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Ahmed M Alzaeem
Beginner

question about the null route

hi ,

im just asking about the AD & metric of Null route .

is AD is  1 and the metric is 0 ????????  i mean it is a static route does it treadted as the stataic routes configs ??

assume i have a directlyconnected route 1.1.1.0/24

and i performed a null route for 1.1.1.0/24

if a packet reahced the router desrined for 1.1.1.0/24 ,

will the router forward or discard the packet  based on the info above ?? & why

with my best regards

2 ACCEPTED SOLUTIONS

Accepted Solutions
Jon Marshall
VIP Community Legend

Ahmed

If you have an interface directly connected to the network then this would be used rather than the null route so packets would be forwarded to the destination ojn the 1.1.1.x network.

It will be forwarded because a directly connected route has an AD of 0 whereas your null0 route has an AD of 1 and so the lower AD route is used.

Jon

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Hi,

AD is only locally significant.

Regards.

Alain

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Don't forget to rate helpful posts.

View solution in original post

6 REPLIES 6
Jon Marshall
VIP Community Legend

Ahmed

If you have an interface directly connected to the network then this would be used rather than the null route so packets would be forwarded to the destination ojn the 1.1.1.x network.

It will be forwarded because a directly connected route has an AD of 0 whereas your null0 route has an AD of 1 and so the lower AD route is used.

Jon

View solution in original post

Alessio Andreoli
Contributor

Ahmed M Alzaeem wrote:

hi ,

im just asking about the AD & metric of Null route .

is AD is  1 and the metric is 0 ????????  i mean it is a static route does it treadted as the stataic routes configs ??

Hi Ahmed,

your understanding is correct, a static route pointing tu NULL is exactly like a static routing pointing to a normal IP address. Indeed, you need to consider null like an interface, or the equivalent in unix systems of "null" directory.

It is not a case that for redistributing summaries or networks into BGP it is common use to write:

ip route 1.1.1.0 255.25.255.0 null0

router bgp 65000

.....

network 1.1.1.0 255.255.255.0

....

If the static route would not be considered like a normal static route, no entry would be present on the IGP routing table and no entry could be detected from the bgp network command.

here another of many uses you can do with a static route to null0:

http://www.cisco.com/en/US/tech/tk364/technologies_tech_note09186a00801c9a6e.shtml

Hope this helps

Alessio

Peter Paluch
Hall of Fame Cisco Employee

Hello Ahmed,

im just asking about the AD & metric of Null route .

is AD is  1 and the metric is 0 ????????

This strongly depends on where the Null0 route came from. If it was statically configured then by default, AD=1 and M=0. Of course, if you specified a different AD in the ip route command then this value would be used as the AD.

For dynamic routing protocols, the Null0 route is installed automatically when route summarization is performed, and the AD differs:

  • EIGRP: AD=5
  • OSPF: AD=110
  • IS-IS: AD=115
  • BGP: AD=200

The metric of these Null0 routes will always be set to the lowest metric of the summarized routes.

assume i have a directlyconnected route 1.1.1.0/24

and i performed a null route for 1.1.1.0/24

Exactly as Jon has explained, a directly connected route has AD=0 while a static Null0 route has AD=1. Therefore, the Null0 route won't be placed into the routing table because of its higher AD, and the directly connected route will be used instead.

Best regards,

Peter

i would like to thnak you all ,

a really good info ,

i could nail it down ,

here is a question

if i was using EIGRP  and configured an autosummary ,

  the AD of the autosummary  is 5  .

why at the neihgbour routers , the AD of the route is 90 not 5 ?

regards

Hi,

AD is only locally significant.

Regards.

Alain

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Don't forget to rate helpful posts.

View solution in original post

thanks very much

thatswhy at the neighbours have 90 .

regards