A network engineer is troubleshooting an issue where hosts on VLAN 10 () can ping their default gateway interface (), but cannot establish TCP connections to an application cluster on VLAN 50 (). Diagnostic commands reveal that the intermediate Core-Router shows its OSPF adjacency with the Distribution Switch stuck in the EXSTART/EXCHANGE state across their interconnect subnet (). Additionally, large packet pings with the Don't Fragment (DF) bit set fail without receiving any diagnostic response. Which TWO of the following root causes or misconfigurations are most likely responsible for this behavior?
- An Maximum Transmission Unit (MTU) size mismatch exists between the connecting interface of the Core-Router and the Distribution Switch.Cevap
- An Access Control List (ACL) on the router interface is explicitly dropping ICMP Type 3 Code 4 (Fragmentation Needed) packets.Cevap
- CThe host workstations on VLAN 10 are configured with a default gateway address residing on an unusable broadcast IP subnet.
- DVLAN 10 and VLAN 50 require an active DHCP relay agent configured on the switchport to allow inter-VLAN IP packet routing.
Cevap
The MTU size mismatch on the interconnecting router interfaces and an ACL filtering ICMP Fragmentation Needed (Type 3, Code 4) packets.
An OSPF neighbor relationship stuck in EXSTART/EXCHANGE is a classic sign of an interface MTU mismatch between adjacent devices, as OSPF includes interface MTU in Database Description (DBD) packets. Furthermore, when large packets with the Don't Fragment (DF) bit set fail without an ICMP response, it indicates an Access Control List (ACL) is dropping ICMP Type 3 Code 4 (Destination Unreachable - Fragmentation Needed) packets, breaking Path MTU Discovery and causing silent packet drops.
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Anahtar Kavram
OSPF Adjacency States, Interface MTU Mismatch, and Path MTU Discovery (PMTUD) ICMP Dependencies