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Home › Reti di Calcolatori 2 › Capitolo III - Inter-domain routing with BGP › III.1Intra-Domain Routing

III.1.1Distance Vector

Roberto Bifulco — Sat, 10/18/2008 - 22:11

Il DV è implementato dal protocollo RIP. Il meccanismo di routing è basato sull'algoritmo di Bellman-Ford per il calcolo del percorso minimo su rete. Sostanzialmente i router si scambiano fra loro un vettore che contiene, per ogni router, le destinazioni che quel router può raggiungere con associato il costo necessario per raggiungerle. Le destinazioni sono espresse in termini di prefissi di rete.

Chiaramente ogni router riceve solo i vettori di tutti i suoi vicini (e manda i vettori soltanto a tutti i suoi vicini). Su tali vettori sono basate le tabelle di routing del router.

Questo algoritmo soffre di alcuni seri problemi quando ci sono variazioni consistenti della topologia di rete, come ad esempio la caduta di un link. In questo caso, infatti, prima che la conoscenza dell'avvenuta caduta del link sia consolidata, i router chiusi in un ciclo si scambiano informazioni incrementando di una unità per volta il costo del link caduto, che è considerato tale soltanto quando l'incremento ennesimo porta il costo di quel link al valore che indica infinito, ossia l'assenza del link. Per tali problemi la rete non può avere molti hop poiché altrimenti l'algoritmo non convergerebbe mai. Solitamente il limite consigliato è 16.

Le varianti che tentano di risolvere questo problema (Split Horizon, Split Horizon Poison Reverse) in realtà risolvono soltanto i problemi derivanti da cicli formati fra due hop. Per queste ragioni gli algoritmi DV sono usati solo in piccoli domini.

‹ III.1Intra-Domain Routing up III.1.2Link State ›
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Reti di Calcolatori 2

  • Introduzione
  • Capitolo I - Networks Evolution
    • Communication models
      • Circuit switching
      • Packet Switching
      • Flow Switching
    • A rapidly changing scenario
    • Data and media taxonomy
  • Capitolo II - Techniques and architectures for QoS
    • II.1QoS problems
    • II.2Service Specification
    • II.3Traffic and Service Characterization
      • II.3.1Token Bucket
      • II.3.2Leaky Bucket
      • II.3.3Queue management
      • II.3.4Scheduling
        • II.3.4.1Scheduling policies
        • II.3.4.1.1GPS
    • II.4Parekh-Gallager Theorem
    • II.5QoS Architectures
      • II.5.1Integrated Services (IntServ)
        • II.5.1.1RSVP
        • II.5.1.2IntServ today
      • II.5.2Differentiated Services (DiffServ)
        • II.5.2.1PHB: Expedited Forwarding
        • II.5.2.2PHB: Assured Forwarding
    • II.6QoS in Fast Interconnect
      • II.6.1Flow Control
        • II.6.1.1Flow Control in IBA
        • II.6.1.2Flow Control in ASI
        • II.6.1.3Flow Control in Ethernet
      • II.6.2Congestion control
        • II.6.2.1Congestion Control in IBA
        • II.6.2.2Congestion Control in ASI
  • Capitolo III - Inter-domain routing with BGP
    • III.1Intra-Domain Routing
      • III.1.1Distance Vector
      • III.1.2Link State
    • III.2Inter-domain routing
      • III.2.1Border Gateway Protocol (BGP)
        • III.2.1.1BGP Messages
        • III.2.1.2BGP Example 1
        • III.2.1.3Route preference
        • III.2.1.4The internet organization
        • III.2.1.5BGP in large networks
        • III.2.1.6Confederations
        • III.2.1.7Route Reflectors
        • III.2.1.8The dynamics of BGP
        • III.2.1.9BGP routing tables
        • III.2.1.10The route selection process
  • Capitolo IV - Asynchronous transfer mode
    • ATM architecture
    • ATM Protocol Stack
    • ATM Addressing
    • ATM Quality of Service
    • ATM Adaptation Layer
    • Call and Connection Control
    • ATM in LAN
    • IP over ATM
  • Capitolo V - Multi Protocol Label Switching
    • MPLS
  • Capitolo VI - Traffic Engineering
    • IP-based Traffic Engineering
    • MPLS-based traffic engineering
  • Capitolo VII - SDH/SONET
  • Capitolo VIII - IP su reti ottiche
    • DWDM
    • Generalized Framing Procedure (GFP)
    • Gigabit Ethernet (GbE)
    • IP-centric control of optical networks
  • Capitolo IX - Network Management
    • Simple Network Management Protocol (SNMP)
    • Network management applications
    • Professional and Business Challanges
    • Service life-cycle
    • Provisioning level Agreement
  • Capitolo X - Network Resiliency
    • Network recovery
    • Recovery Mechanisms Control
  • Capitolo XI - Network security
    • Types of Attack
    • Firewall
    • NAT
    • Intrusion Prevention Systems
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