EIGRP
(Enhanced Interior Gateway Routing Protocol)
Important points about EIGRP
- EIGRP is a
Cisco proprietary protocol which means it will work only on Cisco routers.
- EIGRP is
also called advanced distance vector or Hybrid routing protocol.
- Multicast or
unicast is used for exchange of information.
- Multiple
network layer protocols are supported.
- 100%
loop-free.
- By default,EIGRP will limit itself to use no more than 50% of the interface bandwidth
- Maximum
Hop-Count is 255[ 100 by default]
- EIGRP is
classless protocol
- EIGRP
supports VLSM and authentication
- EIGRP uses
Diffusing Update Algorithm (DUAL) to determine the best path among all
“feasible” paths. DUAL also helps ensure a loop-free routing environment.
- EIGRP will
form neighbor relationships with adjacent routers in the same Autonomous System
(AS)
- EIGRP uses
RTP (Reliable Transport Protocol) and its function is to deliver EIGRP packets
between neighbors in a reliable and ordered way.
- All EIGRP
routing information are exchanged between neighbors via multicast using the address 224.0.0.10
- EIGRP
routers do not send periodic, full-table routing updates. Updates are sent when a change occurs, and
include only the change
- K-Values are
used for calculating metric. By default EIGRP consider k1 and k3 only
- EIGRP having
internal Administrative distance as 90 and external AD as 170
- EIGRP summary route AD value is 5
- In EIGRP Summarization is enabled by default."No auto-summary" command is needed because by default EIGRP will behave like a classfull routing protocol which means it won’t advertise the subnet mask along the routing information. For example 1.1.1.0/24 and 2.2.2.0/24 will be advertised as 1.0.0.0/8 and 2.0.0.0/8. Disabling auto-summary will ensure EIGRP sends the subnet mask along.
- EIGRP converges rapidly after link failure
- EIGRP can load balance on both equal and unequal cost paths."Variance" command is used to configure load balancing
- EIGRP supports up to 6 loop-free paths with a metric lowers than the product of variance and the best metric
- Manual summarization can be enabled at any interface
- If a packet is not acknowledged,EIGRP re transmit the packet to the non responding neighbor as a unicast.No other traffic is sent to this neighbor until it responds.After 16 unacknowledged re-transmissions,the neighbor is removed from the neighbor table
- The process of neighbor discovery and route exchange between two EIGRP router is as follows
- Router A sends out a hello
- Router B sends back a hello and an update.The update contains routing information
- Router A acknowledges the update
- Router A sends its update
- Router B acknowledges
- A neighbor is considered as lost if no hello is relieved within 3 hello periods
Why do we
call EIGRP an advanced distance vector or hybrid routing protocol?
Because it
shares features of both distance vector and link state protocols. For
example EIGRP advertise routes to
directly connected neighbors like a
distance vector protocols and it uses a series of tables like link state
protocols
Advantages of EIGRP
- EIGRP uses
AS (Autonomous system) number ranging from
1-65535 to identify collection of routers that share same information.
- EIGRP have
less convergent time and is more efficient
- EIGRP
supports both auto and manual route summarization
- Supports
multiple routed protocols like IP ,IPX and apple talks
- EIGRP
converges rapidly in the event of link failure
- EIGRP can
load balance equal and unequal cost path.By default EIGRP supports 4 load
balancing path.It can be extended to 6 paths
EIGRP Packet types
There are 5 packet types
- Hello Packet
– In normal scenarios Hello packets are
send in 5 seconds .Hello packets are send between directly connected neighbors.
Hello packets are send as Multicast
- Update –
Update packet are used to send triggered updates. EIGRP won’t send periodic
updates like distance vector protocols. They
send triggered updates only when a chance is occurred .Update packets
are send as Multicast or unicast
- Query –
Query packets are send when the successor path is failed and there is no
feasible successor (back up path).Query packets are send as multicast
- Reply –
Reply for query packets. Reply packets are send as unicast
- ACK – Ack
packets are send as Acknowledgement for update and Reply packet. Send as
Unicast
NOTE : Hello
packets don’t have to be acknowledged since EIGRP uses a holddown time. If a
router
doesn’t receive hello packets in an X amount of time it will drop the neighbor adjacency.
Stuck in Active: When a route (current successor) goes down, the router first
checks its topology table for a feasible successor. If backup path(feasible successor) is not
present, it goes active(actively checking to find new route) on the that route to find a new successor by
sending queries out to its neighbors requesting a path to the lost route. Such
state of router while waiting for a reply for a query packet is called Stuck in Active. In normal working
condition router running EIGRP protocols is in passive state(P).If there is a
failure in successor path and there is no back up path, then router will be in
Active state(A).
Conditions to form EIGRP neighbors
Must receive
HELLO PACKET from neighbor router
AS number
must be same
Must have
identical K-values or Metric
Adjacencies
will not form unless the primary IP addresses on connecting interfaces are on
the same subnet.
EIGRP maintain 3 tables
• Neighbor table – list of all neighboring
routers. Neighbors must belong to the same Autonomous System
• Topology table – list of all routes in the
Autonomous System
• Routing table – contains the best route for
each known network
EIGRP Neighbors
EIGRP forms
neighbor relationships, called adjacencies, with other routers in the same AS
by exchanging Hello packets. Routers share routing information after forming
adjacencies. Hello packets are sent as
multicasts
to address 224.0.0.10. By default, EIGRP
Hellos are sent every 5 seconds. On slower connections, EIGRP Hellos are sent
every 60 seconds by default.
The EIGRP
Hello timer can be adjusted on a per interface basis:
Router(config)#Interface
fa 0/1
Router(config-if)# ip hello-interval eigrp 20 15
The above
command allows us to change the hello timer to 15 seconds for Autonomous System
20.
In addition
to the Hello timer, EIGRP neighbors have a Hold timer. The Hold timer indicates
how long a router should wait before marking a neighbor inactive, if it stops
receiving hello packets from that neighbor.
By default,
the Hold timer is three times the Hello timer. On high-speed links the timer is
set to 15 seconds, and on slower links the timer is set to 180 seconds.
The Hold
timer can also be adjusted on a per interface basis:
Router(config)#Interface fa 0/1
Router(config-if)# ip hold-interval
eigrp 20 21
The above
command allows us to change the hold timer to 21 seconds for Autonomous System
20.
Changing the
Hello timer does not automatically change the Hold timer. Additionally, Hello
and Hold timers do not need to match between routers for an EIGRP neighbor
relationship to form.
Successor Path and Feasible Successor path
EIGRP
enabled routers will send HELLO Packets to each other and become neighbors.
EIGRP neighbors will exchange routing information which will be saved in the
topology table. The best path from the topology table will be copied in the
routing table. Such path are called Successor. Metric of the successor path is
called Feasible distance.
After
finding the successor path ,EIGRP will find another path as a back up path to
successor to give redundancy is called Feasible successor. The metric used to
find feasible successor is called
Advertised distance (AD) or Reported distance (RD).This Advertised distance
(AD) is the distance between neighbor of the source and destination .
In short,
Advertised distance: How far the
destination is away for your neighbor.
Feasible distance: The total distance
to the destination.
successor : The best path to the
destination
Condition for choosing Feasible successor
Advertised
distance (AD) must be less than the metric of successor path
Advertised
distance of feasible successor < Feasible distance of successor.
Note : While
calculation best paths, one thing you must keep in mind is Split horizon rule which states “whatever
you learn on an interface you don’t advertise back out of the same interface.”
Feasible
Successors provide EIGRP with redundancy, without forcing routers to
re-converge when a topology change occurs. If no Feasible Successor exists and
a link fails, a route will enter an Active (converging) state until an
alternate route is found.
EIGRP Route States
An EIGRP
route can exist in one of two states, in the topology table:
• Active state
• Passive State
A Passive
state indicates that a route is reachable, and that EIGRP is fully converged. A
stable EIGRP network will have all routes in a Passive state.
A route is
placed in an Active state when the Successor and any Feasible Successors fail,
forcing the EIGRP to send out Query packets and re-converge. Multiple routes in
an Active state indicate an unstable EIGRP network. If a Feasible Successor
exists, a route should never enter an Active
state.
You can
check the status of states by using
Router#
show ip eigrp topology
To view only
active routes in the topology table:
Router#
show ip eigrp topology active
EIGRP
Metrics
EIGRP can
utilize 5 separate metrics to determine the best route to a
destination:
- Bandwidth (K1)
- Load (K2
- Delay of the Line (K3
- Reliability (K4)
- MTU (K5)
By default,
only Bandwidth and Delay of the Line are used
K1 = 1, K2 =
0, K3 = 1, K4 = 0, K5 = 0
Graceful Shutdown
When an
EIGRP process is shut down, the router sends out “goodbye” messages to its
neighbors (in the form of hello packets).
The neighbors can then immediately begin recalculating paths to destinations
that went through the shutdown router without having to wait for the hold timer
to expire.
GNS3 Labs:
Interview Questions
What is
EIGRP?
What are the
different tables in EIGRP?
Why EIGRP is
called hybrid protocol
What are the
different packets in EIGRP?
What are the
advantages of EIGRP other routing protocol ?
What type of
Authentication is supported by EIGRP ?
What is the
use of "variance" Command in EIGRP?
Internal and
external Administrative distance in EIGRP ?
What is
Feasible successor ?
What is
Advertised distance ?
What is
successor ?
What is the
muticast address used by EIGRP to send Hello packets ?
What is
"Stuck in Active" ?
what is
"Graceful shutdown" ?
what is
"Goodbye" message recieved in EIGRP ?
Maximum path
load balanced by EIGRP ?
How EIGRP
support unequal load balancing ?
What happen
when we enable passive interface in EIGRP ?
Conditions
for EIGRP neigbours
what is
meant by active and passive states in EIGRP ?
What are the
different K-values used in EIGRP ?
