--- tags: - networking - l1 - flashcard-deck - dhcp --- [Portal](../../../../library/portal/index.md) | **Level:** [L1: Foundations](../../../../library/portal/levels.md) | **Topics:** [DHCP & IP Address Management](../../../../library/portal/topics.md) | **Domain:** Networking id category difficulty tags question answer source_path dhcp/a1b2c3d4e5f7 dhcp easy dhcp, protocol, dora What are the four steps of the DHCP handshake (DORA), and what transport protocol do they use? Discover, Offer, Request, Acknowledge — all four messages use UDP. The client sends on port 68, the server listens on port 67.\n\nName origin: DORA is the mnemonic for the handshake. The client starts at 0.0.0.0 and broadcasts to 255.255.255.255.\n\nFun fact: DHCP (RFC 2131, 1997) replaced BOOTP, which replaced RARP — each generation added more configuration capability. training/library/topics/dhcp-ipam/primer.md dhcp/b2c3d4e5f7a8 dhcp easy dhcp, leases, renewal At what percentage of lease time does a DHCP client begin unicast renewal (T1)? At 50% of the lease time (T1). If that fails, at 87.5% (T2) the client begins broadcast rebinding to any available server.\n\nUnder the hood: for a 24-hour lease, T1 fires at 12 hours (unicast to original server), T2 at 21 hours (broadcast to any server).\n\nGotcha: if both renewals fail, the client releases its IP at lease expiry and restarts DORA, briefly losing connectivity. training/library/topics/dhcp-ipam/primer.md dhcp/c3d4e5f7a8b9 dhcp easy dhcp, options What three essential network parameters does a DHCP server typically provide beyond just an IP address? Subnet mask (option 1), default gateway/router (option 3), and DNS servers (option 6). Additional common options include domain name, NTP servers, and lease time.\n\nFun fact: DHCP has 255 option codes across various RFCs. Option 150 (TFTP for Cisco phones) and option 66 (TFTP server) are common in VoIP.\n\nRef: https://www.iana.org/assignments/bootp-dhcp-parameters/ training/library/topics/dhcp-ipam/primer.md dhcp/d4e5f7a8b9c0 dhcp medium dhcp, relay, giaddr Why are DHCP relay agents needed, and what field does the relay agent set to help the server pick the correct scope? DHCP uses broadcast, and broadcasts do not cross routers. A relay agent receives the broadcast, sets the giaddr (gateway IP address) field to its own IP on that subnet, and forwards the request as unicast to the DHCP server. The server uses giaddr to determine which pool to allocate from. training/library/topics/dhcp-ipam/primer.md dhcp/e5f7a8b9c0d1 dhcp medium dhcp, dora, broadcast Why does the DHCP REQUEST message use broadcast instead of unicast? Because there may be multiple DHCP servers that sent offers. The broadcast REQUEST tells all servers which offer was accepted, so the others can release their reserved addresses back to the pool. training/library/topics/dhcp-ipam/primer.md dhcp/f7a8b9c0d1e2 dhcp medium dhcp, ipam, reservations Why is using DHCP reservations preferred over static IP configuration on servers? DHCP reservations (MAC-to-IP bindings) keep all IP assignments in one system — the DHCP server — instead of splitting between DHCP for dynamic clients and static configs on individual hosts. This provides a single source of truth for IP management. training/library/topics/dhcp-ipam/primer.md dhcp/a8b9c0d1e2f3 dhcp medium dhcp, failover, redundancy What are the two main approaches to DHCP redundancy, and what is the downside of split scope? Split scope divides the pool between two servers (e.g., 60/40) with no inter-server communication. Active failover shares state between servers so both can serve the full pool. The downside of split scope is that if one server fails, its portion of the pool is unavailable until it returns. training/library/topics/dhcp-ipam/primer.md dhcp/b9c0d1e2f3a4 dhcp hard dhcp, lease-times, design How should DHCP lease times be chosen for different environments, and why? Match lease time to device turnover: guest WiFi 1-4 hours (high turnover, reclaim fast), office 8-24 hours (stable devices), server VLAN 7-30 days (stability, fewer renewals), lab/test 1-2 hours (frequent reprovisioning). Too short wastes bandwidth on renewals; too long exhausts the pool when devices leave. training/library/topics/dhcp-ipam/primer.md dhcp/c0d1e2f3a4b5 dhcp hard dhcp, cisco, helper-address On a Cisco router, what does the "ip helper-address" command do, and what is the caveat about its scope? It configures the interface as a DHCP relay agent, forwarding broadcast DHCP requests to the specified server IP via unicast. The caveat is that ip helper-address forwards not just DHCP but also TFTP, DNS, TACACS, and other UDP services. If you only want DHCP relayed, use "ip forward-protocol" to limit which protocols are forwarded. training/library/topics/dhcp-ipam/primer.md dhcp/d1e2f3a4b5c6 dhcp hard dhcp, scope, design What elements make up a well-designed DHCP scope, and why are exclusion ranges important? A scope includes: network and broadcast addresses, gateway (excluded from pool), exclusion range for infrastructure IPs (e.g., first 10 addresses), dynamic pool range, reservation range for servers, options (router, DNS, domain), and lease time. Exclusion ranges prevent the DHCP server from assigning infrastructure addresses (routers, switches, management IPs) to dynamic clients. training/library/topics/dhcp-ipam/primer.md --- ## Wiki Navigation ### Related Content - [DHCP & IP Address Management](../../../../library/topics/dhcp-ipam/index.md) (Topic Pack, L1) — DHCP & IP Address Management