing the same remote network, the first thing the router checks is the AD. If one of the advertised routes has a lower AD than the other, then the route with the lowest AD will be placed in the routing table. If both advertised routes to the same network have the same AD, then routing protocol metrics will be used to find the best path to the remote network. The routing tables include information about the total path cost as defined by the metrics.
Distance vector routing loop issues
Routing loops can occur when inconsistent routing tables are not updated due to slow convergence in a changing network. Just before the failure of Network 1, all routers have consistent knowledge and correct routing tables. When Network 1 fails, Router E sends an update to Router A. Router A stops routing packets to Network 1, but Routers B, C, and D continue to do so because they have not yet been informed of the failure. When Router A sends out its update, Routers B and D stop routing to Network 1. However, Router C has not received an update, so Network 1 is still reachable via Router B. Now Router C sends a periodic update to Router D, indicating a path to Network 1 by way of Router B. Router D changes its routing table to reflect this good, but incorrect, information, and propagates the information to Router A. Router A propagates the information to Routers B and E, and so on. Any packet destined for Network 1 will now loop from Router C to B to A to D and back to again to C.
Defining a maximum count The invalid updates of Network 1 will continue to loop until some other process stops the looping. This condition, called count to infinity
Defining a maximum count
Elimination loops by split horizon Split horizon reduces incorrect routing information and routing overhead in a distance-vector network by enforcing the rule that routing information cannot be sent back in the direction from which it was received.
Route poisoning Route poisoning is used to overcome large routing loops and offer explicit information when a subnet or network is not accessible. This is usually accomplished by setting the hop count to one more than the maximum.
Holddown Timers A holddown prevents regular update messages from reinstating a route that is going up and down (called flapping).
Routing Information Protocol Characteristics. RIP Timers. Configuring RIP Routing. Verifying the RIP Routing Tables. Holding Down RIP Propagations.
RIP Characteristics Vector Distance Protocol. Open standard protocol. RIP works well in small networks. Default AD (administrative distance) is 120. Hop count is (number of routers) used as Metric for path selection. Maximum Hop count is 15. RIP v2RIP v1 Classless RoutingClassful Routing Support for VLSMNo support for VLSM Support for discontiguous networks No support for discontiguous networks Use broadcast or multicast to deliver routing updates. Use broadcast to deliver routing updates.
RIP Timers Route update timer: Sets the interval (typically 30 seconds) between periodic routing updates, in which the router sends a complete copy of its routing table out to all neighbors. Route invalid timer: Determines the length of time that must elapse (180 seconds) before a router determines that a route has become invalid. Route flush timer: Sets the time between a route becoming invalid and its removal from the routing table (240 seconds). Holddown timer: routes will enter into the holddown state when an update packet is received that indicated the route is unreachable. The default is 180 seconds.
Configuring RIP Routing
Verifying the RIP Routing Tables Two of the most common commands are the show ip route command and the show ip protocols command. The show ip protocols command shows which routing protocols are carrying IP traffic on the router. Another command use to for troubleshooting, debug ip rip. To cancel this opertaon, use this command undebug all.
Holding Down RIP Propagations You probably don’t want your RIP network advertised everywhere on your LAN and WAN. The easiest way is through the passive-interface command. This command prevents RIP update broadcasts from being sent out a defined interface, but that same interface can still receive RIP updates. Router#config t Router(config)#router rip Router(config-router)#network Router(config-router)#passive-interface s0/0
IGRP Characteristics Vector Distance Protocol. Cisco-proprietary protocol. IGRP can works well in large networks. Default AD (administrative distance) is 100. Uses an autonomous system number. Uses bandwidth and delay of the line as metric. Maximum Hop count is 255. Classful Routing. Don’t support VLSM and discontiguous networks.
IGRP Timers Update timers These specify how frequently routing-update messages should be sent. The default is 90 seconds. Invalid timers These specify how long a router should wait before declaring a route invalid if it doesn’t receive a specific update about it. The default is three times the update period (270 sec). Flush timers The default is seven times the routing update period. If the update timer is 90 seconds by default, then 7 90 = 630 seconds elapse before a route will be flushed from the route table. Holddown timers These specify the holddown period. The default is three times the update timer period plus 10 seconds (280 sec).
Configuring IGRP routing The command used to configure IGRP is the same as the one used to configure RIP routing with one important difference: you use an autonomous system (AS) number. Router#config t Router(config)#router igrp 10 Router(config-router)#net To Delete routing table built by IGRP, use this command. Router(config)# no router igrp 10
Verifying IGRP routing table To verify that IGRP has been configured properly, enter the show ip route command and look for IGRP routes signified by an "I". Additional command show ip protocols debug ip igrp events debug ip igrp transactions