Administrative Distance of a Default Route

[jagadishbabu_thota]

I was told in the training sessions that the Administrative Distance for Default Route is 255 and is the least preferred route. My interviewer says its 1. Which is correct?

[attrgautam]

THe max admin distance for any static route (including default) is 255. Default is the least preferred route as long as there is a better IP match in the routing table. The default admin distance for a static route is 1.

Hope this clarifies

[jagadishbabu_thota]

Thanks attrgautam

My question is not about the Administrative distance of Static Routing, but about the default Administrative Distance of Default Routing where the destinations are unknown.

[Georg Pauwen]

Hello,

as Gautam said, the default administrative distance of static routes is 1. You might be confusing the concept of least specific and administrative distance. If you have a static default route:

ip route 0.0.0.0 0.0.0.0 Dialer1

this static route has an administrative distance of 1, and packets would only be routed out interface Dialer 1 if there is not a more specific route in the routing table. So, let's say you have packet destined for 192.168.1.2/24. If there is nothing else in the routing table except for the default static route, the packet would be routed out interface Dialer 1. If there is a more specific route in the routing table, either a static route, or a route learned through a routing protocol, the packet would be routed out using that route, regardless of the administrative distance. That is:

ip route 192.168.1.0 255.255.255.0 Ethernet0

would always be preferred over:

ip route 0.0.0.0 0.0.0.0 Dialer1

regardless of administrative distance. The administrative distance would be a tie breaker in case of two identical routes, e.g.:

ip route 0.0.0.0 0.0.0.0 Dialer1

ip route 0.0.0.0 0.0.0.0 Ethernet0

If you do not specify an administrative distance for these routes, they would load balance. By adding an administrative distance, one will be preferred over the other.

You might want to have a look at the following document:

Route Selection in Cisco Routers

http://www.cisco.com/warp/public/105/21.html

HTH,

GP

[duahimanshu816]

Hello George, 

Please i have the similar question, I am using multiple static route in my router i.e-

0.0.0.0 0.0.0.0 1.1.1.1

192.168.10.0 255.255.255.0 2.2.2.2

192.168.20.0 255.255.255.0 3.3.3.3

192.168.30.0 255.255.255.0 4.4.4.4

my router will give preference to which route? i am facing some issue thats y i am asking you kindly help me to understand it . i am not using exit interface as a gateway

thanks in advance.  

[Harold Ritter]

The most specific route is always preferred. The default, as its name indicates is only used if no more specific route exists in the routing table.

Regards,

[Richard Burts]

The original post (in this very old thread) asks a confusing question about the administrative distance for default routing. The answer to that question really is that it depends on how the default route was learned. Was it a static default route, was it learned from EIGRP, was it learned from BGP? Each of those would have a different administrative distance.

The recent post to which Harold is responding has 4 static routes, one static default route, and 3 static routes for /24 networks. It asks which one will have preference and the answer is that if the destination is in either 192.168.10.0, or 192.168.20.0, or 192.168.30.0 then that static route will have preference and otherwise the default route will have preference.

HTH

Rick

[spremkumar]

hi

The admin distance which you have mentioned as 255 is given only for the unknown routes and they arent installed in the routing table at all.

As mentioned by Gautam its 1 for the static routes whichs most preferred when it comes for route selection.

If i read ur second post properly i feel you are asking about the admin distance attached to the default route which you mention ..

if you are using a static default route pointing any ip or interface it will have the admin distance of 1 and will be preferred .

But if you have some kinda routing protocols like ospf to generate the default route and inject it onto the other boxes then you will have the OSPFs admin distance attached to it.

for an instance the default route generated by OSPF will have 110 as its admin distance.

hope this helps.

regds

[jackko]

you mentioned, "Administrative Distance for Default Route". i guess it's not very accurate in saying that, the reason being ad is used to prioritize the routing protocol. so the ad will depends on how the router yields the default route in the first place.

e.g. if the default route was yielded by static route, then the ad is 1; whereas if the default route was yielded by ospf, then the ad is 110.

http://www.cisco.com/en/US/tech/tk365/technologies_tech_note09186a0080094195.shtml

[jagadishbabu_thota]

Thanks gpauwen, spremkumar, jackko..got the answer.

[paul driver]

Hello

---

@jagadishbabu_thota wrote:

My question is not about the Administrative distance of Static Routing, but about the default Administrative Distance of Default Routing where the destinations are unknown.

---

One thing that hasn't been mentioned here is if the default route from lets say ISDN PPOE/A interface, Which is usually assigned dynamically to a routers dialer interface.

As such without any static route being added, This default route or gateway of last resort will have an metric of 254 and not 1.

Hence why in such cases it preferable to add default static route manually which will then have a admin distance of 1

[mohamedabouhegaza]

In my opinion you are the right Pearson That is Impossible That the gateway of the last resort have Admin Distance 1