The OSPF neighbor relationship is not establishing due to a mismatch in MTU (Maximum Transmission Unit) settings on the interfaces of PE-A and PE-B routers. On PE-A, there is no MTU configuration under the interface FastEthernet0/0, which means it is using the default MTU setting. However, on PE-B, the MTU is explicitly set to 1400 bytes with “ip mtu 1400” command under interface FastEthernet0/0. OSPF requires that both ends of a point-to-point link have matching MTU sizes to form a neighbor relationship; otherwise, OSPF adjacency will not be established.
References:
Implementing and Operating Cisco Service Provider Network Core Technologies (SPCOR) - Cisco SPCOR Resources
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