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Routing issue: aggregate auto-summary of an advertised received route

Answered Question

Dear,


If 2 routers are eBGP neighbours, and:


- Router A advertises a class B subnet: 10.26.0.0/16

and

- Router B has an aggregate statement for this subnet: aggregate-address 10.26.0.0 255.255.0.0 summary-only


> Is router B injecting a NULL0 route (pref. 200) due to this 10.26.0.0/16 being received from router A ?

> Or will router B only inject the NULL0 route just in case a more specific route is learned from router A ?



This quesiton to known better the working of BGP in this configuration. Because last time we connected a new router C to router B,

which was advertising more specific (router C) routes within subnet 10.26.0.0/16.

And router B's routing table included this 10.26.0.0/16 to NULL0 interface.


As a concequense the more specific networks in range 10.26.0.0/16 to which router A is connected, where NOT KNOWN anymore (or reachable) from router B point of view.


Thank you!


Kind regards,

Wim

Correct Answer by milan.kulik about 3 years 11 months ago

Hi,


if a BGP router receives only the same class B for which it has an aggregate/summary defined, it will not create NULL0 route for this aggregate/summary.


An interesting case is when it receives both that prefix plus some more specific.

In that case it creates the NULL0 route which beats the same aggregate prefix recieved!


Best regards,

Milan

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Lei Tian Mon, 09/16/2013 - 04:57
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  • Cisco Employee,

Hi Wim,


Routing protocol by default injects null0 route when a router advertise summary/aggregate route. This null0 route is used to prevent routing loop caused by summary route in certain scenario.


HTH,

Lei Tian

Hi Lei,


Thanks for your quick reply!


The key question in this story is:

Whether router B will inject this NULL0 route when it receives the same class B for which it has an aggregate/summary defined.

Or, does it need a more specific in order to inject a NULL0 route for this aggregate/summary?


You known what I mean?



Thanks!

Wim

Lei Tian Mon, 09/16/2013 - 06:47
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  • Cisco Employee,

Hi Wim,


Router B needs more specifics to advertise aggregate and injects the null0. Router shouldn't aggregate and inject null0 route if it only receives route with same prefix length as aggregate route, as this could cause routing loop.


HTH,

Lei Tian

Correct Answer
milan.kulik Mon, 09/16/2013 - 06:52
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  • Red, 2250 points or more

Hi,


if a BGP router receives only the same class B for which it has an aggregate/summary defined, it will not create NULL0 route for this aggregate/summary.


An interesting case is when it receives both that prefix plus some more specific.

In that case it creates the NULL0 route which beats the same aggregate prefix recieved!


Best regards,

Milan

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