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Eigrp Network command


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#1 vin1801

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Posted 20 November 2008 - 07:44 PM

Hi Friends,

I have a question from the BSCI Official Exam Certification Guide, chapter 4 :

3. What happens immediately after the network command is configured?
a. Updates are sent.
b. The routing table is created.
c. Hellos are sent on appropriate interfaces.
d. Networks are advertised.

The answer mentioned in the book is option "b" which I think is wrong, because even if we don't run eigrp and its network command, the routing table is already created, for example when you enter the "sh ip route" command all the directly connected networks and any static networks configured are displayed.
The correct answer should be "c". Please someone clarify on this.


- Vin.
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#2 jafar2048

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Posted 20 November 2008 - 08:14 PM

Hello there,

Yes, I think you're right,

A is wrong, because updates are exchanged between routers only after they have become neighbors and when there is a change in network topology. When a new router joins the EIGRP AS, other routers will send/receive updates but the new router will first of all form adjacency (sending/receiving Hello/ACK packets) with its directly connected neighbors (according to EIGRP adjacency conditions).

B is definitely wrong for exactly the same reason you mentioned.

D is wrong, because for being able to advertise any network with a neighbor router, first of all the EIGRP adjaceny conditions
(same K values, exchange of Hello/ACK packets, same AS) must be satisfied.

C is the correct answer according to EIGRP specification.

Besides in the book "EIGRP network design solutions" written by Ivan Pepelnjak at the page 9 you can read:

When a major network is specified in the EIGRP process with the network command, all the directly connected subnets of that major network are entered in the EIGRP topology table, EIGRP neighbors are discovered on all the interfaces belonging to the specified major network, and the routing information is exchanged with those neighbors.


So you can see that first we start with the neighbor discovery and then the routing information exchange.

Regards,

:rolleyes:

Edited by jafar2048, 20 November 2008 - 08:16 PM.

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#3 raaki_88

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Posted 20 November 2008 - 08:17 PM

then when would option 'b' will be correct ?
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#4 jafar2048

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Posted 20 November 2008 - 08:36 PM

then when would option 'b' will be correct ?


Hello dear Raaki,

Well, I think (please correct me if I'm wrong) from the moment an IOS is installed on a router, we can say that the router has
a routing table which is of course empty. Maybe I'm wrong in understanding the routing terminologies, but for me using a routing protocol such as EIGRP, OSPF, ... on a router, doesn't create the routing table but it just populates the routing table by installing appropriate routes (with best metrics). Therefore the option b in my personal point of view is not relevant to the fact that whether or not the network command has been invoked on the router, as the routing table already exists.

Regards,

:rolleyes:

Edited by jafar2048, 20 November 2008 - 08:38 PM.

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#5 raaki_88

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Posted 20 November 2008 - 09:44 PM

thanks for the explanation jafar .. it did clear off something mystical ... the explanation was cool

i found these points in addition

* IP Eigrp module is responsible for sending and receiving eigrp pakects that are encapsulated in IP

* The ip-eigrp also responsible for parsing Eigrp Packets and informing DUAL of the new Information received

* Ip-eigrp asks Dual to make routing decisions , the results ofwhich are stored in ip routing table

* Ip-eigrp is responsible for re-distributing routes learned by other ip routing protocols

so seeing the third point theres is nothing a network command is doing its only the pdm protocol dependant modules which are doing it ..

regards
raaki

Edited by raaki_88, 20 November 2008 - 09:45 PM.

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#6 AsianDub

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Posted 29 November 2008 - 10:35 AM

Its a very badly worded question.

Check this out https://cisco.hosted...com/thread/2234
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#7 bilgisayar

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Posted 30 November 2008 - 02:35 AM

I have done a little lab for this ... Just two routers

Before i configured eigrp ..........

Router#show ip route
Codes: C - connected, S - static, I - IGRP, R - RIP, M - mobile, B - BGP
D - EIGRP, EX - EIGRP external, O - OSPF, IA - OSPF inter area
N1 - OSPF NSSA external type 1, N2 - OSPF NSSA external type 2
E1 - OSPF external type 1, E2 - OSPF external type 2, E - EGP
i - IS-IS, L1 - IS-IS level-1, L2 - IS-IS level-2, ia - IS-IS inter area
* - candidate default, U - per-user static route, o - ODR
P - periodic downloaded static route

Gateway of last resort is not set

10.0.0.0/24 is subnetted, 1 subnets
C 10.10.10.0 is directly connected, Serial2/0


Router# we have already got routing table .....

AND i configured EIGRP

Router#sh ip eigrp traffic
IP-EIGRP Traffic Statistics for process 1
Hellos sent/received: 19/0 Updates sent/received: 0/0
Queries sent/received: 0/0
Replies sent/received: 0/0
Acks sent/received: 0/0
Input queue high water mark 1, 0 drops
SIA-Queries sent/received: 0/0
SIA-Replies sent/received: 0/0


as I can see this router started to send HELLO packets as soon as network command applied .... but not UPDATEs

Edited by bilgisayar, 30 November 2008 - 02:38 AM.

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#8 jafar2048

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Posted 30 November 2008 - 03:10 AM

...
as I can see this router started to send HELLO packets as soon as network command applied .... but not UPDATEs


I agree with you 100%! :D
As I said in my precedent comment, from the moment that there is an IOS installed on a router, there is a routing table (although may be empty at the beginning).

Thanks for this nice reply!

Regards,

:rolleyes:
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#9 bilgisayar

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Posted 01 December 2008 - 04:42 AM

That is right bro ..

...
as I can see this router started to send HELLO packets as soon as network command applied .... but not UPDATEs


I agree with you 100%! :D
As I said in my precedent comment, from the moment that there is an IOS installed on a router, there is a routing table (although may be empty at the beginning).

Thanks for this nice reply!

Regards,

:rolleyes:


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#10 in10se

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Posted 02 December 2008 - 05:08 PM

When EIGRP is configured on a router, it starts to send, and listen, to multicast packets out to IP 224.0.0.10. These multicast packets will get propigated throught the network. The router will decide based on the "network 0.0.0.0" command which interfaces it needs to send EIGRP hello packets out. Now if Router A sends a EIGRP packet out its interface, based on the "network 0.0.0.0" command. This behavior is shown here;

RouterA#show ip eigrp 1 traffic
IP-EIGRP Traffic Statistics for AS 1
Hellos sent/received: 10/0
Updates sent/received: 0/0
Queries sent/received: 0/0
Replies sent/received: 0/0
Acks sent/received: 0/0
Input queue high water mark 0, 0 drops
SIA-Queries sent/received: 0/0
SIA-Replies sent/received: 0/0
Hello Process ID: 192
PDM Process ID: 157

So as soon as the EIGRP AS is configured wit the "network 0.0.0.0" command, it will start to advertise Hello packets on the interfaces. In this case FastEthernet0/0.

Now the first thing that Router B is going to look at when it receives a Hello is to look to see whether it has EIGRP running. Then, it is going to look to see if the hello it recieved is within the same AS as itself. Next, it will check its EIGRP configuration to see if the packet came from a network that it is configured to listen for, "network 0.0.0.0". If it is, it will process the packet, then form the adjacency.

This behavior is shown here;

*Mar 1 00:12:58.767: IP-EIGRP(Default-IP-Routing-Table:1): Processing incoming UPDATE packet
*Mar 1 00:12:58.767: IP-EIGRP(Default-IP-Routing-Table:1): 1.1.1.0/24 - do advertise out FastEthernet0/0

So, as soon as Router B is configured with the "network 0.0.0.0" command, it will start to process packets. The router then recognises that it is receiving a packet for a network it is configured for. The EIGRP adjacency is then formed.

RouterB#show ip eigrp neigh
IP-EIGRP neighbors for process 1
H Address Interface Hold Uptime SRTT RTO Q Seq
(sec) (ms) Cnt Num
0 1.1.1.1 Fa0/0 11 00:08:31 156 936 0 3


Hope this helps clear it up a bit, if someone was still not in the loop
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#11 divinjohn

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Posted 04 December 2008 - 09:30 AM

When EIGRP is configured on a router, it starts to send, and listen, to multicast packets out to IP 224.0.0.10. These multicast packets will get propigated throught the network. The router will decide based on the "network 0.0.0.0" command which interfaces it needs to send EIGRP hello packets out. Now if Router A sends a EIGRP packet out its interface, based on the "network 0.0.0.0" command. This behavior is shown here;

RouterA#show ip eigrp 1 traffic
IP-EIGRP Traffic Statistics for AS 1
Hellos sent/received: 10/0
Updates sent/received: 0/0
Queries sent/received: 0/0
Replies sent/received: 0/0
Acks sent/received: 0/0
Input queue high water mark 0, 0 drops
SIA-Queries sent/received: 0/0
SIA-Replies sent/received: 0/0
Hello Process ID: 192
PDM Process ID: 157

So as soon as the EIGRP AS is configured wit the "network 0.0.0.0" command, it will start to advertise Hello packets on the interfaces. In this case FastEthernet0/0.

Now the first thing that Router B is going to look at when it receives a Hello is to look to see whether it has EIGRP running. Then, it is going to look to see if the hello it recieved is within the same AS as itself. Next, it will check its EIGRP configuration to see if the packet came from a network that it is configured to listen for, "network 0.0.0.0". If it is, it will process the packet, then form the adjacency.

This behavior is shown here;

*Mar 1 00:12:58.767: IP-EIGRP(Default-IP-Routing-Table:1): Processing incoming UPDATE packet
*Mar 1 00:12:58.767: IP-EIGRP(Default-IP-Routing-Table:1): 1.1.1.0/24 - do advertise out FastEthernet0/0

So, as soon as Router B is configured with the "network 0.0.0.0" command, it will start to process packets. The router then recognises that it is receiving a packet for a network it is configured for. The EIGRP adjacency is then formed.

RouterB#show ip eigrp neigh
IP-EIGRP neighbors for process 1
H Address Interface Hold Uptime SRTT RTO Q Seq
(sec) (ms) Cnt Num
0 1.1.1.1 Fa0/0 11 00:08:31 156 936 0 3


Hope this helps clear it up a bit, if someone was still not in the loop


its the HELLO Messages only. EIGRP does establish adjacency with neighbours.! and it does it with the HELLo Mesages.!
Neighbor Relationship

A router discovers a neighbor when it receives its first hello packet on a directly connected network. The router requests DUAL to send a full route update to the new neighbor. In response, the neighbor sends its full route update. Thus, a new neighbor relationship is established in the following steps:

1. When a router A receives a hello packet from a new neighbor B, A sends its topology table to router B in unicast updates with the initialization bit turned on.

2. When router B receives a packet with the initialization bit on, it sends its topology table to router A.

The interval between hello packets from any EIGRP-speaking router on a network is five seconds (by default) on most media types. Each hello packet advertises hold-time--the length of time the neighbor should consider the sender up. The default hold-time is 15 seconds. If no hellos are received for the duration of the hold-time, DUAL is informed that the neighbor is down. Thus, in addition to detecting a new neighbor, hello packets are also used to detect the loss of a neighbor.

The hello-interval can be changed with the following command in interface configuration mode:

ip hello-interval eigrp autonomous-system-number seconds

Lengthening the hello-interval will also lengthen the route convergence time. However, a longer hello-interval may be desirable on a congested network with many EIGRP routers.

If the hello-interval is changed, the hold-time should also be modified. A rule of thumb is to keep the hold-time at three times the hello-interval.

ip hold-time eigrp autonomous-system-number seconds

Note that the hello-interval and hold-time need not be the same for all routers on a network. Each router advertises its own hold-time, which is recorded in the neighbor's neighbor table.

The default hello-interval is 60 seconds (with a hold-time of 180 seconds) on multipoint interfaces (such as ATM, Frame Relay, and X.25) with link speeds of T-1 or less. Hello packets are multicast; no acknowledgments are expected.

The following output shows NewYork's neighbors. The first column--labeled H--is the order in which the neighbors were learned. The hold-time for 172.16.251.2 (Ames) is 10 seconds, from which we can deduce that the last hello was received 5 seconds ago. The hold-time for 172.16.250.2 (Chicago) is 13 seconds, from which we can deduce that the last hello was received 2 seconds ago. The hold-time for a neighbor should not exceed 15 seconds or fall below 10 seconds (if the hold-time fell below 10 s, that would indicate the loss of one or more hello packets).

NewYork#sh ip eigrp neighbor
IP-EIGRP neighbors for process 10
H Address Interface Hold Uptime SRTT RTO Q Seq
(sec) (ms) Cnt Num
1 172.16.251.2 Se0/1 10 00:17:08 28 2604 0 7
0 172.16.250.2 Se0/0 13 00:24:43 12 2604 0 14.

After a neighbor relationship has been established between A and B the only EIGRP overhead is the exchange of hello packets, unless there is a topological change in the network.
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#12 talent pk

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Posted 04 December 2008 - 04:22 PM

then when would option 'b' will be correct ?


Hello dear Raaki,

Well, I think (please correct me if I'm wrong) from the moment an IOS is installed on a router, we can say that the router has
a routing table which is of course empty. Maybe I'm wrong in understanding the routing terminologies, but for me using a routing protocol such as EIGRP, OSPF, ... on a router, doesn't create the routing table but it just populates the routing table by installing appropriate routes (with best metrics). Therefore the option b in my personal point of view is not relevant to the fact that whether or not the network command has been invoked on the router, as the routing table already exists.

Regards,

:rolleyes:


Yes you are 100% correct. The Router has Already Routing Table created inside of it, its empty in a way that as soon as you give ip addresses to interfaces it will start adding those interfaces as directly connected Networks inside its routing table. So this means that the routing table starts populating first with its directly connected networks with the AD of 0. Nothing beats the Directly attached Interfaces.
The reason of being how the routing table is there is that in routers IP ROUTING is by default enabled. if you turned off ip routing with NO in its premble. the routing table will be deleted and the router starts acts as a bridge or host.

Further anaylzing the original question my knowledge aswell says that the Answer B is not correct. and the reason why is that its explained well by JAFAR.

using NETWORK command the first thing that routing protocol will do is to look inside the router for particular match for interface ip addresses, and include them under its process, now as soon as Routing protocol adds specific router interfaces, the interfaces start flooding Small Hello Packets.

before Updates will be sent, adjacencies must be formed, before adjacencies must be formed, the the valid ip address and subnet informations across neighbors, their various hello timers etc must be matched depending on routing protcol used. and all these information is exchanged inside small hello packets.

so Answer C is correct. Small Hello packets are sent and received to see wether remote router is able to become adjacent or not.

Edited by talent pk, 04 December 2008 - 04:31 PM.

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