There are some common services (such as database servers and application servers) that multiple virtual networks need to access. Shared services are beneficial because:

• Services are usually not duplicated for each group.
• Sharing services is economical.
• Sharing services is efficient and manageable.
• Policies can be centrally deployed.

To achieve route separation, you could replicate the service, either physically or virtually, one service for each virtual network. However, that solution might not be cost effective or feasible. For a router that supports EVN, the solution is to perform route replication and route redistribution.

Route replication allows shared services because when routes are replicated between virtual networks, clients who reside in one virtual network can reach prefixes that exist in another virtual network.

A shared services approach works best for DNS, DHCP, and corporate communications. It is not a solution for sharing access to an internet gateway.

Sharing servers in VRF-Lite requires route distinguishers (RDs), route targets with importing and exporting, and configuring BGP.

In an EVN environment, shared services are achieved with route replication, which is a simple deployment. Route replication requires no BGP, no RD, no route targets, and no import or export.

In summary, the BGP import and export method of copying routes between VRFs works with both VRF-Lite and EVN. However, route replication is the simpler alternative to enable sharing of common services across multiple virtual networks.

With shared service, clients and servers are located in different virtual networks. To achieve connectivity between clients and servers, routes must be exchanged among virtual networks. Such route exchanges among VRFs are accomplished in two different ways, depending on whether VRF-Lite or EVN is implemented:

• In VRF-Lite, route leaking is achieved, via BGP, by using the route import/export feature.
• In EVN, route replication is supported directly by the RIB; there is no dependency on BGP. After routes are replicated from a different virtual network, those routes are propagated across each virtual network through existing redistribution into the Interior Gateway Protocol (IGP).

In the following route replication scenario, a router has two VRFs named Services and User-A. OSPF is configured:

router ospf 99 vrf services
 network 126.1.0.0 0.0.255.255 area 0
!
router ospf 98 vrf user-a
 network 126.1.0.0 0.0.255.255 area 0

Furthermore, route replication is configured for VRF User-A:

vrf definition user-a
 !
 address-family ipv4
 route-replicate from vrf services unicast ospf 99
 exit-address-family

In the scenario, the following RIB for the VRF Services contains four routes, three of which are replicated to the RIB for VRF User-A. Route replication creates a link to the source RIB, as shown in the figure below.

Configuring route replication allows mutual redistribution between virtual IP networks. In the case of shared services, you would configure route replication within the VRF that needs access to the shared services. Within each route-replicate command, you can optionally filter out routes with a route map to prevent a routing loop. That is, you do not want to redistribute routes back into the original routing protocol. You do not want a native route to show up as a replicated route.

• Where to Implement Route Replication
  

This section describes the behavior of route replication for EVN, which differs from the behavior for Multi-Topology Routing. In the EVN environment:

• The route-replicate command is accepted only under the address-family ipv4 command, which is configured under the vrf definition command.
• The route-replicate command replicates routes into the base topology within the specified address family.
• If all is specified as a source protocol, only one route-replicate command is allowed per VRF for a given destination topology.
• The no route-replicate command is allowed to exclude a source protocol.
• If all is specified as a source protocol, then connected routes are replicated (unlike in the Multi-Topology Routing version of the route-replicate command).
• A replicated route inherits the administrative distance and source protocol of the source route.

If a route is replicated, the following rules determine route preference:

1. If two routes are owned by the same protocol (for example, both are EIGRP routes), and both routes have the same source VRF and one of the routes is NOT replicated, then the non-replicated route is preferred.

If the above rule does not apply, the following rules determine route preference, in this order:

1. Prefer the route with smaller administrative distance.
2. Prefer the route with smaller default administrative distance.
3. Prefer a non-replicated route over a replicated route.
4. Compare original vrf-names. Prefer the route with the lexicographically smaller vrf-name.
5. Compare original sub-address-families: Prefer unicast over multicast.
6. Prefer the oldest route.

Perform this task to replicate routes from one VRF to another. The examples within this task table are based on the figure below.

In this particular task, routes from VRF SERVICES are replicated to both VRF RED and VRF GREEN, and VRF RED and VRF GREEN are not allowed to share routes between them. In order to allow bidirectional traffic, routes from VRF RED and VRF GREEN are also replicated to VRF SERVICES.

Note	In a real EVN environment, there would also be route replication between VRF SERVICES and a third VRF, and maybe more VRFs. Such replication is left out of the following configuration task for the sake of brevity.

1.    enable

2.    configure terminal

3.    vrf definition vrf-name

4.    vnet tag number

5.    description string

6.    address-family ipv4

7.    exit

8.    exit

9.    vrf definition vrf-name

10.    vnet tag number

11.    description string

12.    address-family ipv4

13.    exit

14.    exit

15.    interface type number

16.    vrf forwarding vrf-name

17.    ip address ip-address mask

18.    no shutdown

19.    exit

20.    router ospf process-id vrf vrf-name

21.    network ip-address wildcard-mask area area-id

22.    exit

23.    router ospf process-id [vrf vrf-name]

24.    network ip-address wildcard-mask area area-id

25.    exit

26.    vrf definition vrf-name

27.    address-family ipv4

28.    route-replicate from [vrf vrf-name] {multicast| unicast} {all| protocol-name} [route-map map-tag]

29.    exit

30.    exit

31.    vrf definition vrf-name

32.    address-family ipv4

33.    route-replicate from [vrf vrf-name] {multicast| unicast} {all| protocol-name} [route-map map-tag]

34.    end

35.    show ip route vrf vrf-name

• Example
  
• What to Do Next
  

This task is based on the assumption that you also performed the task, Configuring Route Replication to Share Services in EVN.

The figure below shows the same networks we used in the figure above. In this task, we perform redistribution on Router 3 so that Router 1 and Router 2 have a route to the shared service residing on 192.168.1.1.

1.    enable

2.    configure terminal

3.    router ospf process-id vrf vrf-name

4.    redistribute vrf vrf-name ospf process-id subnets

5.    redistribute vrf vrf-name ospf process-id subnets

6.    exit

7.    router ospf process-id vrf vrf-name

8.    redistribute vrf vrf-name ospf process-id subnets

9.    exit

10.    router ospf process-id vrf vrf-name

11.    redistribute vrf vrf-name ospf process-id subnets

12.    end

13.    show ip route vrf vrf-name