Section 4.2: Routing Protocols

There are two types of dynamic routing protocols: Interior Gateway Protocols (IGP) and External Gateway Protocols (EGP). IGPs are used to exchange routing information within an autonomous system (AS), which is a collection of routing domains under the same administrative control the same routing domain. An EGP, on the other hand, is used to exchange routing information between different ASs.

IGPs can be broken into two classes: distance-vector and link-state, and can also be broken into two categories: classful routing protocols and classless routing protocols.

4.2.1: Distance-Vector Routing

Distance-vector routing is consists of two parts: distance and vector. Distance is the measure of how far it is to reach the destination and vector is the direction the packet must travel to reach that destination. The latter is determined by the next hop of the path. Distance-vector routing protocols will learn routes from its neighbors. This is called routing by rumor. Examples of distance-vector routing protocols are: Routing Information Protocol (RIP), Interior Gateway Routing Protocol (IGRP), and Enhanced Interior Gateway Routing Protocol (EIGRP).

4.2.1.1: Route Poisoning

Route poisoning is a feature that distance vector protocols use to reduce the chance of routing loops. Route poisoning begins when a router notices that a connected route is no longer valid. The router then advertises that route out all its interfaces and with a very large metric so that other routers consider the metric infinite and the route invalid. However, route poisoning does not solve the counting-to-infinity problem.

4.2.1.2: Split Horizon

As mentioned earlier, route poisoning does not solve the counting-to-infinity problem. Counting-to-infinity can occur when one router has a valid metric that points to an address that is reachable through an intermediate router while the intermediate router has an infinite-distance route to the same address (see Figure 4.1). If routing table updates are sent by both routers at the same in time, the intermediate router will advertise that the route to the destination address is an infinite-distance route while the other router will advertise that the route has a valid metric. Because the two routers use the same update interval between updates, this process repeats itself with the next routing update, with the difference that the valid metric will be incremented by 1 each time until an infinite metric is reached, hence this phenomenon is called counting to infinity.

Figure 4.1: Count To Infinity

Split horizon solves the counting-to-infinity problem by preventing a router from sending routing updates out the same interface on which it learnt the route. Thus, in Figure 4.1, the router would have learnt the route to the destination address across the link from the intermediate router. With split horizon, that router cannot then send advertisements about the route to the destination address out across the same link. Therefore the intermediate router will not receive the valid metric from the route to the destination address from the other router ad the count to infinity problem will not occur, solving the count-to-infinity problem on a single link.

4.2.1.3: Split Horizon with Poison Reverse

Split horizon with poison reverse, or simply poison reverse combines the two features. When a route fails the router uses route poisoning, i.e., the router advertises an infinite-metric route about that subnet out all interfaces, including interfaces previously prevented by split horizon. This ensures that all routers know for sure that the route has failed, while split horizon prevents counting to infinity.

4.2.1.4: Hold-Down Timer

Split horizon solves the counting-to-infinity problem over a single link but the counting to infinity problem can also occur in networks with multiple or redundant paths because there are more than one path to a router. In such networks, the hold-down timer feature prevents the counting-to-infinity problem.

With the Hold-down timer feature, a router ignores any information about an alternative route to a destination address for a time equal to the hold-down timer once it has learnt that a route to the destination address has failed.

4.2.1.5: Triggered Updates

Distance vector protocols typically send updates based on a regular update interval. However, most looping problems occur when a route fails. Therefore, some distance vector protocols send triggered updates as soon as a route fails. This causes the information about the route whose status has changed to be forwarded more quickly and also starts the hold-down timers more quickly on the neighboring routers.

4.2.2: Link-State Routing

Link-state routing differs from distance-vector routing in that each router knows the exact topology of the network. This reduces the number of bad routing decisions that can be made because every router in the process has an identical view of the network. Each router in the network will report on its state, the directly connected links, and the state of each link. The router will then propagate this information to all routers in the network. Each router that receives this information will take a snapshot of the information. This ensures all routers in the process have the same view of the network, allowing each router to make its own routing decisions based upon the same information.

In addition, link-state routing protocols generate routing updates only when there is a change in the network topology. When a link, i.e., a point on a route, changes state, a link-state advertisement (LSA) concerning that link is created by the device that detected the change and propagated to all neighboring devices using a multicast address. Each routing device takes a copy of the LSA, updates its topological database and forwards the LSA to all neighboring devices. An LSA is generated for each link on a router. Each LSA will include an identifier for the link, the state of the link, and a metric for the link. With the use of LSAs, link-state protocols reduces routing bandwidth usage.

Examples of link-state routing protocols are: Open Shortest Path First (OSPF) and Integrated Intermediate System to Intermediate System (IS-IS). Another protocol, Enhanced Interior Gateway Routing Protocol (EIGRP) is considered a hybrid protocol because it contains traits of both distance-vector and link-state routing protocols. Most link-state routing protocols require a hierarchical design, especially to support proper address summarization. The hierarchical approach, such as creating multiple logical areas for OSPF, reduces the need to flood an LSA to all devices in the routing domain. The use of areas restricts the flooding to the logical boundary of the area rather than to all devices in the OSPF domain. In other words, a change in one area should only cause routing table recalculation in that area, not in the entire domain.

OSPF is discussed in more detail in Section 5 and EIGRP is discussed in more detail in Section 6. IS-IS is not covered in the CCNA 640-822 exam and is thus not discussed in this Study Guide.

4.2.3: Classful Routing

Classful routing is used in routing packets based upon the class of IP address. IP addresses are divided into five classes: Class A, Class B, Class C, Class D, and Class E. Class A, Class B and Class C are used to private and public network addressing; Class D is used for multicast broadcasting; and Class E is reserved by the Internet Assigned Numbers Authority (IANA) for future use. IP Address classes were discussed in detail in Section 3.1.3.

Classful routing is a consequence of the fact that routing masks are not advertised in the periodic, routine, routing advertisements generated by distance vector routing protocols. In a classful environment, the receiving device must know the routing mask associated with any advertised subnets or those subnets cannot be advertised to it. There are two ways this information can be gained:

Share the same routing mask as the advertising device
If the routing mask does not match, this device must summarize the received route a classful boundary and send the default routing mask in its own advertisements.

Classful routing protocols, such as Routing Information Protocol version 1 (RIPv1) and Interior Gateway Routing Protocol (IGRP), exchange routes to subnetworks within the same network if network administrator configured all of the subntworks in the major network have the same routing mask. When routes are exchanged with foreign networks, subnetwork information from this network cannot be included because the routing mask of the other network is not known. As a result, the subnetwork information from this network must be summarized to a classful boundary using a default routing mask prior to inclusion in the routing update. The creation of a classful summary route at major network boundaries is handled automatically by classful routing protocols. However, summarization at other points within the major network address is not allowed by classful routing protocols.

4.2.4: Classless Routing

One of the most serious limitations in a classful network environment is that the routing mask is not exchanged during the routing update process. This requires the same routing mask be used on all subnetworks. The classless approach advertises the routing mask for each route and therefore a more precise lookup can be performed in the routing table. Classless routing, which is also known as Classless Interdomain Routing (CIDR), is thus not dependent on IP address classes but, instead, allows a variable-length subnet mask (VLSM), which extends IP addressing beyond the limitations of using fixed-length subnet masks (FLSM),to be sent in the routing update with the route. This allows you to conserve IP addresses, extending the use of IP addresses. Classless routing protocols also addressed the need to summarize to a classful network with a default routing mask at major network boundaries. In the classless environment, the summarization process is manually controlled and can be invoked at any point within the network. VLSM was discussed in Section 3.1.5.

The routing protocols that support classless routing protocols are: Routing Information Protocol version 2 (RIPv2) ; Enhanced Interior Gateway Routing Protocol (EIGRP); Open Shortest Path First (OSPF) ; and Integrated Intermediate System to Intermediate System (IS-IS).