topic Re: Route with gateway 0.0.0.0 in Routing

Route with gateway 0.0.0.0

Christian Jorge — Wed, 30 Nov 2022 15:12:39 GMT

Good morning

Checking an ACI leaf switch I found these static routes

ip route 10.10.0.0/16 0.0.0.0 200 (gateway 0.0.0.0 ?)
ip route 10.10.0.0/16 10.1.0.1 210

Checking the routing table, it's been considered only the route number 2 (distance 210)

10.10.0.0/16, ubest/mbest: 1/0
*via 10.1.0.1, vlan10, [210/0], 32w05d, static

I don't know (or have history about) why someone has created these routes, but I'm wondering:

- what the reason that a switch disregard the primary route (distance 200)?

- how a switch/router understands a static route with gateway 0.0.0.0?

- route with gateway 0.0.0.0 would be different than a route with a gateway Null0?

*ps: I have already seen before a route with gateway 0.0.0.0 only in BGP routing tables

Regards

Re: Route with gateway 0.0.0.0

Giuseppe Larosa — Wed, 30 Nov 2022 15:35:26 GMT

Hello @Christian Jorge ,

A static route with next-hop 0.0.0.0 is probably treated as invalid with a not specified next-hop.

This would explain why the second static route with AD 210 is installed because it has a valid IP next-hop hopefully pointing to another device.

A BGP next-hop of 0.0.0.0 applies to locally generated BGP prefixes.

Finally a static route to null0 is a valid static route that silently discards packets. null0 is a defined logical interface the waste bin an IP next-hop of 0.0.0.0 is not equivalent to it, it should just be treated as an undefined IP nv4 next-hop.

Hope to help

Giuseppe

Re: Route with gateway 0.0.0.0

Christian Jorge — Wed, 30 Nov 2022 16:45:48 GMT

Hello Giuseppe

Thanks for your answer.

I found something about this matter in Internet, even in another topic in Cisco Community:
https://community.cisco.com/t5/switching/static-route-next-hop-0-0-0-0-quot-ip-route-192-168-1-0-255-255/td-p/663840

but I think your explanation is clearer, makes sense and é exactly what I'd like to confirm

Regards

Re: Route with gateway 0.0.0.0

MHM Cisco World — Wed, 30 Nov 2022 16:53:19 GMT

sorry can you more elaborate where exact you see next-hop 0.0.0.0 for static route in ACI ?

Re: Route with gateway 0.0.0.0

Christian Jorge — Wed, 30 Nov 2022 16:57:37 GMT

Actually I can see next-hop 0.0.0.0 only in static route configuration:

ip route 10.10.0.0/16 0.0.0.0 200

but exactly as explained before, this route/gateway is considered invalid and not input in routing table