EIGRP Network Command

Answered Question
Aug 7th, 2007
User Badges:

Hi Experts,

What will happen if the EIGRP for one router is having larger network range then the other?


Eg:

Router A


Router EIGRP 2

Network 16.11.0.0 0.7.255.255


Router B

Router EIGRP 2

Network 16.12.0.0


Any impacts on the routing?


Thanks,


Correct Answer by Richard Burts about 9 years 10 months ago

Cindy


I believe that your essential question was: IS this indicating the network the router will advertise?

and the simple answer is no it is not indicating what the router will advertise.


It is common to the IGPs (but not to BGP) that the function of the network statement is to instruct the router which interfaces to include in the routing protocol. So the router reads the network statement (including any mask that may be configured) and looks for connected interfaces that are included within the network or subnet shown in the network statement. Once the router has selected the interface to include then the router looks at the address and mask of the interface and this is what determines what the router will advertise.


With EIGRP the mask is optional. If no mask is given then EIGRP will assume the default mask for the class of the network given. Using the mask in EIGRP network statements allows you to supply more specificity about what interfaces should be or should not be included with the EIGRP protocol.


HTH


Rick

  • 1
  • 2
  • 3
  • 4
  • 5
Overall Rating: 3 (3 ratings)
Loading.
hardiklodhia Tue, 08/07/2007 - 21:41
User Badges:
  • Bronze, 100 points or more

hi,

when u advertise major network in eigrp it will then look for the connected subnetworks and propogate it as it is classless routing protocol and will not cause adverse effect normally unless there were discontiguous networks.one more thing is auto summary is by default enabled so good general practice is to disable auto summarization.

for more info on eigrp: http://cisco.com/en/US/tech/tk365/technologies_white_paper09186a0080094cb7.shtml#summarization

hope this will help

rgrds,

cindylee27 Wed, 08/08/2007 - 00:57
User Badges:

Hi,

Thanks for ur explaination. but what is the command -> "Network xx.xx.xx.xx" for?


IS this indicating the network the router will advertise?


Thanks in advanced.



hardiklodhia Wed, 08/08/2007 - 01:26
User Badges:
  • Bronze, 100 points or more

hi

network x.x.x.x y.y.y.y where x=ip y=wildcard mask


These two fields in combination specify which interfaces, based on their IP address, or range of interfaces, based on the range of their IP addresses, will run EIGRP.

hope this will help.

rgrds,


smothuku Wed, 08/08/2007 - 03:06
User Badges:
  • Silver, 250 points or more

Hi ,


When the "network" command is configured for an EIGRP routing process, the router matches one or more local interfaces. The network command will match only local interfaces that are configured with addresses that are within the same subnet as the address that has been configured with the network command. The router will then establish neighbors through the matched interfaces. There is no limit to the number of network statements (network commands) that can be configured on a router.


Examples

The following example configures EIGRP autonomous system 1 and establishes neighbors through network 172.16.0.0 and 192.168.7.0:


router eigrp 1

network 172.16.0.0

network 192.168.7.0


Thanks :)

satish




Correct Answer
Richard Burts Wed, 08/08/2007 - 05:49
User Badges:
  • Super Silver, 17500 points or more
  • Hall of Fame,

    Founding Member

  • Cisco Designated VIP,

    2017 LAN, WAN

Cindy


I believe that your essential question was: IS this indicating the network the router will advertise?

and the simple answer is no it is not indicating what the router will advertise.


It is common to the IGPs (but not to BGP) that the function of the network statement is to instruct the router which interfaces to include in the routing protocol. So the router reads the network statement (including any mask that may be configured) and looks for connected interfaces that are included within the network or subnet shown in the network statement. Once the router has selected the interface to include then the router looks at the address and mask of the interface and this is what determines what the router will advertise.


With EIGRP the mask is optional. If no mask is given then EIGRP will assume the default mask for the class of the network given. Using the mask in EIGRP network statements allows you to supply more specificity about what interfaces should be or should not be included with the EIGRP protocol.


HTH


Rick

cindylee27 Wed, 08/08/2007 - 07:01
User Badges:

Rick,

You really understand my question very well.. :)

Time to rate!

Rick = 5 (for giving such a wonderful answer and really understand it very well! )


Satish/Hardi=2 (For assistance to my question) :)


Richard Burts Wed, 08/08/2007 - 07:08
User Badges:
  • Super Silver, 17500 points or more
  • Hall of Fame,

    Founding Member

  • Cisco Designated VIP,

    2017 LAN, WAN

Cindy


Thanks for the kind words (and thanks for the rating).


I find that the role of the network statements is frequently not well understood by many people. I am glad that we have had the opportunity to discuss this and to improve people's understanding of this functionality. Thanks for asking an interesting question.


HTH


Rick

cindylee27 Wed, 08/08/2007 - 07:11
User Badges:

Rick,

By the way..left out one more..;) last question of my eigrp topic ya..

How do Auto Summary/no auto summary command perform in the eigrp process?

it is used for route summarization?


Thanks once again..



Richard Burts Wed, 08/08/2007 - 07:55
User Badges:
  • Super Silver, 17500 points or more
  • Hall of Fame,

    Founding Member

  • Cisco Designated VIP,

    2017 LAN, WAN

Cindy


Yes it is used for route summarization. By default EIGRP will summarize at network boundaries. This is sort of a hold over from its derivation from a classful routing protocol (auto summarization at network boundaries is a classful behavior). The nice thing about EIGRP is that we have the ability to turn off auto summarization if we want to. And we have the ability to summarize at any point in the network that we choose (not limited to network boundaries) where OSPF is limited to summarize at Area Border Routers (or as ASBR for redistributed routes).


If this is not quite the information that you were looking for please clarify the question and I will try again.


HTH


Rick

cindylee27 Thu, 08/09/2007 - 02:46
User Badges:

Rick,


Any examples that can point to auto-summarization as how it being used in real scenario?


For eg, when we suppose to use "No Auto summary" or when we should go for "Auto Summary" for a particular EIGRP..


Thanks once again..




Richard Burts Thu, 08/09/2007 - 03:10
User Badges:
  • Super Silver, 17500 points or more
  • Hall of Fame,

    Founding Member

  • Cisco Designated VIP,

    2017 LAN, WAN

Cindy


The decision about whether to leave auto summarization on or to disable it would reflect the local environment and I am not sure that there is a simple description of how you would decide. The one simple rule that I do know is that if your network has discontiguous subnets then you should turn auto summarization off. For example if your network had a class B address (perhaps 160.160.0.0) which is used on LAN interfaces and on end stations and uses addresses in 10.0.0.0 on the links that connect routers and for loopback addresses, then you would certainly want to turn off auto summarization.


In some cases it does not matter whether you leave auto summarization on or turn it off. For example if all the interfaces of all the routers are in the same network (perhaps 10.0.0.0) and the Internet facing router is doing NAT then it does not matter since there is no network boundary where EIGRP would be running.


For an example where auto summarization would be used (and be useful) lets think about an enterprise that has 2 divisions (divisionA and divisionB) and the enterprise was formed in a merger of two existing companies. One company (now divisionA) used network 192.168.30.0 and the other company (now divisionB) used network 192.168.66.0. The merger did not renumber and just put a router (or maybe a couple of routers) that connect the 2 existing networks. So the Border router(s) have a couple of interfaces in 192.168.30.0 and a couple of interfaces in 192.168.66.0. The routers in divisionA do not need to know any detail of how divisionB is subnetted and only need to know that to get to divisionB they need to forward to the Border router. So in this case auto summary is a good thing. The Border router has auto summary turned on and for all the interfaces in 192.168.30.0 it only advertises a summary for 192.168.66.0. And it would automatically summarize the subnets of 192.168.30.0 and advertise a single /24 into divisionB for 192.168.30.0.


I will close this with a slightly different way of looking at this question. Do you want the routers in the network to automatically decide where summarization is helpful or do you want to manually control this. auto summarization allows the routers to decide where summarization makes sense. Turning off auto summarization means that you now must manually make that decision for each router in your network.


HTH


Rick

cindylee27 Thu, 08/09/2007 - 03:43
User Badges:

Rick,

Thanks for the explaination..Get a bit of idea there..:)

I believe for my case is to let the routers in the network to automatically decide...So,will go with auto-summary..



Actions

This Discussion