EIGRP Discontiguous network

[chiragom2341]

Hi,

why EIGRP could not support Discontiguous network ?

we have to use no auto-summary command in order to support discontinuous network in eigrp, why is that ? what exactly triggers through no auto-summary command that enables to use discontiguous network...

the answer will me to understand thoroughly, please reply if any one know.

Thank you !

Chirag

[InayathUlla Sharieff]

EIGRP DOES NOT support discontiguous networks by default. Because EIGRP does automatic summarization by default. And ideally you would want a contiguous network to prevent routing issues when using route summarization. And the fact that you have to disable the default auto summarization to route properly using a discontiguous network ip scheme, would make me think the following, EIGRP Default -- automatic summarization, contiguous network ip scheme. Disable automatic summarization, discontiguous network ip scheme.

HTH

REgards

Inayath

&***Please dont forget to rate usefull posts***

[chiragom2341]

Thank you for your reply !

I understand what your have explained, Except :  fact that you have to disable the default auto summarization to route properly using a discontiguous network ip scheme

Why is that ? why we have disable default auto-summarization ? 

suppose a router contains summary address of 10.0.0.0/8 at port 1 , 172.16.0.0/16 at port 2 and 192.168.0.0/16 at port 3. Now a packet arrives at port 1 and it needs to go to 172.16.32.1 host, then router just check the entries into routing table and immediately find out the summary address of 172.16.0.0, well in this case it should then send the packet at port 2.

the eigrp theory says the summary should not work in discontiguous network but what I have just mentioned above, it looks like it need to work... it is confusing me...

I would appreciate if you or any one clear my point or understanding.... 

thanks,

Chirag

[Richard Burts]

Chirag

You ask a simple question "why do we need to disable automatic summarization when the network is discontiguous". And the answer is quite simple - we must disable automatic summarization or the network will not function correctly.

Let me describe a simple example which may help you to understand this. Think about a network with 3 routers, routerA, routerB, and routerC. On routerA there are multiple interfaces including 172.16.1.0, 172.16.2.0, 172.16.3.0. The connection from routerA to routerB is 192.168.1.0. On routerC are networks 172.16.13.0, 172.16.14.0, 172.16.15.0. The connection from routerB to routerC is 192.168.2.0. It does not matter what subnets are on routerB. So we have created a discontiguous network where parts of 172.16 are separated by 192.168. On these 3 routers we have configured EIGRP on all of the interfaces. routerA knows its subnets in 172.16.1/2/3 and since its connection to routerB is in 192.168.1.0 and auto summarization is enabled it will advertise the summary of 172.16.0.0/16 and not advertise the individual subnets. Similar situation is on routerC where it knows its subnets of 172.16.13/14/15 but will advertise to routerB only the summary of 172.16.0.0/16.

The result of this is that routerA has no knowledge of 172.16.13.0 or 172.16.14.0 or 172.16.15.0. The only way to get this network to function is to disable autosummarization.

So your simple question has a simple answer. Perhaps the question that you should be asking is why is the default auto summarization?

HTH

Rick

[Richard Burts · Accepted solution]

Chirag

In reading through the thread again I realize that I have not addressed the example that you use where 10.0.0.0/8 is on port 1, 172.16.0.0/16 is on port 2 and 192.168.0.0/16 is on port 3. You have not described anything that makes this network discontiguous. If it were discontiguous then some part of 10.0.0.0 should be reached by some way other than port 0. But your explanation depends on the fact that 10.0.0.0 is reached only by port 0 which makes this network contiguous and not discontiguous.

HTH

Rick

[chiragom2341]

Hi Rick,

Thank you for reply ! you explain really well.

The concept still catching me up: 

As you explain above router A has no knowledge of 172.16.13.0 or 172.16.14.0 or 172.16.15.0, It is true because of summarization of 172.16.0.0 network but it has a knowledge of 172.16.0.0/16 network. So, my question is why router can't send just by looking the summary address of 172.16.0.0.

The networks are there, it is just summarized. in this case actually router A should try to send packet if the packet has destiny for network 172.16.13.0/1/2... 

please help me to clear this point.

Thanks,

Chirag

[Richard Burts · Accepted solution]

Chirag

There are several ways to explain why using the summary from its neighbor will not work.

- routerA may not see the summary from routerC.  routerB will receive advertisement of the summary of 172.16.0.0/16 from both routerA and from routerC and we can not predict which one it will put into its routing table. But split horizon will prevent routerB from advertising the summary back to the router from which it learned the summary. So either routerA or routerC will never see the summary from the other router and this creates a problem.

- even more important is the fact that when routerA creates the summary advertisement (using auto summarization) it inserts a summary route into its routing table. Then if routerB sends the summary from routerC to routerA the router will not accept the advertisement because it already has the route in its routing table as a locally generated route with a better administrative distance.

So the summary route from remote routers does not help to resolve the problem created by autosummarization.

HTH

Rick

[chiragom2341]

Understand it  !

many thanks,

Chirag

[Richard Burts]

Chirag

I am glad that you understand it now and that my explanations were helpful.  Thank you for using the rating system to mark this question as answered.

HTH

Rick