Static Route (Next hop 0.0.0.0) "ip route 192.168.1.0 255.255.0.0 0.0.0.0"

[kfarrington]

Guys,

Has anyone ever used a recurive static route for a network to point out to a next hop of the default route?

Would just be interested as the requiredment has come up and I have never seen it in action.

Any woes to warn me about?

Many thx,

Ken

[smothuku · Accepted solution]

Hi Ken ,

It is very good question.The IOS allows the use of 0.0.0.0 as a next-hop. This is to allow the static route to point to the default as the next-hop.

*****IMP: This configuration creates problems in the CEF switching path.

When a route is entered with a next-hop of 0.0.0.0, it shows up in the routing table as a static/connected route, with no interface:

RTR2#sh ip route 100.0.0.2

Routing entry for 100.0.0.2/32

Known via "static", distance 1, metric 0

Routing Descriptor Blocks:

* directly connected

Route metric is 0, traffic share count is 1

RTR2#sh run | inc ^ip route

ip route 0.0.0.0 0.0.0.0 2.2.2.2

ip route 100.0.0.2 255.255.255.255 0.0.0.0

ip route 200.0.0.2 255.255.255.255 Serial2/0

RTR2#sh ip route 0.0.0.0

Routing entry for 0.0.0.0/0, supernet

Known via "static", distance 1, metric 0, candidate default path

Routing Descriptor Blocks:

* 2.2.2.2

Route metric is 0, traffic share count is 1

RTR2#sh ip route 200.0.0.2

Routing entry for 200.0.0.2/32

Known via "static", distance 1, metric 0 (connected)

Routing Descriptor Blocks:

* directly connected, via Serial2/0

Route metric is 0, traffic share count is 1

Using it will only put another, more specific route into the routing table for a packet which would use the default route anyways (ip classless).

This is a nice functionality which is essential for DMVPN configuration for DHCP Spokes.When deploying DMVPN configuration, there is a need for specifying static routes pointing to default gateway. In the case of DHCP this is not possible right now.

Hope it helps you.Plz rate it.

Thanks,

satish

[kfarrington]

Many thx for that Satish. That is really great and good to know.

I Aggree, this is a very interesting question.

I am a bit concerned as you say about CEF and if this could cause any issues. Could you elaborate further, if this is a show stopper.

If you look at the attached jpeg, these static routes with the default next hop is being looked at as a tactical fix for a small problem I have and would like to know the views on this.

1.0.0.0 /8 cannot for legacy reasons cannot be sent to R2 and R5 from R1 and R4

R1 and R4 only send a default to R2 and R5 from AS100 to AS101

R3 requires network 1.0.0.0/8 in AS102 to be received from AS101 but AS101 does not have this prefix

Solution, have a static route point towards the default network for 1.0.0.0/8 and the static redistributed into BGP

If link 1 fails, the static route follows the default via link 2

If both links fail, the static disappears from the routing table for 1.0.0.0/8 and thus gets withdrawn from BGP and stops being advertised to R3 **No need for the BGP conditional advertisement feature which current has bug CSCsb54969 **

Could anyone let me know if they see any major issues with this.

Many thx

Ken

[kfarrington]

Sorry, pls see attached jpeg

missed off last post