Which of the five EIGRP packets are sent reliably?

Answer 1

EIGRP is an enhanced version of IGRP. The same distance vector technology found in IGRP is also used in EIGRP, and the underlying distance information remains unchanged. The convergence properties and the operating efficiency of this protocol have improved significantly. This allows for an improved architecture while retaining existing investment in IGRP.

The convergence technology is based on research conducted at SRI International. The Diffusing Update Algorithm (DUAL) is the algorithm used to obtain loop-freedom at every instant throughout a route computation. This allows all routers involved in a topology change to synchronize at the same time. Routers that are not affected by topology changes are not involved in the recomputation. The convergence time with DUAL rivals that of any other existing routing protocol.

EIGRP has been extended to be network-layer-protocol independent, thereby allowing DUAL to support other protocol suites.

This paper is an introduction to the Interior Gateway Routing Protocol (IGRP) suite of routing protocols designed and developed by Cisco Systems. This paper should be used as an information-only document intended as a technology introduction and does not represent a protocol specification or product description.

Conventions
For more information on document conventions, see the Cisco Technical Tips Conventions.

Prerequisites
There are no specific prerequisites for this document.

Components Used
This document is not restricted to specific software and hardware versions.

What is IGRP?
IGRP is used in TCP/IP and Open System Interconnection (OSI) Internets. The original IP version was designed and deployed successfully in 1986. It is regarded as an IGP but has also been used extensively as an Exterior Gateway Protocol (EGP) for inter-domain routing. IGRP uses distance vector routing technology. The concept is that each router need not know all the router/link relationships for the entire network. Each router advertises destinations with a corresponding distance. Each router hearing the information adjusts the distance and propagates it to neighboring routers.

The distance information in IGRP is represented as a composite of available bandwidth, delay, load utilization, and link reliability. This allows fine tuning of link characteristics to achieve optimal paths.

EIGRP has four basic components:

Neighbor Discovery/Recovery

Reliable Transport Protocol

DUAL Finite State Machine

Protocol Dependent Modules

Neighbor Discovery/Recovery is the process that routers use to dynamically learn of other routers on their directly attached networks. Routers must also discover when their neighbors become unreachable or inoperative. This process is achieved with low overhead by periodically sending small hello packets. As long as hello packets are received, a router can determine that a neighbor is alive and functioning. Once this is determined, the neighboring routers can exchange routing information.

The reliable transport is responsible for guaranteed, ordered delivery of EIGRP packets to all neighbors. It supports intermixed transmission of multicast or unicast packets. Some EIGRP packets must be transmitted reliably and others need not. For efficiency, reliability is provided only when necessary. For example, on a multi-access network that has multicast capabilities, such as Ethernet, it is not necessary to send hellos reliably to all neighbors individually. So EIGRP, sends a single multicast hello with an indication in the packet informing the receivers that the packet need not be acknowledged. Other types of packets, such as updates, require acknowledgment and this is indicated in the packet. The reliable transport has a provision to send multicast packets quickly when there are unacknowledged packets pending. This helps insure that convergence time remains low in the presence of varying speed links.

The DUAL finite state machine embodies the decision process for all route computations. It tracks all routes advertised by all neighbors. The distance information, known as a metric, is used by DUAL to select efficient loop free paths. DUAL selects routes to be inserted into a routing table based on feasible successors. A successor is a neighboring router used for packet forwarding that has a least cost path to a destination that is guaranteed not to be part of a routing loop. When there are no feasible successors but there are neighbors advertising the destination, a recomputation must occur. This is the process where a new successor is determined. The amount of time it takes to recompute the route affects the convergence time. Even though the recomputation is not processor-intensive, it is advantageous to avoid recomputation if it is not necessary. When a topology change occurs, DUAL will test for feasible successors. If there are feasible successors, it will use any it finds in order to avoid any unnecessary recomputation. Feasible successors are defined in more detail later in this document.

The protocol-dependent modules are responsible for network layer, protocol-specific requirements. For example, the IP-EIGRP module is responsible for sending and receiving EIGRP packets that are encapsulated in IP. IP-EIGRP is responsible for parsing EIGRP packets and informing DUAL of the new information received. IP-EIGRP asks DUAL to make routing decisions and the results of which are stored in the IP routing table. IP-EIGRP is responsible for redistributing routes learned by other IP routing protocols.

EIGRP Concepts
This section describes some details about Cisco's EIGRP implementation. Both data structures and the DUAL concepts are discussed.

Is configuring EIGRP as easy as configuring IGRP?
Yes, you configure EIGRP just like you configure IGRP. You configure a routing process and which networks the protocol should run over. Existing configuration files can be used.

Do I have debugging capabilities like IGRP?
Yes, there are both protocol independent and dependent debug commands that inform you what the protocol is doing. There is a suite of show commands that give you neighbor table status, topology table status, and EIGRP traffic statistics.

Are the same features available in IP-EIGRP as are available in IP-IGRP?
All of the features that you have used in IGRP are available in EIGRP. One feature to point out is multiple routing processes. You can use a single process that runs both IGRP and EIGRP. You can use multiple processes that run both. You can use one process that runs IGRP and another to run EIGRP. You can mix and match. This can help customize your routing to a particular protocol as your needs change.

How much bandwidth and processor resources does EIGRP use?
The bandwidth utilization issue has been addressed by implementing partial and incremental updates. Therefore, only when a topology change occurs does routing information get sent. Regarding processor utilization, the feasible successor technology greatly reduces the total processor utilization of an AS by requiring only the routers that were affected by a topology change to perform the route recomputation. Furthermore, the route recomputation only occurs for routes that were affected. Only those data structures are accessed and used. This greatly reduces search time in complex data structures.

Does IP-EIGRP support aggregation and variable length subnet masks?
Yes it does. IP-EIGRP performs route aggregation the same way IGRP does. That is, subnets of an IP network are not advertised over another IP network. The subnet routes are summarized into a single network number aggregate. In addition, IP-EIGRP will allow aggregation on any bit boundary in an IP address and can be configured at network interface granularity.

Does EIGRP support areas?
No, a single EIGRP process is analogous to an area of a link-state protocol. However, within the process, information can be filtered and aggregated at any interface boundary. If one wants to bound the propagation of routing information, multiple routing processes can be configured to achieve a hierarchy. Since DUAL itself limits route propagation, multiple routing processes are typically used to define organizational boundaries.

http://www.cisco.com/en/US/tech/tk365/technologies_tech_note09186a0080093f07.shtml#is_configuring_easy

www.it.jcu.edu.au/Subjects/cp2240/2006-2/Summaries/Chapter%203-3.doc

www.mdh.se/netcenter/ct3790/ct3790_HT2004_p2/F2_EIGRP_CT3790.ppt

www.blackroute.net/eigrp.pdf

DESCRIPTION
EIGRP Enhanced Interior Gateway Routing Protocol (EIGRP) is an enhanced version of IGRP. IGRP is Cisco's Interior Gateway Routing Protocol used in TCP/IP and OSI internets. It is regarded as an interior gateway protocol (IGP) but has also been used extensively as an exterior gateway protocol for inter-domain routing. IGRP uses distance vector routing technology. The same distance vector technology found in IGRP is also used in EIGRP, and the underlying distance information remains unchanged. The convergence properties and the operating efficiency of this protocol have improved significantly.

Here are some of EIGRP's improvements over IGRP:

DUAL (Diffusing Update Algorithm)

Incremental updates

Loop-free networks

Reduced bandwidth usage

Support for multiple network layer protocols (IP, IPX, AppleTalk)

Support for variable-length subnet masks (VLSMs), discontiguous networks, and classless routing

Advanced distance vector capabilities

Automatic route summarization on major network boundaries


Enhanced IGRP provides compatibility and seamless interoperation with IGRP routers. An automatic-redistribution mechanism allows IGRP routes to be imported into Enhanced IGRP, and vice versa, so it is possible to add Enhanced IGRP gradually into an existing IGRP network. Because the metrics for both protocols are directly translatable, they are as easily comparable as if they were routes that originated in their own autonomous systems (***). In addition, Enhanced IGRP treats IGRP routes as external routes and provides a way for the network administrator to customize them.

EIGRP uses bandwidth and delay by default to calculate its metric. It can also be configured to use reliability, load, and MTU. EIGRP's metric is the same as IGRP's metric, except that it is multiplied by 256 for improved granularity.

Processes and Technologies
To provide superior routing performance, Enhanced IGRP employs four key technologies that combine to differentiate it from other routing technologies: neighbor discovery/recovery, reliable transport protocol (RTP), DUAL finite-state machine, and protocol-dependent modules.

The neighbor discovery/recovery mechanism enables routers to dynamically learn about other routers on their directly attached networks. Routers also must discover when their neighbors become unreachable or inoperative. This process is achieved with low overhead by periodically sending small hello packets. As long as a router receives hello packets from a neighboring router, it assumes that the neighbor is functioning, and the two can exchange routing information.

The DUAL finite-state machine embodies the decision process for all route computations by tracking all routes advertised by all neighbors. DUAL uses distance information to select efficient, loop-free paths and selects routes for insertion in a routing table based on feasible successors. A feasible successor is a neighboring router used for packet forwarding that is a least-cost path to a destination that is guaranteed not to be part of a routing loop. When a neighbor changes a metric, or when a topology change occurs, DUAL tests for feasible successors. If one is found, DUAL uses it to avoid recomputing the route unnecessarily. When no feasible successors exist but neighbors still advertise the destination, a recomputation (also known as a diffusing computation) must occur to determine a new successor. Although recomputation is not processor-intensive, it does affect convergence time, so it is advantageous to avoid unnecessary recomputations.

Protocol-dependent modules are responsible for network layer protocol-specific requirements. The IP-Enhanced IGRP module, for example, is responsible for sending and receiving Enhanced IGRP packets that are encapsulated in IP. Likewise, IP-Enhanced IGRP is also responsible for parsing Enhanced IGRP packets and informing DUAL of the new information that has been received. IP-Enhanced IGRP asks DUAL to make routing decisions, the results of which are stored in the IP routing table. IP-Enhanced IGRP is responsible for redistributing routes learned by other IP routing protocols.

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Processes and Technologies
To provide superior routing performance, Enhanced IGRP employs four key technologies that combine to differentiate it from other routing technologies: neighbor discovery/recovery, reliable transport protocol (RTP), DUAL finite-state machine, and protocol-dependent modules.

The neighbor discovery/recovery mechanism enables routers to dynamically learn about other routers on their directly attached networks. Routers also must discover when their neighbors become unreachable or inoperative. This process is achieved with low overhead by periodically sending small hello packets. As long as a router receives hello packets from a neighboring router, it assumes that the neighbor is functioning, and the two can exchange routing information.

The DUAL finite-state machine embodies the decision process for all route computations by tracking all routes advertised by all neighbors. DUAL uses distance information to select efficient, loop-free paths and selects routes for insertion in a routing table based on feasible successors. A feasible successor is a neighboring router used for packet forwarding that is a least-cost path to a destination that is guaranteed not to be part of a routing loop. When a neighbor changes a metric, or when a topology change occurs, DUAL tests for feasible successors. If one is found, DUAL uses it to avoid recomputing the route unnecessarily. When no feasible successors exist but neighbors still advertise the destination, a recomputation (also known as a diffusing computation) must occur to determine a new successor. Although recomputation is not processor-intensive, it does affect convergence time, so it is advantageous to avoid unnecessary recomputations.

Protocol-dependent modules are responsible for network layer protocol-specific requirements. The IP-Enhanced IGRP module, for example, is responsible for sending and receiving Enhanced IGRP packets that are encapsulated in IP. Likewise, IP-Enhanced IGRP is also responsible for parsing Enhanced IGRP packets and informing DUAL of the new information that has been received. IP-Enhanced IGRP asks DUAL to make routing decisions, the results of which are stored in the IP routing table. IP-Enhanced IGRP is responsible for redistributing routes learned by other IP routing protocols.

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Capabilities and Attributes
Key capabilities that distinguish Enhanced IGRP from other routing protocols include fast convergence, support for variable-length subnet mask, support for partial updates, and support for multiple network layer protocols.

A router running Enhanced IGRP stores all its neighbors' routing tables so that it can quickly adapt to alternate routes. If no appropriate route exists, Enhanced IGRP queries its neighbors to discover an alternate route. These queries propagate until an alternate route is found.

Its support for variable-length subnet masks permits routes to be automatically summarized on a network number boundary. In addition, Enhanced IGRP can be configured to summarize on any bit boundary at any interface.

Routing Concepts

Neighbor Tables
When a router discovers a new neighbor, it records the neighbor's address and interface as an entry in the neighbor table. One neighbor table exists for each protocol-dependent module. When a neighbor sends a hello packet, it advertises a hold time, which is the amount of time that a router treats a neighbor as reachable and operational. If a hello packet is not received within the hold time, the hold time expires and DUAL is informed of the topology change.


Topology Tables
The topology table contains all destinations advertised by neighboring routers. The protocol-dependent modules populate the table, and the table is acted on by the DUAL finite-state machine. Each entry in the topology table includes the destination address and a list of neighbors that have advertised the destination. For each neighbor, the entry records the advertised metric, which the neighbor stores in its routing table. An important rule that distance vector protocols must follow is that if the neighbor advertises this destination, it must use the route to forward packets.


Route States
A topology-table entry for a destination can exist in one of two states: active or passive. A destination is in the passive state when the router is not performing a recomputation; it is in the active state when the router is performing a recomputation. If feasible successors are always available, a destination never has to go into the active state, thereby avoiding a recomputation.


Route Tagging
Enhanced IGRP supports internal and external routes. Internal routes originate within an Enhanced IGRP AS. Therefore, a directly attached network that is configured to run Enhanced IGRP is considered an internal route and is propagated with this information throughout the Enhanced IGRP AS. External routes are learned by another routing protocol or reside in the routing table as static routes. These routes are tagged individually with the identity of their origin.


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Packet Types
Enhanced IGRP uses the following packet types: hello and acknowledgment, update, and query and reply.

Hello packets are multicast for neighbor discovery/recovery and do not require acknowledgment. An acknowledgment packet is a hello packet that has no data. Acknowledgment packets contain a nonzero acknowledgment number and always are sent by using a unicast address.


Update packets are used to convey reachability of destinations. When a new neighbor is discovered, unicast update packets are sent so that the neighbor can build up its topology table. In other cases, such as a link-cost change, updates are multicast. Updates always are transmitted reliably.


Query and reply packets are sent when a destination has no feasible successors. Query packets are always multicast. Reply packets are sent in response to query packets to instruct the originator not to recomputed the route because feasible successors exist. Reply packets are unicast to the originator of the query. Both query and reply packets are transmitted reliably.

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Answer 2

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