Multiprotocol BGP MPLS VPN

A Multiprotocol Label Switching (MPLS) virtual private network (VPN) consists of a set of sites that are interconnected by means of an MPLS provider core network. At each site, there are one or more customer edge (CE) devices, which attach to one or more provider edge (PE) devices. PEs use the Multiprotocol-Border Gateway Protocol (MP-BGP) to dynamically communicate with each other.

Prerequisites for Multiprotocol BGP MPLS VPN

Configure MPLS virtual private networks (VPNs) in the core.

Information About Multiprotocol BGP MPLS VPN

MPLS Virtual Private Network Definition

Before defining a Multiprotocol Label Switching virtual private network (MPLS VPN), you must define a VPN in general. A VPN is:

  • An IP-based network delivering private network services over a public infrastructure

  • A set of sites that are allowed to communicate with each other privately over the Internet or other public or private networks

Conventional VPNs are created by configuring a full mesh of tunnels or permanent virtual circuits (PVCs) to all sites in a VPN. This type of VPN is not easy to maintain or expand, because adding a new site requires changing each edge device in the VPN.

MPLS-based VPNs are created in Layer 3 and are based on the peer model. The peer model enables the service provider and the customer to exchange Layer 3 routing information. The service provider relays the data between the customer sites without the customer’s involvement.

MPLS VPNs are easier to manage and expand than conventional VPNs. When a new site is added to an MPLS VPN, only the service provider’s edge device that provides services to the customer site needs to be updated.

The different parts of the MPLS VPN are described as follows:

  • Provider (P) device—Device in the core of the provider network. P devices run MPLS switching, and do not attach VPN labels to routed packets. The MPLS label in each route is assigned by the provider edge (PE) device. VPN labels are used to direct data packets to the correct egress device.

  • PE device—Device that attaches the VPN label to incoming packets based on the interface or subinterface on which they are received. A PE device attaches directly to a customer edge (CE) device.

  • Customer (C) device—Device in the ISP or enterprise network.

  • CE device—Edge device on the network of the ISP that connects to the PE device on the network. A CE device must interface with a PE device.

The figure below shows a basic MPLS VPN.

Figure 1. Basic MPLS VPN Terminology

How an MPLS Virtual Private Network Works

Multiprotocol Label Switching virtual private network (MPLS VPN) functionality is enabled at the edge of an MPLS network. The provider edge (PE) device performs the following:

  • Exchanges routing updates with the customer edge (CE) device.

  • Translates the CE routing information into VPNv4 routes.

  • Exchanges VPNv4 routes with other PE devices through the Multiprotocol Border Gateway Protocol (MP-BGP).

The following sections describe how MPLS VPN works:

How Virtual Routing and Forwarding Tables Work in an MPLS Virtual Private Network

Each virtual private network (VPN) is associated with one or more virtual routing and forwarding (VRF) instances. A VRF defines the VPN membership of a customer site attached to a PE device. A VRF consists of the following components:

  • An IP routing table

  • A derived Cisco Express Forwarding table

  • A set of interfaces that use the forwarding table

  • A set of rules and routing protocol parameters that control the information that is included in the routing table

A one-to-one relationship does not necessarily exist between customer sites and VPNs. A site can be a member of multiple VPNs. However, a site can associate with only one VRF. A site’s VRF contains all the routes available to the site from the VPNs of which it is a member.

Packet forwarding information is stored in the IP routing table and the Cisco Express Forwarding table for each VRF. A separate set of routing and Cisco Express Forwarding tables is maintained for each VRF. These tables prevent information from being forwarded outside a VPN, and they also prevent packets that are outside a VPN from being forwarded to a device within the VPN.

How VPN Routing Information Is Distributed in an MPLS Virtual Private Network

The distribution of virtual private network (VPN) routing information is controlled through the use of VPN route target communities, implemented by Border Gateway Protocol (BGP) extended communities. VPN routing information is distributed as follows:

  • When a VPN route that is learned from a customer edge (CE) device is injected into BGP, a list of VPN route target extended community attributes is associated with it. Typically the list of route target community extended values is set from an export list of route targets associated with the virtual routing and forwarding (VRF) instance from which the route was learned.

  • An import list of route target extended communities is associated with each VRF. The import list defines route target extended community attributes that a route must have in order for the route to be imported into the VRF. For example, if the import list for a particular VRF includes route target extended communities A, B, and C, then any VPN route that carries any of those route target extended communities—A, B, or C—is imported into the VRF.

MPLS Forwarding

Based on routing information stored in the virtual routing and forwarding (VRF) IP routing table and VRF Cisco Express Forwarding table, packets are forwarded to their destination using Multiprotocol Label Switching (MPLS).

A provider edge (PE) device binds a label to each customer prefix learned from a customer edge (CE) device and includes the label in the network reachability information for the prefix that it advertises to other PE devices. When a PE device forwards a packet received from a CE device across the provider network, it labels the packet with the label learned from the destination PE device. When the destination PE device receives the labeled packet, it pops the label and uses it to direct the packet to the correct CE device. Label forwarding across the provider backbone is based on either dynamic label switching or traffic engineered paths. A customer data packet carries two levels of labels when traversing the backbone:

  • The top label directs the packet to the correct PE device.

  • The second label indicates how that PE device should forward the packet to the CE device.

BGP Distribution of VPN Routing Information

A provider edge (PE) device can learn an IP prefix from the following sources:

  • A customer edge (CE) device by static configuration

  • A Border Gateway Protocol (BGP) session with the CE device

  • A Routing Information Protocol (RIP) exchange with the CE device

The IP prefix is a member of the IPv4 address family. After the PE device learns the IP prefix, the PE converts it into a VPN-IPv4 prefix by combining it with an 8-byte route distinguisher (RD). The generated prefix is a member of the VPN-IPv4 address family. It uniquely identifies the customer address, even if the customer site is using globally nonunique (unregistered private) IP addresses. The route distinguisher used to generate the VPN-IPv4 prefix is specified by a configuration command associated with the virtual routing and forwarding (VRF) instance on the PE device.

BGP distributes reachability information for VPN-IPv4 prefixes for each VPN. BGP communication occurs at two levels:

  • Within an IP domains, known as an autonomous system (interior BGP [IBGP])

  • Between autonomous systems (external BGP [EBGP])

PE-PE or PE-RR (route reflector) sessions are IBGP sessions, and PE-CE sessions are EBGP sessions. In an Enhanced Interior Gateway Routing Protocol (EIGRP) PE-CE environment, when an EIGRP internal route is redistributed into BGP by one PE, and then back into EIGRP by another PE, the originating router ID for the route is set to the router ID of the second PE, replacing the original internal router ID.

BGP propagates reachability information for VPN-IPv4 prefixes among PE devices by means of the BGP multiprotocol extensions (refer to RFC 2283, Multiprotocol Extensions for BGP-4), which define support for address families other than IPv4. Using the extensions ensures that the routes for a given VPN are learned only by other members of that VPN, enabling members of the VPN to communicate with each other.

Major Components of an MPLS Virtual Private Network

An Multiprotocol Label Switching (MPLS)-based virtual private network (VPN) has three major components:

  • VPN route target communities—A VPN route target community is a list of all members of a VPN community. VPN route targets need to be configured for each VPN community member.

  • Multiprotocol BGP (MP-BGP) peering of VPN community provider edge (PE) devices—MP-BGP propagates virtual routing and forwarding (VRF) reachability information to all members of a VPN community. MP-BGP peering must be configured on all PE devices within a VPN community.

  • MPLS forwarding—MPLS transports all traffic between all VPN community members across a VPN service-provider network.

A one-to-one relationship does not necessarily exist between customer sites and VPNs. A given site can be a member of multiple VPNs. However, a site can associate with only one VRF. A customer-site VRF contains all the routes available to the site from the VPNs of which it is a member.

How to Configure Multiprotocol BGP MPLS VPN

Configuring Multiprotocol BGP Connectivity on the PE Devices and Route Reflectors

SUMMARY STEPS

  1. enable
  2. configure terminal
  3. router bgp as-number
  4. no bgp default ipv4-unicast
  5. neighbor {ip-address | peer-group-name} remote-as as-number
  6. neighbor {ip-address | peer-group-name} activate
  7. address-family vpnv4 [unicast]
  8. neighbor {ip-address | peer-group-name} send-community extended
  9. neighbor {ip-address | peer-group-name} activate
  10. end

DETAILED STEPS

  Command or Action Purpose
Step 1

enable

Example:


Device> enable

Enables privileged EXEC mode.

  • Enter your password if prompted.

Step 2

configure terminal

Example:


Device# configure terminal

Enters global configuration mode.

Step 3

router bgp as-number

Example:


Device(config)# router bgp 100

Configures a Border Gateway Protocol (BGP) routing process and enters router configuration mode.

  • The as-number argument indicates the number of an autonomous system that identifies the device to other BGP devices and tags the routing information passed along. The range is 0 to 65535. Private autonomous system numbers that can be used in internal networks are 64512 to 65535.

Step 4

no bgp default ipv4-unicast

Example:


Device(config-router)# no bgp default ipv4-unicast

(Optional) Disables the IPv4 unicast address family on all neighbors.

  • Use the no bgp default ipv4-unicast command if you are using this neighbor for Multiprotocol Label Switching (MPLS) routes only.

Step 5

neighbor {ip-address | peer-group-name} remote-as as-number

Example:


Device(config-router)# neighbor 10.0.0.1 remote-as 100

Adds an entry to the BGP or multiprotocol BGP neighbor table.

  • The ip-address argument specifies the IP address of the neighbor.

  • The peer-group-name argument specifies the name of a BGP peer group.

  • The as-number argument specifies the autonomous system to which the neighbor belongs.

Step 6

neighbor {ip-address | peer-group-name} activate

Example:


Device(config-router)# neighbor 10.0.0.1 activate

Enables the exchange of information with a neighboring BGP device.

  • The ip-address argument specifies the IP address of the neighbor.

  • The peer-group-name argument specifies the name of a BGP peer group.

Step 7

address-family vpnv4 [unicast]

Example:


Device(config-router)# address-family vpnv4

Enters address family configuration mode for configuring routing sessions, such as BGP, that use standard VPNv4 address prefixes.

  • The optional unicast keyword specifies VPNv4 unicast address prefixes.

Step 8

neighbor {ip-address | peer-group-name} send-community extended

Example:


Device(config-router-af)# neighbor 10.0.0.1 send-community extended

Specifies that a communities attribute should be sent to a BGP neighbor.

  • The ip-address argument specifies the IP address of the BGP-speaking neighbor.

  • The peer-group-name argument specifies the name of a BGP peer group.

Step 9

neighbor {ip-address | peer-group-name} activate

Example:


Device(config-router-af)# neighbor 10.0.0.1 activate

Enables the exchange of information with a neighboring BGP device.

  • The ip-address argument specifies the IP address of the neighbor.

  • The peer-group-name argument specifies the name of a BGP peer group.

Step 10

end

Example:


Device(config-router-af)# end

(Optional) Exits to privileged EXEC mode.

Troubleshooting Tips

You can enter a show ip bgp neighbor command to verify that the neighbors are up and running. If this command is not successful, enter a debug ip bgp ip-address events command, where ip-address is the IP address of the neighbor.

Configuring BGP as the Routing Protocol Between the PE and CE Devices

SUMMARY STEPS

  1. enable
  2. configure terminal
  3. router bgp as-number
  4. address-family ipv4 [multicast | unicast | vrf vrf-name]
  5. neighbor {ip-address | peer-group-name} remote-as as-number
  6. neighbor {ip-address | peer-group-name} activate
  7. exit-address-family
  8. end

DETAILED STEPS

  Command or Action Purpose
Step 1

enable

Example:


Device> enable

Enables privileged EXEC mode.

  • Enter your password if prompted.

Step 2

configure terminal

Example:


Device# configure terminal

Enters global configuration mode.

Step 3

router bgp as-number

Example:


Device(config)# router bgp 100

Configures a Border Gateway Protocol (BGP) routing process and enters router configuration mode.

  • The as-number argument indicates the number of an autonomous system that identifies the device to other BGP devices and tags the routing information passed along. The range is 0 to 65535. Private autonomous system numbers that can be used in internal networks range from 64512 to 65535.

Step 4

address-family ipv4 [multicast | unicast | vrf vrf-name]

Example:


Device(config-router)# address-family ipv4 vrf vpn1

Specifies the IPv4 address family type and enters address family configuration mode.

  • The multicast keyword specifies IPv4 multicast address prefixes.

  • The unicast keyword specifies IPv4 unicast address prefixes.

  • The vrf vrf-name keyword and argument specify the name of the VRF to associate with subsequent IPv4 address family configuration mode commands.

Step 5

neighbor {ip-address | peer-group-name} remote-as as-number

Example:


Device(config-router-af)# neighbor 10.0.0.1 remote-as 200

Adds an entry to the BGP or multiprotocol BGP neighbor table.

  • The ip-address argument specifies the IP address of the neighbor.

  • The peer-group-name argument specifies the name of a BGP peer group.

  • The as-number argument specifies the autonomous system to which the neighbor belongs.

Step 6

neighbor {ip-address | peer-group-name} activate

Example:


Device(config-router-af)# neighbor 10.0.0.1 activate

Enables the exchange of information with a neighboring BGP device.

  • The ip-address argument specifies the IP address of the neighbor.

  • The peer-group-name argument specifies the name of a BGP peer group.

Step 7

exit-address-family

Example:


Device(config-router-af)# exit-address-family

Exits address family configuration mode.

Step 8

end

Example:


Device(config-router)# end

(Optional) Exits to privileged EXEC mode.

Verifying the Virtual Private Network Configuration

A route distinguisher must be configured for the virtual routing and forwarding (VRF) instance, and Multiprotocol Label Switching (MPLS) must be configured on the interfaces that carry the VRF. Use the show ip vrf command to verify the route distinguisher (RD) and interface that are configured for the VRF.

SUMMARY STEPS

  1. show ip vrf

DETAILED STEPS

show ip vrf

Displays the set of defined VRF instances and associated interfaces. The output also maps the VRF instances to the configured route distinguisher.

Verifying Connectivity Between MPLS Virtual Private Network Sites

To verify that the local and remote customer edge (CE) devices can communicate across the Multiprotocol Label Switching (MPLS) core, perform the following tasks:

Verifying IP Connectivity from CE Device to CE Device Across the MPLS Core

SUMMARY STEPS

  1. enable
  2. ping [protocol] {host-name | system-address}
  3. trace [protocol] [destination]
  4. show ip route [ip-address [mask] [longer-prefixes]] | protocol [process-id]] | [list [access-list-name | access-list-number]

DETAILED STEPS

Step 1

enable

Enables privileged EXEC mode.

Step 2

ping [protocol] {host-name | system-address}

Diagnoses basic network connectivity on AppleTalk, Connectionless-mode Network Service (CLNS), IP, Novell, Apollo, Virtual Integrated Network Service (VINES), DECnet, or Xerox Network Service (XNS) networks. Use the ping command to verify the connectivity from one CE device to another.

Step 3

trace [protocol] [destination]

Discovers the routes that packets take when traveling to their destination. The trace command can help isolate a trouble spot if two devices cannot communicate.

Step 4

show ip route [ip-address [mask] [longer-prefixes]] | protocol [process-id]] | [list [access-list-name | access-list-number]

Displays the current state of the routing table. Use the ip-address argument to verify that CE1 has a route to CE2. Verify the routes learned by CE1. Make sure that the route for CE2 is listed.

Verifying That the Local and Remote CE Devices Are in the PE Routing Table

SUMMARY STEPS

  1. enable
  2. show ip route vrf vrf-name [prefix]
  3. show ip cef vrf vrf-name [ip-prefix]

DETAILED STEPS

Step 1

enable

Enables privileged EXEC mode.

Step 2

show ip route vrf vrf-name [prefix]

Displays the IP routing table associated with a virtual routing and forwarding (VRF) instance. Check that the loopback addresses of the local and remote customer edge (CE) devices are in the routing table of the provider edge (PE) devices.

Step 3

show ip cef vrf vrf-name [ip-prefix]

Displays the Cisco Express Forwarding forwarding table associated with a VRF. Check that the prefix of the remote CE device is in the Cisco Express Forwarding table.

Configuration Examples for Multiprotocol BGP MPLS VPN

Example: Configuring an MPLS Virtual Private Network Using BGP

PE Configuration

CE Configuration 


ip vrf vpn1
 rd 100:1
 route-target export 100:1
 route-target import 100:1
!
ip cef
mpls ldp router-id Loopback0 force
mpls label protocol ldp
!
interface Loopback0
 ip address 10.0.0.1 255.255.255.255
!
interface 
 ip vrf forwarding vpn1
 ip address 192.0.2.3 255.255.255.0
 no cdp enable
!
interface 
ip address 192.0.2.2 255.255.255.0
mpls label protocol ldp
mpls ip
!
router ospf 100
network 10.0.0. 0.0.0.0 area 100
network 192.0.2.1 255.255.255.0 area 100
!
router bgp 100
 no synchronization
 bgp log-neighbor changes
 neighbor 10.0.0.3 remote-as 100
 neighbor 10.0.0.3 update-source Loopback0
no auto-summary
 !        
address-family vpnv4
 neighbor 10.0.0.3 activate
 neighbor 10.0.0.3 send-community extended
 bgp scan-time import 5
 exit-address-family
 !        
address-family ipv4 vrf vpn1
 redistribute connected
 neighbor 198.51.100.1 remote-as 200
 neighbor 198.51.100.1 activate
 neighbor 198.51.100.1 as-override
 neighbor 198.51.100.1 advertisement-interval 5
 no auto-summary
 no synchronization
 exit-address-family
 

ip cef
mpls ldp router-id Loopback0 force
mpls label protocol ldp
!
interface Loopback0
 ip address 10.0.0.9 255.255.255.255
!
interface 
 ip address 198.51.100.1 255.255.255.0
 no cdp enable
!
router bgp 200
 bgp log-neighbor-changes
 neighbor 198.51.100.2 remote-as 100
 !        
address-family ipv4
 redistribute connected
 neighbor 198.51.100.2 activate
 neighbor 198.51.100.2 advertisement-interval 5
 no auto-summary
 no synchronization
 exit-address-family

Additional References

Related Documents

Related Topic

Document Title

Cisco IOS commands

Cisco IOS Master Command List, All Releases

Description of commands associated with MPLS and MPLS applications

Cisco IOS Multiprotocol Label Switching Command Reference

Configuring MPLS virtual private networks

“MPLS Virtual Private Networks” module in the MPLS Layer 3 VPNs Configuration Guide

Standards and RFCs

RFC

Title

RFC 2283

Multiprotocol Extensions for BGP-4

RFC 2547

BGP/MPLS VPNs

Technical Assistance

Description

Link

The Cisco Support and Documentation website provides online resources to download documentation, software, and tools. Use these resources to install and configure the software and to troubleshoot and resolve technical issues with Cisco products and technologies. Access to most tools on the Cisco Support and Documentation website requires a Cisco.com user ID and password.

http://www.cisco.com/cisco/web/support/index.html

Feature Information for Multiprotocol BGP MPLS VPN

For information on compatibility of this feature with route processors (RP), see Cisco ASR 900 Series Aggregation Services Routers Feature Compatibility Matrix.

Table 1. Feature Information for Multiprotocol BGP MPLS VPN

Feature Name

Releases

Feature Information

Multiprotocol BGP MPLS VPN

12.0(11)ST

12.2(9)S

12.2(17b)SXA

12.2(27)SBB

12.3(8)T

15.2(1)S

Cisco IOS XE Release 2.1

Cisco IOS XE Release 3.5S

An MPLS VPN consists of a set of sites that are interconnected through the MPLS provider core network. At each site, there are one or more CEs, which attach to one or more PEs. The Multiprotocol BGP MPLS VPN feature allows PEs to use the MP-BGP to dynamically communicate with each other.

In Cisco IOS Release 12.0(11)ST, this feature was introduced.

In Cisco IOS Release 12.2(9)S, 12.2(17b)SXA, 12.2(27)SBB, 12.3(8)T, and 15.2(1)S, this feature was integrated.

In Cisco IOS XE Release 2.1, this feature was implemented on Cisco ASR 1000 Series Routers.

In Cisco IOS XE Release 3.5S, support was added for the Cisco ASR 903 Router.

No commands were introduced or modified.