Default Route using 128.0.0.0

[kradjesh13]

Hi,

While going to a config i came across the following line of configs and i know there will/may be a reason for using 128.0.0.0 in their static routes pointing to the default gateway/firewall. I am pretty keen to know in what situation this kind of routes will be added on to the  router.

ip route 0.0.0.0 0.0.0.0 vlan 10 192.168.10.254

ip route 0.0.0.0 128.0.0.0 vlan 10 192.168.10.254

ip route 128.0.0.0 128.0.0.0 vlan 10 192.168.10.254

If some one can explain me, that will be grate.

Thanks

Rajesh

[Shri612]

Hi Rajesh ,

As per mu understanding these are split static routes which were used in the case of equal load balancing and unequal load balancing are required.

Regards.

A.

[kradjesh13]

Thanks for the reply.

These static routes have been applied towards the interface facing the firewalls. I will be more convinced if they have used split static routes in a server environment.

But still, i dont know how this will perform load balancing since both the follwoing lines are found at the same router.

ip route 0.0.0.0 128.0.0.0 vlan 10 192.168.10.254

ip route 128.0.0.0 128.0.0.0 vlan 10 192.168.10.254

Thanks

Rajesh

[jeross219]

I found a good use case. Let's say that you have a default originated into OSPF and some remote device has a static default pointing to a backup next hop not running OSPF. The remote device will never use the OSPF originated default it received because of the AD. Maybe the remote device needs a gateway of last resort on it's primary link or a supernet to prevent assymetric routing or black holes over the backup link.

Using 0.0.0.0/1 & 128.0.0.0/1 solves this!

[markjenks]

We are looking at this exact thing right now.

0.0.0.0/1 route over eigrp to the data center.

0.0.0.0/1 route over ospf (AD 180) for alternate path to the same data center.  Lower speed, over subscribed.

0.0.0.0/0 summarized route over eigrp (AD 170) over a backup satellite link to the data center.

Satellite will never get used unless the other 2 "more specific" routes are not available.

-Mark

[cis-sic]

Exactly  Mark, it is useful after all! 

Jeremy 

[Lee Smitherman · Accepted solution]

Hi,

I cannot see the 0.0.0.0 0.0.0.0 route ever being used as all routes will fall into one of the more significant routes below

ip route 0.0.0.0 128.0.0.0 vlan 10 192.168.10.254

ip route 128.0.0.0 128.0.0.0 vlan 10 192.168.10.254

So you could have either

ip route 0.0.0.0 0.0.0.0 vlan 10 192.168.10.254

or 

ip route 0.0.0.0 128.0.0.0 vlan 10 192.168.10.254

ip route 128.0.0.0 128.0.0.0 vlan 10 192.168.10.254

Both will exclusively do the same job on that router.

[Peter Paluch · Accepted solution]

Hello Rajesh,

I do not personally see any logic behind these routes. As Lee has correctly pointed out, because of longest-prefix matching, the /1 routes will be always used and the 0.0.0.0/0 route will thus remain unused. Contrary to what Alessio suggests, I do not believe this has anything to do with load balancing or load splitting - because the next hop IP address is identical in all three routes, so there are no more next hops to load balance among.

However, I remember reading about a bug on Catalyst 2960 switches immediately after introducing their limited static routing functionality. This bug caused the 2960 to ignore the 0.0.0.0/0 default route in its routing table, preventing the switch from routing packets towards the default gateway. All other routing entries were used correctly. This lead to an immediate workaround of specifying the default route as a pair of routes 0.0.0.0/1 and 128.0.0.0/1 towards the same next hop.

Is perhaps your device here a Cat 2960?

Best regards,

Peter

[kradjesh13]

Peter & Lee,

You are right, I don’t see a point in using /1 routes along with 0.0.0.0/0 route. But when I saw those routes, I was interested to know the reason why it was applied and in which situation can be used and I did my own research. I was not convinced with my finding since all the 3 routes are pointing to the same gateway IP which cannot do load balancing. From this discussion, I am kind of convinced and know this is not used for any load balancing scenarios.

Peter, the device is Cisco 6513 and this is not from Cat 2960

Thanks for the input guys and If I find any thing interesting I will update you guys.

Rajesh

[Peter Paluch]

Hello Rajesh,

Thank you for your generous ratings! Sure, if you learn about anything new please let us know.

Best regards,

Peter