OSPF

OSPF

1. 1. OSPF Basics

   • 1. Open standard (developed by the IETF (Internet Engineering Task Force))
   • 2. Establishes adjacencies with other routers
   • 3. Sends Link State Advertisements (LSAs) to other routers in an area
   • 4. Constructs a link state database from received LSAs
   • 5. Runs the Dijkstra Shortest Path First (SPF) algorithm to determine the shortest path to a network
   • 6. Attempts to inject the best path for each network into a router’s IP routing table

2. 10. Dijkstra Algorithm – Considers a topology and calculates the shortest path to a destination

3. 2. OSPF Terminology

   • 1. Hello: Protocol to discover OSPF neighbors and confirm reachability to them (also used in election of Designated Router)
   • 2. Link State Advertisement (LSA): Info a router sends and receives about network reachability (used to construct the Link State Database)
   • 3. Link State Update (LSU): A packet that carries LSAs
   • 4. Link State Request (LSR): Used by a router to request specific LSA information from a neighbor
   • 5. Link State Acknowledgment (LSAck): Used by a router to confirm it received a LSU

4. 3. Neighborship versus Adjacency

   • Neighbors
     • Routers that reside on the same network link
     • Exchange Hello messages
   • Adjacencies
     • Routers that are neighbors
     • Have exchanged Link State Updates (LSU) and Data Description (DD) packets

5. 4. Designated Routers

   • 2. DR (Designated Router)
   • 1. BDR (Backup Designated Router)
   • 3. Adjacencies only need to be formed with the DR and BDR

6. 8. Electing a Designated Router

   • 1. The Hello protocol is used to elect a DR
   • 2. During a DR election, the router with the highest OSPF priority wins
   • 3. The OSPF priority value is associated with an interface and can be a value in the range 0 -255
   • 4. An OSPF priority value of 0 means that the router will not become the DR
   • 5. The default OSPF priority value of an interface is 1
   • 6. The OSPF priority of an interface can be configured using ip ospf priority value command in interface configuration mode
   • 7. If the priorities tie, the router with the highest router ID (RID) becomes the DR
   • 8. A router’s RID can be configured in router configuration mode, with the command router-id id
   • 9. If a RID is not configured, the highest IP address of a loopback interface (that is currently up) becomes the RID
   • 10. If a router has no loopback interfaces, the highest IP address of a non-loopback interface (that is currently up) becomes the RID

7. 5. Measuring cost

   • 1. Default reference bandwidth is 100 Mbps
   • 2. Cost = reference BW / Interface BW

8. 11. Set a higher reference bandwidth

   • auto-cost reference-bandwidth value

9. 6. OSPF Areas

   • 2. We always have a backbone area
   • 1. Different interfaces can live in different areas
   • 4. Area Border Routers (ABRs) – interfaces in different areas
   • 3. Virtual link – Links up areas to area 0

10. 7. OSPF Network Type

    • 1. Broadcast is the default network type on Ethernet networks
    • Point-to-Point is the default network type on Frame Relay point-to-point subinterfaces
    • 1. Broadcast Network
      • 2. Elects a DR and BDR: Yes
      • 3. Default Hello Interval: 10 seconds
      • 4. Uses the neighbor Command: No
    • 2. Point-to-Point
      • Elects a DR and BDR: No
      • Default Hello Interval: 10 seconds
      • Uses the neighbor Command: No
    • 3. Non-Broadcast (NBMA)
      • Elects a DR and BDR: Yes
      • Default Hello Interval: 30 seconds
      • Uses the neighbor Command: Yes
    • 4. Point-to-Multipoint (each virtual connection is treated as a separate connection)
      • Elects a DR and BDR: No
      • Default Hello Interval: 30 seconds
      • Uses the neighbor Command: No

11. 9. Single-Area OSPF

    • Benefits
      • 2. Configuration is simplified
      • 3. Routers can handle large SPF calculations
      • 1. Consider multiple areas for hosts and loadbalancers running OSPF
    • OSPFv2 configuration and verification
      • R1#show ip protocols (shows whether any are running, eg OSPF) R1#conf t R1(config)#router ospf 1 R1(config-router)#network 192.168.1.0 0.0.0.255 area 0 R1(config-router)#network 10.1.1.0 0.0.0.3 area 0 R1(config-router)#network 1.1.1.1 0.0.0.0 area 0 R1(config-router)#passive-interface fa0/0 (don’t send hello messages) R1(config-router)#auto-cost reference-bandwidth 10000 R1(config) int fa 0/1 R1(config-if)#ip ospf priority 2 (makes this the DR)
      • R2#conf t R2(config)#router ospf 1 R2(config-router)#network 0.0.0.0 255.255.255.255 area 0 (everyone belongs) R2(config-router)#auto-cost reference-bandwidth 10000
      • R3#conf t R3(config)#router ospf 1 R3(config-router)#network 0.0.0.0 255.255.255.255 area 0 R3(config-router)#auto-cost reference-bandwidth 10000 R3(config-router)#passive interface fa 0/0
    • OSPFv3 configuration
      • R1#conf t R1(config)#ipv6 unicast-routing R1(config)#ipv6 cef R1(config)#int fa 0/0 R1(config-if)#ipv6 ospf 1 area 0 R1(config-if)#int fa 0/1 R1(config-if)#ipv6 ospf 1 area 0 R1(config-if)#int fa lo0 R1(config-if)#ipv6 ospf 1 area 0
      • R1(config)#router ospfv3 1 R1(config-router)#auto-cost reference-bandwidth 10000 R1(config-router)#passive-interface fa 0/0

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