Interior gateway protocols (IGPs) are used to dynamically configure routers in an autonomous system (AS). Examples of IGPs are Routing Information Protocol (RIP), Cisco’s Interior Gateway Routing Protocol (IGRP), and Enhanced Interior Gateway Routing Protocol (EIGRP). Exterior gateway protocols (EGPs) are used to communicate between IGPs; one example is Border Gateway Protocol (BGP).
The three IGPs discussed in this book are RIP, IGRP, and EIGRP. To pass the CCNA exam, you must know the protocols discussed in this objective.
Before we can see the problems each routing type encounters, we must have a good understanding of routing and the three routing protocols. Let’s first take a look at routing and then the problems with each of the routing protocols. We will also look at static routes, and other techniques that can be used to resolve routing issues or can serve as alternatives to routing protocols.
Warning RIP, IGRP, and EIGRP are only briefly covered in this book. You should obtain the CCNA Study Guide, 3rd ed. (Sybex, 2002) or the CCNP: Routing Study Guide (Sybex, 2001) for a more in-depth look at these protocols
Critical Information
Dynamic routing is the process of using protocols to find and update routing tables on routers. Using protocols to do the work of mapping the network is easier than static or default routing, but it’s not always accurate. Let’s review some of the techniques used in distancevector routing protocols to overcome inconsistent routing tables and to prevent routing loops, which can cause serious problems. For the exam, you need to know the following techniques:
Split Horizon
One of the problems with early dynamic-routing protocols was the fact they were designed to advertise to every router on the network, including the routers from which they learned the same information. This created a problem of readvertising a route that no longer exists, making the router that believed the route was gone to start believing the route exists again
Route Poisoning with Poison Reverse
One of the best fixes to overcome a downed route from being readvertised is route poisoning, which sets a downed link to infinity. When a router poisons a downed route, neighboring routers are not susceptible to incorrect updates about the downed route because the information they receive back contains a maximum hop-count allocation. When a neighboring router receives a route poison, they send an update, called a poison reverse, back to the router with the downed link. This ensures that all routes on the segment have received the poisoned route information.
Hold-Downs
Hold-downs tell routers to restrict, for a specific time period, any changes that might affect recently removed routes
Additional Routing Protocols
Two protocols which you should have a good understanding for this objective are Routing Information Protocols version 2 (RIPv2) and Enhanced Interior Gateway Routing Protocol (EIGRP). Both of these are improvements from other protocols: RIPv2 is a new and improved version of RIP, and EIGRP has improvements over IGRP.
Routing Information Protocols version 2 (RIPv2)
Routing Information Protocol version 2 (RIPv2) is similar to version 1, but it enables the support of subnet masks, a critical feature that is not available in RIP. It also supports variable-length subnet masks (VLSMs), with which you can use different masks for the same network number on different interfaces. This allows you to conserve IP addresses and use available address space more efficiently.
Another advantage of using RIPv2 is the availability of support for classless interdomain routing (CIDR).
Enhanced Interior Gateway Routing Protocol (EIGRP)
Enhanced Interior Gateway Routing Protocol (EIGRP) is another Cisco proprietary distance-vector routing protocol, but it uses Diffusing Update Algorithm (DUAL) to provide a loop-free operation throughout the network using EIGRP protocol for routing. DUAL grants routers involved in a topology revision the ability to synchronize simultaneously, while routers unaffected by this change are not involved in receiving updates and other unnecessary updates. EIGRP uses both bandwidth and delay of the line by default to determine the best paths through the network. However, maximum transmission unit (MTU), load, and reliability of a link can be used as well.
EIGRP is also a classless routing protocol that sends subnet mask information in the DUAL updates. Classless routing allows for VLSMs and supernetting.
Note Routing protocols that support classless routing are RIPv2, EIGRP, and OSPF.
Verifying Your Configurations
Several commands apply to RIP and IGRP, but let’s review the commands you need to know for this objective and also those that apply to RIPv2 and EIGRP:
show ip route
Shows the routes the router knows about.
show protocols
Shows the routed protocol information configured on the router.
show ip protocol
Shows the routing protocol information configured on the router.
debug ip rip
Shows routing updates as they are sent and received on the router to the console session.
debug ip igrp events
Summarizes the IGRP routing information that is running on the network.
debug ip igrp transactions
Shows message requests from neighboring routers asking for an update and the broadcasts sent from your router toward those neighbors.
debug ip eigrp events
Summarizes the EIGRP routing information that is running on the network.
debug ip eigrp transactions
Shows message requests from neighboring EIGRP-enabled routers asking for an update and the broadcasts sent from your router toward those neighbors.
Necessary Procedures
The following lists the router command procedures to configure both RIPv2 and EIGRP routing protocols.
Configuring RIPv2
To configure RIPv2, just turn on the protocol with the router rip command, and then use the version 2 command:
2621A(config)#router rip
2621A(config)#version 2
2621A(config-router)#network 172.16.0.0
2621A(config-router)#^Z
Configuring EIGRP Routing
The command used to configure EIGRP is the same as the one used to configure RIP routing. As with IGRP, you still use an AS number for EIGRP. Here is an example of how to
turn on EIGRP routing, using an AS number of 10:
RouterA#config t
RouterA(config)#router eigrp 10
RouterA(config-router)#network 172.16.0.0
Related posts:
- Problems in Routing Topology
- IGRP Routing Protocol
- RIP Routing Protocol
- Initial Configuration in Router Using the Setup Command
- Commands to Monitor Frame Relay Operation in the Router
- TCP-IP Network-layer protocol
- Benefits of Network Segmentation With Routers
- Manage Router Configuration Files from the Privileged EXEC Mode
- Function of the MAC Address
- Commands to Configure Frame Relay LMIs, Maps, and Subinterfaces
