RIP Configuration - No auto-summary

kcn
kcn used Ask the Experts™
on
Understand that Auto-summarization has number of advantages and it is default turn on in RIP configuration .

But , I cannot understand in the REAL WORLD , when we configure RIPv2 , why we need to turn off auto-sumarization ??? example :-  Router(config-router)#no auto-summary ...

I did go through below  article , I think below link tried to explain why need to turn off summarization on RIPv2 but I still cannot catch it ??

http://www.freeccnaworkbook.com/labs/section-7-configuring-routing-information-protocol/lab-7-8-configuring-rip-route-summarization/

Can someone use simple way to explain why we need to turn off ( disable ) summarization on RIPv2 configuration ??
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Instructor
Top Expert 2015
Commented:
If you have a scenario where having the routes summarized causes problems (discontiguous subnets) or you simply want to see all the routes in the routing table.


Top Expert 2007

Commented:
Try this scenaria,

2 routers connected by a t1. The F0/0 port on router A is 10.1.1.1 and on Router B it's 10.2.2.2.

Rip would summarize the route to 10.0.0.0. Router A will see 10.0.0.0 as a connected network therefore never learning about router b's routes.
Don JohnstonInstructor
Top Expert 2015

Commented:
That would only be true if the link between the two routers was NOT a 10.0.0.0 network.
Commented:
I quote from your link: "Auto summary will summary routes on a router to their classful networks".

With the use of VLSM and CIDR, it is very rare that summarisation to classful network masks will not produce some unintended black-holing of traffic:  Only in the simplest of network set-ups will this be beneficial, if not problematic.

The control that manual summarisation provide by being able to determine what ranges will be summarised at what points in the network is powerful tools to reduce network variability, and increase network stability, even in failure conditions.

If, on a rare occasion, you come across a network that has a full classful network allocated to it, you can leave auto-summarisation turned on at the edge of the network.  However, within the network itself, auto-summarisation will either have no effect, or, if other subnets from different networks (such as private addresses) are present, auto-summary could incorrectly indicate that the entire classful network is available through a specific router, causing inefficient routing or black-holing of traffic.  For the possibility of creating chaos, the auto-summary feature is usually disabled in practise, and replaced with manual (configurable, therefore controllable) summarisation.

Commented:
kcn, have we answered your question, or do you want us to put this in another way?

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