* EIGRP operation can be subdivided into four discrete steps..
- Discover EIGRP Neighbors
- Exchange Topology Information
- Choose Best Path
- Neighbor and Topology Table Maintenance
### Discover EIGRP Neighbors
* EIGRP neighbors discovered through multicast
- Implies bi-directional multicast transport of IP protocol 88 to 224.0.0.10 is needed.
- Possible NBMA pseudo-broadcast support issues (Frame Relay or ATM)
- Possible filtering issues (firewall,vlan access-list)
* If neighbor statement configured, only unicast hellos are accepted.
- If not upon adjacency cannot continue.
* Attributes that must match to proceed.
- Common subnet (Must be primary IP address, not secondary)
- ASN (Autonomous System Number )
- Authentication
- K Values (Metric weights)
### Exchange Topology Information
* Topology info exchanged through unicast, not multicast.
- Imolies bi-directional unicast transport of IP protocol 88 needed.
- Multicast still required unless neighbor statement used.
* EIGRP only advertises what it installs in routing table.
- Troubleshooting considerations
- Auto-summary
- Split-horizon
- Duplicate Router-IDs for external routes
- No seed metric for external routes
- Distribute-list filters
### EIGRP Path Selection
* Feasible distance must be finite to use and advertise a path
- show ip eigrp topology
* Unequal cost load balancing only supported for feasible successors.
- If Advertise Distance < Feasible Distance, Feasible Successor = TRUE
* Modifying bandwidth for path selection can starve EIGRP updates of bandwidth.
- ip bandwidth percent eigrp
* Common EIGRP Verifications
- show ip route
- show ip eigrp neighbor
- show ip eigrp topology
- show ip eigrp topology all-links
- debug eigrp packet hello
- debug eigrp packet query replay
