T-Shoot EIGRP INE

* 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

Split Horizon EIGRP

**Note The Serial Link between R3-R1 is down. R2 cannot reach Loopback 0 of R4 Network (10.1.4.0/24)

Split-horizon route advertisement is a method of preventing routing loops in distance-vector routing protocols by prohibiting a router from advertising a route back onto the interface from which it was learned.

# Issue Split Horizon 

R2#show ip route eigrp              
     10.0.0.0/24 is subnetted, 6 subnets
D       10.1.3.0 [90/409600] via 10.1.23.3, 00:03:26, FastEthernet0/0
D       10.1.5.0 [90/2297856] via 10.1.245.5, 00:07:56, Serial0/0
D       10.1.50.0 [90/2195456] via 10.1.245.5, 00:07:56, Serial0/0


# Desable The Split-Horizon
R5#conf terminal 
R5(config)#in serial 0/0
R5(config-if)#no ip split-horizon eigrp 1 

*Mar  1 07:03:57.838: %DUAL-5-NBRCHANGE: IP-EIGRP(0) 1: Neighbor 10.1.245.2 (Serial0/0) is resync: split horizon changed
*Mar  1 07:03:57.842: %DUAL-5-NBRCHANGE: IP-EIGRP(0) 1: Neighbor 10.1.245.4 (Serial0/0) is resync: split horizon changed

R2#
*Mar  1 07:03:53.442: %DUAL-5-NBRCHANGE: IP-EIGRP(0) 1: Neighbor 10.1.245.5 (Serial0/0) is resync: peer graceful-restart

R2#show ip route eigrp 
     10.0.0.0/24 is subnetted, 11 subnets
D       10.1.3.0 [90/409600] via 10.1.23.3, 00:00:09, FastEthernet0/0
D       10.1.1.0 [90/2835456] via 10.1.245.5, 00:00:09, Serial0/0
D       10.1.6.0 [90/2835456] via 10.1.245.5, 00:00:09, Serial0/0
D       10.1.5.0 [90/2297856] via 10.1.245.5, 00:08:43, Serial0/0
D       10.1.4.0 [90/2809856] via 10.1.245.5, 00:00:09, Serial0/0
D       10.1.60.0 [90/2733056] via 10.1.245.5, 00:00:09, Serial0/0
D       10.1.50.0 [90/2195456] via 10.1.245.5, 00:08:43, Serial0/0
D       10.1.146.0 [90/2707456] via 10.1.245.5, 00:00:09, Serial0/0