Home » EIGRP Deep Dive: Dual Algorithm, Feasible Successor, Stuck-in-Active, Named Mode และ Migration
EIGRP Deep Dive: Dual Algorithm, Feasible Successor, Stuck-in-Active, Named Mode และ Migration
EIGRP Deep Dive: Dual Algorithm, Feasible Successor, Stuck-in-Active, Named Mode และ Migration
EIGRP (Enhanced Interior Gateway Routing Protocol) เป็น advanced distance-vector routing protocol ที่ Cisco พัฒนา DUAL Algorithm คำนวณ loop-free paths อย่างรวดเร็ว, Feasible Successor ให้ backup route พร้อมใช้ทันที, Stuck-in-Active เป็นปัญหาที่ต้องเข้าใจและแก้ไข, Named Mode เป็น modern configuration approach และ Migration จาก EIGRP ไป OSPF/BGP ต้องวางแผนอย่างดี
EIGRP ถูกใช้ใน enterprise networks ทั่วโลก เพราะ: convergence เร็วกว่า OSPF (milliseconds vs seconds), ใช้ bandwidth น้อยกว่า (partial updates), configuration ง่ายกว่า (auto-summary, split horizon), รองรับ unequal-cost load balancing (variance command) แม้ว่า EIGRP จะเป็น open standard แล้ว (RFC 7868) แต่ยังนิยมใช้ใน Cisco-dominant environments เป็นหลัก
EIGRP vs OSPF
| Feature |
EIGRP |
OSPF |
| Type |
Advanced distance-vector (hybrid) |
Link-state |
| Algorithm |
DUAL (Diffusing Update Algorithm) |
Dijkstra (SPF) |
| Convergence |
Very fast (feasible successor = instant) |
Fast (SPF recalculation) |
| Updates |
Partial, bounded (only changes, only affected routers) |
LSA flooding (entire area) |
| Load Balancing |
Equal + Unequal cost (variance) |
Equal cost only (ECMP) |
| Multi-Vendor |
Mainly Cisco (open standard but limited adoption) |
All vendors (true open standard) |
| Scalability |
Good (stub, summarization) |
Better (areas, LSA filtering) |
DUAL Algorithm
| Concept |
Description |
| Feasible Distance (FD) |
Best metric (lowest composite cost) จาก local router ไป destination |
| Reported Distance (RD) |
Metric ที่ neighbor report ว่าตัวเองไป destination (neighbor’s best metric) |
| Successor |
Best route (lowest FD) — installed ใน routing table |
| Feasible Successor (FS) |
Backup route: RD < FD ของ successor → guaranteed loop-free → instant failover |
| Feasibility Condition |
RD of alternate path < FD of current best path → ถ้าเป็นจริง = feasible successor |
| Active State |
ไม่มี FS → router sends queries → wait for replies → route ใน active state จนกว่าจะได้ reply |
| Passive State |
Route stable — successor known, optional FS available → normal operation |
EIGRP Metric (Composite)
| Component |
Default K-Value |
Used? |
| Bandwidth (K1) |
K1 = 1 |
Yes (10^7 / min bandwidth in Kbps) |
| Load (K2) |
K2 = 0 |
No (unstable — changes frequently) |
| Delay (K3) |
K3 = 1 |
Yes (sum of delays in 10s of microseconds) |
| Reliability (K4) |
K4 = 0 |
No (unstable) |
| MTU (K5) |
K5 = 0 |
No (carried but not used in calculation) |
| Wide Metrics |
Named Mode: 64-bit metrics, throughput + latency + jitter + energy |
Supports modern high-speed links |
Feasible Successor Deep Dive
| Scenario |
Action |
Convergence Time |
| Successor fails, FS exists |
Instant switchover to FS → no queries sent |
Sub-second (< 200ms) |
| Successor fails, no FS |
Go active → send queries → wait for replies → calculate new successor |
Seconds to minutes (depends on network) |
| Multiple FS exist |
Best FS becomes successor, others remain as FS |
Sub-second |
| Unequal-cost LB |
variance command: include FS paths within variance × FD → traffic sharing |
Already active (no failover needed) |
Stuck-in-Active (SIA)
| Feature |
รายละเอียด |
| คืออะไร |
Router sends query → neighbor ไม่ reply ภายใน 3 minutes → neighbor relationship reset |
| Cause |
Query propagation too wide, slow neighbor, CPU overloaded, unidirectional link, packet loss |
| Impact |
Neighbor reset → all routes through that neighbor lost → cascading failures possible |
| SIA Timer |
Default 3 minutes (180 seconds) — configurable with timers active-time |
| SIA Query |
At 50% of active timer → send SIA-Query → if neighbor replies → reset timer |
| Prevention: Summarization |
Summarize routes at boundaries → queries stop at summary → limit query scope |
| Prevention: Stub |
eigrp stub: router won’t be queried for transit routes → reduce query domain |
| Prevention: Distribute List |
Filter routes → fewer routes = fewer queries |
Named Mode (EIGRP Named Configuration)
| Feature |
Classic Mode |
Named Mode |
| Config |
router eigrp [AS] → network commands |
router eigrp [name] → address-family → af-interface |
| IPv4 + IPv6 |
Separate config: router eigrp [AS] + ipv6 router eigrp [AS] |
Single config: address-family ipv4/ipv6 under same instance |
| Interface Config |
Under interface (ip hello-interval eigrp) |
Under af-interface (centralized) |
| Wide Metrics |
32-bit (classic metrics) |
64-bit (RIB scale factor, supports 10G+ links) |
| Per-AF Config |
ไม่ชัดเจน |
Clean separation: topology base → per-AF settings |
| Migration |
– |
Can coexist with classic mode (same AS number) |
ทิ้งท้าย: EIGRP = Fast Convergence with Feasible Successors
EIGRP Deep Dive DUAL: FD (best metric), RD (neighbor’s metric), successor (best route), FS (backup, RD < FD = loop-free) Metric: bandwidth + delay (default K1=1, K3=1), wide metrics in named mode (64-bit) FS: instant failover (sub-second), no queries needed — key advantage over OSPF SIA: query timeout (3 min) → neighbor reset | prevent: summarization, stub, distribute-list Named Mode: single config for IPv4+IPv6, af-interface, wide metrics, clean hierarchy Unequal-cost LB: variance command → include FS within variance × FD → unique EIGRP feature Key: EIGRP excels in Cisco environments — fast convergence via FS, unequal LB, easy config | OSPF better for multi-vendor
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