CCDA 640-864 Official Cert Guide - Chapter 10 Summary

Routing Protocol Characteristics

• Distance-vector, link-state, or hybrid: How routes are learned
• Interior or exterior: For use in private networks or the public Internet
• Classless (classless interdomain routing [CIDR] support) or classful: CIDR enables aggregation of network advertisements (supernetting) between routers
• Fixed-length or variable-length subnet masks (VLSM): Conserve addresses within a network
• Flat or hierarchical: Addresses scalability in large internetworks
• IPv4 or IPv6: Newer routing protocols are used for IPv6 networks

IPv4 Routing Protocols	IPv6 Routing Protocols
RIPv2	RIPng
EIGRP	EIGRP for IPv6
OSPFv2	OSPFv3
IS-IS	IS-IS for IPv6
BGP	Multiprotocol BGP

Default Administrative Distances for IP Routes

Routing Loop-Prevention Schemes:

Some routing protocols employ schemes to prevent the creation of routing loops in the network. These schemes are:

• Split horizon

Split Horizon

Split horizon helps reduce convergence time by not allowing routers to advertise networks in the direction from which those networks were learned. The only information sent in RIP announcements are for those networks that are beyond the neighboring router in the opposite direction. Networks learned from the neighboring router are not included.

Split horizon eliminates count-to-infinity and routing loops during convergence in single-path internetworks and reduces the chances of count-to-infinity in multi-path internetworks. Figure 6 illustrates how split horizon keeps the RIP router from advertising routes in the direction from which they were learned.

• Poison reverse

Split Horizon with Poison Reverse

Split horizon with poison reverse differs from simple split horizon because it announces all networks. However, those networks learned in a given direction are announced with a hop count of 16, indicating that the network is unreachable. In a single-path internetwork, split horizon with poison reverse has no benefit beyond split horizon. However, in a multi-path internetwork, split horizon with poison reverse greatly reduces count-to-infinity and routing loops. Count-to-infinity can still occur in a multi-path internetwork because routes to networks can be learned from multiple sources.

In Figure 7, split horizon with poison reverse advertises learned routes as unreachable in the direction from which they are learned. Split horizon with poison reverse does have the disadvantage of additional RIP message overhead because all networks are advertised.

• Counting to infinity

The Count-to-Infinity Problem

The classic distance vector convergence problem is known as the count-to-infinity problem and is a direct result of the asynchronous announcement scheme. When RIP for IP routers add routes to their routing table, based on routes advertised by other routers, they keep only the best route in the routing table and they update a lower cost route with a higher cost route only if is being announced by the same source as the current lower cost route. In certain situations, as illustrated in Figures 1 through 5, this causes the count-to-infinity problem.

Assume that the internetwork in Figure 1 has converged. For simplicity, the announcements sent by Router 1 on Network 1 and Router 2 on Network 3 are not included.

Figure 1: Converged Internetwork

Now assume that the link from Router 2 to Network 3 fails and is sensed by Router 2. As shown in Figure 2, Router 2 changes the hop count for the route to Network 3 to indicate that it is unreachable, an infinite distance away. For RIP for IP, infinity is 16.

Figure 2: Link to Network 3 Fails

However, before Router 2 can advertise the new hop count to Network 3 in a scheduled announcement, it receives an announcement from Router 1. The Router 1 announcement contains a route to Network 3 which is 2 hops away. Because 2 hops away is a better route than 16 hops, Router 2 updates its routing table entry for Network 3, changing it from 16 hops to 3 hops, as shown in Figure 3.

Figure 3: Router 2 After Receiving Announcement From Router 1

When Router 2 announces its new routes, Router 1 notes that Network 3 is available 3 hops away through Router 2. Because the route to Network 3 on Router 1 was originally learned from Router 2, Router 1 updates its route to Network 3 to 4 hops. (See Figure 4.)

Figure 4: Router 1 After Receiving Announcement From Router 2

When Router 1 announces its new routes, Router 2 notes that Network 3 is available 4 hops away through Router 1. Because the route to Network 3 on Router 2 was originally learned from Router 1, Router 2 updates its route to Network 3 to 5 hops. (See Figure 5.)

Figure 5: Router 2 After Receiving Another Announcement from Router 1

The two routers continue to announce routes to Network 3 with higher and higher hop counts until infinity (16) is reached. Then, Network 3 is considered unreachable and the route to Network 3 is eventually timed out of the routing table. This is known as the count-to-infinity problem.

The count-to-infinity problem is one of the reasons why the maximum hop count of RIP for IP internetworks is set to 15 (16 for unreachable). Higher maximum hop count values would make the convergence time longer when count-to-infinity occurs. Also note that during the count-to-infinity in the previous example, the route from Router 1 to Network 3 is through Router 2. The route from Router 2 to Network 3 is through Router 1. A routing loop exists between Router 1 and Router 2 for Network 3 for the duration of the count-to-infinity problem.

RIPv2 Summary

The characteristics of RIPv2 follow:

• Distance-vector protocol.
• Uses UDP port 520.
• Classless protocol (support for CIDR).
• Supports VLSMs.
• Metric is router hop count.
• Low scalability: maximum hop count is 15; infinite (unreachable) routes metric of 16.
• Periodic route updates are sent every 30 seconds to multicast address 224.0.0.9.
• 25 routes per RIP message (24 if you use authentication).
• Supports authentication.
• Implements split horizon with poison reverse.
• Implements triggered updates.
• Subnet mask included in route entry.
• Administrative distance for RIPv2 is 120.
• Not scalable. Used in small, flat networks or at the edge of larger networks.

RIPng Summary

• The characteristics of RIPng are as follows:
• Distance-vector protocol for IPv6 networks only.
• Uses UDP port 521.
• Metric is router hop count.
• Maximum hop count is 15; infinite (unreachable) routes have a metric of 16.
• Periodic route updates are sent every 30 seconds to multicast address FF02::9.
• Uses IPv6 functions for authentication.
• Implements split horizon with poison reverse.
• Implements triggered updates.
• Prefix length included in route entry.
• Administrative distance for RIPv2 is 120.
• Not scalable. Used in small networks.

EIGRP for IPv4 Summary

The characteristics of EIGRP for IPv4 networks follow:

• Hybrid routing protocol (a distance-vector protocol that has link-state protocol characteristics).
• Uses IP protocol number 88.
• Classless protocol (supports VLSMs).
• Default composite metric uses bandwidth and delay.
• You can factor load and reliability into the metric.
• Sends partial route updates only when there are changes.
• Supports MD5 authentication.
• Uses DUAL for loop prevention.
• Fast convergence.
• By default, equal-cost load balancing with equal metrics. Unequal-cost load sharing with the variance command.
• Administrative distance is 90 for EIGRP internal routes, 170 for EIGRP external routes, and 5 for EIGRP summary routes.
• High scalability; used in large networks.
• Multicasts updates to 224.0.0.10.
• Does not require a hierarchical physical topology.
• Provides routing for IPv4, plus legacy protocols such as AppleTalk and IPX.

  

EIGRP for IPv6 Summary

The characteristics of EIGRP for IPv6 are as follows:

• Uses the same characteristics and functions as EIGRP for IPv4.
• Hybrid routing protocol (a distance-vector protocol that has link-state protocol characteristics).
• Uses Next Header protocol 88.
• Routes IPv6 prefixes.
• Default composite metric uses bandwidth and delay.
• You can factor load and reliability into the metric.
• Sends partial route updates only when there are changes.
• Supports EIGRP MD5 authentication.
• Uses DUAL for loop prevention and fast convergence.
• By default, equal-cost load balancing. Unequal-cost load balancing with the variance command.
• Administrative distance is 90 for EIGRP internal routes, 170 for EIGRP external routes, and 5 for EIGRP summary routes.
• Uses IPv6 multicast FF02::A for EIGRP updates.
• High scalability; used in large networks.