OSPF Network Architecture, Metro Ethernet, and VPN Basics

OSPF Network Elements

OSPF networks are structured into the following hierarchy:

• Autonomous Systems (AS): The primary network division.
• Areas: Subdivisions of an AS. All areas must connect to Area 0 (Backbone).
• Router Types:
  • IR (Interior Router): Operates within a single area.
  • ABR (Area Border Router): Connects different areas.
  • ASBR (AS Border Router): Connects OSPF to external autonomous systems.

OSPF Network Types

• BMA: Broadcast Multiaccess.
• Point-to-Point: Direct connection between two nodes.
• NBMA: Non-Broadcast Multiaccess.

OSPF Packet Types

• Hello: Discovers neighbors, establishes adjacencies, and elects DR/BDR. Sent every 10s (BMA/P2P) or 30s (NBMA).
• Database Description (DBD): Synchronizes databases between routers.
• Link-State Request (LSR): Requests specific link-state data.
• Link-State Update (LSU): Sends requested information containing LSAs.
• Link-State Acknowledgment (LSAck): Confirms receipt of LSAs.

OSPF LSA Types

• Router LSA (RL): Link-state info generated by IRs.
• Network LSA (NL): Summary of link-state info generated by DRs.
• Summary LSA (SL): Area summaries generated by ABRs.
• AS External LSA (EL): External AS summaries generated by ASBRs.

Metro Ethernet WAN Links

Main Components

• UNI (User-to-Network Interface): The physical demarcation point.
• EVC (Ethernet Virtual Connection): Association of two or more UNIs (P2P or MP2MP tunnel).

Service Types

• E-Line Service: Point-to-Point (P2P).
• E-LAN Service: Multipoint-to-Multipoint (MP2MP).

Key Parameters

• Interface Speed: 10 Mbps, 100 Mbps, 1 Gbps, 10 Gbps.
• CIR (Committed Information Rate): Guaranteed bandwidth per VLAN.
• PIR (Peak Information Rate): Maximum non-guaranteed bandwidth.

VPN Fundamentals

A VPN provides an encrypted connection between private networks over a public network using IP encapsulation.

• Benefits: Cost savings, security, and scalability.
• Disadvantage: No guaranteed bandwidth.
• Types: Site-to-site and Remote-access.

RIP Protocol: Hold-Down Timer

The hold-down timer (180 seconds) prevents routing loops when a network becomes unreachable:

• Case 1: Update from same neighbor with reachable network: Update table.
• Case 2: Update from different neighbor with better distance: Update table.
• Case 3: Update from different neighbor with worse distance: Ignore.

IPv6 Unicast Address Types

• Global Unicast
• Link-Local
• Loopback
• Unspecified Address
• Unique Local
• Embedded IPv4