CompTIA Network+ N10-006 Cert Guide (2015)

Similarly, router R2 can reach the Internet by sending traffic out of its Serial 1/0 interface. However, router R2 does need information about how to reach the 192.168.1.0/24 network available off of router R1. To educate router R2 as to how this network can be reached, a static route, pointing to 192.168.1.0/24, can be statically added to router R2’s routing table.

Dynamic Routing Protocols

If you want to add routing information to routers in more complex networks, such as the topology shown in Figure 6-7, static routing does not scale well. Fortunately, a variety of dynamic routing protocols are available that allow a router’s routing table to be updated as network conditions change.

Figure 6-7 Dynamic Routes

In Figure 6-7, router R2 is advertising a default route to its neighbors (routers R1, R3, and R4). What happens if PC1 wants to send traffic to the Internet? PC1’s default gateway is router R3, and router R3 has received three default routes. Which one does it use?

Router R3’s path selection depends on the dynamic routing protocol being used. As you see later in this chapter, a routing protocol such as Routing Information Protocol (RIP) would make the path selection based on the number of routers that must be transited to reach the Internet (that is, hop count). Based on the topology presented, router R3 would select the 128-Kbps link (where Kbps stands for kilobits per second, meaning thousands of bits per second) connecting to router R2 because the Internet would be only one hop away. If router R3 had instead selected a path pointing to either router R1 or R4, the Internet would be two hops away.

However, based on the link bandwidths, you can see that the path from router R3 to router R2 is suboptimal. Unfortunately, RIP does not consider available bandwidth when making its route selection. Some other protocols (for example, Open Shortest Path First [OSPF]) can consider available bandwidth when making their routing decisions.

Dynamic routes also allow a router to reroute around a failed link. For example, in Figure 6-8, router R3 had preferred to reach the Internet via router R4. However, the link between routers R3 and R4 went down. Thanks to a dynamic routing protocol, router R3 knows of two other paths to reach the Internet, and it selects the next-best path, which is via router R1 in this example. The process of failing over from one route to a backup route is called convergence.

Figure 6-8 Route Redundancy

Routing Protocol Characteristics

Before examining the characteristics of routing protocols, an important distinction to make is the difference between a routing protocol and a routed protocol. A routing protocol (for example, RIP, OSPF, or EIGRP) is a protocol that advertises route information between routers.

Conversely, a routed protocol is a protocol with an addressing scheme (for example, IP) that defines different network addresses. Traffic can then be routed between defined networks, perhaps with the assistance of a routing protocol.

This section looks at routing protocol characteristics, such as how believable a routing protocol is versus other routing protocols. Also, in the presence of multiple routes, different routing protocols use different metrics to determine the best path. A distinction is made between Interior Gateway Protocols (IGP) and Exterior Gateway Protocols (EGP). Finally, this section discusses different approaches to making route advertisements.

Believability of a Route

If a network is running more than one routing protocol (maybe as a result of a corporate merger), and a router receives two route advertisements from different routing protocols for the same network, which route advertisement does the router believe? Interestingly, some routing protocols are considered to be more believable that others. An example would be a Cisco router considering EIGRP to be more believable than RIP.

The index of believability is called administrative distance (AD). Table 6-1 shows the AD for various sources of routing information. Note that lower AD values are more believable than higher AD values.

Table 6-1 Administrative Distance

Metrics

Some networks might be reachable via more than one path. If a routing protocol knows of multiple paths to reach such a network, which route (or routes) does the routing protocol select? Actually, it varies depending on the routing protocol and what that routing protocol uses as a metric. A metric is a value assigned to a route; lower metrics are preferred over higher metrics.

If a routing protocol knows of more than one route to reach a destination network and those routes have equal metrics, some routing protocols support load balancing across equal-cost paths. EIGRP can even be configured to load balance across unequal-cost paths.

Different routing protocols can use different parameters in their calculation of a metric. The specific parameters used for a variety of routing protocols are presented later in this chapter.

Interior Versus Exterior Gateway Protocols

Routing protocols can also be categorized based on the scope of their operation. Interior Gateway Protocols (IGPs) operate within an autonomous system, where an autonomous system is a network under a single administrative control. Conversely, Exterior Gateway Protocols (EGPs) operate between autonomous systems.

Consider Figure 6-9. Routers R1 and R2 are in one autonomous system (AS 65002), and routers R3 and R4 are in another autonomous system (AS 65003). Within those autonomous systems, an IGP is used to exchange routing information. However, router ISP1 is a router in a separate autonomous system (AS 65001) run by a service provider. EGP (typically, Border Gateway Protocol [BGP]) is used to exchange routing information between the service provider’s autonomous system and each of the other autonomous systems.

Figure 6-9 IGPs Versus EGPs

Route Advertisement Method

Another characteristic of a routing protocol is how it receives, advertises, and stores routing information. The two fundamental approaches are distance vector and link state.

Distance Vector

A distance-vector routing protocol sends a full copy of its routing table to its directly attached neighbors. This is a periodic advertisement, meaning that even if there have been no topological changes, a distance-vector routing protocol will, at regular intervals, readvertise its full routing table to its neighbors.

Obviously, this periodic advertisement of redundant information is inefficient. Ideally, you want a full exchange of route information to occur only once and subsequent updates to be triggered by topological changes.

Another drawback to distance-vector routing protocols is the time they take to converge, which is the time required for all routers to update their routing table in response to a topological change in a network. Hold-down timers can speed the convergence process. After a router makes a change to a route entry, a hold-down timer prevents any subsequent updates for a specified period of time. This approach helps stop flapping routes (which are routes that oscillate between being available and unavailable) from preventing convergence.

Yet another issue with distance-vector routing protocols is the potential of a routing loop. To illustrate, consider Figure 6-10. In this topology, the metric being used is hop count, which is the number of routers that must be crossed to reach a network. As one example, router R3’s routing table has a route entry for network 10.1.1.0/24 available off of router R1. For router R3 to reach that network, two routers must be transited (routers R2 and R1). As a result, network 10.1.1.0/24 appears in router R3’s routing table with a metric (hop count) of 2.

Figure 6-10 Routing Loop: Before Link Failure

Continuing with the example, imagine that interface Ethernet 1/0 on router R3 goes down. As shown in Figure 6-11, router R3 loses its directly connected route (with a metric of 0) to network 10.1.4.0/24. However, router R2 had a route to 10.1.4.0/24 in its routing table (with a metric of 1), and this route was advertised to router R3. Router R3 adds this entry for 10.1.4.0 to its routing table and increments the metric by 1.

Figure 6-11 Routing Loop: After Link Failure

The problem with this scenario is that the 10.1.4.0/24 entry in router R2’s routing table was due to an advertisement router R2 received from router R3. Now, router R3 is relying on that route, which is no longer valid. The routing loop continues as router R3 advertises its newly learned route of 10.1.4.0/24 with a metric of 2 to its neighbor, router R2. Because router R2 originally learned the 10.1.4.0/24 network from router R3, when it sees router R2 advertising that same route with a metric of 2, the network gets updated in router R2’s routing table to have a metric of 3, as shown in Figure 6-12.

Figure 6-12 Routing Loop: Routers R2 and R3 Incrementing the Metric for 10.1.4.0/24

The metric for the 10.1.4.0/24 network continues to increment in the routing tables for both routers R2 and R3, until the metric reaches a value considered to be an unreachable value (for example, 16 in the case of RIP). This process is referred to as a routing loop.

Distance-vector routing protocols typically use one of two approaches for preventing routing loops:

Split horizon: The split-horizon feature prevents a route learned on one interface from being advertised back out of that same interface.

Poison reverse: The poison-reverse feature causes a route received on one interface to be advertised back out of that same interface with a metric considered to be infinite.

Having either approach applied to the previous example would have prevented router R3 from adding the 10.1.4.0/24 network into its routing table based on an advertisement from router R2.

Link State

Rather than having neighboring routers exchange their full routing tables with one another, a link-state routing protocol allows routers to build a topological map of the network. Then, similar to a global positioning system (GPS) in a car, a router can execute an algorithm to calculate an optimal path (or paths) to a destination network.

Routers send link-state advertisements (LSAs) to advertise the networks they know how to reach. Routers then use those LSAs to construct the topological map of a network. The algorithm run against this topological map is Dijkstra’s shortest path first algorithm.

Unlike distance-vector routing protocols, link-state routing protocols exchange full routing information only when two routers initially form their adjacency. Then routing updates are sent in response to changes in the network, as opposed to being sent periodically. Also, link-state routing protocols benefit from shorter convergence times, as compared to distance-vector routing protocols.

Routing Protocol Examples

Now that you understand some of the characteristics that distinguish one routing protocol from another, this section contrasts some of the most popular routing protocols found in modern networks:

Routing Information Protocol (RIP): A distance-vector routing protocol that uses a metric of hop count. The maximum number of hops between two routers in an RIP-based network is 15. Therefore, a hop count of 16 is considered to be infinite. Also, RIP is an IGP.

Open Shortest Path First (OSPF): A link-state routing protocol that uses a metric of cost, which is based on the link speed between two routers. OSPF is a popular IGP because of its scalability, fast convergence, and vendor-interoperability.

Intermediate System-to-Intermediate System (IS-IS): This link-state routing protocol is similar in its operation to OSPF. It uses a configurable, yet dimensionless, metric associated with an interface and runs Dijkstra’s shortest path first algorithm. Although IS-IS as an IGP offers the scalability, fast convergence, and vendor-interoperability benefits of OSPF, it has not been as widely deployed as OSPF.

Enhanced Interior Gateway Routing Protocol (EIGRP): A Cisco proprietary protocol. So, although EIGRP is popular in Cisco-only networks, it is less popular in mixed-vendor environments. Like OSPF, EIGRP is an IGP with fast convergence and is very scalable. EIGRP is more challenging to classify as a distance-vector or a link-state routing protocol.

By default, EIGRP uses bandwidth and delay in its metric calculation; however, other parameters can be considered. These optional parameters include reliability, load, and maximum transmission unit (MTU) size. Using delay as part of the metric, EIGRP can take into consideration the latency caused from the slowest links in the path.

Some literature calls EIGRP an advanced distance-vector routing protocol, and some literature calls it a hybrid routing protocol (mixing characteristics of both distance-vector and link-state routing protocols). EIGRP uses information from its neighbors to help it select an optimal route (like distance-vector routing protocols). However, EIGRP also maintains a database of topological information (like a link-state routing protocol). The algorithm EIGRP uses for its route selection is not Dijkstra’s shortest path first algorithm. Instead, EIGRP uses diffusing update algorithm (DUAL).

Border Gateway Protocol (BGP): The only EGP in widespread use today. In fact, BGP is considered to be the routing protocol that runs the Internet, which is an interconnection of multiple autonomous systems. Although some literature classifies BGP as a distance-vector routing protocol, it can more accurately be described as a path-vector routing protocol, meaning that it can use as its metric the number of autonomous system hops that must be transited to reach a destination network, as opposed to a number of required router hops. BGP’s path selection is not solely based on autonomous system hops, however. BGP has a variety of other parameters that it can consider. Interestingly, none of those parameters are based on link speed. Also, although BGP is incredibly scalable, it does not quickly converge in the event of a topological change.

A network can simultaneously support more than one routing protocol through the process of route redistribution. For example, a router could have one of its interfaces participating in an OSPF area of the network and have another interface participating in an EIGRP area of the network. This router could then take routes learned via OSPF and inject those routes into the EIGRP routing process. Similarly, EIGRP-learned routes could be redistributed into the OSPF routing process.

Address Translation

As described in Chapter 5, some IP addresses are routable through the public Internet, and other IP addresses are considered private and are intended for use within an organization. Network Address Translation (NAT) allows private IP addresses (as defined in RFC 1918) to be translated into Internet-routable IP addresses (public IP addresses). This section examines the operation of basic NAT and a variant called Port Address Translation (PAT). Address translation can also be done for specific ports associated with an IP address. When this is done, it’s often referred to as port forwarding.

NAT

Consider Figure 6-13, which shows a basic NAT topology. Note that, even though the IP networks of 172.16.1.0/24 and 192.168.1.0/24 are actually private IP networks, for this discussion, assume that they are publicly routable IP addresses. The reason for the use of these private IP addresses to represent public IP addresses is to avoid using an entity’s registered IP addresses in the example.

Figure 6-13 Basic NAT Topology

In Figure 6-13’s topology, two clients with private IP addresses of 10.1.1.1 and 10.1.1.2 want to communicate with a web server on the public Internet. The server’s IP address is 192.168.1.1. Router R1 is configured for NAT. As an example, router R1 takes packets coming from 10.1.1.1 destined for 192.168.1.1 and changes the source IP address in the packets’ headers to 172.16.1.101 (which we assume is a publicly routable IP address for the purposes of this discussion). When the server at IP address 192.168.1.1 receives traffic from the client, the server’s return traffic is sent to a destination address of 172.16.1.101. When router R1 receives traffic from the outside network destined for 172.16.1.101, the router translates the destination IP address to 10.1.1.1 and forwards the traffic to the inside network, where client 1 receives the traffic. Similarly, client 2’s IP address of 10.1.1.2 is translated into an IP address of 172.16.1.102.

Table 6-2 introduces the terminology used when describing the various IP addresses involved in a translation.

Table 6-2 Names of NAT IP Addresses

As a memory aid, remember that inside always refers to an inside device, and outside always refers to an outside device. Also, think of the word local being similar to the Spanish word loco, meaning crazy. That is what a local address could be thought of. It is a crazy, made-up address (a private IP address that is not routable on the Internet). Finally, let the g in global remind you of the g in good, because a global address is a good (routable on the Internet) IP address.

Based on these definitions, Table 6-3 categorizes the IP addresses previously shown in Figure 6-13.

Table 6-3 Classifying the NAT IP Addresses in Figure 6-13

NAT does not always have to be between private and public addresses. For example, NAT could be implemented between two private address ranges or two public address ranges as well.

Whether an inside local address is randomly assigned an inside global address from a pool of available addresses or is assigned an address from a static configuration determines the type of NAT you are using. These two approaches to NAT are called DNAT and SNAT:

DNAT: In the preceding example, the inside local addresses were automatically assigned an inside global address from a pool of available addresses. This approach to NAT is referred to as Dynamic NAT (DNAT).

SNAT: Sometimes, you want to statically configure the inside global address assigned to a specific device inside your network. For example, you might have an e-mail server inside your company, and you want other e-mail servers on the Internet to send e-mail messages to your server. Those e-mail servers on the Internet need to point to a specific IP address, not one that was randomly picked from a pool of available IP addresses. In such a case, you can statically configure the mapping of an inside local address (the IP address of your internal e-mail server) to an inside global address (the IP address to which e-mail servers on the Internet will send e-mail for your company). This approach to NAT is referred to as Static NAT (SNAT).

PAT

A challenge with basic NAT, however, is that there is a one-to-one mapping of inside local addresses to inside global addresses, meaning that a company would need as many publicly routable IP addresses as it had internal devices needing IP addresses. This does not scale well because, often, a service provider will provide a customer with only a single IP address or a small block of IP addresses.

Fortunately, many routers support Port Address Translation (PAT), which allows multiple inside local addresses to share a single inside global address (a single publicly routable IP address). In Chapter 2, “The OSI Reference Model,” you learned about how IP communications rely on port numbers. As a review, when a client sends an IP packet, not only does that packet have a source and destination IP address, it has a source and destination port number. PAT leverages these port numbers to track separate communication flows.

For instance, consider Figure 6-14. Unlike the example shown in Figure 6-13, in which each inside local address was translated to its own inside global address, the example shown in Figure 6-14 has only one inside global address. This single inside global address is shared among all the devices inside a network. The different communication flows are kept separate in router R1’s NAT translation table by considering port numbers.

Figure 6-14 PAT Topology

When client 1 sends a packet to the web server (with an IP address of 192.168.1.1), the client’s ephemeral port number (its source port selected, which is greater than 1023) is 1025. Router R1 notes that port number and translates the inside local address of 10.1.1.1 with a port number of 1025 to an inside global address of 172.16.1.100 with a port number of 2025. When client 2 sends a packet to the same web server, its inside local address of 10.1.1.2 with a port number of 1050 is translated into an outside local address of 172.16.1.100 with a port number of 2050.

Notice that both client 1 and client 2 had their inside local addresses translated into the same inside global address of 172.16.1.100. Therefore, when the web server sends packets back to client 1 and client 2, those packets are destined for the same IP address (172.16.1.100). However, when router R1 receives those packets, it knows to which client each packet should be forwarded based on the destination port number. For example, if a packet from the web server (192.168.1.1) arrived at router R1 with a destination IP address of 172.16.1.100 and a destination port number of 2050, router R1 would translate the destination IP address to 10.1.1.2 with a port number of 1050, which would be forwarded to client 2.

Multicast Routing

Chapter 5 introduced the concept of multicast transmission, where a multicast sender could send traffic destined for a Class D IP address, known as a multicast group, and devices on a network wanting to receive that transmission could join that multicast group. Let us now consider how a client joins a multicast group and how routers route multicast traffic. Keep in mind that one of the main goals with multicast traffic is to send that traffic only to devices in a network wanting to receive that traffic. Two primary protocols used for multicast are Internet Group Management Protocol (IGMP) and Protocol Independent Multicast (PIM).