Notes on Networking

A blog to share my study notes on Cisco networking and more




  • EIGRP Process
  • EIGRP Terminology
  • EIGRP Packets
  • Neighborship
  • EIGRP Tables
  • EIGRP Timers
  • Passive Interface
  • EIGRP Re-convergence Process
  • Stub Routers
  • Split Horizon
  • EIGRP Bandwidth Control
  • EIGRP Load Balancing
  • Route Filtering
  • Route Summarization
  • Default Routing
  • Configuration
  • Verification
  • EIGRP Feature Summary

EIGRP Process

  • Neighbor Discovery
    • Routers multicast ( hello messages on EIGRP enabled interfaces.
    • Routers check parameter to form neighborship.
    • Neighbor table is formed.
  • Topology Exchange
    • Neighbor routers exchange full topology table using Update/Ack packets.
    • Topology table is formed.
  • Best Route Calculation
    • Routers analyze the topology table and choose the lowest metric route and place it in the routing table
    • Routing table is formed.
  • Continuous Hello
  • Partial Updates
    • Routers send partial updates on changes.


  • EIGRP uses a reliable protocol called RTP to exchange packets.


EIGRP Terminology

Feasible Distance (FD): Metric for the best route from the local router perspective to each prefix

Reported Distance (RD): Metric for the route from neighbor router perspective also known as Advertised Distance (AD).

Successor: For each prefix, the path with the lowest FD is called successor route and placed in the routing table.

Feasible Successor (FS): For each prefix, the path with the second lowest FD could be feasible successor if it complies with feasibility condition:

Reported Distance (RD) of alternate route must be less than Feasible Distance of Successor Route. Feasibility condition ensures that this route does not lead to routing loop.

Feasible Successor routes if available are installed in routing table immediately in case the Successor Route fails.

When a feasible successor route is placed in the routing table, the metric (FD) of the route does not change. The feasible distance of the route never raises until the route has been actively queried.

Feasibility Condition

Not always redundant routes comply with feasibility condition. It’s a good idea to check whether there are Feasible Successors for all prefixes.


Route R1 to R3 is the Successor Route. Although route from R1 to R2 to R3 does not lead to routing loop, it does not comply with feasibility condition.

The following configuration adds a value to RD, FD of prefix on interface S1, so alternate route will pass feasibility condition.


Access-list 1 permit
router eigrp
offset-list 1 in 3 serial 1

EIGRP Packets

  • Hello
  • Update
    • {Prefix, Prefix Length, Bandwidth, Delay, Reliability, Load, MTU, Hop Count}
    • EIGRP sends updates to multicast address ( on multi access interfaces and expect to receive ACK from all neighbors by unicast.
  • Query
  • Reply
  • ACK



Neighborship criteria

  • Neighbor routers should be able to send and receive packets.
  • AS number must match on neighbor routers.
  • K values must match on neighbor routers.
  • Interface primary IP addresses in the same subnet.
  • Routers should pass authentication.
  • Connected interface between neighbors must not be passive.

But it’s not necessary that

  • Hello interval /Hold timer to match.
  • MTU to match.
  • Router ID to be unique.


Static Neighborship

  • Static neighborship is used to reduce EIGRP multicast overhead.
  • No Hello Packet is send and processed on the interface when static neighborship is configured on that interface.
  • No dynamic neighborship on the interface when static neighborship is configured on that interface.
  • Should be configured on both neighbors.
  • Good for Frame Relay since Frame Relay should replicate multicast frames.


EIGRP Metric

  • EIGRP calculate metric using bandwidth and delay by default.
  • Metric = ((10^7 / (minimum-bandwidth) ) + cumulative-delay ) * 256
  • EIGRP metric can include load reliability and MTU but it’s not recommended.
  • K value manipulation is not recommended!


EIGRP Tables

Neighbor Table


Topology Table

EIGRP adds the following prefixes to the topology table.

  • prefixes of connected subnets for interfaces that EIGRP has been enabled on them using network command
  • prefixes of connected subnet for interfaces referenced by neighbor command
  • Prefixes learned by redistribution in to EIGRP


Routing Table

EIGRP uses DUAL algorithm to calculate the best route.

Paths with the lowest FD are placed in routing table.

“H” column in the routing table shows the order in which the neighbors become adjacent.


EIGRP Timers

  • Hello Interval
  • Hold Timer

EIGRP timers may be adjusted to reduce re-convergence time.

Timers can be adjusted per interface or for all the process.

Timers may be different for neighbors on the same interface.


Passive Interface

  • EIGRP do not send hello packets on passive interfaces.
  • EIGRP do not process hello packets if received on passive interfaces.
  • EIGRP include the passive interfaces’ IP addresses in the routing updates


EIGRP Re-convergence Process

If successor route fails and there is no feasible successor for that prefix, active process will happen for that particular prefix.

Routers send Query to neighbors (except failed router) and ask if neighbors have a loop free route to that prefix.

If neighbors have a passive route for that prefix send Reply and do not forward the query.

If neighbors are also active for that prefix, flood the query to their neighbors and wait until receive Reply for all those Queries.

Neighbors send back Reply to the active originating router when Replies are received from all their neighbors.

Stuck in Active

If routers go active for a prefix, they send Query to their neighbors. If they do not receive Reply for a long time (active-time) routers put that prefix in Stuck-in-Active status.

If a prefix goes to Stuck-in-Active status, older versions of IOS bring the non-replying router down.

Newer versions of IOS send SIA-Query to non-replying neighbor at the half of active-time. If SIA-Reply is received from that neighbor, it means that the neighbor is waiting for its own Queries.

If router receives no SIA-Reply from neighbor, brings the non-replying router down.


Stub Routers

  • Stub routers do not advertise EIGRP-learned routes from one neighbor to another.
  • Non-stub routers do not query stub routers.

Stub Router Types

  • Connected

Router advertises connected prefixes but only for interfaces that match in network command.

  • Summary

Router advertises summary routes.

  • Static

Router advertises static routes but only if redistribute static has been configured.

  • Redistributed

Router advertises redistributed routes.

  • Receive Only

Router does not advertise any routes.

If stub keyword is configured with no parameter connected and summary are the default parameters.

Stub Scenario

When there are many branches, a link failure causes many Queries to be sent all branches. We also do not like branch routers to become transit router for other branches when their link fails. So we should limit the Query scope by configuring branch routers as stub.



  • EIGRP calculate MD5 for every packets and it with the pack. No encryption!
  • If authentication fails, neighborship will not form.
  • Neighbor routers should be synchronized by NTP.


Split Horizon

If the best route for a prefix contains an interface (is learned via that interface) EIGRP does not advertise that prefix on that particular interface.

Split Horizon and NBMA interfaces

R2 and R3 cannot become neighbors because they don’t have direct connection.

R1 does not advertise routes learned from R2 to R3 and vice versa because of Split Horizon.

Split Horizon can be disabled on NBMA interfaces.


EIGRP Bandwidth Control

Routers cannot multicast on NBMA interfaces. Therefore multicast packets should be replicated. When there are multiple connections on a NBMA interface – e.g. multiple PVC – EIGRP traffic may saturate the interface bandwidth.

EIGRP at most uses 50% of interface bandwidth by default. By bandwidth we mean the value that has been configured on the interface using bandwidth command.

Bandwidths of serial interfaces are considered 1544 kbps by default. 

Remember to configure real bandwidth on the sub-/interfaces.

  • For point-to-point sub-/interfaces configure the CIR of PVC on the sub-/interface.
  • For multipoint sub-/interfaces configure the sum of all CIR of PVCs on that interface.
    • For multipoint sub-/interfaces it’s also recommended to configure the multiply of PVCs numbers by minimum CIR. Because on multipoint sub interface IOS divides bandwidth into number of PVCs and then calculate the percent of EIGRP bandwidth usage.


Load Balancing

  • Up to 6 equal metric routes can be installed in the routing table at the same time. Default is 4.
  • Unequal load balancing by defining variance multiplier.
    If any Feasible Successor routes have a better metric than product of variance * FD (the metric of the Successor Route), those routes are installed in the routing table. Only Feasible Successor!!!
  • Load balancing cab be Evenly, Balanced, Only on the lowest metric route or across the equal lowest metric paths.


Route Filtering

Route filtering allows to determine which prefixes are permitted to be advertised /received on different interfaces on different directions in Update messages.

Route Filtering Methods

  • Access List
  • Route Map
  • IP Prefix
  • Distribution List


access-list 1 deny w.x.y.z w.w.w.w
router eigrp …
distribute-list 1 out serial 1/0

access-list 1 deny w.x.y.z w.w.w.w

route-map RMAP deny …
match list 1

router eigrp …
distribute-list route-map RMAP out
Route Summarization

Auto Summarization

  • At class full boundaries.
  • If auto-summary is used, EIGRP does not support discontiguous IP networks like class full routing protocols.


Manual Summarization

  • EIGRP can perform summarization at all routes.
  • Smaller Routing Table
  • Reduce Query Scope: EIGRP Queries stop at a router that has a summary route including the prefix listed in the Query but not the specific route.
  • Summary routes have the best metric of all those subnets that have been summarized.
  • Sub optimal routing
  • Configuration at interface level
  • Summarized route has an administrative distance of 5 by default to ensure that summarizing router adds the summary routes to its routing table.
  • Summary route administrative distance of 5 prevents from routing loop.


Route Summarization Scenario:

router eigrp …

interface serial 1
   ip summary-address eigrp … 5

interface serial 1
   ip summary-address eigrp … 5

R1 and R2 add a discard route to Null0 interface for summarized subnet.


Default Routing

Method 1

This method redistribute the default route as external route with metric=170.

  • Create a static default route.
  • Redistribute static route.


Method 2

  • Network !!!

This method redistribute the default route as internal route with metric=90.


Method 3

  • Define a class full network the router (e.g. internet router). It can be a loopback interface.
  • Configure that network as default network. (ip default-network)



router eigrp …
  network … …
  passive-interfaces …
  no auto-summary
  neighbor … serial …
  metric weight … … … … …
  maximum-path … {1..6}
  variance …
  [no] traffic-share [balanced | [min |[across-interfaces]] ]
 interface …
 ip hello-interval eigrp … …
 ip hold-time eigrp … …
ip authentication mode eigrp … md5
ip authentication key-chain eigrp … …
no ip split-horizon eigrp … (kbps)
delay … (tens of microsecond)
bandwidth …
ip bandwidth-percent eigrp … …

key-chain …
  key 1
    key-string …
    accept-lifetime …



#show ip eigrp interfaces

  • Shows active interfaces that eigrp is running on them.
  • Passive interfaces are omitted.


#show ip protocol

  • Shows network command configuration.
  • Shows routing process and neighbors.
  • Shows passive interfaces.


#show ip eigrp neighbors

  • Shows neighbor table


#show ip eigrp topology [all-links]

  • Shows all subnets / links and successor and feasible successor.


#show ip route

  • Routes installed by eigrp are labeled with “D”

#show ip eigrp interface … detail

  • Shows Hello time


EIGRP Feature Summary


Standard Cisco proprietary
Type Distance Vector
Administrative Distance (Internal / External) 90 / 170 , 5 (Summary)
VLSM / Classless Yes
Algorithm DUAL
Multicast Address

(Unicast retransmission to neighbor address!)

Transport Protocol RTP ( IP protocol type 88), Not TCP/UDP
Routed Protocols IP, IPv6, IPX, Appletalk
Updates Full update at first,

Partial update on changes.

Timers Hello Interval= 5 / 60 s

Hold Timer= 15 / 180 s

Active Time=  3 min

Authentication MD5
Split Horizon Yes
Auto Summarization Yes (default),
At class full network boundaries.
Manual Summarization Yes.
At any point.
Load balancing Yes,
Equal and unequal load balancing
Next Hop Field Yes
Route Tag Yes
Terminology Reported Distance
Feasible Distance
Feasible Successor

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