Sybex CCNA 640-802

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Sybex CCNA 640-802 Chapter 7 EIGRP and OSPF
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• Enhanced IGRP (EIGRP)
• EIGRP tables
• Configuring EIGRP
• Verifying EIGRP
• Open Shortest Path First (OSPF)
• Configuring OSPF
• Verifying OSPF
• Configuring OSPF with wildcards

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• EIGRP is an advanced distance-vector routing
  protocol that relies on features commonly
  associated with link-state protocols.
• EIGRP uses Link State's partial updates and
  neighbor discovery.
• EIGRP's advanced features supports IP, IPX and
  AppleTalk.
• EIGRP uses RTP (Reliable Transport Protocol) to
  transport its routing updates

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IP RoutingProtocols
IP RoutingProtocols
AppleTalk Routing Protocol
EnhancedIGRP
AppleTalk Routing Protocol
IPX RoutingProtocols
IPX RoutingProtocols

• Enhanced IGRP supports
• Rapid convergence
• Reduced bandwidth usage
• Multiple network-layer support
• EIGRP includes support for AppleTalk, IP, and
  Novell NetWare as well as IP and IP v.6. The
  AppleTalk implementation redistributes routes
  learned from the Routing Table Maintenance
  Protocol (RTMP). The IP implementation
  redistributes routes learned from OSPF, RIP,
  IS-IS, EGP and BGP. The Novell implementation
  redistributes routes learned from Novell RIP or
  Service Advertisement Protocol (SAP).

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IP RoutingProtocols
IP RoutingProtocols
AppleTalk Routing Protocol
EnhancedIGRP
AppleTalk Routing Protocol
IPX RoutingProtocols
IPX RoutingProtocols

• Enhanced IGRP supports
• Uses Diffused Update Algorithm (DUAL) to select
  loop-free routes and enable fast convergence.
• DUAL enables EIGRP routers to determine whether a
  path advertised by a neighbor is looped or
  loop-free, and
• Allows a router running EIGRP to find alternate
  paths without waiting on updates from other
  routers.
• Up to 6 unequal paths to remote network, default
  4

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• Both IGRP and EIGRP
• Use Autonomous Systems (AS) to divide the
  internetwork
• This is the number that you include when
  configuring the protocol e.g. router (e)igrp
  1
• All routers in the same AS
• use at least one common protocol,
• share the same routing information and
• are contiguous.
• They also use the same metrics bandwidth,
  delay, load and reliability with MTU as a
  tiebreaker.
• Same load balancing properties
• Maximum hop count of 255 (100 default)

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• But EIGRP
• Includes the subnet mask information in its
  routing updates, which allows the use of VLSM.
• And helps EIGRP to differentiate between internal
  (within an AS) and external (between ASs) routes
• Also, it does not send any periodic updates
  (which IGRP sends every 90 seconds)
• EIGRP has improved convergence time much faster
  than IGRP
• EIGRP also sharply reduces network overhead

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• No updates Route updates sent only when a
  change occurs multicast on 224.0.0.10
• Hello messages sent to neighbors every 5
  seconds (60 seconds in most WANs)

hello
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• EIGRP and PDMs (Protocol-dependent modules )
• Supports IP (and through IP (v.4 6), IPX, OSPF,
  IS-IS, RIP and RIP v.2, EGP (Exterior Gateway
  Protocol), and BGP (Border Gate Protocol),
  AppleTalk which gives you RTMP (Routing Table
  Maintenance Protocol), and Novell NetWare,
  supporting IPX, Novell RIP and SAP (Service Ad
  Protocol).
• EIGRP supports more protocols than any other
  routing protocol, (only IS-IS comes close), by
  using PDMs.
• Each PDM keeps its own set of routing tables.
• PDMs are responsible for network layer
  protocol-specific requirements.
• The IP-EIGRP module, for example, is responsible
  for sending and receiving EIGRP packets that are
  encapsulated in IP.

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• EIGRP (other features)
• Is Classless
• Supports VLSM and CIDR.
• Supports discontiguous networks
  summarization
• Uses RTP (Reliable Transport Protocol) (see ff)
  for communication uses multicasts and unicasts
  for quick updates with receipts for tracking
  data.
• Uses the DUAL algorithm unique and efficient.

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• EIGRP sends out five different types of packets
• hello,
• update,
• query,
• reply, and
• acknowledge (ACK)
• that establish the initial adjacency between
  neighbors and to keep the topology and routing
  tables current.
• When troubleshooting an EIGRP network, network
  administrators must understand what EIGRP packets
  are used for and how they are exchanged.
• For example, if routers running EIGRP do not form
  neighbor relationships, those routers cannot
  exchange EIGRP updates with each other and cannot
  connect to services across the internetwork.

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• The following terms are related to EIGRP
• Neighbor table (contains neighbors)
• EIGRP routers use hello packets to discover
  neighbors.
• When a router discovers and forms an adjacency
  with a new neighbor, it includes the neighbor's
  address and the interface through which it can be
  reached in an entry in the neighbor table.
• This table is comparable to the neighborship
  (adjacency) database used by link-state routing
  protocols.
• It serves the same purposeensuring bidirectional
  communication between each of the directly
  connected neighbors.
• EIGRP keeps a neighbor table for each network
  protocol supported in other words, the following
  tables could exist an IP neighbor table, an IPX
  and an AppleTalk neighbor table.

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• Topology table (contains updates re all routes)
• When the router dynamically discovers a new
  neighbor, it sends an update about the routes it
  knows to its new neighbor and receives the same
  back.
• These updates populate the topology table.
• The topology table contains all destinations
  advertised by neighboring routers
• in other words, each router stores its neighbors'
  routing tables in its EIGRP topology table.
• If a neighbor is advertising a destination, it
  must be using that route to forward packets
• this rule must be strictly followed by all
  distance vector protocols.
• An EIGRP router maintains a topology table for
  each network protocol configured (IP, IPX, and
  AppleTalk).

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• Advertised distance (AD) feasible distance (FD)
  
• DUAL uses distance information, known as a metric
  or cost, to select efficient, loop-free paths.
• The lowest-cost route is calculated by adding the
  cost between the next-hop router and the
  destinationreferred to as the advertised
  distanceto the cost between the local router and
  the next-hop router. The sum of these costs is
  referred to as the feasible distance.
• Successor
• Is a neighboring router that has a least-cost
  path to a destination (the lowest FD) that is
  guaranteed not to be part of a routing loop
• Successors are used for forwarding packets.
• Multiple successors can exist if they have the
  same FD.

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• Routing table (contains only the best routes)
• Holds the best routes to each destination and is
  used for forwarding packets.
• Successor routes are offered to the routing
  table.
• If a router learns more than one route to exactly
  the same destination from different routing
  sources, it uses the administrative distance to
  determine which route to keep in the routing
  table.
• By default, up to 4 routes to the same
  destination with the same metric can be added to
  the routing table (the table can hold up to 6
  unequal cost paths).
• The router maintains one routing table for each
  network protocol configured.

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• Feasible successor (FS)
• Along with keeping least-cost paths, DUAL keeps
  backup paths to each destination.
• The next-hop router for a backup path is called
  the feasible successor.
• To qualify as a feasible successor, a next-hop
  router must have an AD less than the FD of the
  current successor route
• in other words, a feasible successor is a
  neighbor that is closer to the destination, but
  it is not the least-cost path and, thus, is not
  used to forward data.
• Feasible successors are selected at the same time
  as successors but are kept only in the topology
  table.
• The topology table can maintain multiple feasible
  successors for a destination.

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• If the route via the successor becomes invalid
  (because of a topology change for example) or if
  a neighbor changes the metric, DUAL checks for
  feasible successors to the destination.
• If a feasible successor is found, DUAL uses it,
  thereby avoiding recomputing the route.
• If no suitable feasible successor exists, a
  recomputation must occur to determine the new
  successor.
• Although recomputation is not processor-intensive,
  it does affect convergence time, so it is
  advantageous to avoid unnecessary recomputations.

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• The composite metric is calculated with the
  following formula
• By default, k1k31 and k2k4k50. The default
  composite metric for EIGRP, adjusted for scaling
  factors, is as follows

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• BWmin is in kbps and the sum of delays are in 10s
  of microseconds.
• Example
• The bandwidth and delay for an Ethernet interface
  are 10 Mbps and 1ms, respectively.
• The calculated EIGRP BW metric is as follows
• 256 107/BW 256 107/10,000,
• 256 x (10,000,000/10,000)
• 256 1,000
• 256000

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• EIGRP routers actively establish relationships
  with their neighbors, similar to what Link State
  routers do.
• EIGRP routers establish adjacencies with
  neighbor routers by using small hello packets.
• The Hello protocol uses a multicast address of
  224.0.0.10, and all routers periodically send
  hellos.

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• On hearing hellos, the router creates a table of
  its neighbors.
• The continued receipt of these packets maintains
  the neighbor table
• To become a neighbor, the following 3 conditions
  must be met
• The router must hear a hello packet or an ACK
  from a neighbor.
• The AS number in the packet header must be the
  same as that of the receiving router.
• The neighbors metric settings must be the same.
• Note
• Each Layer 3 protocol has its own neighbor table.

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• EIGRP updates are set only when necessary and are
  sent only to neighboring routers. There is no
  periodic update timer.
• EIGRP use hello packets to learn of neighboring
  routes.
• The holdtime to maintain a neighbor adjacency is
  three times the hello time.
• For hello is not received with the holdtime, the
  neighbor is removed from the table.

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Default Hello Intervals and Hold Time for EIGRP
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• Topics now considered in more detail
• EIGRP relies on four fundamental concepts
• neighbor tables,
• topology tables,
• route states, and
• route tagging.
• Each of these is summarized in the slides that
  follow.

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SRTT Smooth Round Trip timer Time for round
trip to neighbor and back. RTO Retransmission
Time Out Time EIGRP waits to send a packet from
its retransmission queue to a neighbor. Q count -
the number of EIGRP Packets that the software is
waiting to send
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• AD (Advertised distance) is the metric that is
  reported by the neighbor router(s).
• FD (Feasible Distance) Feasible distance is the
  metric that is reported by neighbor router(s),
  plus the cost associated with the forwarding link
  from the local interface to the neighbor
  router(s).
• When multiple paths exist, the local FD is the
  lowest-cost metric to a remote network.
• Feasibility Condition If the AD from a given
  neighbor is less than the locally calculated FD,
  that neighbor meets the criteria to become the
  feasible successor.

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• Successor - A successor is a neighboring router
  that is currently being used for packet
  forwarding it provides the least-cost route to
  the destination and is not part of a routing loop
• Feasible successor - A feasible successor is a
  backup route. Feasible successors provide the
  next lowest-cost path without introducing routing
  loops.
• Feasible successor routes can be used in case the
  existing route fails.
• Packets to the destination network are
  immediately forwarded to the feasible successor,
  which at that point is promoted to the status of
  successor

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• Successor route is used by EIGRP to forward
  traffic to a destination
• A successor routes may be backed up by a feasible
  successor route
• Successor routes are stored in both the topology
  table and the routing table

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• In the previous slide, EIGRP's composite metric
  is replaced by a link cost to simplify
  calculations.
• RTA's topology table includes a list of all
  routes advertised by neighbors.
• For each network, RTA keeps the real (computed)
  cost of getting to that network and also keeps
  the advertised cost (reported distance) from its
  neighbor.

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• RTY is the successor to network 24, by virtue of
  its lowest computed cost 31. This value is also
  the FD to Network 24.
• RTA follows a three-step process to select a
  feasible successor to become a successor for
  Network 24
• Determine which neighbors have a reported
  distance (RD) (AD) to Network 24 that is less
  than 31.
• RTX's RD is 30 lt 31, meet FC and is a feasible
  successor.
• RTZ's RD is 220 gt 31, not meet FC, and cannot be
  a FS.

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(a) Is the Destination Network
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• In this example, (a) is the destination network,
• From Cs point of view, if it goes to (a) via B,
  the FD is 3 and the AD is 1. Others entries are
  computed in the same manner.
• Note in the example that router D does not have a
  feasible successor identified. The FD for router
  D to router A is 2 and the AD via router C is 3.
  Because the AD is larger than the FD, no feasible
  successor is placed in the topology table.

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• Router C has a feasible successor identified
  because the AD for the next hop router is less
  than the FD for the successor.
• How about router E?

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• In the context of routing protocols, convergence
  refers to the speed and ability of a group of
  internetworking devices running a specific
  routing protocol to agree on the topology of an
  internetwork after a change in that topology.
• DUAL results in EIGRP's exceptionally fast
  convergence. Why?
• The FS provides the capability to make an
  immediate switchover to a backup route!

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• The most important table in EIGRP is the neighbor
  table and relationships tracked in the neighbor
  table are the basis for all the EIGRP routing
  update and convergence activity.
• The neighbor table contains information about
  adjacent neighboring EIGRP routers.
• A neighbor table is used to support reliable,
  sequenced delivery of packets.
• An EIGRP router can maintain neighbor tables, one
  for each PDM running (e.gmultiple ., IP, IPX, and
  AppleTalk) routed protocols.

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• Hello packets assist in the discovery of EIGRP
  neighbors.
• The packets are multicast to 224.0.0.10.
• An acknowledgment packet acknowledges the
  reception of an update packet.
• An acknowledgment packet is a hello packet with
  no data.
• Acknowledgment packets are sent to the unicast
  address of the sender of the update packet.

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• Update packets contain the routing information of
  destinations.
• Update packets are unicast to newly discovered
  neighbors otherwise, update packets are
  multicast to 224.0.0.10 when a link metric
  changes.
• Update packets are acknowledged to ensure
  reliable transmission.
• Query packets are sent to ?nd feasible successors
  to a destination.
• Query packets are always multicast.

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• Reply packets are sent to respond to query
  packets.
• Reply packets provide a feasible successor to the
  sender of the query.
• Reply packets are unicast to the sender of the
  query packet.

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• The neighbor table and topology table are held in
  ram and are maintained through the use of hello
  and update packets.

hello
To see all feasible successor routes known to a
router, use the show ip eigrp topology command
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IP
IP
A
B
19.2
AppleTalk
AppleTalk
T1
T1
IPX
IPX
T1
C
D

• EIGRP uses a composite metric to pick the best
  path bandwidth and delay of the line by default.
• EIGRP can load balance across six unequal cost
  paths to a remote network (4 by default)

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AS10
C
A
B
172.16.10.0
10.110.1.0
192.168.0.0
192.168.0.0
Router(config)router eigrp 10Router(config-route
r)network 10.0.0.0Router(config-router)network
172.16.0.0
Enable EIGRP Assign networks
If you use the same AS number for EIGRP as IGRP,
EIGRP will automatically redistribute IGRP into
EIGRP
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• Redistribution is translating one type of routing
  protocol into another.

EIGRP
IGRP
Router B
Router A
Router D
Router C
IGRP and EIGRP translate automatically, as long
as they are both using the same AS number. See
another example - next slide
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• Assuming all default parameters, which route will
  RIP (v1 and v2) take, and which route(s) will
  IGRP and EIGRP take to get from Routers A to B?

T1
T1
56K
10BaseT
Router B
Router A
100BaseT
100BaseT
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• Displays the neighbors discovered by IP Enhanced
  IGRP
• Displays the IP Enhanced IGRP topology table
• Displays current Enhanced IGRP entries in the
  routing table
• Displays the parameters and current state of the
  active routing protocol process
• Displays the number of IP Enhanced IGRP packets
  sent and received

show ip eigrp neighbors
Router
show ip eigrp topology
Router
show ip route eigrp
Router
show ip protocols
Router
show ip eigrp traffic
Router
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P1R1sh ip route output cut Gateway of last
resort is not set D 192.168.30.0/24 90/2172
via 192.168.20.2,000436, Serial0/0 C
192.168.10.0/24 is directly connected,
FastEthernet0/0 D 192.168.40.0/24 90/2681
via 192.168.20.2,000436, Serial0/0 C
192.168.20.0/24 is directly connected,
Serial0/0 D 192.168.50.0/24 90/2707 via
192.168.20.2,000435, Serial0/0 P1R1

• -D is for DUAL
• -90/2172 is the administrative distance and
  cost of the route. The cost of the route is a
  composite metric comprised from the bandwidth and
  delay of the line

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• Large Network support
• Support for multiple Autonomous Systems This is
  one way to break up a large number of hosts.
• VLSM Support and Summarization
• Support for discontiguous networks
• This is a network in which 2 subnets of a
  classful network say, 10.1.0.0 and 10.2.0.0,
  which are both part of the classful 10.0.0.0
  network,
• are separated by a different classful network
  say 172.16.0.0, or any subnet in that network.
• By default, EIGRP does not handle this
  configuration, (only OSPF can), but it can be
  configured to do so.

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• Load Balancing
• EIGRP can handle equal or unequal load balancing
• By default, up to 4 links up to 6 links can be
  configured with the maximum paths command.

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• Initial Setup (pg 426, and Cisco command
  reference)
• Step Command Purpose
• 1 router eigrp autonomous-system Enable an
  EIGRP routing process
  in global config mode.
• 2 network network-number
  Associate networks with an EIGRP
  routing process in router config mode.
• Create a Passive Interface This prohibits an
  interface from sending or
• Router(config-router)passive-interface serial
  0/1 receiving Hellos so it will never form
  
  
  adjacencies.
• Redistribution and Set Metric values
• The following example takes redistributed Routing
  Information Protocol (RIP) metrics and translates
  them into EIGRP metrics with values as follows
  bandwidth 1000, delay 100, reliability 250,
  loading 100, and MTU 1500.
• router eigrp 1
• network 172.16.0.0
  Command Syntax
• redistribute rip redistribute (IP)
• default-metric 1000 100 250 100 1500
  default-metric bandwidth delay reliability
  loading mtu

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• Load Balancing
• This is automatic with EIGRP the only time you
  need to configure it is when you want to vary the
  load over each of several links.
• In this case you would use the traffice-share
  balanced or the variance command
• To control how traffic is distributed among
  routes when there are multiple routes for the
  same destination network that have different
  costs, use the traffic-share balanced command in
  router configuration mode. To disable this
  function, use the no form of the command.
• traffic-share balanced
• To control load balancing in an Enhanced Interior
  Gateway Routing Protocol (EIGRP) based
  internetwork, use the variance command in router
  configuration mode. To reset the variance to the
  default value, use the no form of this command.
• variance multiplier

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• Open standard
• Shortest path first (SPF) algorithm
• Link-state routing protocol (vs. distance vector)
• Can be used to route between ASs

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• Consists of areas and autonomous systems
• Minimizes routing update traffic
• Supports VLSM
• Unlimited hop count

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• Link State
• Provides common view of entire topology
• Calculates shortest path
• Utilizes event-triggered updates
• Can be used to route between ASs

• Distance Vector
• Exchanges routing tables with neighbors
• Utilizes frequent periodic updates

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Area 2
Backbone Area 0
Area 1
ABR and BackboneRouter
Backbone/InternalRouters
InternalRouters
InternalRouters
ASBR andBackbone Router
ABR and BackboneRouter

• External AS

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Feature RIP OSPF
Algorithm Vector-distance Link-state
Maximum Hops 15 hops. 16 hops is considered to be infinity, implying that the destination is unreachable Limited only by size of routing tables within routers
Subsystem Segmentation Treats the autonomous system as a single subsystem Breaks the autonomous system into one or more areas with two levels of routing algorithms, intra-area, and inter-area.
Metric Destination/hop Destination/cost/link identifier
Integrity No authentication in RIP-1, Authentication has been added to RIP-2 Supports Authentication. Several authentication algorithms are available ranging from simple password operations to more complex cryptographic algorithms.
Complexity Relatively Simple More Complex. Several more PDUs and exchanges are defined in the protocol. Routing tables are large and include not only destinations, but also a tree representation of local network.
Acceptance Widely Available, BSD routed supports RIP Newer, published in RFCs
Route Options Identifies a single route to a destination Supports multiple routes to a single destination. Facilitates load-balancing traffic distribution
Types of Routes Host, network. RIP-2 adds the ability to transfer subnetwork route entries Host, network, and subnetwork routes
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Router(config)router ospf ltprocess-idgt
Defines OSPF as the IP routing protocol. Note
The process ID is locally significant and is
needed to identify a unique instance of an OSPF
database
Router(config-router)network address mask area
ltarea-idgt
Assigns networks to a specific OSPF area
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hostname R3 router ospf 10 network 10.1.2.3 0.0.0.0 area 0 network 10.1.3.1 0.0.0.0 area 0 hostname R2 router ospf 20 network 10.0.0.0 0.255.255.255 area 0 hostname R1 router ospf 30 network 10.1.0.0 0.0.255.255 area 0 network 10.5.5.0 0.0.0.0 area 0
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Routershow ip protocols
Verifies that OSPF is configured
Routershow ip route
Displays all the routes learned by the router
Routershow ip ospf interface
Displays area-ID and adjacency information
Routershow ip ospf neighbor
Displays OSPF-neighbor information on a
per-interface basis
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• OSPF uses hello packets to create adjacencies and
  maintain connectivity with neighbor routers
• OSPF uses the multicast address 224.0.0.5

Hello? 224.0.0.5

• Hello packets provides dynamic neighbor discovery
• Hello Packets maintains neighbor relationships
• Hello packets and LSAs from other routers help
  build maintain the topological database

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• Neighbor
• Two routers that have an interface on a common
  network
• Usually discovered by hellos but can also be
  configured administratively
• Adjacency
• Relationship formed between selected neighbors in
  which routing information is exchanged. Not all
  neighbors are adjacent
• Only Broadcast and Non-Broadcast network types
  have Designated and Backup Designated Routers!!!

Neighbors
ABR
DR
Adjacencies
Non-DR
Cost6
BDR
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Term Description
Link state Information is shared between directly connected routers. This information propagates throughout the network unchanged and is also used to create a shortest path first (SPF) tree.
Area A group of routers that share the same area ID. All OSPF routers require area assignments.
Autonomous system (AS) A network under a common network administration.
Cost The routing metric used by OSPF. Lower costs are always preferred. You can manually configure the cost with the ip ospf cost command. By default, the cost is calculated by using the formula cost 108 / bandwidth.
Router ID Each OSPF router requires a unique router ID, which is the highest IP address configured on a Cisco router or the highest numbered loopback address. You can manually assign the router ID.
Adjacency When two OSPF routers have exchanged information between each other and have the same topology table. An adjacency can have the following different states or exchange states
1. Init state When Hello packets have been sent and are awaiting a reply to establish two-way communication. 1. Init state When Hello packets have been sent and are awaiting a reply to establish two-way communication.
2. Establish bi-directional (two-way) communication Accomplished by the discovery of the Hello protocol routers and the election of a DR. 2. Establish bi-directional (two-way) communication Accomplished by the discovery of the Hello protocol routers and the election of a DR.
3. Exstart Two neighbor routers form a master/slave relationship and agree upon a starting sequence to be incremented to ensure LSAs are acknowledged. 3. Exstart Two neighbor routers form a master/slave relationship and agree upon a starting sequence to be incremented to ensure LSAs are acknowledged.
4. Exchange state Database Description (DD) packets continue to flow as the slave router acknowledges the master's packets. OSPF is operational because the routers can send and receive LSAs between each other. DD packets contain information, such as the router ID, area ID, checksum, if authentication is used, link-state type, and the advertising router. LSA packets contain information, such as router ID also but in addition include MTU sizes, DD sequence numbering, and any options. 4. Exchange state Database Description (DD) packets continue to flow as the slave router acknowledges the master's packets. OSPF is operational because the routers can send and receive LSAs between each other. DD packets contain information, such as the router ID, area ID, checksum, if authentication is used, link-state type, and the advertising router. LSA packets contain information, such as router ID also but in addition include MTU sizes, DD sequence numbering, and any options.
5. Loading state Link-state requests are sent to neighbors asking for recent advertisements that have not yet been discovered. 5. Loading state Link-state requests are sent to neighbors asking for recent advertisements that have not yet been discovered.
6. Full state Neighbor routers are fully adjacent because their link-state databases are fully synchronized. Routing tables begin to be populated. 6. Full state Neighbor routers are fully adjacent because their link-state databases are fully synchronized. Routing tables begin to be populated.
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Topology table Also called the link-state table. This table contains every link in the whole network.
Designated router (DR) This router is responsible for ensuring adjacencies between all neighbors on a multiaccess network (such as Ethernet). This ensures all routers do not need to maintain full adjacencies with each other.
The DR is selected based on the router priority. In a tie, the router with the highest router ID is selected. The DR is selected based on the router priority. In a tie, the router with the highest router ID is selected.
Backup DR A backup router designed to perform the same functions in case the DR fails.
Link-state advertisement (LSA) A packet that contains all relevant information regarding a router's links and the state of those links.
Priority Sets the router's priority so a DR or BDR can be correctly elected.
Router links Describe the state and cost of the router's interfaces to the area. Router links use LSA type 1.
Summary links Originated by area border routers (ABRs) and describe networks in the AS. Summary links use LSA types 3 and 4.
Network links Originated by DRs. Network links use LSA type 2.
External links Originated by autonomous system boundary routers (ASBRs) and describe external or default routes to the outside (that is, non- OSPF) devices for use with redistribution. External Links use the LSA type 5.
Area border router (ABR) Router located on the border of one or more OSPF areas that connects those areas to the backbone network.
Autonomous system boundary router (ASBR)   ABR located between an OSPF autonomous system and a non-OSPF network.
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• Each router in OSPF needs to be uniquely
  identified to properly arrange them in the
  Neighbor tables.

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Multicast Hellos are sent and compared Router
with Highest Priority is Elected as DR Router
with 2nd Highest Priority is Elected as BDR

• OSPF sends Hellos which elect DRs and BDRs
• Routers form adjacencies with DRs and BDRs in a
  multi-access environment
• The next slide covers loopback interfaces. The
  reason you would configure a loopback (a logical
  interface) is to assign it the highest priority
  interface on the router, thus ensuring that it
  will become the DR.
• This avoids the router selecting a physical
  interface as DR, which is sometimes undesirable
  because physical interfaces can go up and down
  and sometimes fail to provide a stable routing
  environment.

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• Router ID (RID)
• Number by which the router is known to OSPF
• Default The highest IP address on an active
  interface at the moment of OSPF process startup
• Can be overridden by a loopback interface
  Highest IP address of any active loopback
  interface also called a logical interface

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• What is the default OSPF interface priority?
• Router show ip ospf interface ethernet0/0
• Ethernet0 is up, line protocol is up
• Internet Address 192.168.1.137/29, Area 4
• Process ID 19, Router ID 192.168.1.137, Network
  Type BROADCAST,
• Cost 10 Transmit Delay is 1 sec, State DR,
  Priority 1
• Designated Router (ID) 192.168.1.137, Interface
  address 192.168.1.137
• No backup designated router on this network
• Timer intervals configured, Hello 10, Dead 40,
  Wait 40, Retransmit 5
• Hello due in 000006
• Index 2/2, flood queue length 0
• Next 0x0(0)/0x0(0)
• Last flood scan length is 0, maximum is 0
• Last flood scan time is 0 msec, maximum is 0 msec
• Neighbor Count is 0, Adjacent neighbor count is 0
• Suppress hello for 0 neighbor(s)

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• ip ospf priority
• To set the router priority, which helps determine
  the designated router for this network, use the
  ip ospf priority command in interface
  configuration mode.
• To return to the default value, use the no form
  of this command.
• ip ospf priority number-value
• no ip ospf priority number-value
• Syntax Description
• number-value ltA number value that specifies the
  priority of the router. The range is from 0 to
  255gt

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• What options can you configure that will ensure
  that R2 will be the DR of the LAN segment?

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• If you want to advertise a partial octet
  (subnet), you need to use wildcards.
• 0.0.0.0 means all octets match exactly
• 0.0.0.255 means that the first three match
  exactly, but the last octet can be any value
• After that, you must remember your block sizes.

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• The wildcard address is always one less than the
  block size.
• 192.168.10.8/30 0.0.0.3
• 192.168.10.48/28 0.0.0.15
• 192.168.10.96/27 0.0.0.31
• 192.168.10.128/26 0.0.0.63
• What the author means is that where, in the first
  line, youve borrowed 6 bits to get a /30 subnet
  mask, and this would give you 64 subnets with 4
  hosts in each! The 4 is the block size that
  the author refers to. So, in the wildcard, the
  last number must be one less than 4, or 3.
• Same thing in line 2 /28 means 4 bits borrowed
  this gives you 16 subnets with 16 hosts in each.
  Block size is 16 and the wildcard is 16-1, or 15.

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• Lab_B
• E0 192.168.40.1/24
• S0 192.168.10.10/30
• S1 192.168.10.6/30

• Lab_C
• E0 192.168.50.1/24
• S1 172.16.10.9/30

• Lab_A
• E0 192.168.30.1/24
• S0 172.16.10.5/30

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• Go through all the written and review questions
• Go over the answers with the class

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