Why would you set ip ospf network point-to-point on an Ethernet link?
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Most routed Ethernet links connect exactly two routers, so a DR adds nothing. Point-to-point removes the election, the 40-second wait timer when the link comes up and the Type 2 LSA for the segment. Adjacencies form faster and the LSDB is smaller. Both ends must use the same type.
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What changes
- No wait timer. On broadcast the router waits the dead interval before the election. Point-to-point goes straight from 2-Way to ExStart.
- No Type 2 LSA. The link appears in each router's Type 1 LSA as a point-to-point link plus a stub network, instead of a transit network pointing at a DR.
- No mask check in the hello.
Config
interface GigabitEthernet0/0/1
description to R2
ip address 10.0.12.1 255.255.255.252
ip ospf network point-to-point
ip ospf 1 area 0Output
R1# show ip ospf neighbor
Neighbor ID Pri State Dead Time Address Interface
2.2.2.2 0 FULL/ - 00:00:38 10.0.12.2 GigabitEthernet0/0/1Priority shows 0 and the role shows a dash, because there is no election on this link.
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What if only one side is changed?
Timers still match (10/40), so they can go Full, but the LSAs disagree about the link and routes through it fail the SPF check. Always change both ends together.
Traps
- Thinking point-to-point needs a serial link. It is an OSPF setting and works on any interface with two routers.
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