MPLS VPN Basic Configuration

2017-11-03 09:05:09

This section outlines the generic configurations required on the routers in the service provider domain to implement MPLS VPN. The configurations of the PE and P routers will be covered in this section. The subsequent sections in this chapter delve into each of the configuration blocks on the PE and P routers alone. The configurations required to implement PE-CE routing sessions are discussed in Chapters 4 through 6, depending on the PE-CE protocol in use.

All configurations outlined in the following sections are performed in the network shown in Figure 3-11. For simplicity, only connected networks that are part of the VRF will be redistributed into the MP-BGP processes.

Figure 3-11. Network Topology: MPLS VPN PE and P Configuration

The topology in Figure 3-11 attempts to implement a simple intranet VPN between two sites belonging to Customer A, site 1 and site 2. The customer network consists of the CE routers CE1-A and CE2-A. In addition, two loopbacks (loopback 1) on PE1-AS1 and PE2-AS1 will be configured as part of the VRF CustomerA and be redistributed into the MP-BGP routing contexts.

Configuration of CE Routers

The configuration of route exchange between PE and CE routers involves the implementation of a routing protocol (or static/default routes) on the CE routers. No specific configuration other than the regular routing protocol configuration is required on the CE routers. On the PE router, VRF routing contexts (or address family contexts) are required for route exchange between the PE and CE. These routes are then mutually redistributed with the MP-BGP process per VRF. Configurations for the above based on protocol choice between PE and CE will be covered in Chapters 4 through 6.

Configuring MPLS Forwarding and VRF Definition on PE Routers

Configuring MPLS forwarding is the first step to provision the service provider's MPLS VPN backbone. This step ensures the service provider's readiness to provide MPLS-related services to prospective customers. At a minimum, the steps to configure MPLS forwarding on PE routers are

Step 1.

Enable CEF.

Step 2.

Configure IGP routing protocol on the PE router.

Step 3.

Configure MPLS or label forwarding on the PE interfaces connected to P.

These steps have already been discussed in Chapters 1 and 2 and thus have not been shown.

In this section, we configure VRFs on the PE routers. Figure 3-12 shows the configuration steps on the PE routers to configure VRF definition.

Figure 3-12. VRF Definition on PE Routers: Configuration Steps

Step 1.	Configure VRF on PE router – Configure the VRF CustomerA on PE1 and PE2-AS1 router. This results in the creation of a VRF routing table and a Cisco Express Forwarding (CEF) table for CustomerA. Example 3-1 shows CustomerA VRF being configured on PE1-AS1 router. Note the VRF name is case sensitive. Example 3-1. VRF Definition PE1-AS1(config)#ip vrf CustomerA Note that creation or deletion of a VRF results in removal of the IP address from the interface. Example 3-2 illustrates the message that occurs on VRF deletion. Example 3-2. VRF Deletion PE1-AS1(config-vrf)#no ip vrf CustomerA % IP addresses from all interfaces in VRF CustomerA have been removed
Step 2.	Configure the RD – The RD creates routing and forwarding tables. The RD is added to the beginning of the customer's IPv4 prefixes to convert them into globally unique VPNv4 prefixes. Example 3-3 shows the configuration for defining the RD under the VRF. Example 3-3. Configuring VRF Parameters: RD PE1-AS1(config-vrf)#rd 1:100 The RD can be used in either of these formats: - 16-bit AS number: Your 32-bit number (for example, 1:100) - 32-bit IP address: Your 16-bit number (for example, 10.10.10.101:1) RD for an existing VRF can be changed only after deletion of that VRF. Example 3-4 illustrates the concept. Example 3-4. Redefining VRF RD Value PE1-AS1(config)#ip vrf CustomerA PE1-AS1(config-vrf)#rd 1:100 % Do "no ip vrf " before redefining the VRF RD has to be unique for that particular VRF. No two VRFs on the same router can have similar RD. Trying to set the same RD on the VRF on the same router results in the message shown in Example 3-5. Example 3-5. RD Uniqueness PE1-AS1(config)#ip vrf CustomerA PE1-AS1(config-vrf)#rd 1:100 % Cannot set RD, check if it's unique
Step 3.	Configure the import and export policy – Configure the import and export policy for the MP-BGP extended communities. The policy is used for filtering routes for that particular RT. Example 3-6 provides the relevant configuration for defining import and export policy. Example 3-6. Configuring VRF Parameters: RT PE1-AS1(config-vrf)#route-target both 1:100 The both keyword in the previous command results in the configuration of import and export policy, and the configuration output is shown in Example 3-7. Example 3-7. RT Configuration Options PE1-AS1#sh run Building configuration... ip vrf CustomerA rd 1:100 route-target export 1:100 route-target import 1:100
Step 4.	Associate VRF with the interface – Associate virtual routing/forwarding instance (VRF) with an interface or subinterface in this CustomerA. Associating the VRF to an interface results in removal of the IP address from that interface. This is only if VRF was associated to an interface that had the IP address already configured. This means that the IP address will have to be reconfigured after the VRF is associated with that interface. Example 3-8 shows the configuration for associating the VRF to an interface. Example 3-9 shows the removal of the IP address when no ip vrf forwarding vrfname is configured on the interface. Example 3-8. Associating VRF with Interface PE1-AS1(config)#interface serial4/0 PE1-AS1(config-if)#ip add 172.16.1.1 255.255.255.252 PE1-AS1(config-if)# ip vrf forwarding CustomerA % Interface Serial4/0 IP address 172.16.1.1 removed due to enabling VRF CustomerA PE1-AS1(config-if)#ip add 172.16.1.1 255.255.255.252 Example 3-9. VRF Association to Interface IP Address PE1-AS1(config-if)#no ip vrf forwarding CustomerA % Interface Serial4/0 IP address 172.16.1.1 removed due to disabling VRF CustomerA

Final VRF Configuration on PE1-AS1 Router

Example 3-10 shows the VRF configuration on the PE1-AS1 router.

Example 3-10. VRF Configuration of PE1-AS1

ip vrf CustomerA rd 1:100 route-target export 1:100 route-target import 1:100 ! interface Serial1/0 description PE-CE link to CE1-A ip vrf forwarding CustomerA ip address 172.16.1.1 255.255.255.0 ! Interface Loopback1 ip vrf forwarding CustomerA ip address 172.16.100.1 255.255.255.255

Verification of VRF Configuration on PE Routers

The show ip vrf command is used to verify if the correct VRF exists on the interface. Example 3-11 indicates that the correct VRF CustomerA is configured on the Serial1/0 interface on the PE1 router.

Example 3-11. show ip vrf on PE1-AS1

PE1-AS1#show ip vrf Name Default RD Interfaces CustomerA 1:100 Se1/0 Lo1

The show ip vrf interfaces command provides the listing of interfaces that are activated for a particular VRF. Example 3-12 shows that Serial1/0 is active for VRF VRF-Static.

Example 3-12. show ip vrf interfaces on PE1-AS1

PE1-AS1#show ip vrf interfaces Interface IP-Address VRF Protocol Serial1/0 172.16.1.1 CustomerA up Lo1 172.16.100.1 CustomerA up

Configuration of BGP PE-PE Routing on PE Routers

Configuring BGP PE-PE routing between the PE routers is the next step in an MPLS VPN deployment. The purpose of this step is to ensure that VPNv4 routes can be transported across the service provider backbone using MP-iBGP. The P router is transparent to this entire process and, therefore, does not carry any customer routes. Figure 3-13 illustrates the steps for configuring BGP PE-PE routing sessions between the PE routers.

Figure 3-13. BGP PE-PE Routing Configuration Steps

Step 1.	Configure BGP routing on PE routers – Enable BGP routing and identify the AS on the PE1-AS1 and PE2-AS1 routers. Example 3-13 highlights the configuration. Example 3-13. Configuring BGP Routing on PE Routers PE1-AS1(config)#router bgp 1 ________________________________________________________________ PE2-AS1(config)#router bgp 1
Step 2.	Configure the MP-iBGP neighbors – Configure the remote MP-iBGP neighbor and use the loopback interface as the source of BGP messages and updates. Note that you have to use the update-source command only when the neighbor is peering to your loopback address. This is irrespective of whether it is an iBGP or eBGP neighbor. Example 3-14 shows the configuration for the PE1-AS1 and PE2-AS1 router. Example 3-14. Configuring MP-iBGP Neighbors PE1-AS1(config-router)#neighbor 10.10.10.102 remote-as 1 PE1-AS1(config-router)#neighbor 10.10.10.102 update-source loopback0 ________________________________________________________________ PE2-AS1(config-router)#neighbor 10.10.10.101 remote-as 1 PE2-AS1(config-router)#neighbor 10.10.10.101 update-source loopback0
Step 3.	Configure the VPNv4 address family – Configure the address family for VPNv4 under the BGP configuration process. This step allows you to enter the VPNv4 address family to activate the VPNv4 neighbors. Activate the iBGP neighbor, which is essential for transporting VPNv4 prefixes across the service provider backbone. Using next-hop-self is optional and is primarily used when the service provider has an eBGP PE-CE routing with the customers, because internal BGP (iBGP) sessions preserve the next-hop attribute learned from eBGP peers, which is why it is important to have an internal route to the next hop. Otherwise, the BGP route is unreachable. To make sure you can reach the eBGP next hop, include the network that the next hop belongs to in the IGP or use the next-hop-self neighbor command to force the router to advertise itself, rather than the external peer, as the next hop. In addition, configure the propagation of the extended communities with BGP routes so as to enable RT propagation, which identifies the VPNs that the routes have to be imported into. The configuration of the VPNv4 address family for PE1-AS1 and PE2-AS1 is shown in Example 3-15. Note that on some versions of IOS, adding the neighbor for VPNv4 route exchange using the neighbor ip-address activate command also automatically adds the neighbor ip-address send-community extended command. If the neighbor needs to be configured for both standard and extended community exchange, you will explicitly have to configure the neighbor ip-address send-community both command under the VPNv4 address family. Example 3-15. Configuring BGP VPNv4 Address Family PE1-AS1(config-router)#address-family vpnv4 PE1-AS1(config-router-af)# neighbor 10.10.10.102 activate PE1-AS1(config-router-af)# neighbor 10.10.10.102 send-community extended _________________________________________________________________________ PE2-AS1(config-router)#address-family vpnv4 PE2-AS1(config-router-af)# neighbor 10.10.10.101 activate PE2-AS1(config-router-af)# neighbor 10.10.10.101 send-community extended
Step 4.	Configure the IPv4 address family – Configure the peer VRF IPv4 address family under the BGP configuration process. This step allows you to enter the IPv4 networks that will be converted to VPNv4 routes in MP-BGP updates. In Chapters 4, 5, and 6, the individual PE-CE routing protocol interaction configuration involving redistribution of PE-CE routing protocol contexts or instances will be configured in the IPv4 address family per VRF under the BGP process. For simplicity, redistribution of all connected networks is configured into the MP-BGP process. Example 3-16 shows the configuration on PE1-AS1 and PE2-AS1 routers. Example 3-16. Configuring BGP per VRF IPv4 Address Family (Routing Context) PE1-AS1(config-router)#address-family ipv4 vrf CustomerA PE1-AS1(config-router-af)# redistribute connected PE1-AS1(config-router-af)# exit-address-family _______________________________________________________________ PE2-AS1(config-router)#address-family ipv4 vrf CustomerA PE2-AS1(config-router-af)# redistribute connected PE2-AS1(config-router-af)# exit-address-family

BGP PE-PE Routing Final Configuration on PE1-AS1 and PE2-AS1 Router

Example 3-17 shows the final BGP PE-PE routing configuration on the PE1-AS1 and PE2-AS1 router.

Example 3-17. BGP PE-PE Configurations of PE1-AS1 and PE2-AS1 Routers

!PE1-AS1 Router: router bgp 1 no synchronization neighbor 10.10.10.102 remote-as 1 no auto-summary ! address-family vpnv4 neighbor 10.10.10.102 activate neighbor 10.10.10.102 send-community extended exit-address-family ! address-family ipv4 vrf CustomerA redistribute connected no auto-summary no synchronization exit-address-family __________________________________________________________________________ !PE2-AS1 Router: router bgp 1 no synchronization bgp log-neighbor-changes neighbor 10.10.10.101 remote-as 1 neighbor 10.10.10.101 update-source Loopback0 no auto-summary ! address-family vpnv4 neighbor 10.10.10.101 activate neighbor 10.10.10.101 send-community extended exit-address-family ! address-family ipv4 vrf CustomerA redistribute connected no auto-summary no synchronization exit-address-family

Verification and Monitoring of BGP PE-PE Routing on PE Routers

After configuring BGP PE-PE routing between the PE routers, you can verify that the MP-iBGP neighbors are operational by issuing any of the following commands:

show ip bgp vpnv4 * summary

show IP bgp vpnv4 all

show ip bgp summary

show ip bgp neighbor ip-address

Example 3-18 shows that the VPNv4 neighbor relationship is formed.

Example 3-18. VPN Neighbor Relationship Verification

PE1#show ip bgp vpnv4 all summary BGP router identifier 10.10.10.101, local AS number 1 BGP table version is 7, main routing table version 7 Neighbor V AS MsgRcvd MsgSent TblVer InQ OutQ Up/Down State/PfxRcd 10.10.10.102 4 1 202 200 7 0 0 00:00:39 0 __________________________________________________________________________________ PE2#show ip bgp vpnv4 all summary BGP router identifier 10.10.10.102, local AS number 1 BGP table version is 1, main routing table version 1 Neighbor V AS MsgRcvd MsgSent TblVer InQ OutQ Up/Down State/PfxRcd 10.10.10.101 4 1 11 11 1 0 0 00:07:16 0

Configuration of P Router

No special configurations need to be performed on the P routers P1-AS1 and P1-AS2 for MPLS VPN support. Because the P routers only participate in MPLS labeled packet forwarding, the only requirements are those of an LSR in an MPLS network, namely, IGP for NLRI exchange and LDP for label assignment and distribution. As always, CEF needs to be enabled on all interfaces configured for MPLS forwarding. Configuration of the P1-AS1 router is shown in Example 3-19.

Example 3-19. P1-AS1 Configuration

mpls ldp router-id loopback0 ! interface Serial0/0 ip address 10.10.10.2 255.255.255.252 mpls ip ! interface Serial1/0 ip address 10.10.10.5 255.255.255.252 mpls ip ! Interface loopback0 ip address 10.10.10.200 255.255.255.255 ! router ospf 1 network 10.0.0.0 0.255.255.255 area 0 !

Label Verification and Control and Data Plane Operation

After configuring devices in the network as per the previous steps, the verification of label allocation and propagation can be performed on the PE and P routers using the commands described in Figure 3-14.

Figure 3-14. Label Allocation Verification and Control/Data Plane Operation

The control plane and data plane operation for network 172.16.100.1 as part of VRF CustomerA is depicted in Figure 3-14. Note that the outgoing label mapped to prefix 172.16.100.1 on PE1-AS1 is aggregate and not untagged. For all networks that are directly connected to the PE router (like loopbacks or interface IP networks) that are part of a VRF, the outgoing label mapped in the LFIB is the aggregate label. If, however, the incoming VPN packet is to be forwarded to a next-hop address (like that of a connected CE router), the outgoing label mapping is untagged. Thus, aggregate and untagged labels that were explained in Chapter 1 are encountered in MPLS VPN implementations.

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