MTU
The IP MTU parameter determines the maximum size of an IPv4 packet that can be forwarded out the interface without fragmentation. If a packet with an IPv4 MTU larger than the maximum arrives at the router interface, it will be either discarded, if the DF bit in the packet header is set, or it will be fragmented. OSPF for IPv4 packets completely relies on IPv4 for the possible fragmentation. Although RFC 2328 does not recommend OSPF packet fragmentation, in some situations the size of the OSPF packet has greater value than the interface IPv4 MTU. If MTUs are mismatched between two neighbors, this could introduce issues with exchange of link-state packets, resulting in continuous retransmissions.
NOTE
The interface command for setting the IPv6 MTU parameter is ipv6 mtu. An IPv6 router does not fragment an IPv6 packet unless it is the source of the packet.
To prevent such issues, OSPF requires that the same IPv4 MTU be configured on both sides of the link. If neighbors have a mismatched IPv4 MTU configured, they will not be able to form full OSPF adjacency. They will be stuck in the ExStart adjacency state.
Forming an OSPF adjacency requires that the following parameters match on OSPF-enabled interfaces:
Hello and dead timers
Network mask (for broadcast network types only)
Need for a DR
+++Matching MTU (unless configured to ignore MTU)+++
Unique RIDs
Interfaces sharing a common subnet with matching OSPF area ID
Authentication type and credentials
Area flags
@Onny and @ zayd_dean, even if MTU is not mentioned compulsory matching conditions, we need it to be matched. Otherwise, routers will be stuck in EXSTART state.
The answer is "D. mtu" for sure.
@zyad_dean
Technically MTU (Maximum Transmission Unit) is not a part of compulsory matching conditions. Still we should match this value. If this value does not match routers may stuck in Exstart/Exchange exchange stage.
Correct answer is MTU but find it odd it is never listed as one of the parameters for OSPF adjacencies.
Hello / dead timers
Area Type
Area
Subnet
Authentication
upvoted 3 times
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3 years, 2 months agoOnny
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