Routes with a 255 administrative distance are considered unreachable?

[MicJameson1]

GIVEN:    "S 0.0.0.0 0.0.0.0 [255/0] via 172.16.1.1, inside tunneled"

My understanding is that routes with a 255 administrative distance (AD) are considered unreachable. Is this correct?

If the above route has an AD of 255, will it ever be used at all?

Thank you.

[balaji.bandi · Accepted solution]

your understanding is correct.

check doc :

https://www.cisco.com/c/en/us/support/docs/ip/open-shortest-path-first-ospf/4170-ospfprocesses.html

[MicJameson1]

Is it a practice to insert such a static route just as a type of engineer note for all who inspect the routing table?

[Harold Ritter · Accepted solution]

Hi @MicJameson1 ,

Let me take a step back. I assumed you were asking about IOS, but it looks like it might be referring to ASA, right? ASA behaves differently and considers an administrative distance (AD) of 255 as the worst AD, but still considers it as valid.

This different ASA behavior only applies to the ASA "tunneled default route" feature. The behavior is exactly the same as IOS if the administrative distance is set to 255 on any other route. 

ciscoasa# sh runn | incl route
route Inside 0.0.0.0 0.0.0.0 192.168.12.1 tunneled

ciscoasa# sh route 0.0.0.0 0.0.0.0
S 0.0.0.0 0.0.0.0 [255/0] via 192.168.12.1, Inside tunneled

Regards,

[MicJameson1]

God bless you sir for saving me indefinite hours and headache!

[Harold Ritter]

You are very welcome @MicJameson1 and thanks for the feedback

[Harold Ritter]

Hi @MicJameson1 ,

You are right.

The following document says:

* If the administrative distance is 255, the router does not believe the source of that route and does not install the route in the routing table.

https://www.cisco.com/c/en/us/support/docs/ip/border-gateway-protocol-bgp/15986-admin-distance.html

Regards,