Route My World!

OSPF defines three different types of networks based on their physical link types.

Physical Link types:

1. Point-to-point
   • A network that joins a single pair of routers
2. Broadcast
   • A multiaccess broadcast network that joins a single pair of routers
3. Nonbroadcast multiaccess (NBMA)
   • A network that interconnects more than two routers but is not capable of sending broadcast traffic.
   • Examples are:
     • Frame Relay
     • ATM
     • X.25
   • There are five modes of operation for NBMA networks:
     • Nonbroadcast (RFC 2328-compliant mode)
     • Point-to-multipoint (RFC 2328-compliant mode)
     • Point-to-multipoint nonbroadcast (CIsco mode)
     • Broadcast (Cisco mode)
     • Point-to-point (Cisco Mode)

Adjacency Behavior for a Point-to-Point Link

• A point to point network consists of two routers connecting end to end. A typical example is a T1 serial line configured with PPP or HDLC.
• The router dynamically detects its neighboring routers by multicasting OSPF hello packets to address 224.0.0.5
• As long as the pair of routers can communicate directly, they can form and adjacency
• There is no need for a DR or BDR since there can only be two routers involved.
• The outgoing interface’s IP address is usually used as the source IP address of the OSPF packets.
• It is possible to use IP unnumbered interfaces with OSPF.
  • In this case, an IP address of another interface on the router is used as the source IP address.
• The default OSPF hello/dead intervals are 10/40 seconds.

Adjacency Behavior for a Broadcast Network

• OSPF routers on a multiaccess broadcast network (Ethernet LAN) forms an adjacency with the DR and BDR on that network.
  • These adjacent routers have synchronized LSDB.
  • When routers first come up on the Ethernet segment, they exchange hello packets and start electing the DR and BDR. The routers then attempt to form adjacencies with the DR and BDR.
• The DR performs the LSA forwarding and LSDB synchronization task
• The BDR receives all information that the DR has but does not perform any DR functions while the DR is up. Only if the DR fails will the BDR take over.
  • If DR fails, the BDR immediately becomes DR and an election is held to pick the new BDR
• The DR and BDR does the following:
  • Reduce routing update traffic
    • Instead of all the routers exchanging information with each and everyone else, they each establish full adjacency with only the DR and BDR.
    • The DR will then send all the information it gathers to each node on the network.
    • This process significantly reduces the flooding process.
  • Manage link-state synchronization
    • The DR and BDR ensure that the other routers on the network have the same link-state information about the network. This process reduces the number of routing errors.

Electing the DR and BDR

• The DR is the router that has the highest priority value.
• The BDR has the second highest priority value.
• The default for the interface OSPF priority is 1
  • When there is a tie on the priority value, the router ID is used.
  • The highest router ID becomes DR
  • The second highest RID becomes the BDR
• A router that has priority 0 can never be a DR or BDR. These are called DROTHER.
• If a router with a higher priority joins the network, it does not preempt the DR or BDR.
  • The only time a DR or BDR changes is if one of them is out of service. If the DR is out of service, the BDR takes over as DR and a new BDR is elected.
  • If a BDR becomes out of service, a new BDR is elected.
    • To determine if the DR is out of service, the BDR uses the wait timer. This timer is a reliability feature.
    • If the BDR does not confirm that the DR is forwarding LSAs before the wait timer expires, the BDR assumes that the DR is out of service.

DR and BDR on Each Segment

• The DR concept happens at the link level.
• Each network segment has its own pair of DR/BDR in a multiaccess broadcast network.
• A router can be a DR on one segment and a regular (DROTHER) router on another segment if it is connected to a multiaccess broadcast network.

Setting Priority for the DR election

• Setting a priority to an interface allows for it to be designated as a DR or BDR on a multiaccess network
• To configure the priority value, use the following interface configuration command:
  • ip ospf priority number
  • The number value can range between 0 to 255.
• The DR is the highest priority interface
• The BDR has the second-highest priority interface
• Interfaces with priority value set to 0 does not participate in the DR/BDR election, therefore cannot become either.

Example ip ospf priority Configuration:

Router(config)#interface FastEthernet 0/0
Router(config-if)#ip ospf priority 10

• A DR will not give up its status just because a new interface is reporting a higher priority value.
• An interface’s priority usually takes effect only if the existing DR fails.
• Setting an interface to 0, however, takes effect immediately and a new election can take place.

Adjacency Behavior for a Nonbroadcast Multiaccess Network

• A single router interface can connect to multiple routers. They do not, however, have broadcast capability like we’ve seen with multiaccess broadcast networks.
• To implement broadcasting or multicasting on a router in a NBMA network, the router replicates the packets to be broadcasts or multicasts and sends them individually on each PVCs to all destinations.
  • This is a CPU-intensive process
  • Additionally, of the NBMA topology is not fully meshed, a broadcast/multicast sent by one router does not reach all the other routers.
• Examples of NBMA networks are:
  • Frame Relay
  • ATM
  • X.25
• The default OSPF hello/dead intervals on NBMA interaces are 30 seconds and 120 seconds, respectively.

DR Election in an NBMA Topology

By, default, OSPF cannot automatically build adjacencies with neighbor routers over NBMA interfaces.

The next blog post will cover different types of NBMA topologies and how DR and BDR election is accomplished

This entry is not an authoritative guide. These are merely notes and rehash of the primary text materials and resources that I use. For a thorough guide of the BSCI course, consider purchasing Building Scalable Cisco Internetworks (BSCI) (Authorized Self-Study Guide) (3rd Edition) by Diane Teare and Catherine Paquet, as well as following the links on the resources section of this entry.