OSPF is the most commonly recommended Underlay Routing Protocol for some overlay technologies, such as VXLAN EVPN. From a design and optimization perspective, one of the OSPF recommendations for the underlay network is to use the Point-To-Point Network Type instead of the default Broadcast Network Type on interlink switches (between leaf and spine).
First, there is no need to perform DR/BDR election in a segment with only two devices, as the size of the Type-1 LSA is not large. Secondly, the election of DR/BDR will occur in each transit segment, meaning that a Type-2 LSA will be generated in each segment, which is unnecessary.
Another reason for this recommendation is OSPF convergence time, as mentioned in RFC 4062 (published in 2005 and updated in 2015). The RFC explains that "a Point-to-Point Link can take less time to converge than a broadcast link of the same speed because it does not have the overhead of DR Election."
So, what does the overhead of DR Election mean?
RFC 4062 " Point-to-Point Links
- Definition
See [OSPF], Section 1.2.
- Discussion
A point-to-point link can take less time to converge than a
broadcast link of the same speed because it does not have
the overhead of DR election. Point-to-point links can be
either numbered or unnumbered. However, in the context of
[BENCHMARK] and [OSPF], the two can be regarded as the
same.
Broadcast Link
- Definition
See [OSPF], Section 1.2.
- Discussion
The adjacency formation time on a broadcast link can be
greater than that on a point-to-point link of the same
speed because DR election has to take place. All routers
on a broadcast network form adjacency with the DR and BDR. "
If we look at the RFC 2328 section 9.The Interface Data Structure, there is a wait timer equal to the dead interval (default cisco routers 40s).
" Wait Timer
A single shot timer that causes the interface to exit the
Waiting state, and as a consequence select a Designated Router
on the network. The length of the timer is RouterDeadInterval
seconds "
When routers are powered up, the Wait Timer is used for DR/BDR election on the multiaccess segment. In the Hello packet, every router includes its Router-ID, the RIDs of other neighbors it discovers, the default Priority of 1, and the DR and BDR IP address 0.0.0.0 indicating that no DR and BDR has been elected yet.
After that, during the wait interval 40s, each router is listening and stays in the 2-WAY State waiting the DR/BDR election to finish before reaching the full state wih the DR/BDR.
In Point-to-Point network type there is no DR/BDR election and no need to wait 40s. This is why it's recommended to configure the network type point-to-point in the leaf-spine links to optimize OSPF convergence.
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