Cisco Certified Network Associate (CCNA) 200-301 · Free study guide

Objective 2.3 — Use Layer 2 discovery protocols

Neighbor discovery answers a narrow question: what directly connected device is advertising information on this link? It is excellent inventory evidence but not proof of end-to-end IP operation or configuration correctness.

Cisco Discovery Protocol

Cisco Discovery Protocol is a Cisco-proprietary Layer 2 protocol. Compatible devices advertise identity, local and remote port information, platform and capabilities, management addressing, and other fields. Because it operates on a direct link, it can reveal a neighbor even when Layer 3 addressing is missing or wrong.

CDP can be enabled globally and controlled per interface. Disable it on exposed links where policy does not permit information advertisement. A discovery record may be stale until its holdtime expires, so confirm current physical state.

Link Layer Discovery Protocol

LLDP is an open vendor-neutral standard with a similar directly connected scope. It advertises structured type-length-value information and supports multi-vendor inventory. Transmit and receive behavior can be controlled separately on platforms that support it.

LLDP and CDP can operate at the same time, producing two views of a link. They do not negotiate VLANs, routes, or security on behalf of the operator.

Reading neighbor evidence

Match the local interface to the neighbor's advertised port. Check device ID, system name, capabilities, and management address. A router capability does not prove routing is enabled on that specific link. A phone capability does not prove the voice VLAN is correct. Treat each field as a clue whose scope is explicit.

Detailed views often expose software and platform information. That helps operations but also increases information disclosure to an unauthorized device connected to the link.

IOS command map

cdp run enables CDP globally, while cdp enable or no cdp enable controls it on an interface. lldp run enables LLDP globally; supported switches also offer interface controls such as lldp transmit and lldp receive. Apply the organization's exposure policy before enabling advertisements on an untrusted edge.

Use show cdp neighbors and show cdp neighbors detail to compare the local port, advertised remote port, device identity, and management data. Use show lldp neighbors and show lldp neighbors detail for the standards-based view. show cdp interface and show lldp interface help explain a missing record. Command availability and exact output fields vary by IOS family and release.

Worked scenario

Documentation says Switch A port Gi1/0/24 connects to Router R1 Gi0/0, but LLDP shows Switch B on that port. Before changing VLANs or routing, correct the physical map and identify where R1 actually connects. Discovery has localized a cabling/documentation discrepancy; it has not tested application reachability.

Verification workflow

Confirm the protocol is enabled globally and on the interface, inspect summary and detailed neighbors, compare both ends, and correlate with interface descriptions and cabling records. If no neighbor appears, check link state, transmit/receive settings, protocol compatibility, and whether an intermediate device breaks the direct Layer 2 relationship.

Common traps

• Calling CDP an IP routing protocol.
• Expecting discovery across routers.
• Treating a neighbor record as proof a trunk carries every VLAN.
• Leaving discovery exposed without considering policy.
• Confusing local port with advertised remote port.

Readiness checklist

• I can compare CDP and LLDP ownership and scope.
• I can map local and remote interfaces from output.
• I know what neighbor discovery cannot verify.
• I include information exposure in the design decision.