ROUTE: EIGRP Quick Study Notes

EIGRP Features:
• fast convergence
• partial triggered updates
• sophisticated metric
• unequal cost load balancing
• VLSM support
• multiple network layer protocol support
• seamless connectivity across data-link layer protocols

EIGRP Reliable Transport Protocol (RTP):
• Why? EIGRP runs directly above the IP layer [88]; needs reliable transmission
• How? Own protocol to ensure guaranteed, ordered packet delivery
❗️Only selected packets are sent reliably, requiring acknowledgement upon receipt

EIGRP Operation Overview:
Based on three tables:
1️⃣ neighbor table: lists all neighbors that the router has formed an adjacency with
2️⃣ topology table: contains all routes advertised by neighbors
3️⃣ routing table: holds the best routes and is used for forwarding packets

EIGRP Updates:
• only routes used by EIGRP (successor routes) are advertised
• topology table contains prefixes learned from neighbors and locally known sources:
🔹subnets of directly connected interfaces enabled in EIGRP
🔹subnets learned via redistribution

EIGRP Metric:
• multiple components:
🔹(default) bandwidth and delay
🔹(optional) reliability and load
• metric weights (k values) determine which components are used in the metric calculation
• MTU is included in the update but is not used for the metric calculation
• bandwidth: the lowest bandwidth on the path
• delay: the sum of all delays on the path
• reliability: the worst reliability on the path, based on keepalives
• load: the worst load on the path, based on packet rate and configured interface bandwidth

EIGRP Metric Calculation:
• default K values: K1 = K3 = 1, K2 = K4 = K5 = 0
• [shortened] metric = (bandwidth + delay) * 256
• bandwidth = 10^7 / (minimum bandwidth along the path in kbps)
• delay = sum of all delays in the path in tens of microseconds
⚠️ [FULL] metric = [(K1 * bandwidth + [(K2 * bandwidth) / (256 - load)] + K3 * delay) * K5 / (K4 + reliability)] * 256

EIGRP Metric Calculation Example:
• bandwidth: minimum bandwidth on the path is 100 mbps = 100,000 kbps ➡️ 10^7 / 100,000 = 100
• delay: sum of all delays on the path in tens of microseconds ➡️ 1000 + 5000 + 3000 = 9000
• metric: (bandwidth + delay) * 256 = 2,329,600

EIGRP metric calculation from the topology table:
• EIGRP metric components for a specific route can be investigated with the 'show ip eigrp topology <network>' command
• metric for example route (10^7 / 500000 + 31) * 256 = 13056 (FD)

🆘 The output shows the total delay in microseconds but the metric formula calculates the total delay in *tens of* microseconds.
⚠️ MTU and hop count (max. 100 by default) are listed as part of the vector metric but they are not actually used in the metric calculation.

EIGRP Wide Metrics:
• classic metric does not scale correctly for high speed interfaces or port channels (beyond 1 Gbps)
• new formula, K6 added
• 32-bit metric ➡️ 64-bit metric
• 64-bit metric is too big for the routing table ➡️ metric rib-scale
• see: RFC 7868

EIGRP Feasibility Condition:
• used to ensure loop-free topology
• reported distance (RD) | advertised distance (AD): the metric from neighboring router to the destination
• feasible distance (FD): the local (lowest) metric to the destination [since A to P]
• RD < FD

EIGRP Queries:
• sent when a route is lost and no feasible successor is available ➡️ going active on a route
• sent to all neighbors except the previous successor
• the query propagates through the network, creating an expanding tree of queries
• when a router answers a query, it stops the spread of the query through that branch of the network
• route summarization and stub routing can be used to optimize how queries are exchanged

EIGRP Stuck in Active (SIA):
• router must receive replies to all queries sent before a new path is calculated
• if any reply is lost or missing:
🔹the route goes into stuck in active, or SIA, state
🔸neighbor status is verified; if no response, neighborship is reset

EIGRP Load Balancing:
• equal-cost load balancing
🔹enabled by default
🔹up to 4 routes by default (maximum-paths)
• unequal-cost load balancing
🔸disabled by default
🔸variance [1-128]
🔸only feasible successors are eligible, if metric < feasible distance * variance

EIGRP Named Configuration:
• all EIGRP configuration in one place
• unified configuration for IPv4 and IPv6
• backward-compatible
• available from 15.0(1)M
⚠️ problem with classic EIGRP: scattered configuration and similar but different commands for IPv4 and IPv6

EIGRP Named Configuration Modes:
🔹address family: general commands (router-id, network, ...)
🔹address family interface: interface specific commands (summary-address, passive-interface, ...)
🔹address family topology: topology related commands (variance, maximum-paths, ...)


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