For specific routing protocols, such as OSPF and IS-IS, using filter-policy to filter routes in the inbound direction can block the transmission of link state information, so that the filtered…

Question

Options

• ATRUE
• BFALSE

• A
  73% (11)
• B
  27% (4)

Explanation

The stated correct answer of A (TRUE) is actually incorrect based on standard networking fundamentals - this appears to be a mislabeled answer key.

Why B (FALSE) is the right answer: OSPF and IS-IS are link-state protocols that flood topology information (LSAs for OSPF, LSPs for IS-IS) independently of the routing table. Applying filter-policy in the inbound (import) direction does not block LSA/LSP flooding - the link-state database still receives and propagates all topology information across the network. What it actually does is filter which routes, after being calculated from the LSDB via SPF, get installed into the local IP routing table.

Why the statement is false: The statement claims a false cause-and-effect: that filter-policy blocks link state transmission, which then prevents routes from being added. In reality, LSAs/LSPs still propagate freely - only the final routing table installation step is filtered.

Contrast with distance-vector protocols: With RIP or BGP, filter-policy inbound does block route propagation, because those protocols only know what neighbors tell them. OSPF/IS-IS routers maintain a full topology database regardless, so filtering at the routing table level doesn't suppress flooding.

Memory tip: Think "link-state = two-step process." Step 1 is LSA/LSP flooding (always happens). Step 2 is SPF calculation → routing table (filter-policy acts here). filter-policy inbound only touches Step 2, never Step 1.

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