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Understanding OSPF Protocol: Hello Packets and Router Election Process

This article explores the intricacies of the OSPF (Open Shortest Path First) protocol, focusing on Hello packets and the election of Designated Routers (DR) and Backup Designated Routers (BDR) within various network types. It discusses the contents of Hello packets, including RID, Area ID, and authentication information, alongside the significance of priorities in router elections. Additionally, learn about different router roles, types, and the concept of OSPF cost based on reference bandwidth. This comprehensive guide is essential for network professionals and enthusiasts.

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Understanding OSPF Protocol: Hello Packets and Router Election Process

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Presentation Transcript

  1. Hello Protocol • Discover Neighbors • Ensure Bidirectional Comm • Keepalives

  2. Hello Packet Contents • RID • AreaID • Mask • Auth Info (MD5) • Hello Interval • Router Dead Interval • Priority 0..255 • DR & BDR • Flags/Options • Active RIDs on this net

  3. Network Types • Point-To-Point (P2P) • Broadcast • Non-Broadcast Multi-Access (NBMA) • Point to Multipoint (P2MP) • VirtualLink

  4. Local Multicast • 224.0.0.5 -- all OSPF routers listen • 224.0.0.6 -- DR & BDR listen

  5. DR & BDR • 1st two capable routers become DR and BDR on on multiaccess media • If DR dies, BDR is promoted to DR and new BDR elected • Routers with Priority 0 are ineligible for DR & BDR • Highest Priority 1..255 wins • If tie, highest RID wins • Elections only held when DR dies

  6. Router Types / Roles • Internal Router • Area Border Router (ABR) • Backbone Router • Autonomous System Boundary Router (ASBR)

  7. OSPF Cost • Reference BW 10^8 bits/sec • Ospf auto-cost reference-bandwidth to adjust • FDDI, FastEther 1 • T3/HSSI 2 • Ethernet 10 • T1 64 • DS0 1562

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