EIGRP

1
EIGRP

• Enhanced interior gateway routing protocol
  (EIGRP) is a Cisco-proprietary routing protocol
  based on IGRP.
• EIGRP supports CIDR, allowing network designers
  to maximize address space by using CIDR and VLSM.
  
• Faster convergence times, improved scalability,
  and superior handling of routing loops.

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EIGRP

• EIGRP can replace Novell RIP and AppleTalk
  Routing Table Maintenance Protocol (RTMP).
• EIGRP is an advanced distance-vector routing
  protocol that relies on features commonly
  associated with link-state protocols.
• Like OSPF, partial updates and neighbor
  discovery, are similarly put to use by EIGRP.

3
EIGRP and IGRP

• EIGRP is an ideal choice for large, multiprotocol
  networks built primarily on Cisco routers. It is
  Cisco proprietary.

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EIGRP and IGRP

• Despite being compatible with IGRP, EIGRP uses a
  different metric calculation and hop-count
  limitation. EIGRP scales IGRP's metric by a
  factor of 256. That is because EIGRP uses a
  metric that is 32 bits long, and IGRP uses a
  24-bit metric. By multiplying or dividing by 256,
  EIGRP can easily exchange information with IGRP.

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EIGRP and IGRP

• EIGRP also imposes a maximum hop limit of 224,
  slightly less than IGRP's generous 255.
• Redistribution, is automatic between IGRP and
  EIGRP as long as both processes use the same
  autonomous system (AS) number.

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EIGRP and IGRP

• EIGRP will tag routes learned from IGRP (or any
  outside source) as external because they did not
  originate from EIGRP routers.
• IGRP cannot differentiate between internal and
  external routes.
• Notice that in the show ip route command, EIGRP
  routes are flagged with D, and external routes
  are denoted by EX.

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EIGRP Design

• EIGRP acts like a link-state protocol when
  updating neighbors and maintaining routing
  information. EIGRP's advantages are

8
EIGRP Design

• Rapid convergence-converges quickly because EIGRP
  relies on a state-of-the-art routing algorithm
  called the Diffusing Update Algorithm (DUAL).
  DUAL guarantees loop-free operation at every
  instant throughout a route computation and allows
  all routers involved in a topology change to
  synchronize at the same time.

9
EIGRP Design

• Efficient use of bandwidth- Partial, bounded
  updates - EIGRP routers make partial, incremental
  updates rather than sending their complete
  tables. EIGRP routers send these partial updates
  only to the routers that need the information,
  not to all routers in an area like OSPF does.

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EIGRP Design

• Minimal consumption of bandwidth when the network
  is stable - Instead of using timed routing
  updates EIGRP routers keep in touch with each
  other using small hello packets.
• Support for VLSM and CIDR.
• Multiple network-layer support - EIGRP supports
  IP, IPX, and AppleTalk through protocol-dependent
  modules (PDMs).

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EIGRP Design and Terminology

• EIGRP keeps routing and topology information in
  several tables, or databases. The following terms
  are related to EIGRP and its tables and are used
  throughout this chapter
• Neighbor table - Each EIGRP router maintains a
  neighbor table that lists adjacent routers.

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EIGRP Design and Terminology

• Topology table - Every EIGRP router maintains a
  topology table for each configured network
  protocol. All learned routes to a destination are
  maintained in the topology table.

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EIGRP Design and Terminology

• Routing table - EIGRP chooses the best routes to
  a destination from the topology table and places
  these routes in the routing table.

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EIGRP Design and Terminology

• Successor - A successor is a route selected as
  the primary route to use to reach a destination.
  Multiple successors for a destination can be
  retained in the routing table.
• Feasible successor - A feasible successor is a
  backup route. These routes are selected at the
  same time the successors are identified, but are
  kept in the topology table.

15
Neighbor Discovery

• EIGRP routers establish adjacencies with neighbor
  routers by using small hello packets. Hellos are
  sent by default every 5 seconds. An EIGRP router
  assumes that, as long as it is receiving hello
  packets from known neighbors, those neighbors
  (and their routes) remain viable. By forming
  adjacencies, EIGRP routers do the following

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Neighbor Discovery

• Dynamically learn of new routes that join their
  network.
• Identify routers that become either unreachable
  or inoperable.
• Rediscover routers that had previously been
  unreachable.

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RTP-Routing Transport Protocol

• RTP is a transport-layer protocol that can
  guarantee ordered delivery of EIGRP packets to
  all neighbors.
• EIGRP is protocol-independent (i.e., it does not
  rely on TCP/IP to exchange routing information
  the way that RIP, IGRP, and OSPF do). To stay
  independent of IP, EIGRP uses its own proprietary
  transport-layer protocol to guarantee delivery of
  routing information RTP.

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DUAL finite-state machine (FSM).

• The centerpiece of EIGRP is the Diffusing Update
  Algorithm (DUAL), EIGRP's route-calculation
  engine. The full name of this technology is DUAL
  finite-state machine (FSM).

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DUAL finite-state machine (FSM).

• An FSM is an abstract machine, not a mechanical
  device with moving parts. FSMs define a set of
  possible states that something can go through,
  what events cause those states, and what events
  result from those states.

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DUAL finite-state machine (FSM).

• Designers use FSMs to describe how a device,
  computer program, or routing algorithm will react
  to a set of input events. The DUAL FSM contains
  all the logic used to calculate and compare
  routes in an EIGRP network.

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DUAL finite-state machine (FSM).

• DUAL tracks all the routes advertised by
  neighbors and uses the composite metric of each
  route to compare them. DUAL also guarantees that
  each path is loop-free. Lowest-cost paths are
  then inserted by the DUAL protocol into the
  routing table.

22
DUAL finite-state machine (FSM).

• DUAL selects alternate routes quickly by using
  the information in the Topology and Routing
  tables. If a link goes down, DUAL looks for a
  feasible successor in its neighbor and topology
  tables.

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PDM-Protocol Dependent Modules

• One of EIGRP's most attractive features is its
  modular design.
• In theory, EIGRP can easily adapt to new or
  revised routed protocols (e.g., IPv6) by adding
  protocol-dependent modules.
• Support for routed protocols such as IP, IPX, and
  AppleTalk is included in EIGRP through
  protocol-dependent modules (PDMs).

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EIGRP Packet Types

• EIGRP relies on several different kinds of
  packets to maintain its various tables and
  establish complex relationships with neighbor
  routers
• Hello PacketsEIGRP relies on hello packets to
  discover, verify, and rediscover neighbor routers.

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EIGRP Packet Types

• If a neighbor is not heard from for the duration
  of the hold time, EIGRP considers that neighbor
  down, and DUAL must step in to re-evaluate the
  routing table. By default, the hold time is three
  times the hello interval, but an administrator
  can configure both timers as desired.

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EIGRP Packet Types

• Acknowledgment PacketsAn EIGRP router uses
  acknowledgment packets to indicate receipt of any
  EIGRP packet during a "reliable" exchange.
• Update PacketsUpdate packets are used when a
  router discovers a new neighbor. Update packets
  are also used when a router detects a topology
  change.

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EIGRP Packet Types

• Query and Reply PacketsEIGRP routers use query
  packets whenever it needs specific information
  from one or all of its neighbors. A reply packet
  is used to respond to a query.
• If an EIGRP router loses its successor and cannot
  find a feasible successor for a route, DUAL
  places the route in the active state.

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EIGRP Packet Types

• At this point, the router multicasts a query to
  all neighbors, searching for a successor to the
  destination network. Neighbors must send replies
  that either provide information on successors or
  indicate that no successor information is
  available.

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EIGRP Tables

• The Neighbor TableThe most important table in
  EIGRP is the neighbor table. An EIGRP neighbor
  table includes the following key elements

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EIGRP Tables

• Neighbor address (Address) - The network-layer
  address of the neighbor router.
• Hold time (Hold Uptime) - The interval to wait
  without receiving anything from a neighbor before
  considering the link unavailable. Originally, the
  expected packet was a hello packet, but in
  current Cisco IOS software releases, any EIGRP
  packets received after the first hello will reset
  the timer.

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EIGRP Tables

• Smooth Round-Trip Timer (SRTT) - The average time
  that it takes to send and receive packets from a
  neighbor. This timer is used to determine the
  retransmit interval (RTO).
• Queue count (Q Cnt) - The number of packets
  waiting in queue to be sent. If this value is
  constantly higher than zero, then there may be a
  congestion problem at the router. A zero means
  that there are no EIGRP packets in the queue.

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EIGRP Tables

• The Routing TableThe routing table contains the
  routes installed by DUAL as the best loop-free
  paths to a given destination.
• EIGRP routers maintain a separate routing table
  for each routed protocol.

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EIGRP Tables

• The Topology TableEIGRP uses its topology table
  to store all the information it needs to
  calculate a set of distances and vectors to all
  reachable destinations. EIGRP maintains a
  separate topology table for each routed protocol.

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EIGRP Tables

• The topology table is made up of all the EIGRP
  routing tables in the autonomous system. By
  tracking this information, EIGRP routers can find
  alternate routes quickly. The topology table
  includes the following fields

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EIGRP Tables

• Feasible distance (FD is xxxx) - The feasible
  distance (FD) is the lowest calculated metric to
  each destination.
• Route source (via xxx.xxx.xxx.xxx) - The source
  of the route is the identification number of the
  router that originally advertised that route.

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EIGRP Tables

• Reported distance (FD/RD) - The reported distance
  (RD) of the path is the distance reported by an
  adjacent neighbor to a specific destination.
• Show IP EIGRP Topology is the command to see the
  above items.

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EIGRP Tables

• EIGRP sorts the topology table so that the
  successor routes are at the top, followed by
  feasible successors. At the bottom, EIGRP lists
  routes that DUAL believes to be loops in the
  topology table.
• Entries in the topology table can be in one of
  two states active or passive.
• A passive route is one that is stable and
  available for use. An active route is a route in
  the process of being recomputed by DUAL.

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Route Tagging

• EIGRP classifies routes as either internal or
  external. EIGRP uses a process called route
  tagging to add special tags to each route. These
  tags identify a route as internal or external,
  and may include other information as well.

39
Route Tagging

• Routes learned (redistributed) from other routing
  protocols, such as RIP, OSPF, and IGRP are
  external. Static routes originating from outside
  the EIGRP AS and redistributed inside are also
  external routes.

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Route Tagging

• External routes can be accepted, rejected, or
  propagated based on any of the route tags,
  including the administrator tag. Because you can
  configure the administrator tag as you see fit,
  the route-tagging feature affords a high degree
  of control. This level of precision and
  flexibility proves especially useful when EIGRP
  networks interact with BGP networks, which
  themselves are policy-based.