ip route 0.0.0.0 0.0.0.0 null0

It's a default route. If for some reason packets are supposed to be bound for an unknown destination, the router will drop them.

It looks like this default null route was added to ensure that there is always a default route in the routing table so that BGP will always advertise the route to its peers. Since this isn’t tied with an interface, there is no chance that it will be removed from the routing table. There is most likely an interior protocol running that will have more specific routes for your environment.

 As mentioned above, if there is no more specific route the traffic will be dropped.

Hub(config)#ip route 0.0.0.0 0.0.0.0 null0

the way I understand the command above is any traffic that leaves the Hub router will be discarded.. Not sure why is configured that way ...

in the other hand the Hub router is permitting default route to go out to its neighbors

Hub(config-router)#neighbor 172.16.123.2 route-map SPOKE_ROUTERS out
Hub(config-router)#neighbor 172.16.123.3 route-map SPOKE_ROUTERS out

Kevin is correct. This is in order for BGP to advertise a default route by forcing it in the RIB with the static route to null.

Soulja

If I understand to advertise , we need to use the command Network.....
above it is : routing traffic :
Hub(config)#ip route 0.0.0.0 0.0.0.0 null0

then  permitting default route to go out to its neighbors

Hub(config-router)#neighbor 172.16.123.2 route-map SPOKE_ROUTERS out
Hub(config-router)#neighbor 172.16.123.3 route-map SPOKE_ROUTERS out

I do not get the meaning of the configuration below, any one can comment it line by line ?

Hub(config)#ip route 0.0.0.0 0.0.0.0 null0

Hub(config)#ip prefix-list DEFAULT_ROUTE permit 0.0.0.0/0

Hub(config)#route-map SPOKE_ROUTERS permit 10
Hub(config-route-map)#match ip address prefix-list DEFAULT_ROUTE

Hub(config)#router bgp 65001
Hub(config-router)#network 0.0.0.0 mask 0.0.0.0
Hub(config-router)#neighbor 172.16.123.2 route-map SPOKE_ROUTERS out
Hub(config-router)#neighbor 172.16.123.3 route-map SPOKE_ROUTERS out

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in other words if the configuration of the router Hub above did not have this line:
Hub(config)#ip route 0.0.0.0 0.0.0.0 null0

it would have made sense to me

I went ahead and went through the config line by line as requested.


Hub(config)#ip route 0.0.0.0 0.0.0.0 null0  

Static route that matches against all traffic and forwards to a null interface

Hub(config)#ip prefix-list DEFAULT_ROUTE permit 0.0.0.0/0

Prefix list that will only match on the default route

Hub(config)#route-map SPOKE_ROUTERS permit 10              

route map with a permit entry of 10

Hub(config-route-map)#match ip address prefix-list DEFAULT_ROUTE

Uses the prefix list from above. Since route map is permit and prefix list is permit, the result will be to permit the default route

Hub(config)#router bgp 65001 .

Specifies a private BGP AS number for the local router

Hub(config-router)#network 0.0.0.0 mask 0.0.0.0

Tells BGP its allowed to advertise out the default route in its RIB-OUT

Hub(config-router)#neighbor 172.16.123.2 route-map SPOKE_ROUTERS out

Uses the route map previously created to control what is sent to the 172.16.123.2 neighbor

Hub(config-router)#neighbor 172.16.123.3 route-map SPOKE_ROUTERS out

Uses the route map previously created to control what is sent to the 172.16.123.3 neighbor

Thanks Kevin

As I stated the configuration above would have made sense without this line:
Hub(config)#ip route 0.0.0.0 0.0.0.0 null0

if I understand the command above routes everything to bit bucket

That is required to ensure that the default route is present in the routing table. Without a route in the routing table, BGP will not advertise that it knows about a route to its peers.

Hub(config)#ip route 0.0.0.0 0.0.0.0 null0

with null0, I thought any traffic coming out of the Hub will be discarded [put in a trash bin]

You are correct that null0 will discard traffic, but only the traffic that does not match on a more specific route will be discarded.

If you were to execute a show ip route on the device, I assume you will see several more specific routes, anything from /1 to /32 will be more specific than the default, learned by an interior routing protocol (OSPF, RIP, EIGRP).

Thanks Kevin,

if we type it without the Null0
Hub(config)#ip route 0.0.0.0 0.0.0.0

will the default route still be advertised ?

Thank you

You are very welcome!