EIGRP

Characteristics of EIGRP
Enhanced Interior Gateway Routing Protocol, and advanced link state routing protocol

• Fast conversion
• scalable
• Load balancing over unequal cost links
• classless (VLSM)
• Communicates via multicast 224.0.0.10
• was cisco-proprietary

Routing Structure

• EIGRP maintains 3 tables (Neighbor, Interface & Topology)
• EIGRP keeps track of its adjacent neighbors command (show ip eigrp neighbors)
• Interface table which contains a list of interfaces participating in EIGRP Autonomous System command (show ip eigrp interfaces)
• EIGRP Topology table which contains routes to network destination, command (show ip eigrp topology)

Path Selection

• How EIGRP makes path selection decision 
• EIGRP  uses DUAL algorithm (diffusing Update Algorithm) a calculation made by EIGRP to determine the best loop-free path to a network
• Advertised Distance (AD) the Distance (metric value) of a neighboring router to reach a network. Advertised Distance is sometimes called the Reported Distance (RD)
• Feasible Distance (FD): a neighbor's Advertised Distance to a network plus the distance to reach that neighbor
• Successor Router: the primary route to have a network, based on the route having the lowest Feasible Distance (FD) of all routes in the EIGRP topology table.
• Feasible Successor Route: a backup route to a network, based on the route having the second lowest feasible distance in the EIGRP topology (the feasibility condition must be met)
• Feasibility condition: before a route can become a Feasible Successor Route, its Advertised Distance (AD) has to be lower than the Feasible Distance of the successor route. 

Metric Calculation

1. Bandwidth
2. Delay
3. Reliability
4. Load
5. MTU

Metric = [(K1*Bandwidth + (K2*Bandwidth)/(256-Load)+K3*Delay)*K5/(K4+Reliability)]*256

Default K values

K1=1

K2=0

K3=1

K4=0

K5=0

By adding K values into metric formula 

Metric=(Bandwidth + Delay)*256

Metric=[(10,000,000/)+(sum of interface Delays/10)]*256

EIGRP Configuration
from Global configuration mode
Router Eigrp followed by Autonomous system number
Enter networks followed by wildcard mask

R2#sho ip eigrp topology 
IP-EIGRP Topology Table for AS(100)/ID(2.2.2.2)

Codes: P - Passive, A - Active, U - Update, Q - Query, R - Reply,
       r - reply Status, s - sia Status

P 3.3.3.3/32, 1 successors, FD is 156160
        via 192.168.1.1 (156160/128256), FastEthernet0/1
P 1.1.1.1/32, 1 successors, FD is 158720
        via 192.168.1.1 (158720/156160), FastEthernet0/1
        via 172.16.1.1 (2297856/128256), Serial0/1
P 2.2.2.2/32, 1 successors, FD is 128256
        via Connected, Loopback0
P 10.1.1.0/24, 1 successors, FD is 30720
        via 192.168.1.1 (30720/28160), FastEthernet0/1
        via 172.16.1.1 (2172416/28160), Serial0/1
P 192.168.1.0/30, 1 successors, FD is 28160
        via Connected, FastEthernet0/1
P 172.16.1.0/30, 1 successors, FD is 2169856
        via Connected, Serial0/1

R2#show ip eigrp interfaces 
IP-EIGRP interfaces for process 100

                        Xmit Queue   Mean   Pacing Time   Multicast    Pending
Interface        Peers  Un/Reliable  SRTT   Un/Reliable   Flow Timer   Routes
Lo0                0        0/0         0       0/10           0           0
Se0/1              1        0/0       173       0/15         863           0

Fa0/1              1        0/0      1793       0/10        5936           0

R2#show ip eigrp neighbors
IP-EIGRP neighbors for process 100
H   Address                 Interface       Hold Uptime   SRTT   RTO  Q  Seq
                                            (sec)         (ms)       Cnt Num
1   192.168.1.1             Fa0/1             11 00:08:21 1793  5000  0  11
0   172.16.1.1              Se0/1             13 00:09:18  173  1038  0  20

R2#sho ip cef
Prefix              Next Hop             Interface
0.0.0.0/0           drop                 Null0 (default route handler entry)
0.0.0.0/32          receive
1.1.1.1/32          192.168.1.1          FastEthernet0/1
2.2.2.2/32          receive
3.3.3.3/32          192.168.1.1          FastEthernet0/1
10.1.1.0/24         192.168.1.1          FastEthernet0/1
10.2.2.0/24         attached             FastEthernet0/0
10.2.2.0/32         receive
10.2.2.1/32         receive
10.2.2.255/32       receive
172.16.1.0/30       attached             Serial0/1
172.16.1.0/32       receive
172.16.1.2/32       receive
172.16.1.3/32       receive
192.168.1.0/30      attached             FastEthernet0/1
192.168.1.0/32      receive
192.168.1.1/32      192.168.1.1          FastEthernet0/1
192.168.1.2/32      receive
192.168.1.3/32      receive
224.0.0.0/4         drop
224.0.0.0/24        receive
255.255.255.255/32  receive

Configure unequal Cost Load Balancing 
By default OSPF and EIGRP can load balance over an equal cost links

Example:

routing table shows only one path to 1.1.1.1

EIGRP topolofy shows it has 2 paths to 1.1.1.1

WHY?
its very simple why, because of the feasible distance is less 158720 < 2297856

by using this command we can inject another path in the routing table which is to be our load balance link
variance             Control load balancing variance

R2(config-router)#variance ?
  <1-128>  Metric variance multiplier

how to figure out the variance number?
Simple way in doing that!

by dividing the worst FD / Best FD
2297856 / 158720 = 14.4 is not enough which we have to round up to 15

R2(config-router)#variance 15

EIGRP Auto-summary
Auto-summary can reduce the individual route in routing table EIGRP can summarize networks at their classfull boundaries using their natural mask which can be a great thing to reduce size of the ip routing table it could a problem for discontiguous that can cause a problem

R1#show ip protocols
Routing Protocol is "eigrp 100"
  Outgoing update filter list for all interfaces is not set
  Incoming update filter list for all interfaces is not set
  Default networks flagged in outgoing updates
  Default networks accepted from incoming updates
  EIGRP metric weight K1=1, K2=0, K3=1, K4=0, K5=0
  EIGRP maximum hopcount 100
  EIGRP maximum metric variance 1
  Redistributing: eigrp 100
  EIGRP NSF-aware route hold timer is 240s
  Automatic network summarization is in effect
  Automatic address summarization:
    172.16.0.0/16 for Loopback0, FastEthernet0/0
      Summarizing with metric 2169856
    10.0.0.0/8 for Loopback0, Serial2/0
      Summarizing with metric 28160
    1.0.0.0/8 for FastEthernet0/0, Serial2/0
      Summarizing with metric 128256
  Maximum path: 4
  Routing for Networks:
    1.1.1.1/32
    10.1.1.0/24
    172.16.1.0/30
  Routing Information Sources:
    Gateway         Distance      Last Update
    (this router)         90      00:02:38
    10.1.1.2              90      00:02:38
    172.16.1.2            90      00:02:38
  Distance: internal 90 external 170


Configure Passive Interface

hello packet passes through this interface which is connected to a switch, which might be a security issue

!
router eigrp 100
 variance 15
 passive-interface FastEthernet0/0
 network 2.2.2.2 0.0.0.0
 network 10.2.2.0 0.0.0.255
 network 172.16.1.0 0.0.0.3
 network 192.168.1.0 0.0.0.3
 no auto-summary
!

Passive interface will not send hello packets

Troubleshooting

1. show ip route
2. show ip protocols
3. show ip eigrp neighbors
4. show ip eigrp topology
5. show ip cef
6. matching K values
7. look for authentication mismatch
8. EIGRP AS# mismatch
9. look for passive interface