VRFs, or ××× Routing and Forwarding instances, are most commonly associated with MPLS service providers. In such networks, MPLS encapsulation is used to isolate individual customers' traffic and an independent routing table (VRF) is maintained for each customer. Most often, MP-BGP is employed to facilitate complex redistribution schemes to import and export routes to and from VRFs to provide Internet connectivity.
However, VRF configuration isn't at all dependent on MPLS (the two components just work well together). In Cisco terminology, deployment of VRFs without MPLS is known as VRF lite, and this article discusses a scenario where such a solution could come in handy.
Assume the topology illustrated below is a network owned by an enterprise. As you would expect, normal company traffic must pass through the firewall so that company policy can be enforced. However, this a secondary Internet connection has been added to this network: an unrestricted ADSL circuit designated for guests visiting the company campus. The 10.0.0.0/16 network is used for trusted traffic, and 192.168.0.0/16 is used for guest traffic.
All router interfaces which provide transport for both types of traffic have been configured with two subinterfaces performing 802.1Q encapsulation; .10 for VLAN 10 (blue) and .20 for VLAN 20 (red). Note that although 802.1Q encapsulation is used to tag frames across the link, each link is a routed segment with an IP interface at either end. For example, here is R1's F2/0 configuration (the complete config of all three core routers is attached at the end of the article if you'd like to skip ahead):
interface FastEthernet2/0 description R2 no ip address ! interface FastEthernet2/0.10 encapsulation dot1Q 10 ip address 10.0.12.1 255.255.255.252 ! interface FastEthernet2/0.20 encapsulation dot1Q 20 ip address 192.168.12.1 255.255.255.252
My note: "encapsulation dot1q xx" should be configured under subinterfaces on routers which only has a physical link to others.
If this were a generic routed network, the network admin would be busy touching up his or her resume right now. Obviously, the addition of a "back door" Internet access link opens a huge security hole, but we can employ VRFs here to segment the single physical infrastructure into two virtual, isolated networks. VRFs employ essentially the same concept as VLANs and trunking, but at layer three.
VRF lite is simple: each routed interface (whether physical or virtual) belongs to exactly one VRF. Unless import/export maps have been applied, routes (and therefore packets) cannot move from one VRF to another, much like the way VLANs work at layer two. Packets entering VRF A can only follow routes in routing table A, as we'll see shortly.
Prior to VRF configuration, all routers have all of their connected routes in the global routing table as you would expect:
R1# show ip route
Codes: C - connected, S - static, R - RIP, M - mobile, B - BGP
D - EIGRP, EX - EIGRP external, O - OSPF, IA - OSPF inter area
N1 - OSPF NSSA external type 1, N2 - OSPF NSSA external type 2
E1 - OSPF external type 1, E2 - OSPF external type 2
i - IS-IS, su - IS-IS summary, L1 - IS-IS level-1, L2 - IS-IS level-2
ia - IS-IS inter area, * - candidate default, U - per-user static route
o - ODR, P - periodic downloaded static route
Gateway of last resort is not set
192.168.12.0/30 is subnetted, 1 subnets
C 192.168.12.0 is directly connected, FastEthernet2/0.20
192.168.13.0/30 is subnetted, 1 subnets
C 192.168.13.0 is directly connected, FastEthernet2/1.20
10.0.0.0/30 is subnetted, 3 subnets
C 10.0.12.0 is directly connected, FastEthernet2/0.10
C 10.0.13.0 is directly connected, FastEthernet2/1.10
C 10.0.0.0 is directly connected, FastEthernet1/1
192.168.0.0/30 is subnetted, 1 subnets
C 192.168.0.0 is directly connected, FastEthernet1/0
To begin, let's create VRFs BLUE and RED on R1:
R1(config)# ip vrf BLUE R1(config-vrf)# description Trusted Traffic R1(config-vrf)# ip vrf RED R1(config-vrf)# description Guest Traffic
Certainly not the most challenging of tasks, eh? Next, we'll add interface F1/0 (which connects to the guest-use ADSL uplink) to VRF RED:
R1(config)# int f1/0 R1(config-if)# ip vrf forwarding RED % Interface FastEthernet1/0 IP address 192.168.0.2 removed due to enabling VRF RED
Wait a tick, what just happened? When assigning an interface to a VRF, IOS automatically deletes any preconfigured IP address to remove that route from the global table. Now when an IP address is assigned to this interface, its network gets added to the specific routing table for that VRF.
So, we reapply our IP to F1/0 and verify that its configuration is complete:
R1(config-if)# ip add 192.168.0.2 255.255.255.252 R1(config-if)# ^Z R1# show run interface f1/0 Building configuration...
Current configuration : 137 bytes ! interface FastEthernet1/0 description RX ip vrf forwarding RED ip address 192.168.0.2 255.255.255.252 duplex auto speed auto end
But look at our routing table now:
R1# show ip route
[...]
192.168.12.0/30 is subnetted, 1 subnets
C 192.168.12.0 is directly connected, FastEthernet2/0.20
192.168.13.0/30 is subnetted, 1 subnets
C 192.168.13.0 is directly connected, FastEthernet2/1.20
10.0.0.0/30 is subnetted, 3 subnets
C 10.0.12.0 is directly connected, FastEthernet2/0.10
C 10.0.13.0 is directly connected, FastEthernet2/1.10
C 10.0.0.0 is directly connected, FastEthernet1/1
The 192.168.0.0/30 route is gone from the global table; it now resides in the VRF RED table, which we have to inspect separately by appending the vrf argument to show ip route:
R1# show ip route vrf RED
[...]
192.168.0.0/30 is subnetted, 1 subnets
C 192.168.0.0 is directly connected, FastEthernet1/0
As you can imagine, this extra step of reapplying an IP address must be repeated for every interface we add to a VRF. I'll spare you the monotony of line-by-line configs and instead present the relevant finished configuration of R1:
interface FastEthernet1/0 description RX ip vrf forwarding RED ip address 192.168.0.2 255.255.255.252 ! interface FastEthernet1/1 description FW ip vrf forwarding BLUE ip address 10.0.0.2 255.255.255.252 ! interface FastEthernet2/0 description R2 no ip address ! interface FastEthernet2/0.10 encapsulation dot1Q 10 ip vrf forwarding BLUE ip address 10.0.12.1 255.255.255.252 ! interface FastEthernet2/0.20 encapsulation dot1Q 20 ip vrf forwarding RED ip address 192.168.12.1 255.255.255.252 ! interface FastEthernet2/1 description R3 no ip address ! interface FastEthernet2/1.10 encapsulation dot1Q 10 ip vrf forwarding BLUE ip address 10.0.13.1 255.255.255.252 ! interface FastEthernet2/1.20 encapsulation dot1Q 20 ip vrf forwarding RED ip address 192.168.13.1 255.255.255.252
As all interfaces now belong to isolated VRFs, our global routing table is completely empty. We can verify that all 10.0.0.0/16 routes are stored in VRF BLUE, and all 192.168.0.0/16 routes are stored in VRF RED:
R1# show ip route vrf BLUE
Routing Table: BLUE
[...]
10.0.0.0/30 is subnetted, 3 subnets
C 10.0.12.0 is directly connected, FastEthernet2/0.10
C 10.0.13.0 is directly connected, FastEthernet2/1.10
C 10.0.0.0 is directly connected, FastEthernet1/1
R1# show ip route vrf RED
Routing Table: RED
[...]
192.168.12.0/30 is subnetted, 1 subnets
C 192.168.12.0 is directly connected, FastEthernet2/0.20
192.168.13.0/30 is subnetted, 1 subnets
C 192.168.13.0 is directly connected, FastEthernet2/1.20
192.168.0.0/30 is subnetted, 1 subnets
C 192.168.0.0 is directly connected, FastEthernet1/0
At this point, although only R1 has been configured with VRFs, it can still route traffic to R2 and R3 with no problem. This is because, like VLANs, VRFs are only locally significant to the router.
After tediously configuring VRFs on the other two routers in the same manner, we can now configure our IGP. For this example, we'll be running an OSPF instance per VRF. We do this by appending the vrf argument to each router statement:
R1(config)# router ospf 1 vrf BLUE R1(config-router)# router-id 0.0.1.1 R1(config-router)# network 10.0.0.0 0.0.255.255 area 0 R1(config-router)# router ospf 2 vrf RED R1(config-router)# router-id 0.0.1.2 R1(config-router)# network 192.168.0.0 0.0.255.255 area 0
These are completely independent OSPF processes; as such, a unique router ID must be used for each. (If you're used to using IPv4 addresses as router ID, the IDs used above might seem strange. Remember that the OSPF router ID is in fact an arbitrary 32-bit value simply expressed in dotted-decimal. Here, the third "octet" represents the router number and the fourth octet represents the VRF.)
After configuring the other two routers with two OSPF processes each, we see two adjacencies formed per link, one per VRF:
R1# show ip ospf neighbor Neighbor ID Pri State Dead Time Address Interface 0.0.3.2 1 FULL/DR 00:00:39 192.168.13.2 FastEthernet2/1.20 0.0.2.2 1 FULL/DR 00:00:39 192.168.12.2 FastEthernet2/0.20 0.0.3.1 1 FULL/DR 00:00:31 10.0.13.2 FastEthernet2/1.10 0.0.2.1 1 FULL/DR 00:00:32 10.0.12.2 FastEthernet2/0.10
Assuming our edge routers aren't running OSPF, we'll also create two static default routes on R1, one for each VRF:
R1(config)# ip route vrf BLUE 0.0.0.0 0.0.0.0 10.0.0.1 R1(config)# ip route vrf RED 0.0.0.0 0.0.0.0 192.168.0.1
By now you've probably deduced that VRF configuration mostly consists of appending a vrf keyword to certain commands where appropriate. Unfortunately the argument isn't inserted at the same point in all commands, so it may take a few queries of the context-sensitive help before you get them all down.
We can verify that our static routes exist along with their OSPF-leanred companions in their respective VRFs:
R1# show ip route vrf BLUE
Routing Table: BLUE
[...]
10.0.0.0/8 is variably subnetted, 7 subnets, 2 masks
C 10.0.12.0/30 is directly connected, FastEthernet2/0.10
C 10.0.13.0/30 is directly connected, FastEthernet2/1.10
O 10.0.2.0/24 [110/2] via 10.0.12.2, 00:04:52, FastEthernet2/0.10
O 10.0.3.0/24 [110/2] via 10.0.13.2, 00:04:52, FastEthernet2/1.10
C 10.0.0.0/30 is directly connected, FastEthernet1/1
O 10.0.1.0/24 [110/2] via 10.0.12.2, 00:04:52, FastEthernet2/0.10
O 10.0.23.0/30 [110/2] via 10.0.13.2, 00:04:52, FastEthernet2/1.10
[110/2] via 10.0.12.2, 00:04:52, FastEthernet2/0.10
S* 0.0.0.0/0 [1/0] via 10.0.0.1
R1# show ip route vrf RED
Routing Table: RED
[...]
192.168.12.0/30 is subnetted, 1 subnets
C 192.168.12.0 is directly connected, FastEthernet2/0.20
192.168.13.0/30 is subnetted, 1 subnets
C 192.168.13.0 is directly connected, FastEthernet2/1.20
192.168.23.0/30 is subnetted, 1 subnets
O 192.168.23.0 [110/2] via 192.168.13.2, 00:04:16, FastEthernet2/1.20
[110/2] via 192.168.12.2, 00:04:16, FastEthernet2/0.20
192.168.0.0/30 is subnetted, 1 subnets
C 192.168.0.0 is directly connected, FastEthernet1/0
O 192.168.1.0/24 [110/2] via 192.168.12.2, 00:04:16, FastEthernet2/0.20
O 192.168.2.0/24 [110/2] via 192.168.12.2, 00:04:16, FastEthernet2/0.20
O 192.168.3.0/24 [110/2] via 192.168.13.2, 00:04:17, FastEthernet2/1.20
S* 0.0.0.0/0 [1/0] via 192.168.0.1
Finally we just need to advertise a default route in both OSPF processes from R1 so R2 and R3 can learn them:
R1(config)# router ospf 1 R1(config-router)# default-information originate R1(config-router)# router ospf 2 R1(config-router)# default-information originate
Note that when entering OSPF process configuration, we no longer need to append the vrf keyword as it has already been applied.
Over on R2, we see that each VRF now has its own complete routing table:
R2# show ip route vrf BLUE
Routing Table: BLUE
[...]
10.0.0.0/8 is variably subnetted, 7 subnets, 2 masks
C 10.0.12.0/30 is directly connected, FastEthernet1/0.10
O 10.0.13.0/30 [110/2] via 10.0.23.2, 00:14:23, FastEthernet1/1.10
[110/2] via 10.0.12.1, 00:13:53, FastEthernet1/0.10
C 10.0.2.0/24 is directly connected, FastEthernet2/1.10
O 10.0.3.0/24 [110/2] via 10.0.23.2, 00:14:23, FastEthernet1/1.10
O 10.0.0.0/30 [110/2] via 10.0.12.1, 00:13:53, FastEthernet1/0.10
C 10.0.1.0/24 is directly connected, FastEthernet2/0.10
C 10.0.23.0/30 is directly connected, FastEthernet1/1.10
O*E2 0.0.0.0/0 [110/1] via 10.0.12.1, 00:03:33, FastEthernet1/0.10
R2# show ip route vrf RED
Routing Table: RED
[...]
192.168.12.0/30 is subnetted, 1 subnets
C 192.168.12.0 is directly connected, FastEthernet1/0.20
192.168.13.0/30 is subnetted, 1 subnets
O 192.168.13.0 [110/2] via 192.168.23.2, 00:36:59, FastEthernet1/1.20
[110/2] via 192.168.12.1, 00:20:54, FastEthernet1/0.20
192.168.23.0/30 is subnetted, 1 subnets
C 192.168.23.0 is directly connected, FastEthernet1/1.20
192.168.0.0/30 is subnetted, 1 subnets
O 192.168.0.0 [110/2] via 192.168.12.1, 00:20:54, FastEthernet1/0.20
C 192.168.1.0/24 is directly connected, FastEthernet2/0.20
C 192.168.2.0/24 is directly connected, FastEthernet2/1.20
O 192.168.3.0/24 [110/2] via 192.168.23.2, 00:41:13, FastEthernet1/1.20
O*E2 0.0.0.0/0 [110/1] via 192.168.12.1, 00:01:41, FastEthernet1/0.20
At this point our two VRFs are fully functional! A packet from a host on the BLUE VLAN on switch 2, for example, enters the BLUE VRF subinterface on R2 and gets routed via R1's BLUE VRF out to the firewall. Note that for troubleshooting actions (like pinging) you must specify a VRF:
R2# ping 10.0.0.1 Type escape sequence to abort. Sending 5, 100-byte ICMP Echos to 10.0.0.1, timeout is 2 seconds: ..... Success rate is 0 percent (0/5) R2# ping vrf BLUE 10.0.0.1 Type escape sequence to abort. Sending 5, 100-byte ICMP Echos to 10.0.0.1, timeout is 2 seconds: !!!!! Success rate is 100 percent (5/5), round-trip min/avg/max = 12/15/20 ms
Below are the final configurations from all three routers.
- R1.txt
- R2.txt
- R3.txt