Cisco IOS Multiprotocol Label Switching Configuration Guide, Release 12.2SR
MPLS VPN--BGP Local Convergence
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Table Of Contents

MPLS VPN—BGP Local Convergence

Finding Feature Information

Contents

Prerequisites for MPLS VPN—BGP Local Convergence

Restrictions for MPLS VPN—BGP Local Convergence

Information About MPLS VPN—BGP Local Convergence

How Link Failures Are Handled with BGP

How Links Are Handled with the MPLS VPN—BGP Local Convergence Feature

How Link Failures Are Detected

How to Enable MPLS VPN—BGP Local Convergence

Configuring MPLS VPN—BGP Local Convergence with IPv4

Prerequisite

Configuring MPLS VPN—BGP Local Convergence with IPv6

Prerequisite

Examples

Troubleshooting Tips

Configuration Examples for MPLS VPN—BGP Local Convergence

Additional References

Related Documents

Standards

MIBs

RFCs

Technical Assistance

Feature Information for MPLS VPN—BGP Local Convergence


MPLS VPN—BGP Local Convergence

First Published: December 31, 2007
Last Updated: October 2, 2009

The MPLS VPN—BGP Local Convergence feature reduces the downtime of a provider edge (PE) to customer edge (CE) link failure. It does so by rerouting PE-egress traffic onto a backup path to the CE before BGP has re-converged.

The MPLS VPN—BGP Local Convergence feature is also referred to as "local protection".

Note that the MPLS VPN—BGP Local Convergence feature only affects traffic exiting the VPN. Therefore, it cannot fully protect traffic end-to-end by itself.

Finding Feature Information

Your software release may not support all the features documented in this module. For the latest feature information and caveats, see the release notes for your platform and software release. To find information about the features documented in this module, and to see a list of the releases in which each feature is supported, see the "Feature Information for MPLS VPN—BGP Local Convergence" section.

Use Cisco Feature Navigator to find information about platform support and Cisco IOS and Catalyst OS software image support. To access Cisco Feature Navigator, go to http://www.cisco.com/go/cfn. An account on Cisco.com is not required.

Contents

•Prerequisites for MPLS VPN—BGP Local Convergence

•Restrictions for MPLS VPN—BGP Local Convergence

•Information About MPLS VPN—BGP Local Convergence

•How to Enable MPLS VPN—BGP Local Convergence

•Configuration Examples for MPLS VPN—BGP Local Convergence

•Additional References

•Feature Information for MPLS VPN—BGP Local Convergence

Prerequisites for MPLS VPN—BGP Local Convergence

•Before this form of link protection can be enabled, the customer site must be connected to the provider site by more than one path.

•Both the main forwarding path and the redundant backup path must have been installed within BGP, and BGP must support lossless switchover between operational paths.

Note Any routing protocol can be used between the PE and CE as long as the path is redistributed into BGP. That includes: eBGP, RIP, EIGRP, IS-IS, OSPF, and static routing. Any next-hop core tunneling technology that is supported by BGP is also supported for protection, including MPLS, IP/L2TPv3, and IP/GRE. Enabling a Carrier's Carrier (CsC) protocol between the PE and CE is also supported. Inter-AS option A (back-to-back VRF) is supported because it is essentially the same as performing the PE-CE link protection in both AS's. However, inter-AS options B and C protection are not supported at this time.

•All Provider Edge routers that are serving as backup to the link must have assigned a unique Route Distinguisher to each Virtual Routing and Forwarding table involved with the link to ensure that the route reflectors advertise all available paths.

•Although not required, it is recommended that the backup PE (shown as "PE2" in Figure 2) also be running the IOS version that is running on the PE ("PE1") whose link with the CE will be protected; that is, Cisco IOS Release 12.2(33) SRC, Cisco  IOS Release 12.2(33)SB, Cisco  IOS Release 15.0(1)M or a more recent version of those products.

Restrictions for MPLS VPN—BGP Local Convergence

•This feature only affects traffic exiting the VPN. Therefore, it cannot fully protect traffic end-to-end by itself.

•Configuration of this feature is not allowed in IPv6.

•Local protection is not applicable with VRF-lite. Although configuration of both features together is not blocked, protection does not occur.

•This link protection cannot be initiated during an HA stateful switchover (SSO). But links already configured with this protection before the switchover begins will remain protected after the switchover.

•When performing an ISSU downgrade from an image that does include this link protection to an image that does not support this feature, active protection will be halted when BGP routes are refreshed.

Information About MPLS VPN—BGP Local Convergence

To configure the MPLS VPN—BGP Local Convergence feature, you should understand the following concepts:

•How Link Failures Are Handled with BGP

•How Links Are Handled with the MPLS VPN—BGP Local Convergence Feature

•How Link Failures Are Detected

How Link Failures Are Handled with BGP

Within a Layer 3 VPN network, the failure of a PE-CE link can cause a loss of connectivity (LoC) to a customer site, which is detrimental to time-sensitive applications. Several factors contribute to the duration of such an outage:

•The time to detect the failure

•The programming of the forwarding

•The convergence of BGP (In large networks, the restored traffic arrival time at its destination varies according to the prefix.)

When BGP detects a PE-CE link failure, it removes all of the BGP paths through the failing link. BGP runs the bestpath algorithm on the affected prefixes and selects alternate paths for each prefix. These new paths (which typically include a remote PE) are installed into forwarding. The local labels are removed and BGP withdrawals are sent to all BGP neighbors. As each BGP neighbor receives the withdrawal messages (typically indirectly using route-reflectors), the bestpath algorithm is called and the prefixes are switched to an alternate path. Only then is connectivity restored.

How Links Are Handled with the MPLS VPN—BGP Local Convergence Feature

The MPLS VPN—BGP Local Convergence feature requires that the prefixes to be protected on a PE-CE link have at least one backup path that does not include that link. (See Figure 1.) The customer site must have backup paths to the provider site.

Figure 1 Figure 1 Network Configured with Primary and Backup Paths

The MPLS VPN—BGP Local Convergence feature reduces LoC time by sending the broken link's traffic over a backup path (as shown in Figure 2) instead of waiting for total network convergence. The local label is maintained for 5 minutes while prefixes switch from the failing local path to the backup path. Because the label is not freed as had been the usual practice, forwarding continues to take place.

The bestpath algorithm selects the backup path. Thus, the local label has been applied in place of the failed BGP bestpath label (which is sometimes called "label swapping"). Traffic is restored locally while the network propagation of the BGP withdrawal messages takes place. Eventually, the egress PE router converges and bypasses the local repair.

Figure 2 Figure 2 Network Using the Backup Path After a PE-CE Link Failure on the Primary Path

Note After the 5-minute label preservation, the local labels are freed. Any BGP prefix that is remote and is not part of a Carrier Supporting Carrier network does not have a local label and is removed. The delay in local label deletion does not modify normal BGP addition and deletion of BGP paths. Rather, BGP re-programs the new backup bestpath into forwarding as usual.

How Link Failures Are Detected

Local protection relies on BGP being notified of the interface failure. Detection can occur using either the interface drivers or the routing tables. If an interface or route goes down, the corresponding path in the routing table is removed and BGP will be notified using the routing APIs.

However, when the routing table cannot detect the failure (as when a Layer 2 switch goes down), BGP determines that a neighbor is down through use of its hold-down timer. However, that determination can be extremely slow because of the 3-minute default for BGP session time-out.

You can reduce the detection delay by either reducing the BGP session time-out interval (as described in the Configuring Internal BGP Features document) or by enabling the Bidirectional Forwarding Detection protocol within eBGP between the PE and CE. For complete instructions to enable BFD, see the Bidirectional Forwarding Detection document.

How to Enable MPLS VPN—BGP Local Convergence

This section contains the following information:

•Configuring MPLS VPN—BGP Local Convergence with IPv4

•Configuring MPLS VPN—BGP Local Convergence with IPv6

•Troubleshooting Tips

Configuring MPLS VPN—BGP Local Convergence with IPv4

Perform the following steps to configure MPLS VPN—BGP Local Convergence for IPv4 MPLS VPNs.

Prerequisite

Ensure that the CE is already connected to the PE by a minimum of two paths.

SUMMARY STEPS

1. enable

2. configure terminal

3. ip vrf vrf-name

4. rd (conditional)

5. protection local-prefixes

6. show ip vrf detail

DETAILED STEPS

 
Command or Action
Purpose

Step 1 

enable

Example:

Router> enable

Enables privileged EXEC mode. Enter your password if prompted.

Step 2 

configure terminal

Example:

Router# configure terminal

Enters global configuration mode.

Step 3 

ip vrf vrf-name


Example:

Router(config)# ip vrf vpn1

Enters VRF configuration mode. If no VRF routing table and Cisco Express Forwarding (CEF) table had been previously created for this named VRF, then this command also creates them, giving both tables the specified vrf-name (in this example, the name is vpn1).

Step 4 

rd route-distinguisher








Example:

Router(config-vrf)# rd 100:3

(Optional). If no route distinguisher had been previously established for the named VRF, then it is necessary to enter this command.

The route-distinguisher value can be either an:

•autonomous system number followed by a colon and an arbitrary number (for example, 100:3)

or

•IP address followed by a colon and an arbitrary number (for example, 192.168.122.15:1).

Step 5 

protection local-prefixes

Example:

Router(config-vrf)# protection local-prefixes

Allows a pre-configured backup path to carry traffic if the PE-CE link breaks by preserving the local prefixes while BGP reconverges.

Step 6 

show ip vrf detail

Example:

Router(config-vrf)# show ip vrf detail

(Optional) Verifies that MPLS VPN—BGP Local Convergence has been configured. (See Examples.)

Configuring MPLS VPN—BGP Local Convergence with IPv6

Perform the following steps to configure MPLS VPN—BGP Local Convergence for IPv6 MPLS VPNs.

Prerequisite

Ensure that the CE is already connected to the PE by a minimum of two paths.

SUMMARY STEPS

1. enable

2. configure terminal

3. vrf definition vrf-name

4. rd (optional)

5. address-family [ipv4 | ipv6]

6. protection local-prefixes

7. show ip vrf detail

DETAILED STEPS

 
Command or Action
Purpose

Step 1 

enable

Example:

Router> enable

Enables privileged EXEC mode. Enter your password if prompted.

Step 2 

configure terminal

Example:

Router# configure terminal

Enters global configuration mode.

Step 3 

vrf definition vrf-name

Example:

Router(config)# vrf definition vrf2

Enters VRF definition configuration mode. If no VRF routing table and Cisco Express Forwarding (CEF) table had been previously created for this named VRF, then this command also creates them, giving both tables the specified vrf-name (in this example, the name is vrf2).

Step 4 

rd route-distinguisher









Example:

Router(config-vrf)# rd 100:3

(Optional) If no route distinguisher had been previously established for the named VRF, it is necessary to enter this command.

The route-distinguisher value can be either an:

•autonomous system number followed by a colon and an arbitrary number (for example, 100:3)

or

•IP address followed by a colon and an arbitrary number (for example, 192.168.122.15:1).

Step 5 

address-family [ ipv4 | ipv6 ]

Example:

Router(config-vrf)# address-family ipv6

Enters address family configuration mode and specifies the IPv4 or IPv6 protocol.

Step 6 

protection local-prefixes

Example:

Router(config-vrf-af)# protection local-prefixes

Allows a pre-configured backup path to carry traffic if the PE-CE link breaks by preserving the local prefixes while BGP reconverges.

Step 7 

show ip vrf detail

Example:

Router(config-vrf)# show ip vrf detail

(Optional) Verifies that MPLS VPN—BGP Local Convergence has been configured. (See Examples.)

Examples

To verify that local link protection has been enabled, enter the VRF detail command show ip vrf detail. If the protection is enabled, the status message "Local prefix protection enabled" will be shown in the display: 

Router# show ip vrf detail 

VRF vpn1 (VRF Id = 1); default RD 100:1; default VPNID <not set>

  Interfaces:

    AT1/0/1.1               

VRF Table ID = 1

  Export VPN route-target communities

    RT:100:1                

  Import VPN route-target communities

    RT:100:1                 RT:100:2                

  No import route-map

  No export route-map

  VRF label distribution protocol: not configured

  VRF label allocation mode: per-prefix

     Local prefix protection enabled

Troubleshooting Tips

•Ensure that a minimum of two paths are present for the protected prefix w.x.y.z in BGP in steady state condition on the PE. The path using the protected PE should be the BGP best-path before failover occurs. To view the configuration, enter the command show ip bgp vpnv4 vrf vpn w.x.y.z

•Ensure that local protection has been enabled in the protected PE by entering the show ip vrf detail command as shown in the "Examples" section.

•When route reflectors exist in the topology, ensure that each VRF has a unique route distinguisher.

Configuration Examples for MPLS VPN—BGP Local Convergence

The following examples show how MPLS VPN—BGP Local Convergence can prevent traffic loss after a link failure. You can display a detailed view of local link protection before, during, and after BGP convergence by using the show bgp vpnv4 and show mpls forwarding-table vrf commands as shown in the following 3-stage example.

Note The show bgp vpnv4 unicast command is equivalent to the show ip bgp vpnv4 command that existed in prior releases of Cisco IOS.

Example 1: Before the Link Failure

Both a primary path and a backup path have been configured: 

PE1# show bgp vpnv4 unicast all 172.16.0.1 

BGP routing table entry for 100:1:172.16.0.1/32, version 2

Paths: (2 available, best #2, table v1)

Flag: 0x820

  Advertised to update-groups:

     1         

  100, imported path from 100:2:172.16.0.1/32

    172.16.0.6 (metric 21) from 172.16.0.7 (172.16.0.7)

      Origin incomplete, metric 0, localpref 100, valid, internal

      Extended Community: RT:100:0

      Originator: 172.16.0.6, Cluster list: 172.16.0.7

      mpls labels in/out 16/17

  100

    172.16.1.1 from 172.16.1.1 (172.16.0.1)

      Origin incomplete, metric 0, localpref 100, valid, external, best

      Extended Community: RT:100:0

      mpls labels in/out 16/nolabel

BGP routing table entry for 100:2:172.16.0.1/32, version 9

Paths: (1 available, best #1, no table)

Flag: 0x820

  Not advertised to any peer

  100

    172.16.0.6 (metric 21) from 172.16.0.7 (172.16.0.7)

      Origin incomplete, metric 0, localpref 100, valid, internal, best

      Extended Community: RT:100:0

      Originator: 172.16.0.6, Cluster list: 172.16.0.7

      mpls labels in/out nolabel/17

Label information for both paths can be displayed: 

PE1# show bgp vpnv4 unicast all labels

Network          Next Hop      In label/Out label

Route Distinguisher: 100:1 (v1)

   172.16.0.1/32    172.16.0.6      16/17

                    172.16.1.1      16/nolabel

   172.16.0.5/32    172.16.0.4      nolabel/23

   172.16.0.22/32   0.0.0.0         17/nolabel(v1)

   172.16.0.44/32   172.16.0.4      nolabel/24

   172.16.0.66/32   172.16.0.6      nolabel/21

   172.16.1.0/24    172.16.1.1      18/nolabel

                    0.0.0.0         18/nolabel(v1)

   172.16.5.0/24    172.16.0.4      nolabel/25

   172.16.8.0/24    172.16.0.6      19/23

                    172.16.1.1      19/nolabel

Route Distinguisher: 100:2

   172.16.0.1/32    172.16.0.6      nolabel/17

   172.16.0.66/32   172.16.0.6      nolabel/21

   172.16.8.0/24    172.16.0.6      nolabel/23

The PE1 (see Figure 1) forwarding table contains BGP bestpath information: 

PE1# show mpls forwarding-table vrf v1 172.16.0.1 detail

Local      Outgoing   Prefix           Bytes Label   Outgoing   Next Hop    

Label      Label      or Tunnel Id     Switched      interface              

16         No Label   172.16.0.1/32[V] 570           Et0/0      172.16.1.1  

        MAC/Encaps=14/14, MRU=1504, Label Stack{}

        AABBCC000B00AABBCC000C000800 

        VPN route: v1

        No output feature configured


PE1#

Example 2: After the Link Failure and Before BGP Convergence

After the link failure on only one path, the backup path remains available (see Figure 2): 

PE1# show bgp vpnv4 unicast all 172.16.0.1                    

BGP routing table entry for 100:1:172.16.0.1/32, version 19

Paths: (1 available, best #1, table v1)

  Not advertised to any peer

  100, imported path from 100:2:172.16.0.1/32

    172.16.0.6 (metric 21) from 172.16.0.7 (172.16.0.7)

      Origin incomplete, metric 0, localpref 100, valid, internal, best

      Extended Community: RT:100:0

      Originator: 172.16.0.6, Cluster list: 172.16.0.7

      mpls labels in/out 16/17

BGP routing table entry for 100:2:172.16.0.1/32, version 9

Paths: (1 available, best #1, no table)

  Not advertised to any peer

  100

    172.16.0.6 (metric 21) from 172.16.0.7 (172.16.0.7)

      Origin incomplete, metric 0, localpref 100, valid, internal, best

      Extended Community: RT:100:0

      Originator: 172.16.0.6, Cluster list: 172.16.0.7

      mpls labels in/out nolabel/17

The label information for the backup path label can be displayed: 

PE1# show bgp vpnv4 unicast all labels      

Network          Next Hop      In label/Out label

Route Distinguisher: 100:1 (v1)

   172.16.0.1/32    172.16.0.6      16/17

   172.16.0.5/32    172.16.0.4      nolabel/23

   172.16.0.22/32   0.0.0.0         17/nolabel(v1)

   172.16.0.44/32   172.16.0.4      nolabel/24

   172.16.0.66/32   172.16.0.6      nolabel/21

   172.16.1.0/24    172.16.0.6      nolabel/22

   172.16.5.0/24    172.16.0.4      nolabel/25

   172.16.8.0/24    172.16.0.6      19/23

Route Distinguisher: 100:2

   172.16.0.1/32    172.16.0.6      nolabel/17

   172.16.0.66/32   172.16.0.6      nolabel/21

   172.16.1.0/24    172.16.0.6      nolabel/22

   172.16.8.0/24    172.16.0.6      nolabel/23

The PE1 (see Figure 1) forwarding table contains new label and next-hop information to direct traffic onto the backup path: 

PE1# show mpls forwarding-table vrf v1 172.16.0.1 detail                

Local      Outgoing   Prefix           Bytes Label   Outgoing   Next Hop    

Label      Label      or Tunnel Id     Switched      interface              

16         17         172.16.0.1/32[V] 0             Et1/0      172.16.3.2  

        MAC/Encaps=14/22, MRU=1496, Label Stack{21 17}

        AABBCC000D00AABBCC000C018847 0001500000011000

        VPN route: v1

        No output feature configured

PE1#

Example 3: After Local Label Expiration and BGP Re-convergence

Because the local label preservation window has expired, the replacement local label is now gone from the PE1 forwarding table information: 

PE1# show mpls forwarding-table vrf v1 172.16.0.1 detail

Local      Outgoing   Prefix           Bytes Label   Outgoing   Next Hop    

Label      Label      or Tunnel Id     Switched      interface              

None       17         172.16.0.1/32[V] 0             Et1/0      172.16.3.2  

        MAC/Encaps=14/22, MRU=1496, Label Stack{21 17}

        AABBCC000D00AABBCC000C018847 0001500000011000

        VPN route: v1

        No output feature configured

The new BGP information reverts to the configuration shown in Figure 1: 

PE1# show bgp vpnv4 unicast all 172.16.0.1 

BGP routing table entry for 100:1:172.16.0.1/32, version 23

Paths: (1 available, best #1, table v1)

  Not advertised to any peer

  100, imported path from 100:2:172.16.0.1/32

    172.16.0.6 (metric 21) from 172.16.0.7 (172.16.0.7)

      Origin incomplete, metric 0, localpref 100, valid, internal, best

      Extended Community: RT:100:0

      Originator: 172.16.0.6, Cluster list: 172.16.0.7

      mpls labels in/out nolabel/17

BGP routing table entry for 100:2:172.16.0.1/32, version 9

Paths: (1 available, best #1, no table)

  Not advertised to any peer

  100

    172.16.0.6 (metric 21) from 172.16.0.7 (172.16.0.7)

      Origin incomplete, metric 0, localpref 100, valid, internal, best

      Extended Community: RT:100:0

      Originator: 172.16.0.6, Cluster list: 172.16.0.7

      mpls labels in/out nolabel/17


PE1# show bgp vpnv4 unicast all labels 

Network          Next Hop      In label/Out label

Route Distinguisher: 100:1 (v1)

   172.16.0.1/32    172.16.0.6      nolabel/17

   172.16.0.5/32    172.16.0.4      nolabel/23

   172.16.0.22/32   0.0.0.0         17/nolabel(v1)

   172.16.0.44/32   172.16.0.4      nolabel/24

   172.16.0.66/32   172.16.0.6      nolabel/21

   172.16.1.0/24    172.16.0.6      nolabel/22

   172.16.5.0/24    172.16.0.4      nolabel/25

   172.16.8.0/24    172.16.0.6      nolabel/23

Route Distinguisher: 100:2

   172.16.0.1/32    172.16.0.6      nolabel/17

   172.16.0.66/32   172.16.0.6      nolabel/21

   172.16.1.0/24    172.16.0.6      nolabel/22

   172.16.8.0/24    172.16.0.6      nolabel/23


PE1#

Additional References

The following sections provide references related to the MPLS VPN—BGP Local Convergence feature.

•Related Documents

•Standards

•MIBs

•RFCs

•Technical Assistance

Related Documents

Related Topic
Document Title

Configuration of VRF under the specific cases of IPv4 and IPv6 situations.

MPLS VPN—VRF CLI for IPv4 and IPv6 VPNs

Protocol for quickly detecting failed forwarding paths.

Bidirectional Forwarding Detection

BGP Configuration

Configuring a Basic BGP Network


Standards

Standard
Title

No new or modified standards are supported by this feature, and support for existing standards has not been modified by this feature.

—


MIBs

MIB
MIBs Link

No new or modified MIBs are supported by this feature, and support for existing MIBs has not been modified by this feature.

To locate and download MIBs for selected platforms, Cisco IOS releases, and feature sets, use Cisco MIB Locator found at the following URL:

http://www.cisco.com/go/mibs


RFCs

RFC
Title

RFC 2547

BGP/MPLS VPNs


Technical Assistance

Description
Link

The Cisco Support website provides extensive online resources, including documentation and tools for troubleshooting and resolving technical issues with Cisco products and technologies.

To receive security and technical information about your products, you can subscribe to various services, such as the Product Alert Tool (accessed from Field Notices), the Cisco Technical Services Newsletter, and Really Simple Syndication (RSS) Feeds.

Access to most tools on the Cisco Support website requires a Cisco.com user ID and password.

http://www.cisco.com/techsupport


Feature Information for MPLS VPN—BGP Local Convergence

Table 1 lists the release history for this feature.

Not all commands may be available in your Cisco IOS software release. For release information about a specific command, see the command reference documentation.

Use Cisco Feature Navigator to find information about platform support and software image support. Cisco Feature Navigator enables you to determine which Cisco IOS and Catalyst OS software images support a specific software release, feature set, or platform. To access Cisco Feature Navigator, go to http://www.cisco.com/go/cfn. An account on Cisco.com is not required.

Note Table 1 lists only the Cisco IOS software release that introduced support for a given feature in a given Cisco IOS software release train. Unless noted otherwise, subsequent releases of that Cisco IOS software release train also support that feature.

Table 1 Feature Information for MPLS VPN—BGP Local Convergence 

Feature Name
Releases
Feature Information

MPLS VPN—BGP Local Convergence

12.2(33)SRC

This feature reduces the downtime of a PE-CE link failure by rerouting PE-egress traffic onto a backup path to the CE before BGP has re-converged.

In 12.2(33)SRC, this feature was introduced on the Cisco 7200 and the Cisco 7600.

The following command was introduced: protection local-prefixes.

MPLS VPN—BGP Local Convergence

12.2(33)SB

This feature became available on the Cisco 7300 series and the Cisco 10000 series routers.

MPLS VPN—BGP Local Convergence

15.0(1)M

This feature was integrated in this release. The following command was introduced: protection local-prefixes.


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