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BGP PE-CE Routing Protocol Overview, Configuration, and VerificationBGP version 4 (BGP4) is the current de facto Internet standard for inter-domain (AS) exterior routing. In MPLS VPN networks, MP-BGP is used and plays a pivotal role in the transportation of VPNv4 prefixes across the service provider network. In traditional environments, customer networks prefer to use BGP in their networks and, therefore, use BGP as a PE-CE routing protocol when migrating from a non-MPLS based to an MPLS VPN based network. This helps the customer establish a consistent end-to-end routing policy. In an MPLS VPN network, BGP attributes for a VPN site are transparently transported across the service provider backbone to another site in the same VPN. Because there is a single routing protocol used across the VPN between service provider core and customer sites, the concept of redistribution does not apply. BGP PE-CE peering in an MPLS VPN environment can be performed in two different ways:
In the MPLS VPN network shown in Figure 6-1, an MPLS VPN service provider plans to provision BGP PE-CE routing protocol for two customers, Customers A and B. CE1-A and CE2-A are CE devices belonging to Customer A located at Site 1 and Site 2, respectively. They are part of the same VPN, VPN-A. These CE devices are connected to provider edge devices PE1-AS1 and PE2-AS1 in the service provider network. CE1-A belongs to AS 65001 and CE2-A belongs to AS 65002. For Customer B network, CE1-A and CE2-A are the CE devices located at Site 1 and Site 2, respectively, and are part of the VPN, VPN-B. CE1-A and CE2-A belong to AS 65001 and are connected to PE1-AS1 and PE2-AS1, respectively. Figure 6-1. MPLS VPN Provider Provisioning BGP PE-CE Routing
Implementing BGP PE-CE routing for the Customer A network is not an issue because the VPN sites use unique AS numbers. However, Customer B plans on using the same AS number on its sites. This causes an issue when migrating from a traditional non-MPLS based network topology, where the customer might use the same AS numbers at all his sites, to an MPLS VPN-based infrastructure due to the BGP loop prevention mechanism. The BGP loop prevention mechanism disallows customer sites having identical AS numbers to be linked by another AS number. In other words, if such a case occurs, routing updates from one site would be dropped when the other site receives them; therefore, connectivity cannot be established between the sites without additional configuration on the SP PE routers. To explain this further, Figure 6-2 shows an MPLS VPN network provisioned for Customer B. As shown in Figure 6-2, BGP loop prevention mechanism on CE2-B finds 65001 in the AS-PATH; therefore, CE2-B rejects the 192.168.10.0/24 update from PE2-AS1 because it finds its own AS in the update. Figure 6-2. BGP Loop Prevention Mechanism
To circumvent the BGP loop prevention mechanism, the AS-PATH update procedure in BGP was modified. The current AS-PATH update procedure allows customer topologies to reuse AS numbers at the sites by using the AS Override functionality. The functionality is made active only when the first AS number in the AS-PATH is equal to the AS number of the receiving BGP router. As shown in Figure 6-2, the first AS number in the AS-PATH is 65001, which is the AS number of CE2-B, the receiving router. Figure 6-3 shows the AS Override functionality when identical AS numbers are used at customer sites. The AS Override function causes all leading occurrences of the AS number of the receiving BGP router to be replaced with the AS number of the sending BGP router. As shown in Step 3 of Figure 6-3, when AS Override is used, AS 65001 in the AS-PATH is replaced with the AS number of the sending BGP router PE2-AS1, which is 1. Any other occurrences (further down the AS-PATH) of the receiving router's AS number are not replaced because they indicate a real routing information loop. In addition, an extra copy of the sending router's AS number is prepended to the AS-PATH (standard AS number prepending procedure that occurs on every eBGP update) to maintain proper AS hop count for proper BGP route selection. Step 3 in Figure 6-3 shows that PE2-AS1 prepends AS 1 to BGP update. Figure 6-3. AS Override Functionality
Configuration Flowchart to Implement BGP PE-CE Routing for VPN Sites with Unique and Same AS NumbersFigure 6-4 shows the configuration flowchart to configure BGP PE-CE routing for VPN sites using unique and same BGP AS. Figure 6-4. Configuration Flowchart for BGP PE-CE VPN Sites Implementing Unique and Similar AS
Implementing BGP PE-CE Routing for VPN Sites With Unique and Same AS NumbersThis configuration scenario demonstrates BGP PE-CE routing for VPN sites using same and different BGP AS numbers. You will use the topology shown in Figure 6-1, in which Customer A is using BGP AS 65001 and 65002 at Sites 1 and 2, respectively, and Customer B uses the same AS 65001 at both the sites. Before configuring, ensure that the provider network is provisioned to deliver MPLS VPN services to Customer A sites. It is also assumed that IP addresses are preconfigured and VRFs defined on PE routers. Example 6-1 provides the configuration steps defining VRF and its attributes on PE routers for BGP PE-CE routing. Example 6-1. Define VRF Cust_A and Cust_B on PE Routers PE1-AS1 and PE2-AS1PE1-AS1(config)#ip vrf Cust_A PE1-AS1(config-vrf)# rd 1:100 PE1-AS1(config-vrf)# route-target both 1:100 PE1-AS1(config)#interface Serial1/0 PE1-AS1(config-if)# description connected to CE1-A PE1-AS1(config-if)# ip vrf forwarding Cust_A PE1-AS1(config-if)# ip address 172.16.1.1 255.255.255.252 PE1-AS1(config)#ip vrf Cust_B PE1-AS1(config-vrf)# rd 1:200 PE1-AS1(config-vrf)# route-target both 1:200 PE1-AS1(config)#interface Serial2/0 PE1-AS1(config-if)# description connected to CE2-B PE1-AS1(config-if)# ip vrf forwarding Cust_B PE1-AS1(config-if)# ip address 192.168.1.1 255.255.255.252 The steps to configure BGP PE-CE routing on PE routers are as follows:
CE Router ConfigurationExample 6-4 shows the configuration on Routers CE CE1-A, CE2-A, CE1-B, and CE2-B. Example 6-4. CE1-A, CE2-A, CE1-B, and CE2-B Configurationhostname CE1-A ! interface Ethernet0/0 description Customer A Site 1 network ip address 172.16.10.1 255.255.255.0 ! interface Serial1/0 description connected to PE1-AS1 ip address 172.16.1.2 255.255.255.252 ! router bgp 65001 no synchronization network 172.16.10.0 mask 255.255.255.0 neighbor 172.16.1.1 remote-as 1 no auto-summary __________________________________________________________________________ hostname CE2-A ! interface Ethernet0/0 description Customer A Site 2 network ip address 172.16.20.1 255.255.255.0 ! interface Serial1/0 description connected to PE2-AS1 ip address 172.16.2.2 255.255.255.252 ! router bgp 65002 no synchronization network 172.16.20.0 mask 255.255.255.0 neighbor 172.16.2.1 remote-as 1 no auto-summary __________________________________________________________________________ hostname CE1-B ! interface Ethernet0/0 description Customer B Site 1 network ip address 192.168.10.1 255.255.255.0 ! interface Serial1/0 description connected to PE1-AS1 ip address 192.168.1.2 255.255.255.252 ! router bgp 65001 no synchronization network 192.168.10.0 neighbor 192.168.1.1 remote-as 1 no auto-summary __________________________________________________________________________ hostname CE2-B ! interface Ethernet0/0 description Customer B Site 2 network ip address 192.168.20.1 255.255.255.0 ! interface Serial1/0 description connected to PE2-AS1 ip address 192.168.2.2 255.255.255.252 ! router bgp 65001 no synchronization network 192.168.20.0 neighbor 192.168.2.1 remote-as 1 no auto-summary Final Configuration for BGP PE-CE VPN Sites Implementing Unique and Same BGP AS NumbersExample 6-5 shows the detailed and final configurations on the PE1-AS1, PE2-AS1, and P1-AS1 routers. Example 6-5. PE1-AS1, PE2-AS1, and P1-AS1 Final Configuration for BGP PE-CE VPN Sites Implementing Unique BGP AShostname PE1-AS1 ! ip cef ! ip vrf Cust_A rd 1:100 route-target export 1:100 route-target import 1:100 ! ip vrf Cust_B rd 1:200 route-target export 1:200 route-target import 1:200 ! mpls ldp router-id Loopback0 ! interface Loopback0 ip address 10.10.10.101 255.255.255.255 ! interface Serial0/0 description connected to P1-AS1 ip address 10.10.10.1 255.255.255.252 mpls ip ! interface Serial1/0 description connected to CE1-A ip vrf forwarding Cust_A ip address 172.16.1.1 255.255.255.252 ! interface Serial2/0 description connected to CE1-B ip vrf forwarding Cust_B ip address 192.168.1.1 255.255.255.252 ! router ospf 1 router-id 10.10.10.101 network 10.0.0.0 0.255.255.255 area 0 ! router bgp 1 no synchronization neighbor 10.10.10.102 remote-as 1 neighbor 10.10.10.102 update-source Loopback0 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 Cust_B neighbor 192.168.1.2 remote-as 65001 neighbor 192.168.1.2 activate neighbor 192.168.1.2 as-override no auto-summary no synchronization exit-address-family ! address-family ipv4 vrf Cust_A neighbor 172.16.1.2 remote-as 65001 neighbor 172.16.1.2 activate no auto-summary no synchronization exit-address-family __________________________________________________________________________ hostname PE2-AS1 ! ip cef ! ip vrf Cust_A rd 1:100 route-target export 1:100 route-target import 1:100 ! ip vrf Cust_B rd 1:200 route-target export 1:200 route-target import 1:200 ! mpls ldp router-id Loopback0 ! interface Loopback0 ip address 10.10.10.102 255.255.255.255 ! interface Serial0/0 description connected to P1-AS1 ip address 10.10.10.5 255.255.255.252 mpls ip ! interface Serial1/0 description connected to CE2-A ip vrf forwarding Cust_A ip address 172.16.2.1 255.255.255.252 ! interface Serial2/0 description connected to CE2-B ip vrf forwarding Cust_B ip address 192.168.2.1 255.255.255.252 ! router ospf 1 router-id 10.10.10.102 network 10.0.0.0 0.255.255.255 area 0 ! router bgp 1 no synchronization 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 Cust_B neighbor 192.168.2.2 remote-as 65001 neighbor 192.168.2.2 activate neighbor 192.168.2.2 as-override no auto-summary no synchronization exit-address-family ! address-family ipv4 vrf Cust_A neighbor 172.16.2.2 remote-as 65002 neighbor 172.16.2.2 activate no auto-summary no synchronization exit-address-family __________________________________________________________________________ hostname P1-AS1 ! ip cef ! interface Loopback0 ip address 10.10.10.200 255.255.255.255 ! interface Serial0/0 description connected to PE1-AS1 ip address 10.10.10.2 255.255.255.252 mpls ip ! interface Serial1/0 description connected to PE2-AS1 ip address 10.10.10.6 255.255.255.252 mpls ip ! router ospf 1 log-adjacency-changes network 10.0.0.0 0.255.255.255 area 0 Verifying BGP PE-CE Routing for VPN Sites Implementing Unique and Different BGP AS NumbersThe steps to verify BGP PE-CE routing for VPN sites implementing unique and different BGP AS numbers are
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