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• 
  Background
• 
  The Enhanced Interior Gateway Routing Protocol (IGRP) represents an evolution from its
  predecessor IGRP  This evolution resulted from changes in networking and the demands of diverse,
• large-scale internetworks.
• 
  Enhanced IGRP integrates the capabilities of link-state protocols into distance-vector protocols. It
  incorporates the Diffusing-Update Algorithm (DUAL) developed at SRI International by Dr. J.J.
  Garcia-Luna-Aceves.
• 
  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 (ASs). In addition, Enhanced IGRP
• treats IGRP routes as external routes and provides a way for the network administrator to customize them.
   
• 
  Enhanced IGRP Capabilities and Attributes
• 
  Key capabilities that distinguish Enhanced IGRP from other routing protocols include fast
  convergence, support 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.
• 
  Enhanced IGRP does not make periodic updates. Instead, it sends partial updates only when the
  metric for a route changes. Propagation of partial updates is automatically bounded so that only
  those routers that need the information are updated. As a result of these two capabilities, Enhanced
  IGRP consumes significantly less bandwidth than IGRP.
• 
  Enhanced IGRP includes support for AppleTalk, IP, and Novell NetWare. The AppleTalk
  implementation redistributes routes learned from the Routing Table Maintenance Protocol (RTMP).
  The IP implementation redistributes routes learned from OSPF, Routing Information Protocol (RIP),
  IS-IS, Exterior Gateway Protocol (EGP), or Border Gateway Protocol (BGP). The Novell
  implementation redistributes routes learned from Novell RIP or Service Advertisement Protocol
  (SAP).
• 
  Underlying 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.
• 
  Neighbor discovery/recovery is used by 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.
• 
  Reliable Transport Protocol (RTP) is responsible for guaranteed, ordered delivery of
  Enhanced IGRP packets to all neighbors. It supports intermixed transmission of multicast or unicast
  packets. For efficiency, only certain Enhanced IGRP packets are transmitted reliably.
•  
• On a multiaccess network that has multicast capabilities, such as Ethernet, it is not necessary to send hello
  packets reliably to all neighbors individually. For that reason, Enhanced IGRP sends a single
  multicast hello packet containing an indicator that informs the receivers that the packet need not be
  acknowledged.
•  
• Other types of packets, such as updates, indicate in the packet that acknowledgment
  is required. RTP contains a provision for sending multicast packets quickly when unacknowledged
  packets are pending, which helps ensure that convergence time remains low in the presence of
  varying speed links.
• 
  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.
• 
  Routing Concepts
• 
  Enhanced IGRP relies on four fundamental concepts: neighbor tables, topology tables, route states,
  and route tagging. Each of these is summarized in the discussions that follow.
  
  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 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.
• 
  The neighbor-table entry also includes information required by RTP. Sequence numbers are
  employed to match acknowledgments with data packets, and the last sequence number received from
  the neighbor is recorded so that out-of-order packets can be detected.
•  
• A transmission list is used to queue packets for possible retransmission on a per-neighbor basis. Round-trip
• timers are kept in the neighbor-table entry to estimate an optimal retransmission interval.
• 
  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.
• 
  The metric that the router uses to reach the destination is also associated with the destination. The
  metric that the router uses in the routing table, and to advertise to other routers, is the sum of the best
  advertised metric from all neighbors, plus the link cost to the best neighbor.
• 
  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, or 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.
• 
  A recomputation occurs when a destination has no feasible successors. The router initiates the
  recomputation by sending a query packet to each of its neighboring routers. The neighboring router
  can send a reply packet, indicating it has a feasible successor for the destination, or it can send a
  query packet, indicating that it is participating in the recomputation.
•  
• While a destination is in the active state, a router cannot change the destination's routing-table information.
• After the router has received a reply from each neighboring router, the topology-table entry for the destination
• returns to the passive state, and the router can select a successor.
• 
  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.
• 
  External routes are tagged with the following information:
• 
  . Router ID of the Enhanced IGRP router that redistributed the route
  . AS number of the destination
  . Configurable administrator tag
  . ID of the external protocol
  . Metric from the external protocol
  . Bit flags for default routing
• 
  Route tagging allows the network administrator to customize routing and maintain flexible policy
  controls. Route tagging is particularly useful in transit ASs, where Enhanced IGRP typically
  interacts with an interdomain routing protocol that implements more global policies, resulting in a
  very scalable, policy-based routing.
• 
  Enhanced IGRP 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
  non-zero 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 recompute 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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