Configure VRF-Aware Software Infrastructure (VASI) NAT on IOS-XE

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Updated:February 4, 2016
Document ID:200255

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Introduction

This document describes the configuration of VASI NAT on routers that run Cisco IOS-XE®.

Contributed by Rohit Nair, Cisco TAC Engineer.

Prerequisites

Requirements

There are no specific requirements for this document.

Components Used

This document is not restricted to specific software and hardware versions. This document applies to all Cisco routers and switches that run Cisco IOS-XE.

The information in this document was created from the devices in a specific lab environment. All of the devices used in this document started with a cleared (default) configuration. If your network is live, make sure that you understand the potential impact of any command.



Background Information

Devices that run on IOS-XE do not support classical inter-vrf nat configurations as those found on IOS devices. Support for Inter-vrf NAT on IOS-XE is achieved via VASI implementation.

VASI provides the ability to configure services such as IPsec, firewall and NAT to traffic that flows between VRF instances.

VASI is implemented by configuring VASI pairs, where each of the interfaces in the pair is associated with a different VRF instance. The VASI virtual interface is the next-hop interface for any packet that needs to be switched between these two VRF instances. The pairing is done automatically based on the two interface indexes such that the vasileft interface is automatically paired to the vasiright interface. Any packet that enters the vasileft interface is automatically forwarded to its paired vasiright interface.

Working of VASI

When an inter-VRF VASI is configured on the same device, the packet flow happens in the following order:

  1. A packet enters the physical interface that belongs to VRF 1.
  2. Before forwarding the packet, a forwarding lookup is done in the VRF 1 routing table. Vasileft1 is chosen as the next hop, and the Time to Live (TTL) value is decremented from the packet. Usually, the forwarding address is selected on the basis of the default route in the VRF. However, the forwarding address can also be a static route or a learned route. The packet is sent to the egress path of vasileft1 and then automatically sent to the vasiright1 ingress path.
  3. When the packet enters vasiright1, a forwarding lookup is done in the VRF 2 routing table, and the TTL is decremented again (second time for this packet).
  4. VRF 2 forwards the packet to the physical interface.

Configure

The following scenarios describe basic inter-vrf NAT configuration.

Network Diagram

  

Initial Configurations

SanJose:

interface GigabitEthernet0/0/0

 ip address 192.168.1.1 255.255.255.0


ip route 0.0.0.0 0.0.0.0 192.168.1.2
[an error occurred while processing this directive]

Bombay:

vrf definition VRF_LEFT

 rd 1:1

 !

 address-family ipv4

 exit-address-family


vrf definition VRF_RIGHT

 rd 2:2

 !

 address-family ipv4

 exit-address-family


interface GigabitEthernet0/0/0

  vrf forwarding VRF_LEFT

  ip address 192.168.1.2 255.255.255.0


interface GigabitEthernet0/0/1

 vrf forwarding VRF_RIGHT

 ip address 172.16.1.2 255.255.255.0
[an error occurred while processing this directive]

Sydney:

interface GigabitEthernet0/0/0

 ip address 172.16.1.1 255.255.255.0
[an error occurred while processing this directive]

VASI Interface configuration

Each VASI interface will be paired to a different VRF instance.

interface vasileft1

 vrf forwarding VRF_LEFT

 ip address 10.1.1.1 255.255.255.252


interface vasiright1

 vrf forwarding VRF_RIGHT

 ip address 10.1.1.2 255.255.255.252
[an error occurred while processing this directive]

NAT Configuration:

In this example, NAT is to be configured with the following requirements:

1. Static NAT      - Source IP of 192.168.1.1 should be translated to 172.16.1.5

2. Dynamic NAT - Source subnet of 192.168.1.0/24 should be translated to 172.16.1.5 

Scenario 1 - NAT on Vasiright

In most cases, the WAN interface would be on the outgoing VRF, VRF_RIGHT in this topology. In such cases, NAT can be configured between the vasiright and the WAN interface; traffic coming in on the vasiright interface from vasileft will be configured as NAT inside, while the WAN interface would be the NAT outside interface.

In this scenario, we use static routes to traffic between the VRFs. A static route for the destination 172.16.0.0 subnet is configured on VRF_LEFT pointing to the vasileft interface and another route for the source subnet 192.168.0.0 is configured on VRF_RIGHT pointing to the vasiright interface.


Note

Do not configure NAT to translate the source IP to the WAN interface IP; the router will treat return traffic to be destined to itself and will not forward traffic to the vasi interface.

Static NAT : 


!--- Interface configuration

interface vasiright1

 vrf forwarding VRF_RIGHT

 ip address 10.1.1.2 255.255.255.252

 ip nat inside


interface GigabitEthernet0/0/1

 vrf forwarding VRF_RIGHT

 ip address 172.16.1.2 255.255.255.0

 ip nat outside



!--- Static route configuration

ip route vrf VRF_LEFT 172.16.0.0 255.255.0.0 vasileft1 10.1.1.2

ip route vrf VRF_RIGHT 192.168.0.0 255.255.0.0 vasiright1 10.1.1.1



!--- NAT configuration

ip nat inside source static 192.168.1.1 172.16.1.5 vrf VRF_RIGHT
[an error occurred while processing this directive]

Verification:

Bombay#sh ip nat translations vrf VRF_RIGHT

Pro        Inside global     Inside local       Outside local    Outside global

---        172.16.1.5        192.168.1.1        ---              ---

icmp       172.16.1.5:8      192.168.1.1:8      172.16.1.1:8     172.16.1.1:8

tcp        172.16.1.5:47491  192.168.1.1:47491  172.16.1.1:23    172.16.1.1:23

Total number of translations: 3
[an error occurred while processing this directive]

Dynamic NAT :


!--- Interface configuration

interface vasiright1

 vrf forwarding VRF_RIGHT

 ip address 10.1.1.2 255.255.255.252

 ip nat inside


interface GigabitEthernet0/0/1

 vrf forwarding VRF_RIGHT

 ip address 172.16.1.2 255.255.255.0

 ip nat outside



!--- Static route configuration

ip route vrf VRF_LEFT 172.16.0.0 255.255.0.0 vasileft1 10.1.1.2

ip route vrf VRF_RIGHT 192.168.0.0 255.255.0.0 vasiright1 10.1.1.1



!--- Access-list configuration

Extended IP access list 100

 10 permit tcp 192.168.1.0 0.0.0.255 host 172.16.1.1

 20 permit udp 192.168.1.0 0.0.0.255 host 172.16.1.1

 30 permit icmp 192.168.1.0 0.0.0.255 host 172.16.1.1



!--- NAT configuration

ip nat pool POOL 172.16.1.5 172.16.1.5 prefix-length 24

ip nat inside source list 100 pool POOL vrf VRF_RIGHT overload
[an error occurred while processing this directive]

Verification: 

Bombay#sh ip nat translations

Pro      Inside global      Inside local         Outside local      Outside global

icmp     172.16.1.5:1       192.168.1.1:15       172.16.1.1:15      172.16.1.1:1

tcp      172.16.1.5:1024    192.168.1.1:58166    172.16.1.1:23      172.16.1.1:23

Total number of translations: 2
[an error occurred while processing this directive]








Scenario 2 - NAT on Vasileft



NAT can also be configured solely on the vasileft side, i.e VRF_LEFT and have traffic NATTED before it is sent to VRF_RIGHT. The incoming interface on VRF_LEFT will be considered as the NAT inside interface, and vasileft 1 will be configured as the NAT outside interface.



In this scenario, we use static routes to traffic between the VRFs. A static route for the destination 172.16.0.0 subnet is configured on VRF_LEFT pointing to the vasileft interface and another route for the source natted IP 172.16.1.5 is configured on VRF_RIGHT pointing to the vasiright interface.




Static NAT:




!--- Interface configuration

interface GigabitEthernet0/0/0

 vrf forwarding VRF_LEFT

 ip address 192.168.1.2 255.255.255.0

 ip nat inside


interface vasileft1

 vrf forwarding VRF_LEFT

 ip address 10.1.1.1 255.255.255.252

 ip nat outside



!--- Static route configuration

ip route vrf VRF_LEFT 172.16.0.0 255.255.0.0 vasileft1 10.1.1.2

ip route vrf VRF_RIGHT 172.16.1.5 255.255.255.255 vasiright1 10.1.1.1



!--- NAT configuration

ip nat inside source static 192.168.1.1 172.16.1.5 vrf VRF_LEFT
[an error occurred while processing this directive]



Verification:



Bombay#sh ip nat translations vrf VRF_LEFT

Pro       Inside global        Inside local         Outside local      Outside global

---       172.16.1.5           192.168.1.1          ---                ---

icmp      172.16.1.5:5         192.168.1.1:5        172.16.1.1:5       172.16.1.1:5

tcp       172.16.1.5:35414     192.168.1.1:35414    172.16.1.1:23      172.16.1.1:23

Total number of translations: 3
[an error occurred while processing this directive]




Dynamic NAT:




!--- Interface configuration

interface GigabitEthernet0/0/0

 vrf forwarding VRF_LEFT

 ip address 192.168.1.2 255.255.255.0

 ip nat inside


interface vasileft1

 vrf forwarding VRF_LEFT

 ip address 10.1.1.1 255.255.255.252

 ip nat outside



!--- Static route configuration

ip route vrf VRF_LEFT 172.16.0.0 255.255.0.0 vasileft1 10.1.1.2

ip route vrf VRF_RIGHT 172.16.1.5 255.255.255.255 vasiright1 10.1.1.1



!--- Access-list configuration

Extended IP access list 100

 10 permit tcp 192.168.1.0 0.0.0.255 host 172.16.1.1

 20 permit udp 192.168.1.0 0.0.0.255 host 172.16.1.1

 30 permit icmp 192.168.1.0 0.0.0.255 host 172.16.1.1



!--- NAT configuration

ip nat pool POOL 172.16.1.5 172.16.1.5 prefix-length 24

ip nat inside source list 100 pool POOL vrf VRF_LEFT overload
[an error occurred while processing this directive]




Verification: 



Bombay#sh ip nat translations vrf VRF_LEFT

Pro      Inside global      Inside local        Outside local    Outside global

icmp     172.16.1.5:1       192.168.1.1:4       172.16.1.1:4     172.16.1.1:1

tcp      172.16.1.5:1024    192.168.1.1:27593   172.16.1.1:23    172.16.1.1:23

Total number of translations: 2 
[an error occurred while processing this directive]










Verify



    

  1. Check if dynamic/static routes are configured to route traffic between the two VRF instances.
    2. 

  2. Check if NAT has been configured for the correct VRF.
    3. 









Troubleshoot



There is currently no specific troubleshooting information available for this configuration.






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Contributed by Cisco Engineers

  
  

  
    
    
  • Rohit Nair
    Cisco TAC Engineer
    • 
    
    
  






            

        
            
            




















  
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