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The MPLS VPN Explicit Null Label Support with BGP IPv4 Label Session feature provides a method to advertise explicit null in a Border Gateway Protocol (BGP) label session for a carrier supporting carrier (CSC) customer edge (CE) device.
You must configure your network for Multiprotocol Label Switching (MPLS) Virtual Private Network (VPN).
You must configure the Border Gateway Protocol (BGP) to distribute labels between the carrier supporting carrier (CSC) customer edge (CE) device (CSC-CE) and CSC-provider edge (PE) devices.
Configure an explicit null label only in a carrier supporting carrier (CSC) customer edge (CE) device (CSC-CE) topology.
Configure an explicit null label only on a per-neighbor basis.
Information About MPLS VPN Explicit Null Label Support with BGP IPv4 Label Session
On a carrier supporting carrier (CSC) customer edge (CE) device (CSC-CE) with Border Gateway Protocol (BGP) IPv4 label distribution, BGP advertises an implicit null label for directly connected routes. This causes the previous hop (penultimate) device to do penultimate hop popping (PHP).
The MPLS VPN Explicit Null Label Support BGP IPv4 Label Session feature makes the penultimate device swap the incoming label for (or impose) the explicit null label. This action forces the egress device to process the explicit null label by popping it and inspecting the packet that remains.
The explicit null label helps to preserve quality of service (QoS) bits from one Service Level Agreement (SLA) to another until the packets reach their carrier supporting carrier (CSC) customer edge (CE) destination.
How to Configure MPLS VPN Explicit Null Label Support with BGP IPv4 Label Session
| Command or Action | Purpose | |
|---|---|---|
| Step 1 |
enable Example: |
Enables privileged EXEC mode.
|
| Step 2 |
configure terminal Example: |
Enters global configuration mode. |
| Step 3 |
router bgp autonomous-system-number Example: |
Enters router configuration mode and configures the device to run a Border Gateway Protocol (BGP) process. |
| Step 4 |
address-family ipv4 [unicast ] Example: |
Enters address family configuration mode for the IPv4 address family from which you can configure routing sessions that use standard IPv4 address prefixes. |
| Step 5 |
neighbor ip-address send-label explicit-null Example: |
Advertises the capability of a device to send Multiprotocol Label Switching (MPLS) labels with BGP routes.
|
| Step 6 |
neighbor {ip-address | peer-group-name} activate Example: |
Enables the neighbor to exchange prefixes for the IPv4 address family with the local device. |
| Step 7 |
end Example: |
Returns to privileged EXEC mode. |
| Command or Action | Purpose | |
|---|---|---|
| Step 1 |
enable Example: |
Enables privileged EXEC mode.
|
| Step 2 |
show ip bgp neighbors [ip-address [advertised-routes | dampened-routes | flap-statistics | paths [regexp] | received prefix-filter | received-routes | routes ]] Example: |
Displays information about the TCP and Border Gateway Protocol (BGP) connections to neighbors including explicit null.
|
Configuration Examples for MPLS VPN Explicit Null Label Support with BGP IPv4 Label Session
In the following example, the carrier supporting carrier (CSC) is configured with the Border Gateway Protocol (BGP) to distribute labels and to advertise explicit null for all its connected routes:
neighbor 10.0.0.0 send-label explicit-null
router bgp 100
bgp log-neighbor-changes
neighbor 10.0.0.0 remote-as 200
!
address-family ipv4
neighbor 10.0.0.0 activate
neighbor 10.0.0.0 send-label explicit-null
no auto-summary
no synchronization
exit-address-family In this example, the show ip bgp neighbors command displays information about connected Border Gateway Protocol (BGP) neighbors, including IP addresses, version numbers, neighbor capabilities, message statistics, and address family statistics that show if explicit null is configured:
Device# show ip bgp neighbors
BGP neighbor is 10.0.0.2, remote AS 300, external link
BGP version 4, remote router ID 10.0.0.20
BGP state = Established, up for 00:45:16
Last read 00:00:16, hold time is 180, keepalive interval is 60 seconds
Neighbor capabilities:
Route refresh: advertised and received(new)
Address family IPv4 Unicast: advertised and received
ipv4 MPLS Label capability: advertised and received
Message statistics:
InQ depth is 0
OutQ depth is 0
Sent Rcvd
Opens: 1 1
Notifications: 0 0
Updates: 1 2
Keepalives: 47 47
Route Refresh: 0 0
Total: 49 50
Default minimum time between advertisement runs is 30 seconds
For address family: IPv4 Unicast
BGP table version 9, neighbor version 9/0
Output queue sizes : 0 self, 0 replicated
Index 1, Offset 0, Mask 0x2
Member of update-group 1
My AS number is allowed for 3 number of times
AF-dependant capabilities:
Outbound Route Filter (ORF) type (128) Prefix-list:
Sending Prefix & Label(advertise explicit-null set) !Explicit null is configured
Sent Rcvd
Prefix activity: ---- ----
Prefixes Current: 3 3 (Consumes 144 bytes)
Prefixes Total: 3 6
Implicit Withdraw: 0 3
Explicit Withdraw: 0 0
.........
.........|
Related Topic |
Document Title |
|---|---|
|
Cisco IOS commands |
|
|
MPLS commands |
|
|
BGP configuration tasks |
IP Routing: BGP Configuration Guide |
|
BGP commands |
|
RFC |
Title |
|---|---|
|
RFC 1163 |
A Border Gateway Protocol |
|
RFC 1164 |
Application of the Border Gateway Protocol in the Internet |
|
RFC 2283 |
Multiprotocol Extensions for BGP-4 |
|
RFC 2547 |
BGP/MPLS VPNs |
|
RFC 3107 |
Carrying Label Information in BGP-4 |
|
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. |
The following table provides release information about the feature or features described in this module. This table lists only the software release that introduced support for a given feature in a given software release train. Unless noted otherwise, subsequent releases of that software release train also support that feature.
Use Cisco Feature Navigator to find information about platform support and Cisco software image support. To access Cisco Feature Navigator, go to www.cisco.com/go/cfn. An account on Cisco.com is not required.|
Feature Name |
Releases |
Feature Information |
|---|---|---|
|
MPLS VPN Explicit Null Label Support with BGP IPv4 Label Session |
12.0(27)S 12.0(27)S1 12.2(27)SBA 12.2(33)SRA 12.2(33)SXH Cisco IOS XE Release 2.3 |
The MPLS VPN Explicit Null Label Support with BGP IPv4 Label Session feature provides a method to advertise explicit null in a BGP label session for a carrier supporting carrier (CSC) customer edge (CE) device. In 12.0(27)S, this feature was introduced. In 12.0(27)S1, support was added for the Cisco 12000 series Internet routers. In 12.2(27)SBA, support was added for the Cisco 10000 series router. This feature was integrated into Cisco IOS Release 12.2(33)SRA. This feature was integrated into Cisco IOS Release 12.2(33)SXH. In Cisco IOS XE Release 2.3, support was added for the Cisco ASR 1000 Series Routers. The following commands were introduced or modified: debug ip bgp , neighbor send-label explicit-null , show ip bgp neighbors , show ip bgp vpnv4 , show mpls forwarding-table |
Border Gateway Protocol. The exterior Border Gateway Protocol used to exchange routing information between devices in separate autonomous systems. BGP uses TCP. Because TCP is a reliable protocol, BGP does not experience problems with dropped or fragmented data packets.
customer edge device. A device on the border between a VPN provider and a VPN customer that belongs to the customer.
external Border Gateway Protocol. A BGP session between devices in different autonomous systems. When a pair of devices in different autonomous systems are more than one IP hop away from each other, an external BGP session between those two devices is called multihop external BGP.
A short, fixed-length data identifier that tells switching nodes how to forward data (packets or cells).
The techniques and processes used to cause routed traffic to travel through the network on a path other than the one that would have been chosen if standard routing methods had been used.
Label Distribution Protocol. The protocol that supports MPLS hop-by-hop forwarding by distributing bindings between labels and network prefixes. The Cisco proprietary version of this protocol is the Tag Distribution Protocol (TDP).
label switched path. A configured connection between two devices, in which MPLS is used to carry packets. A path created by the concatenation of one or more label switched hops, allowing a packet to be forwarded by swapping labels from an MPLS node to another MPLS node.
Multiprotocol Label Switching. A method for directing packets primarily through Layer 2 switching rather than Layer 3 routing. In MPLS, packets are assigned short, fixed-length labels at the ingress to an MPLS cloud by using the concept of forwarding equivalence classes. Within the MPLS domain, the labels are used to make forwarding decisions mostly without recourse to the original packet headers; formerly known as tag switching.
Network Layer Reachability Information. BGP sends routing update messages containing NLRI, which describes the route. In this context, an NLRI is a prefix. A BGP update message carries one or more NLRI prefixes and the attributes of a route for the NLRI prefixes. The route attributes include a BGP next hop gateway address, community values, and other information.
provider edge device. A device on the border between a VPN provider and a VPN customer that belongs to the provider.
quality of service. A measure of performance for a transmission system that reflects its transmission quality and service availability.
A network layer device that uses one or more metrics to determine the optimal path along which network traffic should be forwarded. Routers forward packets from one network to another based on network layer information.
Virtual Private Network. A secure IP-based network that shares resources on one or more physical networks. A VPN contains geographically dispersed sites that can communicate securely over a shared backbone.
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