Overview
This lecture explains Enhanced Interior Gateway Routing Protocol (EIGRP), focusing on its operation, benefits, metric calculations, convergence, redistribution, summarization, load balancing, and key configuration commands.
EIGRP Basics and Advantages
- EIGRP is an advanced distance-vector routing protocol using Cisco’s Diffused Update Algorithm (DUAL).
- It offers fast convergence, low network resource usage, and sends only necessary routing updates.
Neighbor Discovery and Maintenance
- Routers become neighbors via hello packets, sent every 5s (high bandwidth) or 60s (low bandwidth).
- The hello interval and hold time can be manually configured per interface.
- EIGRP forms neighbor relationships on primary interface addresses only.
Topology Table and Metrics
- EIGRP routers maintain a topology table separate from the routing table.
- Metrics are based on minimum bandwidth and total delay along the path.
- Default metric formula: metric = ([K1 * bandwidth + K3 * delay]) * 256; typically bandwidth and delay are weighted.*
Feasible Distance, Reported Distance, and Feasible Successors
- Feasible distance: lowest calculated metric to a destination.
- Reported distance: metric reported from a neighbor to a destination.
- Feasible successor: alternate loop-free path with reported distance less than the feasible distance.
Loop Prevention: Split Horizon and Poison Reverse
- Split horizon: routes learned on an interface are not advertised back out that interface.
- Poison reverse: routes are advertised as unreachable out the interface they were learned from to prevent loops.
Query Process and Stuck In Active (SIA)
- Queries are used to find new paths when a route fails; SIA occurs if a query remains unanswered.
- SIA timers can be adjusted, but reducing query range (using summarization or boundaries) is preferable.
Redistribution
- EIGRP supports redistribution between EIGRP ASes and other protocols (e.g., RIP, OSPF).
- Route tags prevent loops during redistribution; external routes have a distance of 170.
Summarization
- EIGRP supports auto-summarization at network boundaries and manual summarization on any interface.
- Summarization can limit the spread of queries.
Load Balancing
- By default, EIGRP load balances over up to four equal-cost paths, but can use variance to include unequal-cost paths.
- Traffic is distributed based on metric ratios.
Default Routing and Bandwidth Management
- Default routes can be injected by redistributing static routes or summarizing to 0.0.0.0/0.
- EIGRP uses up to 50% of interface bandwidth by default, adjustable with ip bandwidth-percent eigrp.
Important Commands
show ip eigrp neighbors — displays EIGRP neighbors and timer statuses.show ip eigrp topology — shows feasible successors and path metrics.show ip eigrp topology all-links — lists all known paths, including those not chosen for routing.show ip eigrp traffic — displays EIGRP packet statistics.
Key Terms & Definitions
- DUAL (Diffused Update Algorithm) — algorithm EIGRP uses for loop-free, fast convergence.
- Feasible Distance — lowest cost from the router to the destination.
- Reported Distance — metric to a destination as advertised by a neighbor.
- Feasible Successor — backup route, loop-free, ready for immediate use.
- Split Horizon — rule preventing route advertisement back on the interface it was learned from.
- SIA (Stuck In Active) — state when a route remains active due to unanswered queries.
- Auto-Summarization — automatic summarization across network boundaries.
- Variance — multiplier to allow unequal-cost load balancing.
Action Items / Next Steps
- Review EIGRP metric calculations and configuration commands.
- Practice configuring EIGRP with neighbor relationships, summarization, and load balancing in lab scenarios.
- Analyze EIGRP command outputs from sample routers for better troubleshooting skills.