5 network types
OSPF defines five network types, as shown in Figures:
- Point-to-point – Two routers interconnected over a common link. No other routers are on the link. This is often the configuration in WAN links.
- Nonbroadcast multiaccess (NBMA) – Multiple routers interconnected in a network that does not allow broadcasts, such as Frame Relay.
- Point-to-multipoint – Multiple routers interconnected in a hub-and-spoke topology over an NBMA network. Often used to connect branch sites (spokes) to a central site (hub).
Challenges in Multiaccess network
- Creation of multiple adjacencies: Creating adjacencies with every router is unnecessary and undesirable. This would lead to an excessive number of LSAs exchanged between routers on the same network.
- Extensive flooding of LSAs: Link-state routers flood their link-state packets when OSPF is initialized, or when there is a change in the topology. This flooding can become excessive.
The number of adjacencies required for any number of routers (designated as n) on a multiaccess network is:
n (n – 1) / 2
The solution to managing the number of adjacencies and the flooding of LSAs on a multiaccess network is the DR.
- On multiaccess networks, OSPF elects a DR to be the collection and distribution point for LSAs sent and received.
- A BDR is also elected in case the DR fails. The BDR listens passively to this exchange and maintains a relationship with all the routers. If the DR stops producing Hello packets, the BDR promotes itself and assumes the role of DR.
- All other non-DR or BDR routers become DROTHER (a router that is neither the DR nor the BDR). DROTHERs only form full adjacencies with the DR and BDR in the network.
Instead of flooding LSAs to all routers in the network, DROTHERs only send their LSAs to the DR and BDR using the multicast address 126.96.36.199 (all DR routers). The DR uses the multicast address 188.8.131.52 (all OSPF routers). The end result is that there is only one router doing all of the flooding of all LSAs in the multiaccess network.
Note: DR/BDR elections only occur in multiaccess networks and do not occur in point-to-point networks.
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The OSPF DR and BDR election decision is based on the following criteria, in sequential order:
1. The routers in the network elect the router with the highest interface priority as the DR. The router with the second highest interface priority is elected as the BDR. The priority can be configured to be any number between 0 – 255. The higher the priority, the likelier the router will be selected as the DR. If the priority is set to 0, the router is not capable of becoming the DR. The default priority of multiaccess broadcast interfaces is 1. Therefore, unless otherwise configured, all routers have an equal priority value and must rely on another tie breaking method during the DR/BDR election.
2. If the interface priorities are equal, then the router with the highest router ID is elected the DR. The router with the second highest router ID is the BDR.
Recall that the router ID is determined in one of three ways:
- The router ID can be manually configured.
- If no router IDs are configured, the router ID is determined by the highest loopback IP address.
- If no loopback interfaces are configured, the router ID is determined by the highest active IPv4 address.
Note: In an IPv6 network, if there are no IPv4 addresses configured on the router, then the router ID must be manually configured with the router-id rid command; otherwise, OSPFv3 does not start.
Note: Serial interfaces have default priorities set to 0; therefore, they do not elect DR and BDRs.
The DR and BDR election process takes place as soon as the first router with an OSPF-enabled interface is active on the multiaccess network. This can happen when the routers are powered on, or when the OSPF network command for that interface is configured. The election process only takes a few seconds. If all of the routers on the multiaccess network have not finished booting, it is possible that a router with a lower router ID becomes the DR. (This can be a lower-end router that takes less time to boot.)