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IS3120 Network Communications Infrastructure Unit 9 Link-State IP Routing, Campus Backbones, WANs, and Classless Inter-Domain Routing (CIDR ). Learning Objective. Formulate and plan the implementation of link-state routing protocols to meet business requirements. Key Concepts.

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IS3120 Network Communications Infrastructure

Unit 9

Link-State IP Routing, Campus Backbones, WANs, and Classless Inter-Domain Routing (CIDR)

learning objective
Learning Objective
  • Formulate and plan the implementation of link-state routing protocols to meet business requirements.
key concepts
Key Concepts
  • Classless Inter-Domain IP Routing (CIDR)
  • Link-state routing protocols
  • OSPF v2
  • OSPF two-tier hierarchical IP backbone network design for an enterprise WAN
  • OSPF, Variable Length Subnet Masking (VLSM) configuration
classless inter domain ip routing cidr review
Classless Inter-Domain IP Routing (CIDR) Review
  • Referred to as “supernetting”
  • An alternative to traditional IP subnetting
  • Assigns IP addresses without using Class A, Class B, or Class C
  • IP address represented as A.B.C.D /n, where "/n" is the number of bits in the mask
ipv4 cidr block syntax
IPv4: CIDR Block Syntax
  • CIDR block: group of IP addresses sharing the same prefix
  • Prefix is determined by
    • Translating dot notation to binary
    • Identifying # of bits that are shared
ipv4 cidr example 1
IPv4: CIDR Example #1
  • Address block
  • Any IP address sharing the same initial 16 bits is in the same network
  • Range is -
ipv4 cidr example 2
IPv4: CIDR Example #2
  • Address Block
  • Any IP address sharing the same initial 20 bits is in the same network
  • Range is -
link state routing protocols
Link-State Routing Protocols
  • Open Shortest Path First (OSPF) for IP
  • Intermediate System to Intermediate System (IS-IS) for Connectionless Network Service and IP
  • Novell NetWare Link Services Protocol (NLSP)
ospf and is is
  • Interior gateway protocols
  • Route IP packets within an autonomous system
  • Use multicast to discover neighbor routers (Hello process)
  • Support variable-length subnet masks and route summarization
ospf v3
  • Based on OSPFv2, updated for IPv6
  • 128-bit addresses
  • Multiple addresses per interface
  • Can run over a link
  • Uses IPSec
  • Generalizes link-state advertisement (LSA) flooding scope
  • Removes authentication from protocol
  • Internal routers
  • Backbone routers
  • Area border router (ABR)
  • Autonomous system boundary router (ASBR)
  • Designated router (DR)
  • Backup designated router (BDR)
broadcast networks
Broadcast Networks
  • DRs and BDRs
  • Only DR and BDR form full adjacencies with other routers
  • Remaining routers remain in a two-way state with each other
  • If DR or BDR missing, re-election of missing router takes place
designated router
Designated Router
  • Adjacencies formed with DR and BDR only
  • LSAs propagate along the adjacencies



ospf equal cost multipath
OSPF Equal Cost Multipath
  • If n paths to same destination have equal cost, OSPF installs n entries in forwarding table
  • Loadsharing over npaths
  • Useful for expanding links across an ISP backbone
  • Don’t need hardware multiplexors or static routing
other protocols vrrp and hsrp
Other Protocols - VRRP and HSRP
  • Virtual Router Redundancy Protocol (VRRP)
    • Open standard protocol
    • Provides router state information
  • Hot Standby Router Protocol (HSRP)
    • Cisco proprietary protocol
    • Does not advertise IP routes or affect routing table
ospf and addressing design
OSPF and Addressing Design
  • Idea behind VLSMs is to offer more flexibility
  • Without VLSM, only one subnet mask applied to a network
  • This restricts the number of hosts given the number of subnets required
ospf and vlsm
  • OSPF can carry multiple subnet information
  • Other protocols such as RIP and IGRP cannot
ospf rip and vlsm




Router B






Router A





Router C

configuring ospf for vlsm
Configuring OSPF for VLSM
  • OSPF RFC (1583) does not specify what is the best way to architect a network
  • When subnetting, the router complains if the combination IP address and mask result in a subnet 0
  • When doing protocol redistribution, it is better to avoid OSPF filtering
design an ospf backbone network
Design an OSPF Backbone Network
  • Use variable subnet addressing for IPv4
  • Plan a two-tier hierarchical addressing scheme
  • Pick an appropriate metric
  • Designate backbone (area 0) at the center
  • Attach (physically) other areas to area 0
design an ospf backbone network1
Design an OSPF Backbone Network
  • Use loopbacks or manually configured router IDs areas
  • Use appropriate address aggregation
  • Use stubby/totally stubby areas where possible
ospf network types
OSPF Network Types
  • Broadcast multi-access
  • Point-to-point
  • Point-to-multipoint broadcast
  • Point-to-multipoint non-broadcast
  • Non-broadcast multi-access (NBMA)
ospf in multi access networks
OSPF in Multi-access Networks




Internal Routers



distance vector or link state
Distance Vector or Link State?
  • When planning a network design, what considerations are made whether to choose distance vector or link-state protocols?
  • Why?
  • In this presentation, the following were covered:
    • Review of CIDR
    • Link-state routing protocols
    • OSPF backbone network design
    • OSPF and VLSM configurations
    • Functions of OSPF v2
    • Differences between distance vector and link- state network planning