Skip to main content

EIGRP algorithm


EIGRP algorithm
3.1.6

This page will describe the DUAL algorithm, which results in the exceptionally fast convergence of EIGRP.
The sophisticated DUAL algorithm results in the exceptionally fast convergence of EIGRP. To better understand convergence with DUAL, consider the example in Figure . Each router has constructed a topology table that contains information about how to route to destination Network A.
Each topology table identifies the following information:
  • The routing protocol or EIGRP
  • The lowest cost of the route, which is called feasible distance (FD)
  • The cost of the route as advertised by the neighboring router, which is called reported distance (RD)
The Topology column identifies the primary route called the successor route (successor), and, where identified, the backup route called the feasible successor (FS). Note that it is not necessary to have an identified feasible successor.
The EIGRP network follows a sequence of actions to allow convergence between the routers, which currently have the following topology information:
  • Router C has one successor route by way of Router B.
  • Router C has one feasible successor route by way of Router D.
  • Router D has one successor route by way of Router B.
  • Router D has no feasible successor route.
  • Router E has one successor route by way of Router D.
  • Router E has no feasible successor.
The feasible successor route selection rules are specified in Figure .
The following example demonstrates how each router in the topology will carry out the feasible successor selection rules when the route from Router D to Router B goes down:
In Router D: 
  • Route by way of Router B is removed from the topology table.
  • This is the successor route. Router D has no feasible successor identified.
  • Router D must complete a new route computation.
In Router C:
  • Route to Network A by way of Router D is down.
  • Route by way of Router D is removed from the table.
  • This is the feasible successor route for Router C.
In Router D:  
  • Router D has no feasible successor. It cannot switch to an identified alternative backup route.
  • Router D must recompute the topology of the network. The path to destination Network A is set to Active.
  • Router D sends a query packet to all connected neighbors to request topology information.
  • Router C does have a previous entry for Router D.
  • Router D does not have a previous entry for Router E.
In Router E:
  • Route to Network A through Router D is down.
  • The route by way of Router D is removed from the table.
  • This is the successor route for Router E.
  • Router E does not have a feasible route identified.
  • Note that the RD cost of routing by way of Router C is 3. That is the same cost as the successor route by way of Router D.
In Router C: 
  • Router E sends a query packet to Router C.
  • Router C removes Router E from the table.
  • Router C replies to Router D with a new route to Network A.
In Router D:
  • Route status to destination Network A is still marked as Active. The computation has not been completed yet.
  • Router C has replied to Router D to confirm that a route to destination Network A is available with a cost of 5.
  • Router D still waits for a reply from Router E.
In Router E:
  • Router E has no feasible successor to reach destination Network A.
  • Router E, therefore, tags the status of the route to destination network as Active.
  • Router E has to recompute the network topology.
  • Router E removes the route by way of Router D from the table.
  • Router E sends a query to Router C, to request topology information.
  • Router E already has an entry by way of Router C. It is at a cost of 3, the same as the successor route.
In Router E: 
  • Router C replies with an RD of 3.
  • Router E can now set the route by way of Router C as the new successor with an FD of 4 and an RD of 3.
  • Router E replaces the Active status of the route to destination Network A with a Passive status. Note that a route will have a Passive status by default as long as hello packets are received. In this example, only Active status routes are flagged.
In Router E: 
  • Router E sends a reply to Router D to inform it of the Router E topology information.
In Router D:
  • Router D receives the reply packed from Router E.
  • Router D enters this data for the route to destination Network A by way of Router E.
  • This route becomes an additional successor route as the cost is the same as routing by way of Router C and the RD is less than the FD cost of 5.
Convergence occurs among all EIGRP routers that use the DUAL algorithm.
This page concludes this lesson. The next lesson will discuss the configuration of EIGRP. The first page will explain how EIGRP is configured.

Comments

Post a Comment

Popular posts from this blog

OSI layers / Peer-to-peer communications / TCP/IP model

OSI layers 2.3.4 This page discusses the seven layers of the OSI model. The OSI reference model is a framework that is used to understand how information travels throughout a network. The OSI reference model explains how packets travel through the various layers to another device on a network, even if the sender and destination have different types of network media. In the OSI reference model, there are seven numbered layers, each of which illustrates a particular network function. - Dividing the network into seven layers provides the following advantages: • It breaks network communication into smaller, more manageable parts. • It standardizes network components to allow multiple vendor development and support. • It allows different types of network hardware and software to communicate with each other. • It prevents changes in one layer from affecting other layers. • It divides network communication into smaller parts to make learning it easier to understand. In the foll...
Post a Comment
Read more

Advantages and disadvantages of link-state routing

Advantages and disadvantages of link-state routing 2.1.5  This page lists the advantages and disadvantages of link-state routing protocols. The following are advantages of link-state routing protocols:  Link-state protocols use cost metrics to choose paths through the network. The cost metric reflects the capacity of the links on those paths. Link-state protocols use triggered updates and LSA floods to immediately report changes in the network topology to all routers in the network. This leads to fast convergence times. Each router has a complete and synchronized picture of the network. Therefore, it is very difficult for routing loops to occur. Routers use the latest information to make the best routing decisions. The link-state database sizes can be minimized with careful network design. This leads to smaller Dijkstra calculations and faster convergence. Every router, at the very least, maps the topology of it...
Post a Comment
Read more

Ports for services

Ports for services 10.2.2  Services running on hosts must have a port number assigned to them so communication can occur. A remote host attempting to connect to a service expects that service to use specific transport layer protocols and ports. Some ports, which are defined in RFC 1700, are known as the well-known ports. These ports are reserved in both TCP and UDP.  These well-known ports define applications that run above the transport layer protocols. For example, a server that runs FTP will use ports 20 and 21 to forward TCP connections from clients to its FTP application. This allows the server to determine which service a client requests. TCP and UDP use port numbers to determine the correct service to which requests are forwarded. The next page will discuss ports in greater detail.
Post a Comment
Read more