2.2Discovery protocols and EtherChannel

To map a network and troubleshoot it, you need to quickly learn "which device, on which port, is this box connected to." CDP and LLDP reveal that through show commands without tracing cables. Meanwhile, when a single inter-switch link becomes a bandwidth bottleneck or single point of failure, EtherChannel bundles several links into one logical link, aggregating bandwidth without STP blocking one of them, and keeping traffic flowing if one link fails. This section ties these two practical tools—neighbor discovery and link aggregation—to their configuration and verification commands.

2.2.1CDP (Cisco-proprietary) and LLDP (industry-standard)

• CDP (Cisco Discovery Protocol) is a Cisco-proprietary L2 neighbor-discovery protocol that automatically gathers a directly connected peer's device ID, connected port, platform, IP address, IOS version, and more. It is enabled by default on Cisco devices. Verify with show cdp neighbors (summary) or show cdp neighbors detail (including IP and IOS version).
• LLDP (Link Layer Discovery Protocol) is the IEEE 802.1AB industry-standard neighbor-discovery protocol. Unlike CDP, it works across a multi-vendor environment—its essential difference. It is disabled by default on Cisco devices, so enable it with lldp run (global config). Verify with show lldp neighbors.
• Both operate at L2, so they only see directly connected peers on the same segment (they cannot discover devices beyond a router). For security, some operators disable them on external or untrusted ports with no cdp enable or no lldp transmit/receive to limit leakage of device information.

2.2.2EtherChannel and LACP

• EtherChannel bundles multiple physical links (up to 8 active) into one logical link (a port-channel). STP treats the bundle as a single link, so it can aggregate bandwidth without blocking the redundant links, and traffic continues on the survivors if one link fails.
• LACP (Link Aggregation Control Protocol) is the IEEE 802.3ad industry-standard dynamic negotiation protocol. Its modes are active (initiates negotiation) and passive (only waits for the peer). Passive-passive forms no bundle, so at least one side must be active (active-active or active-passive brings the link up). Cisco's proprietary dynamic version is PAgP (desirable/auto). mode on is static and runs no negotiation.
• Every bundled port must have matching speed, duplex, trunk/access mode, and allowed VLANs. A mismatch keeps the port out of the bundle, showing as (s) (suspended) or (D) (down) in show etherchannel summary (a healthy port shows (P) = bundled in port-channel).

In a data center, a single 1G link between a core switch and a distribution switch began saturating under daytime backup traffic. The administrator added a second physical link for bandwidth, but as-is STP simply blocks the second link, so bandwidth does not increase (to prevent an L2 loop, one of the redundant links is always in blocking). The fix is to bundle the two into an EtherChannel so STP treats them as one logical link, keeping both active and aggregating bandwidth. Both switches are Cisco, so either LACP (standard) or PAgP (Cisco-proprietary) would work, but anticipating future multi-vendor gear the admin chose standard LACP, configuring the core side as channel-group 1 mode active and the distribution side as channel-group 1 mode passive. Active on one side and passive on the other still forms a bundle in LACP, so the port-channel comes up. However, show etherchannel summary shows one of the two links stuck at (s) (suspended). The cause: one member port's trunk allowed-VLAN list differed from the other's, and EtherChannel refuses a member whose speed, duplex, trunk mode, or allowed VLANs do not match the rest. Aligning the allowed VLANs on both ports makes both show (P) (bundled), aggregating bandwidth and providing redundancy. Two classic EtherChannel pitfalls also apply here: had both sides been passive, neither would initiate negotiation and no bundle would form; and using mode on (static) on only one side leaves negotiation unmatched and the link down.

2.2.3Section summary

• CDP = Cisco-proprietary, on by default (show cdp neighbors); LLDP = IEEE 802.1AB industry standard, off by default (enable with lldp run). Both discover only directly connected L2 neighbors
• EtherChannel bundles multiple links into one logical link, gaining bandwidth and redundancy without STP blocking; all members must match in speed, duplex, and trunk/VLAN settings
• LACP (standard, active/passive) needs at least one active side; PAgP (Cisco, desirable/auto); on is static. Verify with (P)/(s)/(D) in show etherchannel summary