Cisco Nexus 7000 Series NX-OS MPLS Configuration Guide

Local Label Allocation Filtering

This MPLS LDP feature enables you to configure filtering policies for selective local label binding assignments to improve LDP scalability and convergence. This section includes the following topics:

• Overview of MPLS LDP Local Label Allocation Filtering
• Prefix Lists for MPLS LDP Local Label Allocation Filtering
• Local Label Allocation Filtering and LDP Actions

Overview of MPLS LDP Local Label Allocation Filtering

LDP allocates a local label for every route that is learned from the Interior Gateway Protocol (IGP). In the absence of inbound and outbound label filtering, these local labels are advertised to and learned by all LDP peers.

In most Layer 3 virtual private network (VPN) configurations, only the label switched paths (LSPs) created to reach the /32 host routes or Border Gateway Protocol (BGP) next hops between the provider edge (PE) routers carry traffic and are relevant to the Layer 3 VPNs. LSPs between the PE routers that are not members of a VPN use more memory and create additional processing in LDP across the core.

With load increases in the enterprise and service provider domains, scalability has become more important in enterprise and service provider networks. Controlling the local label allocation could offload LDP processing of non-VPN LSPs in the network core routers.

The MPLS LDP local label allocation filtering feature enables you to configure LDP to selectively allocate local labels for a subset of the prefixes learned from the IGP. You can select the LDP allocate local labels for prefixes configured in a prefix list in the global table or for host routes in the global table.

Local label allocation filtering reduces the number of local labels allocated and therefore the number of messages exchanged with peers, which improves LDP scalability and convergence. Figure 8-1 and Figure 8-2 show how controlling local label allocation can reduce the local label space size and greatly reduce the number of advertisements to peers. Figure 8-1 shows the label allocation behavior when LDP allocates a local label for every route and advertises a label binding for every route that is learned from the IGP.

Figure 8-1 Default LDP Local Label Allocation Behavior

Figure 8-2 shows the LDP behavior with local label allocation control configured. The size of the local label space and the number of label binding advertisements are reduced with local label allocation filtering through the use of a prefix list. The decrease in the number of local labels and label binding advertisement messages reduces the amount of memory used and improves the convergence time for LDP. The MPLS LDP local label allocation filtering feature also allows for more efficient use of the label space.

Figure 8-2 LDP Behavior with Local Label Allocation Controls

Figure 8-2 shows that router R1 learns the number of routes from its IGP neighbors on routers R2, R3, and R4. A prefix list that is defined on router R1 specifies the prefixes for which LDP allocates a local label.

Note The number of label information base (LIB) entries remains the same regardless of the kind of label filtering used because the remote label bindings for the prefixes that are filtered are kept in the LIB. Memory use is reduced because local label filtering decreases the number of local labels allocated and the number of label bindings advertised to and stored by the peers of an LSR. When all neighboring LSRs are configured with the same local label allocation filter, the number of LIB entries can be reduced (that is, no, or few, LIB entries are created to store remote label bindings).

Prefix Lists for MPLS LDP Local Label Allocation Filtering

The local label allocation filtering feature allows you to configure LDP to allocate local labels for a subset of the learned prefixes. LDP accepts the prefix and allocates a local label if the prefix is permitted by a prefix list. If the prefix list is not defined, LDP accepts all prefixes and allocates local labels based on its default mode of operation.

The benefits of using prefix lists for LDP local label allocation filtering are as follows:

• Prefix lists provide more flexibility for specifying a subset of prefixes and masks.
• Prefix lists use a tree-based matching technique, which is more efficient than evaluating prefixes or host routes sequentially.
• Prefix lists are easy to modify.

Note Prefix lists are also used for outbound label filtering and inbound label filtering. For information on configuring prefix lists, see the “Creating a Prefix List for MPLS LDP Label Filtering” section.

Local Label Allocation Filtering and LDP Actions

Local label allocation filtering modifies the LDP’s local label allocation handling. This feature supports local label allocation filtering through the specification of a prefix list or host routes.

With this feature, LDP determines whether a prefix filter is already configured to control the local label allocation on the local node. If a prefix list exists, the local label allocation is confined to the list of prefixes permitted by the configured prefix list.

LDP also responds to local label allocation configuration changes and to configuration changes that affect the prefix list that is used by LDP. Any of the following configuration changes can trigger LDP actions:

• Creating a local label allocation configuration
• Deleting or changing a local label allocation configuration
• Creating a new prefix list for a local label allocation configuration
• Deleting or changing a prefix list for a local label allocation configuration

LDP responds to local label allocation configuration changes by updating the LIB and the forwarding table in the global routing table. To update the LIB after a local label filter configuration change without a session reset, LDP keeps all remote bindings.

If you create a local label allocation configuration without defining a prefix list, no LDP action is required. The local label allocation configuration has no effect because the prefix list is created and permits all prefixes.

If you create or change a prefix list and prefixes that were previously allowed are rejected, LDP goes through a label withdraw and release procedure before the local labels for these prefixes are deallocated.

If you delete a prefix, LDP goes through the label withdraw and release procedure for the LIB local label. If the associated prefix is one for which no LIB entry should be allocated, LDP bypasses this procedure.

Note Local label allocation filtering has no impact on inbound or outbound label filtering because they all provide LDP filtering independently.

Outbound Label Filtering

MPLS LDP supports outbound label binding filtering. You can use this feature to control which label bindings are advertised to LDP neighbors.

Inbound Label Filtering

MPLS LDP supports inbound label binding filtering. You can use this feature to configure prefix lists for controlling the label bindings that an LSR accepts from its peer LSRs. You can limit LDP to accept a set of prefixes from a given LDP neighbor. By default, LDP accepts all labels for all prefixes from all LDP neighbors.

You can use the inbound label binding filtering feature to control the amount of memory used to store LDP label bindings advertised by other routers. For example, in a simple MPLS VPN environment, the VPN PE routers might require an LSP only to their peer PE routers (that is, they do not need LSPs to core routers). Inbound label filtering enables a PE router to accept labels only for other PE routers.

Creating a Prefix List for MPLS LDP Label Filtering

You can create a prefix list for MPLS LDP local label allocation filtering, outbound filtering, or inbound filtering. A prefix list allows LDP to selectively allocate local labels for a subset of the routes learned from the IGP, restrict the advertisement of local labels to specific LDP peers, or control the label bindings that an LSR accepts from its peer LSRs.

Prerequisites

Ensure that you are in the correct VDC (or use the switchto vdc command).

SUMMARY STEPS

1. configure terminal

2. ip prefix-list prefix-list { description description | seq number [ deny network / length [ eq eq-length | ge ge-length | le le-length ] | permit network / length [ eq eq-length | ge ge-length | le le-length]] | deny network / length [ eq eq-length | ge ge-length | le le-length ] | permit network / length [ eq eq-length | ge ge-length | le le-length] }

3. (Optional) show ip prefix-list [ prefix-list ]

4. (Optional) copy running-config startup-config

DETAILED STEPS

Configuring MPLS LDP Local Label Allocation Filtering

You can configure the Cisco NX-OS device for MPLS LDP local label allocation filtering. You can configure a prefix list, host routes, or all routes as a filter for local label allocation.

Prerequisites

Ensure that you are in the correct VDC (or use the switchto vdc command).

Ensure that LDP is enabled.

SUMMARY STEPS

1. configure terminal

2. mpls ldp configuration

3. label allocate global {prefix-list prefix-list | host-routes | all-routes}

4. (Optional) show mpls ldp bindings detail

5. (Optional) copy running-config startup-config

DETAILED STEPS

Configuring MPLS LDP Outbound Label Filtering

You can configure the Cisco NX-OS device for MPLS LDP outbound label filtering.

Prerequisites

Ensure that you are in the correct VDC (or use the switchto vdc command).

Ensure that MPLS LDP is enabled.

SUMMARY STEPS

1. configure terminal

2. ip prefix-list prefix-list permit network/length

3. ip prefix-list prefix-list permit network/length

4. mpls ldp configuration

5. advertise-labels [ for prefix-list [ to prefix-list ] | interface interface number ]

6. (Optional) show mpls ldp bindings detail

7. (Optional) copy running-config startup-config

DETAILED STEPS

Configuring MPLS LDP Inbound Label Filtering

You can configure the Cisco NX-OS device for MPLS LDP inbound label filtering.

Prerequisites

Ensure that you are in the correct VDC (or use the switchto vdc command).

Ensure that MPLS LDP is enabled.

SUMMARY STEPS

1. configure terminal

2. ip prefix-list prefix-list permit network/length

3. mpls ldp configuration

4. neighbor nbr-address labels accept prefix-list

5. (Optional) show mpls ldp neighbor [ address | interface ] [ detail ]

6. (Optional) copy running-config startup-config

DETAILED STEPS

Examples: Creating a Prefix List for MPLS LDP Local Label Allocation Filtering

The following examples show how to configure a prefix list for MPLS LDP local label allocation filtering.

In the following example, prefix list List1 permits only 192.168.0.0/16 prefixes. LDP accepts 192.168.0.0/16 prefixes but does not assign local labels for the following prefixes: 192.168.0.0/24 and 192.168.2.0/24.

In the following example, prefix list List2 permits a range of prefixes from 192.168.0.0/16 to /20. LDP accepts 192.168.0.0/16 prefixes but does not assign local labels for the following prefixes: 192.168.0.0/24 and 192.168.2.0/24.

In the following example, prefix list List3 permits a range of prefixes greater than /18. LDP accepts 192.168.17.0/20 and 192.168.2.0/24 prefixes but does not assign a local label for 192.168.0.0/16.

Examples: Configuring MPLS LDP Local Label Allocation Filtering

The following examples show how to configure an MPLS LDP local label allocation filter using a prefix list or host routes.

In the following example, a prefix list is configured as the local label allocation filter. Prefix list List3, which permits a range of prefixes greater than /18, is configured as the local label allocation filter for the router. LDP allows 192.168.17.0/20 and 192.168.2.0/24 prefixes but withdraws labels for prefixes not in the allowed range.

In the following example, host routes are configured as the local label allocation filter:

In the following example, all local label allocation filters are removed, and the default LDP local label allocation is restored without a session reset:

Sample MPLS LDP Local Label Allocation Filtering Configuration Example

Figure 8-3 is a sample configuration used to show how MPLS LDP local label allocation filtering works:

• Routers R1, R2, and R3 have loopback addresses 10.1.1.1, 10.2.2.2, and 10.3.3.3 defined and advertised by the IGP, respectively.
• 10.1.1.1 is the router ID of Router R1, 10.2.2.2 is the router ID of Router R2, and 10.3.3.3 is the router ID of Router R3.
• A prefix list is defined on Router R1 to specify the local labels for which LDP allocates a local label.

Router RI learns a number of routes from its IGP neighbors on Routers R2 and R3.

Figure 8-3 Sample MPLS LDP Local Label Allocation Filtering Configuration Example

You can use LDP commands to verify the following:

• Router R1 has allocated a local label for the correct subset of the prefixes.
• Routers R2 and R3 did not receive any remote bindings for the prefixes for which Router R1 did not assign a local label.

Local Label Bindings on Router R1, Router R2, and Router R3

In the following examples, LDP uses the default behavior of allocating a local label for every route and advertising a label binding for every route learned from the IGP.

The following example shows the contents of the LIB on Router R1 based on the configuration in Figure 8-3:

For the first prefix list listed (10.1.1.1/32), Router R1 has received remote labels from Routers R2 and R3 (17 and 16, respectively). The local labels assigned to 10.2.2.2 and 10.3.3.3 on Router R1 (1000 and 1002, respectively) have been advertised to Routers R2 and R3.

The following example shows the contents of the LIB on Router R2 based on the configuration in Figure 8-3:

For the second prefix list listed (10.2.2.2/32), Router R2 has received remote labels from Routers R1 and R3 (1000 and 18, respectively). The local labels assigned to 10.1.1.1 and 10.3.3.3 on Router R2 (17 and 18, respectively) have been advertised to Routers R1 and R3.

The following example shows the contents of the LIB on Router R3 based on the configuration in Figure 8-3:

For the third prefix list listed (10.3.3.3/32), Router R3 has received remote labels from Routers R1 and R2 (1002 and 18, respectively). The local labels assigned to 10.1.1.1 and 10.2.2.2 on Router R3 (16 and 18, respectively) have been advertised to Routers R1 and R2.

Local Label Allocation Filtering Configuration on Router R1

The following examples show how to configure local label allocation filtering.

The following example shows the selection of host routes as the only filter on Router R1:

The following example shows how to configure a local label allocation filter that allows or denies prefixes based on prefix list ListA:

Local Label Allocation Filtering Changes Label Bindings on Router R1, Router R2, and Router R3

After configuring a local label allocation filter on Router R1, you can verify the changes in the local label bindings in the LIB on each router. Changes to the output in the LIB entries are highlighted in bold text.

The following example shows how the configuration of a local label allocation prefix-list filter changes the contents of the LIB on Router R1:

The following example shows how the configuration of a local label allocation prefix-list filter on Router R1 changes the contents of the LIB on Router R2:

The 10.10.7.0/24, 10.10.8.0/24, and 10.10.9.0/24 prefixes are no longer assigned with local labels. Therefore, Router R1 sends no label advertisement for these prefixes.

The following example shows how the configuration of a local label allocation prefix-list filter on Router R1 changes the contents of the LIB on Router R3:

The 10.10.7.0/24, 10.10.8.0/24, and 10.10.9.0/24 prefixes are no longer assigned with local labels. Again, Router R1 sends no label advertisement for these prefixes.

Examples: Configuring MPLS LDP Outbound Label Filtering

The following example shows how to configure the device to advertise the label for network 10.0.0.0 only to LSR 35.0.0.55 and the labels for all other prefixes to all LSRs:

Examples: Configuring MPLS LDP Inbound Label Filtering

The following example shows how to configure a prefix list to filter label bindings received on sessions with the neighbor 10.0.0.31.

Label bindings for prefixes that match 10. b . c . d are accepted, where b is less than or equal to 63, and c and d can be any integer between 0 and 128. Other label bindings received from 10.0.0.31 are rejected.

The following example shows label bindings that were learned from 10.0.0.31. It verifies that the LIB does not contain label bindings for prefixes that have been excluded.

remote binding: lsr: 10.0.0.31:0, label: imp-null

Related Documents

MIBs