EIGRP Questions And Answers [CCNP]
What is EIGRP?
Enhanced Interior Gateway Routing Protocol (EIGRP) is an
enhanced distance vector routing protocol which Uses Diffused Update Algorithm
(DUAL) to calculate the shortest path. It is also considered as a Hybrid
Routing Protocol because it has characteristics of both Distance Vector and
Link State Routing Protocols.
EIGRP supports classless routing and VLSM, route
summarization, incremental updates, load balancing and other features.
What are the requirements
for neighborship in EIGRP?
The following fields in a hello packet must match for
routers to become neighbors:-
1.Autonomous System number.
2.K-values.
3.Authentication.
4.Primary address should be used.
5.If static neighborship than should be define on both
sides.
What are the metric of EIGRP protocol & its default
values?
1.Bandwidth (K1=1)
2.Load (K2=0)
3.Delay (K3=1)
4.Reliability (K4=0)
5.Maximum Transmission Unit (K5=0)
By default, EIGRP only uses bandwidth (K1) and delay (K3) to
calculate metric.
Give the formula by which EIGRP calculates metric?
Metric = 256 * [(10^7 / lowest-bandwidth) +
cumulative-delay]
The lowest bandwidth is the lowest-bandwidth link in the
route, using a unit of kilobits per second.
The cumulative-delay value used in the formula is the sum of
all the delay values for all links in the route, with a unit of tens of
microseconds.
What are the four
basic components of EIGRP?
The four basic components of EIGRP are -
1. The Protocol Dependent Module - It supports IP, IPv6, IPX,
Apple Talk.
2. The Reliable Transport Protocol - RTP is used in EIGRP
for detecting packet loss and to ensure ordered delivery of the packets.
3. The Neighbor Discovery and Recovery Module - Hello
messages are used for Neighbor Discovery and Recovery.
4. The Diffusing Update Algorithm - It is an algorithm used
by EIGRP for selecting lowest cost loop free path for each possible
destination.
What are the
different packet types used by EIGRP?
The packet types used by EIGRP are:-
1. Hello - Neighborship is discovered and maintained by
Hello Packets.
2. Acknowledgment - ACK packets are used to acknowledge the
receipt of update, query and reply packets. Acknowledgment packets are Unicast.
3. Update - EIGRP uses Update messages to send Routing
information to neighbors. Update packets can be sent to a single neighbor using
unicast or to a group of neighbors using multicast.
4. Query - Query
packets are used when EIGRP router has lost path (Successor) to a certain
network and does not have any backup paths (Feasible Successor). Router sends
query packets to its neighbors asking them if they have information about this
particular network. Query packets are multicast.
5. Reply - Reply
packets are used in response to the query packets. Reply packets are unicast to
the originator of the query.
What is the feasibility
condition?
The feasibility condition
is the rule by which feasible successors are chosen for a destination. The
feasibility condition is satisfied if a neighbor's advertised distance to a
destination is lower than the receiving router's feasible distance to the
destination. In other words, a router's neighbor meets the feasibility
condition if the neighbor is metrically closer to the destination than the
router. Another way to describe this is that the neighbor is
"downstream" relative to the destination.
What does
stuck-in-active mean?
A: If a router does not receive a reply from a
queried neighbor within the active time (3 minutes, by default), the route is
declared stuck-in-active. A response with an infinite metric is entered on the
neighbor's behalf to satisfy DUAL, and the neighbor is deleted from the
neighbor table.
What is the
difference between subnetting and address aggregation?
Subnetting is the practice of creating a group
of subnet addresses from a single IP network address.Address aggregation is the
practice of summarizing a group of network or subnet addresses with a single IP
network address.
What is Reliable
Transport Protocol?
EIGRP uses RTP (Reliable Transport Protocol) to deliver
EIGRP packets between neighbors in a reliable and ordered way. If the packet
with RTP enable sent, gets lost in the transit it will be send again (resend).
What packets are RTP
enabled?
1.Update Packet.
2.Query Packet.
3.Reply Packet.
Explain what will
happen if the packet is not acknowledged?
If a packet is not acknowledged, EIGRP will retransmit 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.
Explain EIGRP Router
ID?
In EIGRP, duplicate RIDs do not prevent routers from
becoming neighbors and two EIGRP routers with the same router ID will still
form a neighbor relationship. The only time the value of EIGRP RIDs consider is
when injecting external (redistributed) routes into EIGRP. In this case, the
routers injecting the external routes should have unique RIDs to avoid
confusion.
To manually configures the router ID
R1(config)# router eigrp 10
R1(config-router)# eigrp router-id 1.1.1.1
Explain Unequal Cost
Load Balancing in EIGRP?
By default, EIGRP will automatically load-balance across
equal-metric routes. EIGRP also supports load-balancing across routes with an
unequal metric. Unequal cost load balancing in EIGRP is the concept by which
load sharing can take place on paths that does not have the equal metric. In
EIGRP variance is used for Unequal cost load balancing. Variance is specified
as an integer in the range of 1 through 128. The router then multiplies the
variance by the successor route’s FD (metric of the best route to reach that
subnet). Any Feasible Successor route whose metric is less than or equal to the
product of the variance by the successors FD are considered to be equal routes
and can be placed into the routing table for load sharing.
Router(config)#
router eigrp 100
Router(config-router)#
variance 2
In this case variance is 2.
What is a feasible successor?
What is a feasible successor?
A feasible successor to a destination is a neighbor that satisfies the feasibility condition for that
destination.
What is a successor?
A successor to a destination is a feasible successor that is currently being used as the next hop to the destination.
What is the difference between an active route and a passive route?
A route is active on a particular router if the router has queried its neighbors for a feasible successor and has not yet received a reply from every queried neighbor. The route is passive when there are no outstanding queries.
Explain Split
Horizon?
The Split Horizon feature prevents a route learned on one
interface from being advertised back out of that same interface. It is used to
prevent loop in EIGRP.
Explain Null Zero?
It is a loop avoidance mechanism entry stored in routing
table only in case of summarization (auto & manual). It terminates or flush
unwanted packets, if any traffic goes towards null0 it will be drop by eigrp.
What is Active State
and Passive State?
Routes for which the successor route fails and no feasible
successor routes exist moves to an active state forcing the EIGRP to send out
query packets and reconverge.
A route is in passive state for which the router has a
successor route, and no failure has yet occurred. A stable EIGRP network will
have all routes in a Passive state.
Explain Stuck in
Active?
When for a certain prefix, successor route fails and no
feasible successor route exists than the router begins a process of finding any
loop-free alternative routes to reach that prefix by sending Query messages to
all of its neighbors requesting path to lost prefix. If the neighbor routers
does not have information about the lost prefix, they will forward the query
message to further routers. Within a large network, particularly when routers
exist several router hops away, the number of Queries might not only be large,
but there also might be a string of routers that all must wait on multiple
Reply messages before they can, in turn, issue a Reply. To deal with this long
time problem, Cisco IOS first sets a limit on how long it should take to
receive all such replies. This timer is called the active timer and is set to 3
minutes by default. Routes for which a router does not receive a Reply within
the active timer are considered to be Stuck-in-Active (SIA) routes. Router
sends an SIA-Query (Stuck-in-Active Query) EIGRP message to each neighbor that
has yet to send back a Reply. The purpose of the message is to either get an
SIA-Reply back indicating that the neighbor really is still waiting for replies
to its own queries meaning the neighbor is alive and still working & there
is no need to kill the neighborship or to get nothing in reply meaning neighbor
was not able to reply, so the action of failing the neighborship is reasonable.
What is Graceful
Shutdown and GoodBye message in EIGRP?
When an EIGRP process is shut down, router sends out
“goodbye” messages to its neighbors. The neighbors can then immediately begin
recalculating paths to all the destinations that went through that shutdown
router without having to wait for the hold timer to expire.
How Passive Interface
command works in EIGRP?
With EIGRP running on a network, the passive-interface
command stops sending outgoing hello packets, hence the router cannot form any
neighbor relationship via the passive interface. This behavior stops both
outgoing and incoming routing updates. However, EIGRP still advertises the
connected subnets if matched with an EIGRP network command.
# router eigrp 1
# passive-interface fastethernet0/0
Command to see list of passive-interfaces
# show ip protocols
How can we change Hello and Hold time in EIGRP?
# interface Fa0/0
# ip hello-interval eigrp 100 3
# ip hold-time eigrp 100 12
These commands will make hello interval 3 seconds and hold
time 12 seconds.
# show ip eigrp interfaces detail (To verify)
What is the Multicast
IP address used by EIGRP?
EIGRP uses the multicast address 224.0.0.10
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