OSPF Network Types
Quick bit of a refresh on OSPF network types, these are specified on a per link basis and it changes the way OSPF behaves on that link specifying the wrong type can cause bad things to happen such as if you mix DR & Non DR types adjacencies wont form correctly.
There are 5 network types,
- Point to Point – Should be used as the name implies on P2P links which support broadcast, nearly all OSPF traffic is sent to the ALLSPFRouters address which is 224.0.0.5, although retransmitted LSAs are sent to the unicast address as such these links should support broadcasting.
- Broadcast – This is the default network type for Ethernet networks, as this network type is capable of having adjacencies with multiple routers. To prevent unnecessary meshing DR & BDR elections take place. Communications from the DR & BDR take place on the ALLDRRouters address 224.0.0.6 whilst all other communications on 224.0.0.5. As the name implies the layer 2 network must support broadcasting.
- Non Broadcast Multiple Access (NBMA) – This is the default configuration on a serial interface, and is classified as a link which is able to send to multiple other routers via the single link but with no broadcasting, therefore neighbour statements must be manually set. A DR & BDR will still be elected but all communication is unicast, the hello and dead timers change from the defaults to 30 and 120 seconds.
- Point to Multipoint networks – All communication is unicast and no DR or BDR election takes place an ideal placement for this would be in the hub in a hub and spoke topology.
- Virtual Links – Packets sent over Virtual Links are unicast and perceived as unnumbered point to point connections
Its pretty simple to change the network type its all done under the interface like so.
Router(config-if)#ip ospf network ? broadcast Specify OSPF broadcast multi-access network non-broadcast Specify OSPF NBMA network point-to-multipoint Specify OSPF point-to-multipoint network point-to-point Specify OSPF point-to-point network
Hi!
to complete your information in the post:
ip ospf network point-to-multipoint : neighbors discovery is enabled and DR & BDR is automaticaly established
ip ospf network point-to-multipoint nobroadcast : neighbors discovery is disabled, and DR & BDR must be manualy configured
ip ospf network non-broadcast is the default type used in serial interfaces but when frame-relay multipoint interface type is in use, in this case ip ospf network point-to-multipoint is used by default (with neighbor discovery)
BSCI official exam certification guide, chapter 6 OSPF topology options
Regards!