Lesson 45 - EIGRP Fundamentals Part 3 - The Lab

In this post I'll put the pieces together in the practice lab. Our topology will be as presented below:

Pic. 1 - Topology Diagram.

Icons designed by: Andrzej Szoblik - http://www.newo.pl

This post is going to collect bits and pieces from lessons 43 and 44 to show you the implementation on the command line interface with verification steps.

Task List

General requirements

• All routers must use Autonomous System 1.
• All networks/subnets must be advertised and reachable.
• EIGRP authentication should be enabled between R1 and R4.

1. Enable EIGRP between R1, R2 and R3. Make sure loopbacks are advertised. Use the most specific wildcard mask.
2. Enable EIGRP between R1 and R4. Make sure loopbacks are advertised. Use the most specific wildcard mask on R1. On R4 the network statement should be classful (no wildcard mask).
3. Enable EIGRP between R2 and R5. Make sure loopbacks are advertised. Use the most specific wildcard mask on R2. On R5 the network statement should be classful (no wildcard mask).
4. Enable EIGRP authentication between R1 and R4. Use the password 'cisco'.

Lab Solution

Step 1
Enable EIGRP between R1, R2 and R3. Make sure loopbacks are advertised. Use the most specific wildcard mask.

Note!
The 'no auto-summary' keyword is not necessary in my topology, but I include it to show you how to disable automatic summarization to the class boundary.
R1 Configuration:

!

R1(config)#router eigrp 1
R1(config-router)#no auto-summary
R1(config-router)#network 172.31.123.1 0.0.0.0
R1(config-router)#network 172.31.1.1 0.0.0.0

R1(config-router)#

!

R2 Configuration:

!

R2(config)#router eigrp 1
R2(config-router)#no auto-summary
R2(config-router)#network 172.31.2.1 0.0.0.0
R2(config-router)#network 172.31.123.2 0.0.0.0
R2(config-router)#

!

Pic. 2 - EIGRP Adjacency Between R1 and R3.

R3 Configuration:

!

R3(config)#router eigrp 1
R3(config-router)#no auto-summary
R3(config-router)#network 172.31.3.1 0.0.0.0
R3(config-router)#network 172.31.3.17 0.0.0.0
R3(config-router)#network 172.31.123.3 0.0.0.0
R3(config-router)#

!

Verification:

Pic. 3 - R3's EIGRP-enabled nterfaces.

Pic. 3 confirms the network statement was correct for each interface.

Pic. 4 - R3's EIGRP Neighbor Table.

Notice!

R3 heard EIGRP hello packets from two neighbors. But to be sure they have exchanged their topology tables, the 'Q Cnt' column must show '0' messages pending (not acknowledged).

Notice!
In the 'output explanation' I show only the most relevant pieces of information. Others are either self-explanatory or not necessary to understand at the CCNA level.

Output Explanation:

• H - Handle; shows order in which the neighbors were discovered and adjacency built between them.
• Address - The IP address of the neighbor.
• Interface - The  local interface where neighbors are discovered.
• Hold - Length of time in seconds how long the router is going to wait for hello packet before declaring the neighbor down.
• Uptime - Elapsed time since the local router first heard from the neighbor.
• SRRT - Smooth Round Trip Timer; amount of time in milliseconds the required to send the packet to the neighbor and receive the acknowledgement.
• RTO - Retransmit Time Out; amount of time in milliseconds the local router waits before retransmitting EIGRP packet if acknowledgement did not arrive.
• Q Cnt - The number of EIGRP packets (update, query, reply) that the router keeps in the queue to be sent. Typically, it implies that some EIGRP reliable packets have not been acknowledged.

Pic. 5 - R3's EIGRP Topology Table.

Output Explanation:

• P - Route is Passive which means that router is not actively looking for a path towards it. It is a required status in stable topology.
• FD - Feasible Distance (metric value towards the destination)
• (156160/128256) - The first number (156160) is FD, the second number (128256) is the Advertised Distance advertised by the neighbor (FD of the neighbor). Recall, the Feasibility Condition from the previous lessons.

Pic. 6 - R3's Routing Table.

Output Explanation:

• D - EIGRP learned prefix; 'show ip route' shows all prefixes including connected networks with explanation of codes.
• [90/156160] - The first number (90) is the Administrative Distance (trustworthiness) of the protocol, the second number (156160) is the metric based on best FD from the topology table.
• via 172.31.2.1 - next-hop router (neighbor that advertised it).
• 00:30:04 - how long ago the prefix was learned.
• FastEthernet1/0 - The outbound interface towards the destination.

Pic. 7 - R3's EIGRP Details.

Your homework:
look at pic. 7 and consult my previous posts about EIGRP and the topology diagram to understand the highlighted parts.

Step 2
Enable EIGRP between R1 and R4. Make sure loopbacks are advertised. Use the most specific wildcard mask on R1. On R4 the network statement should be classful (no wildcard mask).

R1 Configuration:

!

R1(config-router)#network 172.31.14.1 0.0.0.0
R1(config-router)#

!

R4 Configuration:

!

R4(config)#router eigrp 1
R4(config-router)#no auto-summary
R4(config-router)#network 172.31.0.0

R4(config-router)#network 192.168.4.0
R4(config-router)#

!

Verification should be performed after this step (look at step 1).

Step 3
Enable EIGRP between R2 and R5. Make sure loopbacks are advertised. Use the most specific wildcard mask on R2. On R5 the network statement should be classful (no wildcard mask).

R2 Configuration:

!

R2(config-router)#network 172.31.25.2 0.0.0.0
R2(config-router)#

!

R5 Configuration:

!

R5(config)#router eigrp 1
R5(config-router)#no auto-summary
R5(config-router)#network 192.168.5.0
R5(config-router)#network 172.31.0.0

R5(config-router)#

!

Verification should be performed after this step (look at step 1).

Step 4
Enable EIGRP authentication between R1 and R4. Use the password 'cisco'.

R1 Configuration:

!

R1(config)#key chain MY_EIGRP_KEY
R1(config-keychain)#key 1
R1(config-keychain-key)#key-string cisco
R1(config-keychain-key)#
R1(config-keychain-key)#int s0/2
R1(config-if)#ip authentication key-chain eigrp 1 MY_EIGRP_KEY
R1(config-if)#ip authentication mode eigrp 1 md5

!

R4 Configuration (without prompts):

!

key chain MY_EIGRP_KEY
 key 1
   key-string cisco
!

interface Serial0/2
 ip address 172.31.14.4 255.255.255.0
 ip authentication mode eigrp 1 md5
 ip authentication key-chain eigrp 1 MY_EIGRP_KEY

!

Verification: 

Pic. 8 - R1's Neighbors.

R1 and R4 have re-established neighbor relationships.

In the next post, I'll present a few troubleshooting techniques based on the topology and configuration used in this lesson.

Lesson 43 - EIGRP Fundamentals Part 1 - Overview

EIGRP is a Cisco Proprietary Routing Protocol. It is classified as an Advanced Distance Vector solution. EIGRP is a fast, modern and robust routing protocol offering features we expect to find in the routing protocol these days (as of the time of writing this tutorial).

EIGRP Characteristics

• Advanced Distance Vector Protocol - EIGRP routers, similarly to RIP, create the database in which only distance (how far) and direction (next-hop) are given. As a result of that, routers do not know the exact topology of the network. However, like in link-state algorithm, EIGRP routers are going to discover their neighbors using EIGRP Hello packets, form adjacency and exchange the routing databases reliably with their neighbors.
• Flexible Design - EIGRP does not use the concept of areas found in OSPF. It is less demanding in terms of the design allowing less experienced engineers implement it quickly. It supports summarization of prefixes on per interface-basis (unlike OSPF which does it on ABRs/ASBRs only). 
• Incremental Updates - Routing updates are not advertised periodically. They are exchanged reliably once (must beacknowledged). In stable network, only hello packets are transmitted between neighbors to maintain the relationship. In case of the topology change, the updates are sent to the routers that should receive it. The routers take into consideration how long it takes to send the reliable packet and get the response from a neighbor. Based on that, they determine how long they should wait to re-send the upate in case they did not get the acknowledgement. 
• VLSM Support - Although EIGRP is primary Distance Vector Protocol, it is a classless protocol which supports variable length subnet masking. In order to turn it on, you must use the 'no auto-summary' keyword in the routing process 'config-router'.
• Rapid Convergence - In case the EIGRP domain is designed with redundant connections and certain criteria are met (Feasibility Condition) a router can keep information about so called Feasible Successor(s) which are the second best path towards the destination (sort of a backup route). The difference between OSPF/RIP backup paths (second best routes) and EIGRP's feasible successor is that in the latter protocol, the traffic is redirected immediately without recomputing the routing table.
• Multicast - EIGRP uses multicast and unicast transmissions. Multicast address reserved for EIGRP is 224.0.0.10.
• 100% Loop Free - The Diffusing Update Algorithm (DUAL) which EIGRP uses, guarantees there are not going to be any loops formed which Distance Vector algorithms are susceptible to (thanks to the Feasibility Condition).
• Equal and Unequal Cost Load Balancing - EIGRP routers can utilize different paths with different metric performing a load balance. The router calculates the appropriate ratio regarding how many packet should be sent out the interface based on the difference in the metric value between these multiple paths available.
• Support for multiple layer 3 protocols - EIGRP can natively support IP,  IPX and Apple-Talk protocols (altough in today's IP world this is no longer a major advantage).

Based on my experience with EIGRP (personal opinion), the two major disadvantages of EIGRP are:

• It is very chatty if the network is not properly designed. Explanation of the design is beyond the scope of this tutorial.
• It is a Cisco proprietary routing protocol which means it won't work on routers from different vendors.

Terminology

EIGRP Databases

EIGRP creates three databases, similarly to OSPF:

• Neighbor Table - Lists all neighbors discovered with which it formed adjacency and exchanged routing information.
• Topology Table - Lists all the paths to all destinations learned. However, it shows only metric and the next-hop router and does not illustrate the real topology like OSPF LSDB.
• Routing Table - The best metric routes are going to be used to send the traffic. 

EIGRP Composite Metric
EIGRP calculates the metric using pretty complex formula which is beyond the scope of the CCNA tutorial. It is referred to as composite metric. EIGRP uses five different components (so called K-values) to determine the value of the metric. Thus, the composite metric. One of them, MTU (Maximu Transmission Unit), is not factored in directly but changes the way the value is calculated.

By default EIGRP uses two parameters in this magical formula:

• Bandwidth
• Delay

All K-values can be seen using: 'show ip protocols' command if EIGRP has been configured. The K-values listed are as follows:

• K1 - Bandwidth
• K2 - Load
• K3 - Delay 
• K4 - Reliability
• K5 - MTU

They can be used for traffic engineering (overriding normal route selection process). This helps to address issues with unde/over utilized links. However, configuring additional K-values to accomplish that is not recommended unless an administrator is experienced with EIGRP and knows their network very well.

Topology Database
Terms to remember:

• Advertised Distance - The best metric a router receives from the neighbor (next-hop device) to a given destination network/subnet. In some documentations it is also referred to as the Reported Distance. Do not confuse this term with Administrative Distance which is the ranking of routing protocols.
• Feasible Distance -  The total metric value which is a sum of the Advertised Distance and the metric to reach the neighbor (next-hop router towards the destination).
• Feasibility Condition - The situation in which the Advertised Distance value provided by a alternate neighbor(s) (their best metric) is lower than the best Feasible Distance (metric) through the best router(s) to reach the destination (successor: look below).
• Feasible Successor - The second best route(s) to a given destination. The router(s) which is/are the second best next-hop router(s) to a given destination meeting the FD (Feasibility Condition).
• Successor - The best route to a given destination (the least cost metric)

In order to understand the above terms and EIGRP behavior better, let's analyze the process of advertising 192.168.1.0 by R5.

R5 advertises its directly connected network with the metric value of 1. R2, R3 and R4 will add their own cost to reach R5 (Pic. 1). The sum of 1 + COST_TO_R5 becomes their successor (best metric). Then we add the R1 to this topology and let's analyze what R1 receives from three neighbors. Take a look at the pic. 1.

Pic. 1 - EIGRP Terminology.

Icons designed by: Andrzej Szoblik - http://www.newo.pl

R1 receives the prefix 192.168.1.0/24 from three neighbors (R2, R3, R4). It calculates the metric value by adding the cost to reach the neighbor (in red) to the metric advertised by this neighbor (in blue aka the advertised distance). The metric (cost if you will, or composite metric to be accurate) that is the lowest becomes the successor route (the best route). If however, other neighbor(s) advertise metric (Advertised Distance) that is lower than the least cost path, this neighbor or these neighbors become the second best next-hop routers towards the destination 192.168.1.0/24 and they are called feasible successors according to the Feasibility Condition:

AD < FD

In such situation, if the best route is not available, the feasible successor (second best) is used immediately without any re-computation of the routing table.

In our Pic.1, R2 is the successor towards 192.168.1.0/24, but R3 becomes the feasible successor (second best gateway) since its advertised distance is 20. This number is lower than the best feasible distance which is 21.

In the next post I will present the basic implementation and verification steps regarding EIGRP.

Lesson 35 - Routing Information Protocol Part1

Now, that you have learned the principles regarding Distance Vector algorithm, it's time to look at the operation of the oldest distance vector routing protocol: RIP.

Routing Information Protocol is seldom used these days. It has been superseded by more sophisticated protocols (OSPF, EIGRP, IS-IS). However, Cisco still keeps it in the curriculum. All professionals up to the CCIE level (as of writing this post) need to know how it works. In order to enable RIP you must choose which version you want to run. By default, if you configure RIP the version used is version 1. You can change it to version 2 which is more preferred if one want to RIP to begin with. The below table summarizes the main feature of both versions.

Table 1- RIPv1 and RIPv2 Features.

Classful, Classless, and Summarization
Routers share their information (routing table) with their neighbors using advertisements. It is important to know that the shorter the routing table is, the less time a router needs to process the incoming packets in order to find the outbound (egress) interface and expedite them.

If your network design uses proper, consecutive IP scheme, a router can advertise fewer prefixes (routing entries) to their neighbors represented by so called summary routes. A summary route represents multiple more specific destinations. But the router which receives this summary route can still send packets towards the subnets that are being summarized. Consider the following picture (pic. 1).

Pic. 1 – Route Summarization.

Icons designed by: Andrzej Szoblik - http://www.newo.pl

In the pic. 1, there are four subnets located behind R1. Router R1 can advertise them ‘as they are’ towards R2. This way, R2 will populate its routing table with all four of them (10.1.0.0/24, 10.1.1.0/24, 10.1.2.0/24, 10.1.3.0/24) with R1 as the gateway to these subnets. However, the subnet addresses behind R1 are designed to be represented by one summary route. This route is going to represent exactly these four subnets. So R1, instead of advertising the four of them, will announce only one address:

10.1.0.0/22

So the question is: how to summarize subnets/networks?

Assuming that IP addresses subnets/networks to be summarized are consecutive numbers when converted to binary, the rule to create the summary route presented in the pic.1 can be applied using three steps.


Step 1
Convert all consecutive summary candidates into binary. List them from the lowest number to the highest like presented in pic.1.

10.1.0.0/24 = 00001010.00000001.00000000.00000000
10.1.1.0/24 = 00001010.00000001.00000001.00000000
10.1.2.0/24 = 00001010.00000001.00000010.00000000
10.1.3.0/24 = 00001010.00000001.00000011.00000000

Step 2
Find the number of bits that is identical and draw the line to see how many bits do not change.

10.1.0.0/24 = 00001010.00000001.00000000.00000000
10.1.1.0/24 = 00001010.00000001.00000001.00000000
10.1.2.0/24 = 00001010.00000001.00000010.00000000
10.1.3.0/24 = 00001010.00000001.00000011.00000000

22 bits do not change = the length of the network mask representing all four entries.

Step 3
Advertise the address with the lowest number in the range using network mask length achieved in the step 2 (bits that never change give you the length of the network mask). The actual commands will be different depending which protocols you use. I will explain them in due time.

10.1.0.0/22 = 00001010.00000001.00000000.00000000
Netmask     = 11111111.11111111.11111100.00000000 = 255.255.252.0

Optionally, you can check if the summary route covers exactly the prefixes. Notice that the two of the bits that change in the third octet will tell you how many addresses have been represented by the summary route (aka aggregate). Look at the bits that do change (in red).

10.1.0.0/24 = 00001010.00000001.00000000.00000000
10.1.1.0/24 = 00001010.00000001.00000001.00000000
10.1.2.0/24 = 00001010.00000001.00000010.00000000
10.1.3.0/24 = 00001010.00000001.00000011.00000000

There are two bits that change in this range in the third byte. This number of bits with the exponent of 2, tells you how many subnets/networks are being summarized. Here: 2 to the power of 2 = 4. Exactly, four subnets have been summarized using prefix 10.1.0.0/22. So this prefix represents exactly the subnets in question.

Now, that you know how to summarize consecutive subnets/networks, let’s see what automatic summarization to the class boundary will look like if you use RIPv1 and have different classes of IP addresses in your desing. Consider the picture presented below (pic. 2).

Pic. 2 – RIPv1 Automatic Summarization to the Class Boundary.

Icons designed by: Andrzej Szoblik - http://www.newo.pl

The routers R1 and R3 are boundary routers between two different classes. They will advertise class A address (10.0.0.0) since RIPv1 does it by default if the outbound interface (the one to send the advertisement) belongs to different class than the subnet being advertised. This will inevitably create confusion as far as R2 is concerned. It will think that both R1 and R3 are gateways to the same class A network 10.0.0.0. As a result of that, the packets destined to the subnets 10.1.1.0 and 10.1.2.0 will not be delivered properly.

In RIPv2 you can disable this automatic summarization by using ‘no auto-summary’ keyword in the ‘config-router’ configuration context. This option makes RIPv2 classless routing protocol (it can advertise subnets of major classes). Classless routing protocols can advertise full prefix (IP address and the network mask) without summarizing it to the class like shown in the pic. 2.

RIPv1 Configuration
In order to configure RIP in our design, we need to perform two steps.

Step 1
Enable RIPv1 protocol process in the global configuration context.

Step 2
Using ‘network’ statement in ‘config-router’ context, instruct the router which interfaces should participate in the RIP process. RIP will run on these interfaces. In RIP protocol the ‘network’ statement has to be followed by the Class A, B, or C address which instructs the router which interfaces should be RIP-enabled.

---

NOTICE!
The ‘network’ statement is one of the most misinterpreted keywords in Cisco IOS. RIP, OSPF, EIGRP use the ‘network’ statement to inform the router which interfaces should be enabled for the routing protocol. The ‘network’ statement does NOT advertise anything. Only when BGP protocol is used, the ‘network’ statement actually advertises the prefixes since BGP is an application using TCP (port 179) and is not run on the interfaces like all IGP protocols are.

---

Having said that, let’s enable RIPv1 in my design (pic. 3).

Pic. 3 – Topology Diagram.

Icons designed by: Andrzej Szoblik - http://www.newo.pl

R1 Configuration:

R1#configurer terminal

R1(config)#router rip

R1(config-router)#network 172.31.0.0

R1(config-router)#

R2 Configuration:

R2#configurer terminal

R2(config)#router rip

R2(config-router)#network 172.31.0.0

R2(config-router)#

R3 Configuration:

R3#configurer terminal

R3(config)#router rip

R3(config-router)#network 172.31.0.0

R3(config-router)#

R4 Configuration:

R4#configurer terminal

R4(config)#router rip

R4(config-router)#network 172.31.0.0

R4(config-router)#network 192.168.4.0

R4(config-router)#

R5 Configuration:

R5#configurer terminal

R5(config)#router rip

R5(config-router)#network 172.31.0.0

R5(config-router)#network 192.168.5.0

R5(config-router)#

I suggest that you analyze the topology, configuration and information in this post. There will be few issues we will come across here using RIPv1 (one of them I have not discussed yet but it will rear its ugly head on R3).

I will explain these problems in my next post. I will also show you how to check the configuration of RIP ('show' and 'debug' commands). Also, I will enable RIPv2 to show you the differences in their operation, and how to optimize and verify its operation.

Lesson 34 - Dynamic Routing Protocols Introduction

If you have read the previous post, you must have noticed that using a static routing method tends to be a bit cumbersome in larger implementations. Using one of the dynamic routing protocols feels like an easier solution in these scenarios.

In this post I will briefly explain the general concepts behind dynamic routing protocols. Then, we can jump to implementation fundamentals.

One way of classifying dynamic routing protocols is based on where they are used. This criterion allows us to distinguish between two major solutions:

1. Interior Gateway Protocols  (IGP) 
2. Exterior Gateway Protocols (EGP)

Common Interior Gateway Protocols are: 

• Routing Information Protocol (RIP), 
• Open Shortest Path First (OSPF), 
• Enhanced Interior Gateway Protocol (EIGRP, Cisco proprietary protocol), 
• Intermediate System to Intermediate System (IS-IS).

Exterior Gateway Protocols (currently there is only one in use)

• Border Gateway Protocol  (BGP)

IGPs are designed to work in private networks. EGPs are used to provide paths in the public network (Internet). 

We can also classify routing protocols based on the algorithm they use to distribute and maintain information (routing table). There are three major algorithms supported by Cisco routers:

1. Distance Vector (DV, aka Bellman-Ford) – example of protocol: RIP.
2. Link-State – example of protocols: OSPF, IS-IS.
3. Advanced Distance Vector – protocols: EIGRP (also BGP is partly distance vector protocol).

Understanding the algorithms helps us determine the proper solution for a given design. There is no one best routing protocol out there, but there could be the best one in a specific design.

In this post I am going to focus in on the first algorithm listed above.

Distance Vector Algorithm Characteristics

This method is sometimes referred to as ‘routing by rumor’. The main characteristics of this approach are:

• Routers do not know the topology of the network. They only know which is the outbound interface and the next-hop router’s IP address (vector) as well as the metric value which describes how far the destination is (distance).
• Routers advertise their full routing table periodically. This method of route distribution creates two problems: routing loops and counting to infinity. Special techniques were created to solve these issues (details later in the post).
• Routers perform automatic summarization if they are connected to different classful (A, B, C) networks.
• No VLSM support. All network masks must be identical if the subnets of a major class are used in the network (RIPv1). RIPv2 is classless (VLSM supported using ‘no auto-summary’ command).
• Routers are slow to converge. It takes a lot of time to invalidate lost routes and pick the new path if one is available as well as to synchronize their routing information.
• Routers use simple metric. The metric number tells a router how many routers the packet has to traverse in order to reach the destination. In modern networks bandwidth of the path is much more important than how many hops will be used.

The above characteristics do not encourage us to use this kind of solution in our modern networks. But knowing the DV rules help us appreciate protocols such as OSPF or EIGRP which are more likely to be used in our designs.

Let’s see how things work when DV algorithm is used. As an example, I will use RIP protocol hoping to explain the principles of operation and how the two design issues have been solved (routing loop and counting to infinity).

Distance Vector Principles of Operation

Consider this simple topology. Without getting into configuration (syntax) details let’s have a quick discussion on how information is distributed using DV algorithm. Initially, the routers recognize only connected subnets. They are populated in the routing table as soon as IP addresses and network masks are configured and they are activated (no shutdown).

Pic. 1 - Connected Subnets.

Icons designed by: Andrzej Szoblik - http://www.newo.pl

Let’s assume that we have enabled RIPv2 protocol in the topology presented above (pic. 1). This version of RIP allows the routers to announce both the subnet IP addresses and the network masks (we’ll put it into practice in the next post). 

The RIP process must be activated in the ‘config’ mode. Then we need to instruct it which interfaces should be activated in the RIP domain. This is configured in the ‘config-router’ mode (‘network’ statement).  The routers begin to ‘chat’ and advertise their routing tables every 30 seconds.

Pretend that R1’s timer of sending the advertisement has just kicked in (pic. 2). R1 is advertising its routing table out of the RIP-enabled interfaces (in my example all interfaces of all routers are in the RIP domain). This way, R2 learns about 10.1.1.0/24 subnet. So from R2’s perspective, R1 router becomes the gateway towards 10.1.1.0/24. 

Now, a word about the metric being advertised. 

Metric used in DV reflects how many routers the packet has to traverse to reach the destination network/subnet (so called 'hop-count'). R1’s routing table’s entries (subnets: 10.1.1.0/24 and 10.1.12.0/24) show the metric of ‘0’ hops (pic. 1) since they are directly connected to F0/0 and F0/1 interfaces respectively (they are local to R1). While advertising them to the neighbors (pic. 2), R1adds 1 hop (itself) to the existing metric found in the routing table.

---

NOTICE!
Bear in mind, that algorithm prompts the router to send the full routing table. Current implementation changes that behavior (split-horizon) but more on this later in the post.

---

Pic. 2 – R1’s RIP advertisements.

Icons designed by: Andrzej Szoblik - http://www.newo.pl

R2 accepts the advertisement about 10.1.1.0/24. It puts this information in the RIP’s database and then it creates the entry in the routing table (purple color). Pay a close attention to what has just happened (pic. 2). The update arrives on R2’s F0/0 interface (RIP-enabled), sourced by the IP address of 10.1.12.1. This way, R2 considers its F0/0 the egress (outbound) interface towards the subnet advertised by R1. The IP address of the sender (10.1.12.1) becomes the next-hop IP address towards the subnet 10.1.1.0/24

Next, let’s imagine R2’s timer has expired and it is sending its routing table out F0/0 and F0/1. Please take a closer look at the picture 3 which shows this process in the graphical form. Just like previously R1 router has done, R2 is sending its routing table adding itself as an additional hop added to the existing metric (existing metric +1).

Pic. 3 – R2’s RIP advertisement.

Icons designed by: Andrzej Szoblik - http://www.newo.pl

Now, R1 and R3 accept the advertisement from R2 and register the information sent in their RIP databases (the interfaces process the update as they RIP-enabled). Appropriate entries in the routing tables also show the egress interfaces and the metric expressed in the number of ‘hops’ (how many routers the packet will have to traverse to reach the destination subnet). Also, the IP address of the gateway (the sender IP address) is registered. Again, take a look at pic.3 which shows the new entries (in purple).

Now is the time for R3 to send its own advertisement. Using the same logic you should be able to tell what is going to happen. Take a look at pic. 4 to see what is going to be advertised and what is going to be learned.

The advertisement sent out R3’s F0/0 interface is useless in our topology because there is no other router listening to it. In my next post, I will show you how to prevent a router from doing it. Advertisement sent out F0/1 interface contains information about R3’s directly connected subnet 10.1.3.0/24. Since the existing metric in R3’s routing table for this subnet is ‘0’ (directly connected to F0/0), R3 will add itself as the hop and advertise it with the metric of ‘1’ (existing metric + 1). R2is going to learn it on its F0/1 interface which becomes the outbound interface to reach the subnet 10.1.3.0/24. It is the interface to reach the advertising router’s IP address 10.1.23.3 after all.

Pic. 4 – R3’s RIP Advertisement.

Icons designed by: Andrzej Szoblik - http://www.newo.pl

Picture 4 shows this process.

This whole process of advertising the routing table out of all RIP-enabled interfaces occurs every 30 seconds but in fact, there is a jitter time introduced so this may vary between 25-30 seconds. WhenR2 advertising timer expires, it will pass the information contained in the its routing table on to R1. By doing this, R1 learns about all subnets R2 can reach, including 10.1.3.0/24 now (pic. 5).

Pic. 5 – R2’s RIP Advertisement.

Icons designed by: Andrzej Szoblik - http://www.newo.pl

The process of spreading the information explained using this method is referred to as the ‘routing by rumor’. The state in which all routers have stable information about all networks/subnets that can be reached is called the ‘convergence’. Do not confuse it with ‘convergent networks’ which allow all sort of packet transmissions (voice, video, and data).

Take a look at picture 6. It shows that all routers can reach all the subnets available in the RIP domain. Convergence has been accomplished since their routing table are synchronized and up-to-date.

Pic. 6 – Convergence Achieved.

Icons designed by: Andrzej Szoblik - http://www.newo.pl

The method of distributing information presented is prone to introduce two problems:

• Routing Loops
• Counting to Infinity

Of course, they have been resolved by using different techniques which I am going to explain later in the post.

Let’s take a look at the downside of using distance vector algorithm.

Routing Loops
In the picture 7, R1’s F0/0 interface. As soon as the IOS detects this fact, the entry in the routing table about 10.1.1.0/24 is immediately flushed (removed from the routing table completely).

Pic. 7 – R1’s F0/0 Interface Goes Down.

Icons designed by: Andrzej Szoblik - http://www.newo.pl

As per the DV algorithm R1 would still wait till its advertisement timer expires. So instead of sending this ‘update’ immediately after it has lost the subnet, it will wait till its timer says: ‘now you can advertise your routing table’. This behavior might create a loop between R1 and R2 as far as the 10.1.1.0/24 subnet is concerned. Consider this situation depicted below.

Pic 8 – R2’s Advertising Timer Expires.

Icons designed by: Andrzej Szoblik - http://www.newo.pl


R2 is advertising its full routing table out of all RIP-enabled interfaces. In this announcement, there is 10.1.1.0/24 subnet. The metric being advertised is: ‘2’ (the existing metric on R2 + 1). By now, you already know that the advertising router is going to add itself as the hop to the metric of the subnet/network it advertises.

Here is the issue. R1 is receiving 10.1.1/0/24 with the metric of 2 hops, the egress interface (the one the ad came on) is F0/1, and the next-hop-address is 10.1.12.2. Look at the pic. 8 and tell me (I can’t hear you though), what would you do if you were R1? Obviously, you would reject this information because by looking at the topology diagram, you already know that 10.1.1.0/24 is inaccessible (down) now, and the only way to reach it is through R1, right?

But the problem is, that routers using DV algorithm do NOT know the topology like explained in the characteristics section. In fact, R1 IS going to accept the information and treat R2 as the gateway towards 10.1.1.0/24 !!!

Wow! As ridiculous as it sounds, it is exactly what would happen according to the rules set by the designers of this algorithm. So R1’s routing table is going to look like shown in the picture 8. Take a look at it now again!

We have a loop between R1 and R2 regarding 10.1.1.0/24. If R2 receives the packets destined to 10.1.1.0/24 subnet, according to its knowledge (current routing table), it is going to send it out F0/0 interface towards R1. This one in turn, will use its F0/1 interface for the destination 10.1.1.0/24, sending it back to R2. The packets will be looped until their TTL values are decremented reaching the value of TTL=0. Then, a router must drop the packet.

Counting To Infinity
A routing loop is not going to be the only problem here. R1 is going to accept advertisements fromR2 regarding 10.1.1.0/24 with the number of hops equal '2’. When R1 advertises its own routing table, it is going to add itself (as the hop) to the metric that already exists in the routing table. Look what is going to happen (pic. 9)

Pic. 9 – R1’s RIP Advertisement.

Icons designed by: Andrzej Szoblik - http://www.newo.pl

Initially, R2 is going to ignore the information about 10.1.1.0/24 from R1 containing the metric of ‘3’ hops since it has much better entry in the routing table (lower metric). However, it was R1 that initially sent the metric of ‘1’ hop. Now, the same R1 router keeps sending the metric of ‘3’ hops. The previous metric of ‘1’ is no longer refreshed. Since it uses the aging timer of 180 seconds (how long the information is valid), it finally accepts the entry with the metric of ‘3’ hops instead.

Then R2 begins to advertise the metric of 4 regarding 10.1.1.0/24 subnet out F0/0 and F0/1. You can predict what is going to happen. Remember, that entries must be refreshed every 30 seconds. If they are not refreshed, the ‘Invalidation Timer’ (180 seconds), allows to accept the entry with worse metric than previously. Take a look at the sequence of events in the picture 10.

Pic. 10 – Larger and Larger Metric Propagation.

Icons designed by: Andrzej Szoblik - http://www.newo.pl

It would last forever despite of the fact that 10.1.1.0/24 is not reachable at all!

The Distance Vector algorithm uses a few techniques to prevent these two problems from happening. Here they are:

• Triggered Update (aka flash update)
• Route Poisoning
• Maximum Metric (RIP considers 16 hops as inaccessible)
• Poison Reverse
• Hold-Down Timer
• Split-Horizon

These methods deserve a few words of explanation.

Triggered Update

IOS uses this method to send the update immediately rather than wait for the advertisement timer to expire. However, there is no guarantee that some router in the chain is not going to send its own information before it receives this update. This might still lead to a situation where the two problems occur. So this method, as the only solution here, is not enough to make it work. Other methods must be used as well in order to avoid routing loops and counting to infinity.

Route Poisoning

Upon losing subnet/network reachability, a router is sending a triggered update. This update is going to include the maximum metric value (RIP=16 hops) which is considered as ‘subnet/network inaccessible’ (cannot be reached).

Maximum Metric (RIP=16)

If a RIP router receives an update about a network/subnet with the metric of 16 hops it is considered as inaccessible. This way, the advertising router is excluded from the list of gateways for the subnet/network advertised with the maximum metric.

Poison Reverse

Once a router receives the advertisement including the maximum metric, if it does not have an alternate path towards the subnet/network lost, it is going to send the same subnet/network prefix with the maximum metric (RIP=16) informing the other routers about it. This will also be sent back to the sender of this information it does not have an alternate path (this might be seen as violation of split-horizon, but remember the metric is the maximum value). Poisoning the path back to the advertising router is the way of informing it that the receiver of this information has no alternate path available either.

Hold-Down Timer

Upon receiving information from a neighbor that a subnet/network is inaccessible, the receiving router is going to enable a hold-down timer for 180 seconds. During this time, the receiving router keeps sending packet to the destination being inaccessible for some time rather than withdrawing the entry from its routing table. Why?

In the past, the routers did not have that much power and the media were unreliable. Interfaces were prone to flaps more often than in today’s reliable networks. An ‘interface flap’ is the condition when it goes down and up subsequently in a very short space of time (1-2 seconds perhaps). Under such circumstances, a router would advertise network as inaccessible and then as accessible again. Since it takes some CPU power to withdraw the entry and put it back in, the designers preferred to wait a bit longer to be absolutely sure (180 seconds by default) that the entry was supposed to be removed from the routing table. In case of an interface flapping, not only would the packets still be delivered but the CPU would not waste its ‘precious’ cycles on removing and putting the entry back in the routing table. 

Split-Horizon

This method prevents the loops from occurring in the scenario we have talked about. This technique prevents a router from sending information it learned back out the interface it was received on. Consider our first example. R2 sent information about 10.1.1.0/24 before R1 had had a chance to send the maximum metric towards R2 (subnet down). Split-Horizon prevents R2 from sending information about 10.1.1.0/24 it learned on its F0/0 interface back out the same interface. As a result of that, R1 is never going to receive information it sent towards R2 (10.1.1.0/24) and believe R2 could be the gateway to 10.1.1.0/24. Thus, there is no loop

In my next post I’m going to show you how to enable RIP and how all these techniques work in practice.