Module 3 : OSPF

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Module 3 OSPF Part2
By Sang Gon Lee Spring 2009
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Contents
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3.6 Multiarea OSPF Operation
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Large OSPF Network Issues

• With single area
• calculations
• Large routing
• Large LSDB
• Hierarchical area
• Reduced frequency of SPF calculations
• - isolate route flapping
• Smaller routing tables
• Reduced LSU overhead

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An advantage of Multiple Areas-FYI

• Question I understand the routing table is
  recalculated every time the router receives an
  new version of an LSA. Does OSPF recalculate its
  routing table when their is a topology change in
  another area? show ip ospf displays no change in
  SPF execution, but show ip ospf database shows a
  change in the topology?
• Answer Good question! OSPF areas are designed to
  keep issues like flapping links within an area.
  SPF is not recalculated if the topology change is
  in another area. The interesting thing is that
  OSPF distributes inter-area (between areas)
  topology information using a distance-vector
  method. OSPF uses link-state principles only
  within an area. ABRs do not announce topological
  information between areas, instead, only routing
  information is injected into other areas. ABRs
  relay routing information between areas via
  distance vector technique similar to RIP or IGRP.
  This is why show ip ospf does not show a change
  in the number of times SPF has been executed when
  the topology change is in another area.
• Note It is still a good idea to perform route
  summarization between areas, announcing multiple
  routes as a single inter-area route. This will
  hide any changes in one area from affecting
  routing tables in other areas.

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OSPF LSA Types
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OSPF LSA Types(??)
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OSPF LSA Types(??)
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OSPF LSA Types(??)
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OSPF LSA Types(??)
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OSPF LSA Types(??)
LSA type source Flooding ?? ? ? Link State ID
1 (router LSA) ?? router Area ?? ?? ??? ?? ?? router
2 (Network LSA) DR Area ?? ?? ????? ???? ? router IP of DR
3(summary LSA) ABR AS ??, ABR? ??? ?? ?? ?? ?? ????, ???? ??. network number
4(summary LSA) ABR AS ??, ABR? ??? ?? ASBR? ?? ? type1 LSA ASBR ??? ID
5(Extenal LSA) ASBR AS ??, ABR? ??? ?? routes to networks outside the OSPF autonomous system. ?? ???? number
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Interpreting the OSPF LSDB and Routing Table

• Link ID Identifies each LSA.
• ADV Router Advertising router
• Age The age in sec, maximum age 3,600 sec.
• Seq Sequence number of the LSA.
• Checksum Checksum of the individual LSA .
• Link count Total number of directly attached
  links,

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Interpreting the Routing Table
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OSPF LSA Types(??)

• Type 6 Multicast OSPF
• Type 7 used in not-so-stubby areas (NSSAs),
  originated by ASBRs, NSSA? ??
• Type 8 used in internetworking OSPF and
  Border Gateway Protocol (BGP)
• Types 9, 10, 11 - opaque LSAs, ?? ??????

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Calculating Costs for E1 and E2 Routes

• ???? ?? E1 ??? ASBR?? ????? ?? ?? ???? ??? ?.
• ???? ?? E2 ?? ? ?? ASBR? ?? ???? ????? ??? ?.

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Configuring OSPF LSDB Overload Protection
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Changing the Cost Metric

• Cost 100 Mbps / (bandwidth in Mbps).

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3.7 Stub, Totally Stubby, and Not-So-Stubby Areas
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Configuring OSPF Area Types
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OSPF Area Types

• Standard area ?? ??? LSA? ??.(default). 
• Backbone area (transit area) area ? ?? ?? ??  
• Stub area - AS ?? ??? ?? ??. - Type 4 and 5
  LSAs are blocked, default ?? ? ?? ?.
• Totally stubby area - ?? AS ??? ?? area?
  summary routes ?? ??. - blocks type 4 and 5
  LSAs as well as all summary LSAs (type 3)
• - Type 3 LSA (advertise the default
  route)? ??.
• Not-so-stubby area - ?? ?? ?? ??. - ASBR
  Type 7 LSAs ? flood ?ABR. Type 5 LSA ? AS ??

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Stub and Totally Stubby Areas Rules
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Configuring Stub Area
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OSPF Stub Area Configuration Example
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Configuring Totally Stub Areas
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Totally Stubby Area Configuration Example
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Interpreting Routing Tables
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Interpreting Routing Tables(??)
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Interpreting Routing Tables(??)
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Interpreting Routing Tables(??)

• No O IA code.

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Configuring Not-So-Stubby Areas

• NSSA ??? ??? ???? ??? stub area? ?????.

• Type 7 LSA in the routing table
• O N1 an external type 1 metric
• O N2 an external type 2 metric
• N stands for NSSA.
• The default is O N2.
• ASBR ?? routes? ?? ???. Type 7?? redistribute.
• ABR Type 7? Type 5? ???? area 0? ??. ????.

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Configuring Not-So-Stubby Areas
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Configuring Not-So-Stubby Areas
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Configuring Not-So-Stubby Area Example
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Configuring an NSSA Totally Stubby Area Example
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Verifying All Stub Area Types
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3.8 Virtual Links
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OSPF Virtual Links

• Hello 10sec
• LSA does not age out

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Configuring Virtual Links
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Verifying a Virtual Link Configuration
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3.9 Route Summarization
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ABR and ASBR Implementation

• area ? route
• ABR
• ??? ?? ?
• AS ? route
• ASBR
• ??? ?? ?

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OSPF Route Summarization Example
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Configuring OSPF Route Summarization
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Configuring Route Summarization at ABR

• ?? ???

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Configuring Route Summarization at ASBR
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Benefits of a Default Route in OSPF

• ??? ??? ?? area? ???? ?? 2??
• (1) advertise 0.0.0.0 into the OSPF domain
  ???? router? ?? default route? ??? ???? ??? ???
  ?.
• (2) advertise 0.0.0.0 (default route ??? ????)
  default information origination command

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Configuring a Default Route in OSPF

• ?? ??? ???? ??? ???? ???, 0.0.0.0 ??? ??
• Always ??? ??? ???? 0.0.0.0 ??? ??
• Metric type external type 1 ?? 2, ???? 2.

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Example of Configuring a Default Route in OSPF

• multihomed to dual Internet service providers
  (ISPs).
• Provider A preferred, provider B backup.

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3.10 OSPF Authentication
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OSPF Authentication Methods

• Simple password authentication
• (also called plain-text authentication)
• Message Digest (MD) 5 authentication

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OSPF Authentication Methods
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Ex. Configuring Simple Password Auth.
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Configuring MD5 Authentication

• A key and key ID must be configured on each
  router.

• Key ??

• ???? ?? - MD5

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Example of Configuring MD5 Authentication
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Troubleshooting Simple Password Authentication
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Troubleshooting Simple Password Authentication
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Troubleshooting MD5 Authentication
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Troubleshooting MD5 Authentication