EIGRP (Enhanced Interior Gateway Routing Protocol) is a routing protocol developed by Cisco. It finds the best paths inside an organization’s network and adapts quickly when links fail. Cisco published its basic functions as informational RFC 7868 in 2016. In practice, however, it still runs almost entirely on Cisco equipment.
This guide explains how EIGRP works, its key terms, the metric, packet types and how to configure it.
Key characteristics
- Advanced distance vector: routers learn routes from neighbours, but EIGRP adds features usually found in link-state protocols.
- DUAL algorithm: the Diffusing Update Algorithm guarantees loop-free paths and fast convergence.
- Partial, bounded updates: routers send updates only when something changes, and only to routers that need them.
- Multicast communication: EIGRP uses the address 224.0.0.10 and its own IP protocol number, 88.
- Unequal-cost load balancing: with the
variancecommand, it can share traffic across paths with different metrics. - Administrative distance: 90 for internal routes, 170 for external routes and 5 for summary routes. See our guide to administrative distance.
Important EIGRP terms
| Term | Meaning |
|---|---|
| Neighbour table | Directly connected EIGRP routers |
| Topology table | All routes learned from neighbours |
| Routing table | The best routes in use |
| Feasible distance (FD) | The best metric to a destination from this router |
| Reported distance (RD) | A neighbour’s own metric to the destination |
| Successor | The neighbour on the best path |
| Feasible successor | A backup neighbour that meets the feasibility condition |
The feasibility condition says a backup is loop-free if its reported distance is lower than the current feasible distance. As a result, a feasible successor can take over instantly when the successor fails.
The EIGRP metric
By default, EIGRP calculates its metric from two values:
- the lowest bandwidth along the path;
- the total delay along the path.
Load and reliability can also be included through K-values, but Cisco recommends leaving the defaults. Neighbours must use matching K-values to form adjacencies.
EIGRP packet types
| Packet | Purpose | Sent as |
|---|---|---|
| Hello | Discover and keep neighbours | Multicast, unreliable |
| Update | Share route information | Multicast or unicast, reliable |
| Query | Ask neighbours about a lost route | Multicast, reliable |
| Reply | Answer a query | Unicast, reliable |
| Acknowledgement | Confirm receipt of reliable packets | Unicast |
EIGRP uses its own Reliable Transport Protocol (RTP) to make sure update, query and reply packets arrive.
Configuration
Classic mode
router eigrp 100
network 192.168.1.0 0.0.0.255
network 10.0.0.0 0.255.255.255
passive-interface GigabitEthernet0/2The number 100 is the autonomous system number, and it must match on neighbours.
Named mode
Newer IOS releases support named mode, which keeps settings in one place and supports wide metrics for fast links.
router eigrp CAMPUS
address-family ipv4 unicast autonomous-system 100
network 192.168.1.0 0.0.0.255
af-interface GigabitEthernet0/0
authentication mode hmac-sha-256 Str0ngKey
exit-af-interfaceUse authentication so rogue routers cannot inject routes.
Verification commands
show ip eigrp neighbors
show ip eigrp topology
show ip route eigrp
show ip protocolsEIGRP vs OSPF
| EIGRP | OSPF | |
|---|---|---|
| Type | Advanced distance vector | Link state |
| Vendor support | Mainly Cisco | All major vendors |
| Configuration | Simple | More design work (areas) |
| Convergence | Very fast with feasible successors | Fast |
For multi-vendor networks, OSPF is usually the safer choice. Read our guide to OSPF.
Sources
Need help with network security? We test routers, firewalls, ACLs and segmentation as part of our VAPT services and hardening audits. Contact us for a quote.
