Invalid route in routing table

[thrtnastrx2]

I have a 3850 switch with SVI's in 172.16.136.0/20 subnets, with EIGRP to our MPLS router, and default static route to a SonicWALL (172.16.141.9).

The issue is that someone accidentally misconfigured the static route to an SonicWALL IP address that lives at another site within our MPLS network.  The invalid static route is in the routing table, and traceroute from the switch (and devices using the switch as their gateway) take the next hop of the MPLS router.  I don't understand why the switch is routing traffic via the MPLS router and not black-holing traffic that isn't specifically in the routing table.

Vlan5 172.16.137.1 YES NVRAM up up
Vlan10 172.16.138.1 YES NVRAM up up
Vlan15 172.16.139.1 YES NVRAM up up
Vlan20 172.16.140.1 YES NVRAM up up

Gateway of last resort is 172.19.141.9 to network 0.0.0.0  (misconfigured IP, should be 172.16.141.9)

S* 0.0.0.0/0 [1/0] via 172.19.141.9

traceroute 8.8.8.8
Tracing the route to 8.8.8.8
VRF info: (vrf in name/id, vrf out name/id)
1 172.16.141.1 2 msec 1 msec 2 msec  (first hop is another router on the network)

2 aaa.161.166.17 32 msec 25 msec 22 msec

3 bbb.225.170.89 40 msec 35 msec 29 msec
4 ccc.225.170.90 28 msec 29 msec 29 msec
5 172.17.191.10 30 msec 31 msec 32 msec
6 172.31.254.233 29 msec 29 msec 31 msec

[Meheretab Mengistu]

Hi,

The switch routes the default traffic through the MPLS router because it learned that it could reach to 172.19.141.9 through the MPLS router. If it could not reach to 172.19.141.9, it will not add 0.0.0.0/0 to the routing table.

HTH,

Meheretab

[Jon Marshall · Accepted solution]

It's called recursive routing ie. the next hop for a static route is usually on shared subnet but it doesn't have to be as long as your router can work out how to get to the next hop. 

So it will look for 172.19.141.9 in it's routing table and there will be a route for that, presumably with the next hop IP of the PE device at your local site so it can still use that route. 

Jon

[thrtnastrx2]

Thank you, for some reason I thought the word "recursive" had to be in the command to make it a recursive route as in PBR but I guess not.