Cisco recently (15.6.3M2 ) resolved CSCve61996, which makes it possible to fail internet access back and forth between two DHCP-managed interfaces in two different front-door VRFs attached to consumer-grade internet service.
Prior to the IOS fix there was a lot of weirdness with route configuration on DHCP interfaces assigned to VRFs.
I'm using a C891F-K9 for this example. The WAN interfaces are Gi0 and Fa8. They're in F-VRF's named ISP_A and ISP_B respectively:
First, create the F-VRFs and configure the interfaces:
ip vrf ISP_A
ip vrf ISP_B
interface GigabitEthernet8
ip vrf forwarding ISP_A
ip dhcp client default-router distance 10
ip address dhcp
interface FastEthernet0
ip vrf forwarding ISP_B
ip dhcp client default-router distance 20
ip address dhcp
The distance commands above assign the AD of the DHCP-assigned default route. Without these directives the distance would be 254 in each VRF. They're modified here because we'll be using the distance to select the preferred internet path when both ISPs are available.
Next, let's keep track of whether or not the internet is working via each provider. In this case I'm pinging 8.8.8.8 via both paths, but this health check can be whatever makes sense for your situation. So, a couple of IP SLA monitors and track objects are in order:
ip sla 1
icmp-echo 8.8.8.8
vrf ISP_A
threshold 500
timeout 1000
frequency 1
ip sla schedule 1 life forever start-time now
track 1 ip sla 1
ip sla 2
icmp-echo 8.8.8.8
vrf ISP_B
threshold 500
timeout 1000
frequency 1
ip sla schedule 2 life forever start-time now
track 2 ip sla 2
Ultimately we'll be withdrawing the default route from each VRF when we determine that the internet has failed. This is introduces a problem: With the default route missing the SLA target will be unreachable. The SLA (and track) will
never recover, so the default route will never be restored. So first let's add a static route to our SLA target in each VRF. The default route will get withdrawn, but the host route for the SLA target will persist in each VRF.
ip route vrf ISP_A 8.8.8.8 255.255.255.255 dhcp 50
ip route vrf ISP_B 8.8.8.8 255.255.255.255 dhcp 60
We used the dhcp keyword as a stand-in for the next-hop IP address. We
could have just specified the interface, but specifying a multiaccess interface without a neighbor ID is an ugly practice and assumes that proxy ARP is available from neighboring devices. Not a safe assumption.
Finally, we can set the default route to be withdrawn when the track object goes down:
interface GigabitEthernet8
ip dhcp client route track 1
interface FastEthernet0
ip dhcp client route track 2
At this point, when everything is healthy, the routing table for ISP_A looks something like this:
S* 0.0.0.0/0 [10/0] via 192.168.1.126
8.0.0.0/32 is subnetted, 1 subnets
S 8.8.8.8 [50/0] via 192.168.1.126
192.168.1.0/24 is variably subnetted, 2 subnets, 2 masks
C 192.168.1.64/26 is directly connected, GigabitEthernet8
L 192.168.1.67/32 is directly connected, GigabitEthernet8
The table for ISP_B looks similar, but with different Administrative Distances. On failure of the SLA/track the default route gets withdrawn but the 8.8.8.8/32 route persists. That looks like this:
8.0.0.0/32 is subnetted, 1 subnets
S 8.8.8.8 [50/0] via 192.168.1.126
192.168.1.0/24 is variably subnetted, 2 subnets, 2 masks
C 192.168.1.64/26 is directly connected, GigabitEthernet8
L 192.168.1.67/32 is directly connected, GigabitEthernet8
When the ISP is healed, the 8.8.8.8/32 ensures that we'll notice, the SLA will recover, and the default route will be restored.
Okay, now it's time to think about leaking these ISP_A and ISP_B routes into the global routing table (GRT). First, we need an interface in the GRT for use by directly connected clients:
interface Vlan10
ip address 10.10.10.1 255.255.255.0
And now the leaking configuration:
ip prefix-list PL_DEFAULT_ONLY permit 0.0.0.0/0
route-map RM_IMPORT_TO_GRT permit
match ip address prefix-list PL_DEFAULT_ONLY
global-address-family ipv4
route-replicate from vrf ISP_A unicast static route-map RM_IMPORT_TO_GRT
route-replicate from vrf ISP_B unicast static route-map RM_IMPORT_TO_GRT
The configuration above leaks only the default route from each F-VRF. The GRT will be offered both routes and will make its selection based on the AD we configured earlier (values 10 and 20).
Here's the GRT with everything working:
S* + 0.0.0.0/0 [10/0] via 192.168.1.126 (ISP_A)
10.0.0.0/8 is variably subnetted, 2 subnets, 2 masks
C 10.10.10.0/24 is directly connected, Vlan10
L 10.10.10.1/32 is directly connected, Vlan10
When the ISP_A path fails, the GRT fails over to the higher distance route via ISP_B:
S* + 0.0.0.0/0 [20/0] via 192.168.1.62 (ISP_B)
10.0.0.0/8 is variably subnetted, 2 subnets, 2 masks
C 10.10.10.0/24 is directly connected, Vlan10
L 10.10.10.1/32 is directly connected, Vlan10
Strictly speaking, it's not necessary to have the SLA monitor, track object and conditional routing in VRF ISP_B. All of those things could be omitted and the GRT would still fail back and forth between the different F-VRFs based only on the tests in "A". But I like the symmetry.
Okay, so now that we've got the GRT's default route flopping back and forth between these two front-door VRFs, we'll need some NAT. First, enable NVI mode on each interface in the transit path:
interface GigabitEthernet8
ip nat enable
interface FastEthernet0
ip nat enable
interface Vlan10
ip nat enable
Next we'll spell out exactly what's going to get NATted. I like to use route-maps rather than ACLs because the templating is easier when we're matching interfaces rather than ip prefixes:
route-map RM_NAT->ISP_A permit 10
match interface GigabitEthernet8
route-map RM_NAT->ISP_B permit 10
match interface FastEthernet0
ip nat source route-map RM_NAT->ISP_A interface GigabitEthernet8 overload
ip nat source route-map RM_NAT->ISP_B interface FastEthernet0 overload
That's basically it. The last thing that might prove useful is to automate purging of NAT translation tables when switching between providers. TCP flows can't survive the ISP switchover, and clearing the NAT translations for active flows should make them fail faster than they might have otherwise.
