A network administrator is connecting a router at a branch office to the central corporate network via a point-to-point IP link (). Ping tests between the local interface () and the remote interface () are successful. However, OSPF dynamic routing fails to establish a neighbor adjacency between the two routers.
An audit of the inbound Access Control List (ACL) applied to the router's interface displays the following rules:
text
Extended IP access list INBOUND_FILTER
10 permit tcp any any eq 80
20 permit tcp any any eq 443
30 permit icmp any any
40 deny ip any any
Which of the following is the root cause of the OSPF adjacency failure?
- OSPF uses IP protocol 89 directly over IP, which is blocked by the explicit deny statement because it matches neither TCP nor ICMP.Cevap
- BOSPF requires TCP port 89 to establish neighbor relationships, but the ACL only allows TCP traffic on ports 80 and 443.
- CThe remote router interface is configured with a default gateway on an incompatible IP subnet, preventing OSPF reply packets from returning.
- DThe permit icmp rule at line 30 overrides OSPF multicast hello messages, triggering an implicit deny action before evaluating protocol numbers.
Cevap
OSPF uses IP protocol 89 directly over IP, which is dropped by the deny statement because it is neither TCP nor ICMP.
OSPF communicates directly over IP using protocol number 89 rather than using transport-layer protocols like TCP or UDP. Pings succeed because ICMP is explicitly permitted in line 30. However, OSPF packets do not match lines 10, 20, or 30, so they are denied by line 40.
Adım Adım Çözüm
Anahtar Kavram
ACL Protocol Filtering and OSPF IP Protocol 89