For this article, let’s continue on a previous one: a basic VRF topology.

VRF

Only this time, at router R1, there’s an extra interface (G0/4), which is used as an internet breakout. You’ve received two public static IP’s on top of the static IP of the router, which you can use. This time, the manager asks that all users connect towards the internet behind one IP address, and that the voice gateway has a static public IP address as well. This gives a problem right away: users and voice server are in a seperate VRF. And an interface can’t be part of more than one VRF. So what do we do with our single internet breakout? Solution: some VRF manipulation, clever placing of the NAT commands, and policy-based routing (PBR). For clarity: towards the ISP we will use 198.15.0.0/30, a point-to-point link, and the two public addresses from the ISP will be 198.15.0.4/31.

First stretching the concept of VRF a bit: an interface can’t be part of more than one VRF, but it can be added to the routing table of multiple VRF. Note that the configuration below is done on a 12.x IOS. 15.x requires different vrf commands (without ‘ip’ in front of it).

R1(config)#interface g0/4
R1(config-if)#ip policy route-map RM-Internet
R1(config-if)#ip vrf receive VOICE
R1(config-if)#ip vrf receive LAN
R1(config-if)#ip address 198.15.0.2 255.255.255.252
R1(config-if)#ip nat outside
R1(config-if)#exit
R1(config)#ip route vrf VOICE 0.0.0.0 0.0.0.0 g0/4 198.15.0.1
R1(config)#ip route vrf LAN 0.0.0.0 0.0.0.0 g0/4 198.15.0.1

The PBR command ‘ip policy’ needs to go first, otherwise ‘ip vrf receive’ will not be accepted and a message “% Need to enable Policy Based Routing on the interface first” will show. The latter places the interface into the VRF routing tables. Together with the default routes for those VRF, it allows for outgoing traffic from both VRF towards the internet.

Now where to place the NAT? Usually this is done in the outside interface. Except, since both VRF have overlapping IP ranges, it can’t always be done there. Instead, on the inside interfaces, inside the VRF, the translation must be made so it’s a unique outside address before it reaches the outside interface.

R1(config)#interface g0/1
R1(config-if)#ip vrf forwarding LAN
R1(config-if)#ip address 172.16.1.1 255.255.255.0
R1(config-if)#ip nat inside
R1(config-if)#exit
R1(config)#interface g0/3
R1(config-if)#ip vrf forwarding VOICE
R1(config-if)#ip address 172.20.1.1 255.255.255.252
R1(config-if)#ip nat inside
R1(config-if)#exit
R1(config)#ip access-list standard AL4-NAT-LAN
R1(config-std-nacl)#permit 172.16.1.0 0.0.0.255
R1(config-std-nacl)#exit
R1(config)#ip nat pool PL-NAT-LAN 198.15.0.4 198.15.0.4 netmask 255.255.255.255
R1(config)#ip nat inside source list AL4-NAT-LAN pool PL-NAT-LAN vrf LAN overload
R1(config)#ip nat inside source static 172.20.1.2 198.15.0.5 vrf VOICE

This gives the users on the LAN an external IP address of 198.15.0.4 and the voice server a static NAT of 198.15.0.5. But while that works for outgoing traffic, incoming traffic still has an issue. How does the router know in which VRF to place the incoming traffic? That’s where the PBR comes into play.

R1(config)#ip access-list extended AL4-IN-LAN
R1(config-ext-nacl)#permit ip any host 198.15.0.4
R1(config-ext-nacl)#exit
R1(config)#ip access-list extended AL4-IN-VOICE
R1(config-ext-nacl)#permit ip any host 198.15.0.5
R1(config-ext-nacl)#exit
R1(config)#route-map RM-IN permit 3
R1(config-route-map)#match ip address AL4-IN-LAN
R1(config-route-map)#set vrf LAN
R1(config-route-map)#route-map RM-IN permit 6
R1(config-route-map)#match ip address AL4-IN-VOICE
R1(config-route-map)#set vrf VOICE
R1(config-route-map)#exit

The above creates a route-map that refers to two ACLs. The goal is simple: if incoming traffic has destination IP addres 198.15.0.4 (NAT for user LAN), then it’s placed in the LAN VRF. If it has 198.15.0.5 (static NAT voice server) as destination IP address, it is placed in the VOICE VRF.

All the above together allow each VRF to use the same internet uplink. The routing table of a VRF looks normal:

R1#show ip route vrf LAN

Gateway of last resort is 198.15.0.1 to network 0.0.0.0

172.16.0.0/24 is subnetted, 1 subnets
C       172.16.1.0 is directly connected, GigabitEthernet0/1
172.20.0.0/30 is subnetted, 1 subnets
C       172.20.1.0 is directly connected, GigabitEthernet0/2
10.0.0.0/16 is subnetted, 1 subnets
O       10.0.0.0 [110/2] via 172.20.1.2, 17:42:38, GigabitEthernet0/2
198.15.0.0/30 is subnetted, 1 subnets
C       198.15.0.0 is directly connected, GigabitEthernet0/4
S*   0.0.0.0/0 [1/0] via 198.15.0.1, GigabitEthernet0/4

This is very useful in an environment with multiple VRF and one internet breakout. However, this can also be used in other cases, such as an MPLS-VPN environment, which Darren covers in detail on his blog.

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