You Are Reviewing the Configuration of a Router That Uses Eigrp for Its Routing Protocol

I used to work every bit a Network Design Specialist at Walt Disney World, in Florida. Their massive network independent over 500 Cisco routers (and thousands of Cisco Catalyst switches). What was the routing protocol keeping all of these routers in understanding almost available routes? It wasEnhanced Interior Gateway Routing Protocol (EIGRP). That'due south the focus of this commodity, which is the first of a series of posts focusing on EIGRP.

If you already have your CCNA R/Due south certification (or higher), you're probably well acquainted with EIGRP. However, unless you have an eidetic memory (similar Sheldon Cooper onThe Large Blindside Theory Television set prove), yous probably don't think every single EIGRP command and concept. Therefore, this series of blog posts is going to review and reinforce those key EIGRP concepts, and even introduce a few other fun facts.

EIGRP Fundamentals

There'southward a long-running contend almost the fundamental nature of EIGRP. At its essence, is EIGRP a link state routing protocol or a distance vector routing protocol? Or, is it a hybrid routing protocol (that is, a combination of both)? You'll find lots of literature supporting the notion that EIGRP is a hybrid routing protocol, arguing that EIGRP neighbors initially substitution their full routing table, much like a distance vector routing protocol, and EIGRP just sends routing updates based on network changes, much like a link land routing protocol.

While I was in the "it's a hybrid routing protocol" camp for many years, recently I converted to the belief that EIGRP is an "advanced distance vector" routing protocol. Here's my reasoning: Consider the fundamental characteristic of a link state routing protocol, which is routers maintaining a topology table, indicating how routers are interconnected. These routers (speaking routing protocols such every bit OSPF and IS-IS) then run the Dijkstra Algorithm on that topology to make up one's mind the "shortest" path to a destination network from the perspective of a detail router. EIGRP does non maintain a view of the network'south topology, nor does it run the Dijkstra Algorithm. Rather, EIGRP's topology tabular array lists available networks, along with information nearly the "distance" to those networks. That'southward my have, merely I certainly won't harbor any ill feelings for y'all if you're on the "hybrid" side of the debate.

EIGRP Characteristics

Let's being our EIGRP review past reminding ourselves of a few basic EIGRP characteristics:

• Fast Convergence: If a link goes down in a network, in many cases EIGRP can rapidly reroute around the link failure. This typically happens in no more than three seconds. This fast convergence is made possible past EIGRP having a standby road to a network, and that standby route is ready to have over in the result the principal route fails.
• Highly Scalable: Whereas a routing protocol such as RIP has a limit of xv router hops, EIGRP can scale to back up very large enterprise networks, such as the Walt Disney World network mentioned earlier.
• Load Balancing Over Unequal Cost Links: By default, both EIGRP and OSPF load balance traffic across more than than 1 link leading to a specific destination network, if the cost (that is, the value of the routing protocol metric) is the same. However, EIGRP can be configured to load balance across links with diff costs. This is fabricated possible past the variance characteristic.
• Variable-Length Subnet Mask (VLSM) Support: Unlike RIP version i, EIGRP sends subnet mask information as part of a route advertisement.
• Communicates via Multicast: An EIGRP-speaking router communicates with other EIGRP-speaking routers via multicast. Specifically, EIGRP for IPv4 uses a multicast address of 224.0.0.10, while EIGRP for IPv6 uses a multicast address of ff02::a.
• Was Cisco-Proprietary: While Cisco originally introduced EIGRP as a Cisco-proprietary routing protocol, in contempo years EIGRP has been opened up to other vendors. Specifically, EIGRP was fabricated an open standard in 2013, and an EIGRP advisory RFC (RFC 7868) was published in 2016.
• Multiprotocol Back up: EIGRP was originally designed to support the routing of multiple protocols, including IPv4, IPX, and AppleTalk. Although modern networks rarely if e'er apply IPX or AppleTalk, EIGRP can now support IPv6, which is chop-chop growing in popularity. This multiprotocol support is made possible past Protocol-Dependent Modules (PDMs), where at that place is a dissever PDM treatment routing decisions for each routed protocol (for example, IPv4 and IPv6).
• Diffusing Update Algorithm (DUAL): EIGRP's algorithm used to continue track of routes known to neighboring routers. DUAL is also used to determine the best path to a destination network (that is, thesuccessor route) and any acceptable backup paths to that destination network (that is,feasible successor routes).
• Summarization: To reduce the number of entries in an EIGRP topology table (or a router'southward IP routing table), EIGRP has the ability to summarize multiple network advertisements into a unmarried network advertisement. This summarization can be manually configured. Nevertheless, EIGRP has an automated route summarization feature that summarizes networks at classful network boundaries.
• Updates: Full EIGRP topology table updates are sent when new neighbors are discovered. Otherwise, partial updates are sent.

Configuration Review

A basic EIGRP configuration is very easy to configure. In fact, it simply requires two commands:

router eigrpasn

networknet-id wildcard-mask

Therouter eigrpasn command starts the EIGRP routing process on a router for an autonomous organisation (Every bit) specified by the asn variable. This control also brings you into router configuration mode. From in that location, you tin can result the 2nd control,networknet-id wildcard-mask. This second command uses a combination of a network accost and a wildcard mask to specify a range of one or more IP addresses, and any router interface whose IP accost belongs to that range of IP addresses then participates in the EIGRP routing procedure. However, there are some rules and behaviors to keep in listen with these commands:

• EIGRP-speaking routers must be in the same As to form a neighborship.
• After a router enables EIGRP on interfaces matched past the EIGRPnetwork command, it attempts to discover neighbors past multicasting EIGRP Hello messages.
• If a wildcard mask is non specified in thenetwork control, the specified network address should be a classful network accost.
• If a wildcard mask is not specified in thenetwork command, and a classful network address is specified, all interfaces whose IP addresses autumn under the classful network (for example, 172.16.one.1 /24 would fall nether 172.sixteen.0.0 /16) will participate in the EIGRP routing procedure.

To illustrate these concepts, consider the following example:

EIGRP Configuration on Routers R1, R2, and R3

The EIGRP configuration on routers R1, R2, and R3 begins with arouter eigrp 1 command. This command tells each router to start an EIGRP routing process within the autonomous arrangement ofane. Since autonomous system numbers must match betwixt EIGRP-speaking neighbors, all three routers use the same autonomous system number of1. Also, notice how the configuration varies in its employ of thenetwork control:

Router R1's network 10.one.1.0 0.0.0.3 Control

On router R1, thenetwork ten.1.one.0 0.0.0.3 control specifies a network accost often.i.1.0 with a wildcard mask of0.0.0.3, which corresponds to a 30-bit subnet mask (that is, a 255.255.255.252 subnet mask). Since router R1's Gig 0/one interface'southward IP address of 10.1.ane.1 /xxx falls in that subnet, that interface is instructed to participate in the EIGRP process.

Router R1'south network 10.i.1.five 0.0.0.0 Command

Thenetwork 10.1.1.5 0.0.0.0control specifies a specific IP address, rather than an entire subnet (or, one might argue that it's a subnet containing a single IP address). We know it's just specifying a single IP address due to the wildcard mask of0.0.0.0. Recollect that in a wildcard mask we have a series of contiguous zeros followed by a serial of contiguous ones (in binary). The binary zeros correspond to fleck positions in the IP address that specify a network address, and the binary ones correspond to chip positions in the IP address that specify the host address. Yet, in an instance were we take all zeros, as in this case, we take a network of one and merely one IP address (that is, the subnet mask is /32). Since the IP accost matches the IP address of router R1'south Gig 0/2 interface, that interface also participates in the EIGRP routing process.

Router R1'southward network 192.0.2.0 Command

The finalnetwork command on router R1 isnetwork 192.0.2.0. Interestingly, this command was really entered every bitnetwork 192.0.2.0 0.0.0.255, but since0.0.0.255 is the wildcard mask respective to the default subnet mask (also known as thenatural mask) of a Class C network (in this example 192.0.2.0 /24), it'southward unsaid but not shown. Router R1'south Gig 0/3 interface's IP address of 192.0.ii.1 /24 does indeed fall within the Class C subnet specified past thenetwork command. Therefore, the Gig 0/3 also starts to participate in router R1'south EIGRP routing process.

Router R2'southward network 10.0.0.0 Command

Thenetwork ten.0.0.0 command on router R2 seems to be missing a wildcard mask. Nonetheless, call back from the previously discussednetwork control (on router R1), a wildcard mask is not displayed if information technology's reflecting the natural mask of the subnet being specified. Building on that logic, we can conclude that if we intentionally omit the wildcard mask statement from anetwork command, the assumed wildcard mask will exist the wildcard mask corresponding to the classful subnet mask of the network specified in thenetwork command. In this example, the get-go octet of the network specified in the network address command is ten. A 10 in the first octet of an address indicates that we're dealing with a Class A address, which has a default subnet mask of255.0.0.0, and therefore a default wildcard mask of0.0.0.255. Since router R2'southward Gig 0/1 and Gig 0/2 interfaces both autumn under this classfulnetwork statement, both interfaces participate in router R2's EIGRP routing process.

Router R2's network 198.51.100.0 Command

Like the previousnetwork command, router R2's network 198.51.100.0 control was entered without specifying a wildcard mask. Since the first octet of the address is a198, nosotros tin can conclude that nosotros have a Class C network, whose default subnet mask is255.255.255.0, and whose default wildcard mask is 0.0.0.255. The IP address (198.51.100.1 /24) of interface Gig 0/3 on router R2 lives within the specified 198.51.100.0 /24 subnet. And then, the interface participates in the EIGRP routing procedure.

Router R3's network 0.0.0.0 Command

Recall that an EIGRPnetwork statement, contrary to popular belief, does not specify a network to annunciate. Rather, it specifies a range of i or more IP addresses, and any interface with an IP address in that range is instructed to participate in the EIGRP routing process. This means if we want all interfaces on a router to participate in the same EIGRP routing process, nosotros could give the commandnetwork 0.0.0.0 to specify all possible IP addresses. Since the IP address of each individual interfaces falls under the category of "all possible IP addresses," all of the interfaces on router R3 are instructed to participate in the EIGRP routing process. Also, the network addresses of those participating interfaces (along with subnet information for those network addresses) are then advertised via EIGRP.

Verification Review

The process of EIGRP verification is more than just making sure neighborships have formed between all routers and that all routers have learned all the routes in the network. The verification procedure needs to assistance united states of america verify our pattern requirements were met. For instance, we need to wait for appropriate routes to appear in the EIGRP topology table, desired interfaces to announced in the EIGRP interface table, and specific neighbors to announced in the EIGRP neighbor table. In one case we accept our design goals and anticipated outcomes in listen, nosotros tin leverage such EIGRP verification commands as those shown in the following tabular array:

Key EIGRP Verification Commands

The post-obit examples show the output from each of these commands subsequently issuing them on router R1 shown in the previous topology.

Output from show ip routeCommand on Router R1

Detect how routes learned via EIGRP are shown with a D in the left column. This D lawmaking indicates a route learned via EIGRP. These routes include 10.1.1.8 /thirty, 198.51.100.0 /24, and 203.0.113.0 /24. Also note the highlighted xc in each EIGRP-learned route. The 90 is EIGRP'southadministrative altitude (that is, it's believability as compared to other routing sources), where lower administrative distance values are preferred over higher values.

Output from show ip protocolsCommand on Router R1

Output from theshow ip protocols command on a router speaking EIGRP, every bit seen higher up, offers us several information points. For example, under theRouting for Networks: section, you see a list of networks specified by thenetwork command under EIGRP configuration way. Under theRouting Information Sources: section, you can determine the IP addresses of EIGRP neighbors, which are 10.ane.1.2 (that is, router R2) and ten.1.1.half dozen (that is, router R3) in our topology. Too nether that department, y'all tin run across the Administrative Distance (AD) for our neighbors. Since those neighbors are EIGRP-speaking routers, they have the default EIGRP Ad of 90. Finally, find theMetric weight K1=i, K2=0, K3=i, K4=0, K5=0 portion of the output. In an upcoming blog post, we'll learn how EIGRP calculates its metric and how that calculation involves Thousand-values. When nosotros discuss those G-values, delight proceed in heed you can view a router's K-values in the output of thetestify ip protocols control.

Output from show ip eigrp interfaces  Command on Router R1

The output of thetestify ip eigrp interfaces, seen above, indicates that router R1's Gig 0/1, Gig 0/2, and Gig 0/3 are participating in an EIGRP routing procedure. Specifically, the process is for EIGRP AS 1. Also find that an EIGRP neighborship has been established with another router continued off of router R1's Gig 0/ane interface, and some other off of interface Gig 0/two. These neighborship establishments are prove by having a number greater than 0 in thePeerscolumn. Since router R1's Gig 0/3 interface did not form a neighborship with whatever other EIGRP-speaking routers, in that location is 0 in its Peers cavalcade.

Output from evidence ip eigrp neighborsControl on Router R1

While output from theshow ip eigrp interfaces command indicated that nosotros had a couple of EIGRP neighbors, output from theshow ip eigrp neighbors command, as seen above, offers more detail nearly those neighbors. Specifically, the neighbor connected off of router R1'southward Gig 0/1 interface has an IP address of ten.1.ane.2, and the neighbor continued off of router R1's Gig 0/two interface has an IP address of 10.1.ane.half dozen.

Output from show ip eigrp topology [all-links]Command on Router R1

One of the nigh common commands used for EIGRP verification and troubleshooting istestify ip eigrp topology, as demonstrated in the example in a higher place. Output from this command shows the successor routes (that is, the preferred routes) and feasible successor routes (that is, backup routes) known to the EIGRP routing procedure. Please proceed in listen that a route's advent in the EIGRP topology tabular array does not guarantee its presence in the router's IP routing table. Specifically, the successor routes present in the EIGRP topology table are but candidates to be injected into the router'southward IP routing table. For example, the router might possess more than believable routing information for a network, such as a statically configured route with an administrative altitude of ane. If EIGRP is indeed the most believable routing source for a specific network, then that network will be injected into the router's IP routing tabular array. Besides, notice how the addition of theall-links argument in the in a higher place instance shows even more routes, which are highlighted. The difference is, theall-links argument instructs thebear witness ip eigrp topology command to display all EIGRP-learned routes, even if some of the routes are not considered successor routes or feasible successor routes.

Well, that'south it for this first in a series of EIGRP blog posts. In the next one, we'll check out some EIGRP internals. For case, we'll dig into those K-values, and meet how they influence EIGRP's path pick.

Take proficient care,

Kevin

trappsearmson.blogspot.com

Source: https://www.linkedin.com/pulse/understanding-eigrp-part-1-kevin-wallace

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