EIGRP

Terms

DUAL — Diffusing update algorithm

Invented by J.J. Garcia-Luna-Aceves
Feasibility conditions

Successor route

AKA, Successful Route
The current best path
Smallest metric

Successor

First next-hop router for the successor route

Feasible distance (FD)

Lowest metric to reach a subnet
Sum of the RD + local cost

Reported distance (RD)

Metric inside a route update from another router
The sending router included its FD, which becomes our RD

Feasibility condition

Easy backup routes
Not all possible backup routes
Mathematical
If backup
- RD is be less than the current FD

Feasible successor

Route
- Satisfies the feasibility condition
- Maintained as a backup route

Split Horizon

Never advertise a network, out the same interface it was learned on

Poison Reverse

If you must advertise a network out the same interface it was received on, advertise the delay as infinity

Count to Infinity

If EIGRP reaches 100 hops, the route is discarded

RTP — Reliable Transport Protocol

Used for EIGRP packets

Topology Table

Metric information for all known routes

Packet types

EIGRP uses a mixture of unicast and multicast while its working.

Multicast is 224.0.0.10

Packet	Reliable	Purpose	Multicast	Unicast
Hello/Ack	No	Neighbor Adjacency	Every 5s, 60s on WAN ≤ 1.544 Mbps	—
Update	Yes	Routing info	Inform neighbors of link/metric changes	New Neighbors
Query/Reply	Yes	Query for feasible successor	Ask neighbors	Reply to query
SIA-Query/SIA-Reply	Yes	Stuck in Active Outstanding	—	Reply to SIA-query

Metrics

See EIGRP Classic Metric.
See EIGRP Wide Metric.

Feasible successor algorithm

Topology

┌────────┐            1000             ┌────────┐    10.0.0.0/24
│   R1   ├─────────────────────────────┤   R2   ├──────────────────
└─────┬──┘                             └─┬──────┘      2000
      │            ┌────────┐            │
      └────────────┤   R3   ├────────────┘
         50        └────────┘      50

R2 sends an update

10.0.0.0/24 - RD is 2000

R3 Sends an update

10.0.0.0/24 - RD is 2050

R1 calculates total path metric

R2 is 2000 + 1000 = 3000
R3 is 2050 + 50 = 2100 < - Successor route

Results

R1 installs the successor route as R1-R2
R1 picks R1-R3 as the feasible successor because the RD (2050) is less than the FD

Results in the CLI

R1# show ip eigrp topology 10.0.0.0/24
EIGRP-IPv4 Topology Entry for AS(1)/ID(1.1.1.1) for 10.0.0.0/24
  State is Passive, Query origin flag is 1, 1 Successor(s), FD is 2100
P 10.0.0.0/24, 1 successors, FD is 2100                <--- Feasible Distance
        via 10.0.13.3 (2100/2050), GigabitEthernet0/3  <--- Successor Route
        via 10.0.12.2 (3000/2000), GigabitEthernet0/2  <--- Feasible Successor
                       |     |
                       |     +-- Reported Distance
                       +-------- Path Metric

                                                             (RD 2000 < FD 2100)

Unequal cost multi path

EIGRP can load balance over the successor and feasible successor routes with a variance command.

Timers

Hello packets are every 5 seconds, on 60 seconds on T1 links
- The deadtime is 3x the hello timer

Initial bringup

Send Hello packets, to 224.0.0.10
- Doesn’t’ require multicast to be on
- Unicast Init from neighbor, set Seq, Set Ack to 0
  - Neighbor Sends back Ack as prior sequence number
  - Update Messages

Stuck in active

The router is too busy to answer the query (generally due to high CPU utilization)
The router has memory problems and cannot allocate the memory to process the query or build the reply packet
The circuit between the two routers is not good; there are not enough packets that get through to keep the neighbor relationship up, but some queries or replies are lost between the routers
unidirectional links (a link on which traffic can only flow in one direction because of a failure)

Update message

AS number
Prefixes
End-of-table Flag

Prefixes

Type (internal, etc)
Reliability
Load
MTU
Hop Count
Delay
Bandwidth
Flags
- Source Withdrawn
- Candidate Default
- Route is Active
- Route is Replicated
Next-hop
Prefix Length

Auto summary

Off by default on versions later than IOS 15.

The summarization done by this command is classful. This should never be turned on.

To enable:

auto-summary

Manual summaries

In EIGRP these go under the interface, on the interface you want the summary to be sent out of.

ethernet 1
  ip summary-address eigrp 100 192.168.0.0/16

Named mode

Name mode supports IPv6 inside a VRF.

Minimum config

router eigrp EIGRP_100
 !
 address-family ipv4 unicast autonomous-system 100
  !
  network 0.0.0.0
  eigrp router-id 1.1.1.1
 exit-address-family

Using the old config, then having the box convert it for you

router eigrp 1
  eigrp upgrade-cli EIGRP_1

RIB scaling

The Cisco RIB can only hold values that are unsigned 4 bytes. The EIGRP named metrics are 64-bit.

This is done automatically (and why the topology values don’t match “show ip route”. In the event you need to modify it, here it is.

router eigrp EIGRP_100
  address-family ipv4 unicast autonomous-system 100
    topology base
      metric rib-scale 100

EIGRP Scaling

Route Summarization
Multiple EIGRP autonomous systems

Variance

Shorter delays

In this example, the delay scale is 1x, 2x, 3x, 4x, 5x, 6x, 7x.

The lowest RIB FD is 433.

With a variance of two, only two interfaces get added to the RIB.

R1# show ip protocols | i eigrp|variance
Routing Protocol is "eigrp 100"
      Maximum metric variance 2

R1# show run | i int|delay
interface GigabitEthernet0/1
 delay 1
interface GigabitEthernet0/2
 delay 2
interface GigabitEthernet0/3
 delay 3
interface GigabitEthernet0/4
 delay 4
interface GigabitEthernet0/5
 delay 5
interface GigabitEthernet0/6
 delay 6
interface GigabitEthernet0/7
 delay 7

R1# show ip route

[output omitted]
!
! sorted to look pretty and be in order
!
D        2.2.2.2 [90/433] via 10.12.1.2, 00:02:35, GigabitEthernet0/1
                 [90/729] via 10.12.2.2, 00:02:35, GigabitEthernet0/2

Longer delays

In this example, the delay scale is: 1x, 1.1x, 1.2x, 1.3x, 1.4x, 1.5x, 1.6x

The lowest FD is 3398.

With a variance of two, all seven interfaces get programmed.

R1# show ip protocols | i eigrp|variance
Routing Protocol is "eigrp 100"
      Maximum metric variance 2
!
! I configured delay, this is the correct way to alter metrics.
!
R1# show run | i int|delay
interface GigabitEthernet0/1
 delay 11
interface GigabitEthernet0/2
 delay 12
interface GigabitEthernet0/3
 delay 13
interface GigabitEthernet0/4
 delay 14
interface GigabitEthernet0/5
 delay 15
interface GigabitEthernet0/6
 delay 16
interface GigabitEthernet0/7
 delay 17

R1# show ip route

[output omitted]

!
! sorted to look pretty and be in order
!
D        2.2.2.2 [90/3398] via 10.12.1.2, 00:00:04, GigabitEthernet0/1
                 [90/3694] via 10.12.2.2, 00:00:04, GigabitEthernet0/2
                 [90/3991] via 10.12.3.2, 00:00:04, GigabitEthernet0/3
                 [90/4288] via 10.12.4.2, 00:00:04, GigabitEthernet0/4
                 [90/4584] via 10.12.5.2, 00:00:04, GigabitEthernet0/5
                 [90/4881] via 10.12.6.2, 00:00:04, GigabitEthernet0/6
                 [90/5177] via 10.12.7.2, 00:00:04, GigabitEthernet0/7

IPv6

Classic Configuration is deprecated.

i.e., do not use these:

!
! Not for v6
!
router eigrp 100
!
! also not for v6
!
ipv6 router eigrp 100

Use named mode.

router eigrp EIGRP-AS-100
  address family ipv4 auton 100
    network 10.0.0.0
  address family ipv6 auton 100

EIGRP for IPv6 uses Link Local hellos.

Commands

show eigrp neighbors
show eigrp interfaces

Network parser

The CLI parser is converting the IP into binary, then comparing it to the wild mask.
The CLI parser will only save the matched bits of the IP.
The CLI parser will not save the zeroth network, anything starting with 0.
The CLI parser will only save the matched bits of an IP if if finds bits that are “on”
Using the “all” mask of 255.255.255.255 creates this statement ‘network 0.0.0.0’ and matches everything.
Using the “unique-ip” mask of 0.0.0.0 means “match this single address”
The wildcard mask only accepts contiguous numbers “Discontiguous mask is not supported.”

192.0.2.5 127.255.255.255 - becomes 128.0.0.0, the rest of the bits get dropped.

References

RFC 7868: Cisco’s Enhanced Interior Gateway Routing Protocol (EIGRP) | RFC Editor

Cisco - Understand and Use the Enhanced Interior Gateway Routing Protocol

Cisco - Configure EIGRP Named Mode

Cisco - Configuring EIGRP Wide Metrics

Cisco - How Does Unequal Cost Path Load Balancing (Variance) Work in IGRP and EIGRP

Cisco - Troubleshooting EIGRP Variance

Cisco Live - EIGRP, The Usual Suspects - Steven Moore - BRKENT-2050

Cisco Live - EIGRP, Introduction and Overview - Steven Moore - BRKENT-1187

Wikipedia - Diffusing update algorithm

Blame The Network - Stuck in Active

Removed

~~BRKENT-2799 is “EIGRP, Introduction and Overview” 2023~~

BRKENT-1187 is “EIGRP, Introduction and Overview” 2024

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