Saturday, February 26, 2011

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
  • 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.

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 by R5.

R5 advertises its directly connected network with the metric value of 1. R2R3 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 -

R1 receives the prefix 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 and they are called feasible successors according to the Feasibility Condition:


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, 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.

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