More OSPF Network Types

• Media Types
  • OSPF works differently depending on the media of interface the protocol is enabled on.
  • Defines network types to deal with specific types of media.
  • Next hop, timers, adjacency formation are different per media.

• All Network types:
  • Broadcast
  • Non-Broadcast
  • Point to Point
  • Point to Multipoint
  • Point to Multipoint Non Broadcast
  • Loopback
• Network types do not need to match.
  • Properties such as timers need to match however.

• LSA Type 2:
  • Type 2 is what makes differing network types compatible.
  • Generated by DR.
  • Enhances performance of OSPF on a shared segment between nodes.
    • Reduces adjacencies.
    • Reduces LSA flooding replication.
    • Simplifies SPF.
  • Network types that use LSA Type 2
    • Broadcast
    • Non-Broadcast
  • Network types that do not:
    • Point to Point
    • Point to Multipoint
    • Point to Multipoint non-Broadcast
  • Default media types are Multiaccess.
    • ie. Ethernet, Token Ring, FDDI
  • Sends hellos and updates on Multicast.
    • 224.0.0.5 – All SPF Routers
    • 224.0.0.6 – DR

• OSPF Network non-Broadcast
  • Examples Frame Relay and ATM
  • Sends hellos as unicast
    • Manually defined neighbors with ‘neighbor’ command.
  • Still uses DR/BDR

• DR and BDR Process:
  • DR
    • Forms adjacency with all routers on the multiaccess network.
    • listens for updates on multicast .6
    • Re-floods updates back on the segment at multicast .5
    • Does not modify next hop value.
  • BDR
    • Used for a DR backup.
    • Does not flood updates.
  • DROTHER
    • Any OSPF speaker not DR or BDR on a multiaccess network.
    • Form full adjacency with DR and BDR
    • Stop at Extart/2-way with each other.
  • DR/BDR are chosen through election
    • Based on interface priority and Router-ID
      • Priority
        • 0-255
        • Higher = Better
        • 0 = Never
      • Router-ID
        • Highest loopback/interface IP.
        • Can be statically set.
        • Higher = Better
    • Uses Wait timer to stop pre-emption of current DR/BDR

• OSPF Network Point-to-Point
  • Default on the following:
    • HDLC, PPP, GRE Tunnel
  • Hellos at 224.0.0.5
  • no DR/BDR
  • Supports only two neighbors on the link.

• OSPF Network Point-to-Multipoint
  • Treats network as collection of point to point networks.
  • Hellos sent to 224.0.0.5
  • No DR/BDR
  • Special Next-Hop processing.

• OSPF Network Point to Multipoint non-Broadcast
  • Same as point to multipoint but sends hellos as unicast.
  • Manually defined neighbors – ‘neighbor’ command under OSPF.
  • Allows for per-VC OSPF cost over NBMA.
  • No DR/BDR
  • Special Next-Hop processing
• OSPF Network type Loopback
  • Advertises link as /32 stub host route.
  • ‘ip ospf network point-to-point’ used to disable this behavior.

Election Modification:

The five routers below are all running OSPF over the shared multiaccess/ethernet network. From R1’s CLI it shows the devices have formed adjacency like they should in a Broadcast network type.

The DR is R5/10.30.5.1, and the is R4/10.30.4.1. This is because the default winner of the election is going to be the highest router-id.

A quick way of changing the DR is going on the active DR and setting OSPF priority to 0. This will remove the Router (R5 in this case) completely from the election process.

The priority change above made R5’s adjacency flap, and now from R1’s perspective R3 is the BDR, R4 is the DR.

Note:

• If running a DMVPN Hub and Spoke while using Broadcast/DR/BDR OSPF network, make sure all spokes have priority 0 setup on interfaces so they do not become the DR. If one of the spokes becomes the DR, none of the other spokes will receive routing updates.