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New Member

Regarding null route

                   Hi.

Just imagine we have three routers A,B and C

A is connected to both B and C.

If we define a blackhole route in A as

ip route 0.0.0.0 0.0.0.0 null0.

Wether it generates a default routes to its downstream router sucha a B and C.

Plelase clarify.

  • WAN Routing and Switching
14 REPLIES

Regarding null route

Without labbing this up, I would say that it would generate the route, but once b and c receive and try to use it, router A will blackhole the traffic. Again though, I would have to lab this up....

HTH,
John

*** Please rate all useful posts ***

HTH, John *** Please rate all useful posts ***
Silver

Regarding null route

You haven't said what routing protocols (if any) you are using betweeen A, B and C.

Hall of Fame Super Blue

Re: Regarding null route

Naveen

Wether it generates a default routes to its downstream router sucha a B and C.

Not possible to say with the information you have provided.

A static route on it's own is not sent to other routers. If you were redistributing statics into your IGP or using "default-information originate" under your IGP then yes it should be sent to the other routers.

But if you were just adding a static and not running a routing protocol then no it won't.

 

Jon

Silver

Regarding null route

Just tried this in GNS3 and BGP DOES advertise the route:

R1:

router bgp 1

no synchronization

bgp log-neighbor-changes

redistribute connected

redistribute static

neighbor 192.168.1.2 remote-as 1

neighbor 192.168.1.2 soft-reconfiguration inbound

default-information originate

no auto-summary

ip route 0.0.0.0 0.0.0.0 Null0

R2:

R2#sh ip ro 0.0.0.0

Routing entry for 0.0.0.0/0, supernet

  Known via "bgp 1", distance 200, metric 0, candidate default path, type internal

  Last update from 192.168.1.1 00:02:26 ago

  Routing Descriptor Blocks:

  * 192.168.1.1, from 192.168.1.1, 00:02:26 ago

      Route metric is 0, traffic share count is 1

      AS Hops 0

Hall of Fame Super Blue

Re: Regarding null route

That's because you have redistribute static under your BGP config.

If you just configure a static and don't redistribute or use "default-information originate" it shouldn't.

Jon

Silver

Regarding null route

Jon,

Yes - all that is true.

I thought the nature of the question was whether a route pointing to null0 would be advertised (rather than a route pointing to a next-hop or an interface) - regardless of how that advertisement took place.

Hall of Fame Super Blue

Regarding null route

Ahh i understand.

A static route is no different whether it points to a next hop IP or an interface and null0 is an interface in this respect (although it behaves differently in other respects).

So there is no reason why it wouldn't be advertised unless you aren't redistributing/default-information etc.

Jon

Regarding null route

Correct me if I'm wrong though - I still haven't labbed this up

Regardless of if it's advertised out, the "unknown" route will be sent to the advertising router, but if that route is also unknown to it, then it will just drop traffic. So, in reality, the only way this configuration would make any sense is if the router that's advertising this null default route has visibility to all subnets either by full bgp table or something like that.

HTH,
John

*** Please rate all useful posts ***

HTH, John *** Please rate all useful posts ***
Hall of Fame Super Blue

Regarding null route

Hi John

Not sure i am following but maybe i misunderstood the original question.

I thought the question was asking if you configured a static default route on router A would it be advertised to B and C. To which,, as far as i know, it won't unless -

a) you are running a routing protocol between A, B and C

and

b) you are redistrbuting that static into the IGP or using "default-information originate"

i'm not sure what you mean when you say -

Regardless of if it's advertised out, the "unknown" route will be sent to the advertising router, but if that route is also unknown to it, then it will just drop traffic

ie. if it is not advertised out then there is no advertising router.

Jon

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