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[J/M/MX/T/EX/QFX-Series] OSPF Ethernet interfaces as Point-to-Point (P2P) interfaces - Junos OS behavior



Article ID: KB23533 KB Last Updated: 24 Dec 2013Version: 2.0

This article describes the behavior of Junos OS when using Ethernet interfaces for Point-to-Point connections, and the resulting adjacency formation.


In certain cases, adjacency issues (INIT state) can occur when using the Point-to-Point Interface type for the Open Shortest Path First (OSPF) protocol. Messages such as this can be seen:

May 10 12:16:17.458702 OSPF rcvd Hello -> (ae0.1 IFL 128 area
May 10 12:16:17.458739 Version 2, length 48, ID, area
May 10 12:16:17.458768 checksum 0x0, authtype 0
May 10 12:16:17.458799 mask, hello_ivl 10, opts 0x2, prio 1
May 10 12:16:17.458828 dead_ivl 40, DR, BDR
May 10 12:16:17.458856 neighbor
May 10 12:16:17.458893 OSPF packet ignored: netmask mismatch from on intf ae0.1 area

As per RFC2328, section 10.5 (Receiving Hello Packets), there is one exception where Point-to-Point interfaces and on virtual links, the network mask in the received hello packets should be ignored.

Ethernet interfaces used as Point-to-Point are widely used for troubleshooting purposes. However, in Junos OS, the network mask will still be considered when Ethernet interfaces are used as Point-to-Point. This is an expected Junos OS behavior.

When Ethernet interfaces are used as Broadcast, Junos OS will still consider the network mask; in case of a mismatch, the OSPF neighbor will not form. The RFC exception applies to only actual Point-to-Point interfaces, such as Frame Relay; so, when configuring the Ethernet as Point-to-Point, it is recommended that the network mask also matches with the peer. 

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