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Point-to-Multipoint LSP Configuration

Point-to-Multipoint LSPs Overview

A point-to-multipoint MPLS LSP is an LSP with a single source and multiple destinations. By taking advantage of the MPLS packet replication capability of the network, point-to-multipoint LSPs avoid unnecessary packet replication at the ingress router. Packet replication takes place only when packets are forwarded to two or more different destinations requiring different network paths.

This process is illustrated in Figure 1. Router PE1 is configured with a point-to-multipoint LSP to Routers PE2, PE3, and PE4. When Router PE1 sends a packet on the point-to-multipoint LSP to Routers P1 and P2, Router P1 replicates the packet and forwards it to Routers PE2 and PE3. Router P2 sends the packet to Router PE4.

This feature is described in detail in the Internet drafts draft-raggarwa-mpls-p2mp-te-02.txt (expired February 2004), Establishing Point to Multipoint MPLS TE LSPs, draft-ietf-mpls-rsvp-te-p2mp-02.txt, Extensions to Resource Reservation Protocol-Traffic Engineering (RSVP-TE) for Point-to-Multipoint TE Label-Switched Paths (LSPs), and RFC 6388, Label Distribution Protocol Extensions for Point-to-Multipoint and Multipoint-to-Multipoint Label Switched Paths (only point-to-multipoint LSPs are supported).

Figure 1: Point-to-Multipoint LSPsPoint-to-Multipoint LSPs

The following are some of the properties of point-to-multipoint LSPs:

  • A point-to-multipoint LSP enables you to use MPLS for point-to-multipoint data distribution. This functionality is similar to that provided by IP multicast.

  • You can add and remove branch LSPs from a main point-to-multipoint LSP without disrupting traffic. The unaffected parts of the point-to-multipoint LSP continue to function normally.

  • You can configure a node to be both a transit and an egress router for different branch LSPs of the same point-to-multipoint LSP.

  • You can enable link protection on a point-to-multipoint LSP. Link protection can provide a bypass LSP for each of the branch LSPs that make up the point-to-multipoint LSP. If any of the primary paths fail, traffic can be quickly switched to the bypass.

  • You can configure branch LSPs either statically, dynamically, or as a combination of static and dynamic LSPs.

  • You can enable graceful Routing Engine switchover (GRES) and graceful restart for point-to-multipoint LSPs at ingress and egress routers. The point-to-multipoint LSPs must be configured using either static routes or circuit cross-connect (CCC). GRES and graceful restart allow the traffic to be forwarded at the Packet Forwarding Engine based on the old state while the control plane recovers. Feature parity for GRES and graceful restart for MPLS point-to-multipoint LSPs on the Junos Trio chipset is supported in Junos OS Releases 11.1R2, 11.2R2, and 11.4.

Understanding Point-to-Multipoint LSPs

A point-to-multipoint MPLS label-switched path (LSP) is an LDP-signaled or RSVP-signaled LSP with a single source and multiple destinations. By taking advantage of the MPLS packet replication capability of the network, point-to-multipoint LSPs avoid unnecessary packet replication at the inbound (ingress) router. Packet replication takes place only when packets are forwarded to two or more different destinations requiring different network paths.

This process is illustrated in Figure 2. Device PE1 is configured with a point-to-multipoint LSP to Routers PE2, PE3, and PE4. When Device PE1 sends a packet on the point-to-multipoint LSP to Routers P1 and P2, Device P1 replicates the packet and forwards it to Routers PE2 and PE3. Device P2 sends the packet to Device PE4.

Figure 2: Point-to-Multipoint LSPsPoint-to-Multipoint LSPs

Following are some of the properties of point-to-multipoint LSPs:

  • A point-to-multipoint LSP allows you to use MPLS for point-to-multipoint data distribution. This functionality is similar to that provided by IP multicast.

  • You can add and remove branch LSPs from a main point-to-multipoint LSP without disrupting traffic. The unaffected parts of the point-to-multipoint LSP continue to function normally.

  • You can configure a node to be both a transit and an outbound (egress) router for different branch LSPs of the same point-to-multipoint LSP.

  • You can enable link protection on a point-to-multipoint LSP. Link protection can provide a bypass LSP for each of the branch LSPs that make up the point-to-multipoint LSP. If any primary paths fail, traffic can be quickly switched to the bypass.

  • You can configure subpaths either statically or dynamically.

  • You can enable graceful restart on point-to-multipoint LSPs.

Point-to-Multipoint LSP Configuration Overview

To set up a point-to-multipoint LSP:

  1. Configure the primary LSP from the ingress router and the branch LSPs that carry traffic to the egress routers.
  2. Specify a pathname on the primary LSP and this same path name on each branch LSP.
Note:

By default, the branch LSPs are dynamically signaled by means of Constrained Shortest Path First (CSPF) and require no configuration. You can alternatively configure the branch LSPs as static paths.

Example: Configuring a Collection of Paths to Create an RSVP-Signaled Point-to-Multipoint LSP

This example shows how to configure a collection of paths to create an RSVP-signaled point-to-multipoint label-switched path (LSP).

Requirements

In this example, no special configuration beyond device initialization is required.

Overview

In this example, multiple routing devices serve as the transit, branch, and leaf nodes of a single point-to-multipoint LSP. On the provider edge (PE), Device PE1 is the ingress node. The branches go from PE1 to PE2, PE1 to PE3, and PE1 to PE4. Static unicast routes on the ingress node (PE1) point to the egress nodes.

This example also demonstrates static routes with a next hop that is a point-to-multipoint LSP, using the p2mp-lsp-next-hop statement. This is useful when implementing filter-based forwarding.

Note:

Another option is to use the lsp-next-hop statement to configure a regular point-to-point LSP to be the next hop. Though not shown in this example, you can optionally assign an independent preference and metric to the next hop.

Topology Diagram

Figure 3 shows the topology used in this example.

Figure 3: RSVP-Signaled Point-to-Multipoint LSPRSVP-Signaled Point-to-Multipoint LSP

Configuration

CLI Quick Configuration

To quickly configure this example, copy the following commands, paste them into a text file, remove any line breaks, change any details necessary to match your network configuration, and then copy and paste the commands into the CLI at the [edit] hierarchy level.

Device PE1

Device CE1

Device CE2

Device CE3

Device CE4

Configuring the Ingress Label-Switched Router (LSR) (Device PE1)

Step-by-Step Procedure

To configure Device PE1:

  1. Configure the interfaces, interface encapsulation, and protocol families.

  2. Enable RSVP, MPLS, and OSPF on the interfaces.

  3. Configure the MPLS point-to-multipoint LSPs.

  4. (Optional) Enable link protection on the LSPs.

    Link protection helps to ensure that traffic sent over a specific interface to a neighboring router can continue to reach the router if that interface fails.

  5. Enable MPLS to perform traffic engineering for OSPF.

    This causes the ingress routes to be installed in the inet.0 routing table. By default, MPLS performs traffic engineering for BGP only. You need to enable MPLS traffic engineering on the ingress LSR only.

  6. Enable traffic engineering for OSPF.

    This causes the shortest-path first (SPF) algorithm to take into account the LSPs configured under MPLS.

  7. Configure the router ID.

  8. Configure static IP unicast routes with the point-to-multipoint LSP name as the next hop for each route.

  9. If you are done configuring the device, commit the configuration.

Configuring the Transit and Egress LSRs (Devices P2, P3, P4, PE2, PE3, and PE4)

Step-by-Step Procedure

To configure the transit and egress LSRs:

  1. Configure the interfaces, interface encapsulation, and protocol families.

  2. Enable RSVP, MPLS, and OSPF on the interfaces.

  3. Enable traffic engineering for OSPF.

    This causes the shortest-path first (SPF) algorithm to take into account the LSPs configured under MPLS.

  4. Configure the router IDs.

  5. If you are done configuring the devices, commit the configuration.

Results

From configuration mode, confirm your configuration by entering the show interfaces, show protocols, and show routing-options commands. If the output does not display the intended configuration, repeat the instructions in this example to correct the configuration.

Device PE1

Device P2

Device P3

Device P4

Device PE2

Device PE3

Device PE4

Configuring Device CE1

Step-by-Step Procedure

To configure Device CE1:

  1. Configure an interface to Device PE1.

  2. Configure static routes from Device CE1 to the three other customer networks, with Device PE1 as the next hop.

  3. If you are done configuring the device, commit the configuration.

Results

From configuration mode, confirm your configuration by entering the show interfaces and show routing-options commands. If the output does not display the intended configuration, repeat the instructions in this example to correct the configuration.

Configuring Device CE2

Step-by-Step Procedure

To configure Device CE2:

  1. Configure an interface to Device PE2.

  2. Configure a static route from Device CE2 to CE1, with Device PE2 as the next hop.

  3. If you are done configuring the device, commit the configuration.

Results

From configuration mode, confirm your configuration by entering the show interfaces and show routing-options commands. If the output does not display the intended configuration, repeat the instructions in this example to correct the configuration.

Configuring Device CE3

Step-by-Step Procedure

To configure Device CE3:

  1. Configure an interface to Device PE3.

  2. Configure a static route from Device CE3 to CE1, with Device PE3 as the next hop.

  3. If you are done configuring the device, commit the configuration.

Results

From configuration mode, confirm your configuration by entering the show interfaces and show routing-options commands. If the output does not display the intended configuration, repeat the instructions in this example to correct the configuration.

Configuring Device CE4

Step-by-Step Procedure

To configure Device CE4:

  1. Configure an interface to Device PE4.

  2. Configure a static route from Device CE4 to CE1, with Device PE4 as the next hop.

  3. If you are done configuring the device, commit the configuration.

Results

From configuration mode, confirm your configuration by entering the show interfaces and show routing-options commands. If the output does not display the intended configuration, repeat the instructions in this example to correct the configuration.

Verification

Confirm that the configuration is working properly.

Verifying Connectivity

Purpose

Make sure that the devices can ping each other.

Action

Run the ping command from CE1 to the interface on CE2 connecting to PE2.

Run the ping command from CE1 to the interface on CE3 connecting to PE3.

Run the ping command from CE1 to the interface on CE4 connecting to PE4.

Verifying the State of the Point-to-Multipoint LSP

Purpose

Make sure that the ingress, transit, and egress LSRs are in the Up state.

Action

Run the show mpls lsp p2mp command on all of the LSRs. Only the ingress LSR is shown here.

Checking the Forwarding Table

Purpose

Make sure that the routes are set up as expected by running the show route forwarding-table command. Only the routes to the remote customer networks are shown here.

Action

Configuring Primary and Branch LSPs for Point-to-Multipoint LSPs

A point-to-multipoint MPLS label-switched path (LSP) is an RSVP LSP with multiple destinations. By taking advantage of the MPLS packet replication capability of the network, point-to-multipoint LSPs avoid unnecessary packet replication at the ingress router. For more information about point-to-multipoint LSPs, see Point-to-Multipoint LSPs Overview.

To configure a point-to-multipoint LSP, you need to configure the primary LSP from the ingress router and the branch LSPs that carry traffic to the egress routers, as described in the following sections:

Configuring the Primary Point-to-Multipoint LSP

A point-to-multipoint LSP must have a configured primary point-to-multipoint LSP to carry traffic from the ingress router. The configuration of the primary point-to-multipoint LSP is similar to a signaled LSP. See Configuring the Ingress Router for MPLS-Signaled LSPs for more information. In addition to the conventional LSP configuration, you need to specify a path name for the primary point-to-multipoint LSP by including the p2mp statement:

You can include this statement at the following hierarchy levels:

You can enable the optimization timer for point-to-multipoint LSPs. See Optimizing Signaled LSPs for more information.

Configuring a Branch LSP for Point-to-Multipoint LSPs

The primary point-to-multipoint LSP sends traffic to two or more branch LSPs carrying traffic to each of the egress provider edge (PE) routers. In the configuration for each of these branch LSPs, the point-to-multipoint LSP path name you specify must be identical to the path name configured for the primary point-to-multipoint LSP. See Configuring the Primary Point-to-Multipoint LSP for more information.

To associate a branch LSP with the primary point-to-multipoint LSP, specify the point-to-multipoint LSP name by including the p2mp statement:

You can include this statement at the following hierarchy levels:

  • [edit protocols mpls label-switched-path lsp-name]

  • [edit logical-systems logical-system-name protocols mpls label-switched-path lsp-name]

    Note:

    Any change in any of the branch LSPs of a point-to-multipoint LSP, either due to a user action or an automatic adjustment made by the router, causes the primary and branch LSPs to be resignaled. The new point-to-multipoint LSP is signaled first before the old path is taken down.

The following sections describe how you can configure the branch LSP as a dynamically signaled path using Constrained Shortest Path First (CSPF), as a static path, or as a combination of dynamic and static paths:

Configuring the Branch LSP as a Dynamic Path

By default, the branch LSP for a point-to-multipoint LSP is signaled dynamically using CSPF and requires no configuration.

When a point-to-multipoint LSP is changed, either by the addition or deletion of new destinations or by the recalculation of the path to existing destinations, certain nodes in the tree might receive data from more than one incoming interface. This can happen under the following conditions:

  • Some of the branch LSPs to destinations are statically configured and might intersect with statically or dynamically calculated paths to other destinations.

  • When a dynamically calculated path for a branch LSP results in a change of incoming interface for one of the nodes in the network, the older path is not immediately torn down after the new one has been signaled. This ensures that any data in transit relying on the older path can reach its destination. However, network traffic can potentially use either path to reach the destination.

  • A faulty router at the ingress calculates the paths to two different branch destinations such that a different incoming interface is chosen for these branch LSPs on a router node common to these branch LSPs.

Configuring the Branch LSP as a Static Path

You can configure the branch LSP for a point-to-multipoint LSP as a static path. See Configuring Static LSPs for more information.

Configuring Inter-Domain Point-to-Multipoint LSPs

An inter-domain P2MP LSP is a P2MP LSP that has one or more sub-LSPs (branches) that span multiple domains in a network. Examples of such domains include IGP areas and autonomous systems (ASs). A sub-LSP of an inter-domain P2MP LSP may be intra-area, inter-area, or inter-AS, depending on the location of the egress node (leaf) with respect to the ingress node (source).

On the ingress node, a name is assigned to the inter-domain P2MP LSP and shared by all constituent sub-LSPs. Each sub-LSP is configured separately, with its own egress node and optionally an explicit path. The location of the egress node of the sub-LSP with respect to the ingress node determines whether the sub-LSP is intra-area, inter-area, or inter-AS.

Inter-domain P2MP LSPs can be used to transport traffic in the following applications in a multi-area or multi-AS network:

  • Layer 2 broadcast and multicast over MPLS

  • Layer 3 BGP/MPLS VPN

  • VPLS

On each domain boundary node (ABR or ASBR) along the path of the P2MP LSP, the expand-loose-hop statement must be configured at the [edit protocols mpls] hierarchy level so that CSPF can extend a loose-hop ERO (usually the first entry of the ERO list carried by RSVP Path message) towards the egress node or the next domain boundary node.

CSPF path computation for inter-domain P2MP LSPs:

  • CSPF path computation is supported on each sub-LSP for inter-domain P2MP LSPs. A sub-LSP may be intra-area, inter-area, or inter-AS. CSPF treats an inter-area or inter-AS sub-LSP in the same manner as an inter-domain P2P LSP.

  • On an ingress node or a domain boundary node (ABR or ASBR), CSPF can perform an Explicit Route Object (ERO) expansion per-RSVP query. The destination queried could be an egress node or a received loose-hop ERO. If the destination resides in a neighboring domain that the node is connected to, CSPF generates either a sequence of strict-hop EROs towards it or a sequence of strict-hop EROs towards another domain boundary node that can reach the destination.

  • If RSVP fails to signal a path through a previously selected domain bounday node, RSVP attempts to signal a path through other available domain boundary nodes in a round-robin fashion.

  • When a sub-LSP is added or removed to or from an inter-domain P2MP LSP, causing its path (branch) to be merged or pruned with or from the current P2MP tree, the paths being taken by the other sub-LSPs should not be affected, helping to prevent traffic disruption on those sub-LSPs.

Be aware of the following when deploying inter-domain P2MP LSPs in your network:

  • Periodic path re-optimization is supported for inter-domain P2MP LSPs on ingress nodes. It can be turned on for an inter-domain P2MP LSP by configuring the optimize-timer statement at the [edit protocols mpls label-switched-path lsp-name] hierarchy level with the same interval for every sub-LSP.

  • Only link protection bypass LSPs are supported for inter-domain P2MP LSPs. To enable it for an inter-domain P2MP LSP, link-protection must be configured for all sub-LSPs and on all of the RSVP interfaces that the P2MP LSP might travel through.

  • Only OSPF areas are supported for inter-domain P2MP LSPs. IS-IS levels are not supported.

Configuring Graceful Restart for Point-to-Multipoint LSPs

You can configure graceful restart on point-to-multipoint LSPs. Graceful restart allows a router undergoing a restart to inform its adjacent neighbors of its condition. The restarting router requests a grace period from the neighbor or peer, which can then cooperate with the restarting router. The restarting router can still forward MPLS traffic during the restart period; convergence in the network is not disrupted. The restart is not apparent to the rest of the network, and the restarting router is not removed from the network topology. RSVP graceful restart can be enabled on both transit routers and ingress routers.

To enable graceful restart on a router handling point-to-multipoint LSP traffic, include the graceful-restart statement:

You can include this statement at the following hierarchy levels:

  • [edit routing-options]

  • [edit logical-systems logical-system-name routing-options]

The graceful restart configuration for point-to-multipoint LSPs is identical to that of point-to-point LSPs. For more information on how to configure graceful restart, see Configuring RSVP Graceful Restart.

Configuring a Multicast RPF Check Policy for Point-to-Multipoint LSPs

You can control whether a reverse path forwarding (RPF) check is performed for a source and group entry before installing a route in the multicast forwarding cache. This makes it possible to use point-to-multipoint LSPs to distribute multicast traffic to PIM islands situated downstream from the egress routers of the point-to-multipoint LSPs.

By configuring the rpf-check-policy statement, you can disable RPF checks for a source and group pair. You would typically configure this statement on the egress routers of a point-to-multipoint LSP, because the interface receiving the multicast traffic on a point-to-multipoint LSP egress router might not always be the RPF interface.

You can also configure a routing policy to act upon a source and group pair. This policy behaves like an import policy, so if no policy term matches the input data, the default policy action is “acceptance.” An accept policy action enables RPF checks. A reject policy action (applied to all source and group pairs that are not accepted) disables RPF checks for the pair.

To configure a multicast RPF check policy for a point-to-multipoint LSP, specify the RPF check policy using the rpf-check-policy statement:

You can include this statement at the following hierarchy levels:

  • [edit routing-options multicast]

  • [edit logical-systems logical-system-name routing-options multicast]

You also must configure a policy for the multicast RPF check. You configure policies at the [edit policy-options] hierarchy level. For more information, see the Routing Policies, Firewall Filters, and Traffic Policers User Guide.

Note:

When you configure the rpf-check-policy statement, the Junos OS cannot perform RPF checks on incoming traffic and therefore cannot detect traffic arriving on the wrong interface. This might cause routing loops to form.

Example: Configuring Multicast RPF Check Policy for a Point-to-Multipoint LSP

Configure a policy to ensure that an RPF check is not performed for sources with prefix 128.83/16 or longer that belong to groups having a prefix of 228/8 or longer:

Configuring Ingress PE Router Redundancy for Point-to-Multipoint LSPs

You can configure one or more PE routers as part of a backup PE router group to enable ingress PE router redundancy. You accomplish this by configuring the IP addresses of the backup PE routers (at least one backup PE router is required) and the local IP address used by the local PE router.

You must also configure a full mesh of point-to-point LSPs between the primary and backup PE routers. You also need to configure BFD on these LSPs. See Configuring BFD for RSVP-Signaled LSPs and Configuring BFD for LDP LSPs for more information.

To configure ingress PE router redundancy for point-to-multipoint LSPs, include the backup-pe-group statement:

For a list of hierarchy levels at which you can include these statements, see the statement summary sections for these statements.

After you configure the ingress PE router redundancy backup group, you must also apply the group to a static route on the PE router. This ensures that the static route is active (installed in the forwarding table) when the local PE router is the designated forwarder for the backup PE group. You can only associate a backup PE router group with a static route that also has the p2mp-lsp-next-hop statement configured. For more information, see Configuring Static Unicast Routes for Point-to-Multipoint LSPs.

Enabling Point-to-Point LSPs to Monitor Egress PE Routers

Configuring an LSP with the associate-backup-pe-groups statement enables it to monitor the status of the PE router to which it is configured. You can configure multiple backup PE router groups using the same router's address. A failure of this LSP indicates to all of the backup PE router groups that the destination PE router is down. The associate-backup-pe-groups statement is not tied to a specific backup PE router group. It applies to all groups that are interested in the status of the LSP to that address.

To allow an LSP to monitor the status of the egress PE router, include the associate-backup-pe-groups statement:

This statement can be configured at the following hierarchy levels:

If you configure the associate-backup-pe-groups statement, you must configure BFD for the point-to-point LSP. For information about how to configure BFD for an LSP, see Configuring BFD for MPLS IPv4 LSPs and Configuring BFD for LDP LSPs.

You also must configure a full mesh of point-to-point LSPs between the PE routers in the backup PE router group. A full mesh is required so that each PE router within the group can independently determine the status of the other PE routers, allowing each router to independently determine which PE router is currently the designated forwarder for the backup PE router group.

If you configure multiple LSPs with the associate-backup-pe-groups statement to the same destination PE router, the first LSP configured is used to monitor the forwarding state to that PE router. If you configure multiple LSPs to the same destination, make sure to configure similar parameters for the LSPs. With this configuration scenario, a failure notification might be triggered even though the remote PE router is still up.

Preserving Point-to-Multipoint LSP Functioning with Different Junos OS Releases

In Junos OS Release 9.1 and earlier, Resv messages that include the S2L_SUB_LSP object are rejected by default. In Junos OS Release 9.2 and later, such messages are accepted by default. To ensure proper functioning of point-to-multipoint LSPs in a network that includes both devices running Junos OS Release 9.1 and earlier and devices running Junos 9.2 and later, you must include the no-p2mp-sublsp statement in the configuration of the devices running Junos 9.2 and later:

You can include this statement at the following hierarchy levels:

  • [edit protocols rsvp]

  • [edit logical-systems logical-system-name protocols rsvp]

Re-merge Behavior on Point-to-Multipoint LSP Overview

This section talks about the benefits and overview of controlling the re-merge behavior on RSVP Point-to-Multipoint (P2MP) LSPs.

Benefits of Controlling the P2MP LSP Re-merge

  • Reduces the RSVP signalling load on the ingress (headend routers) by avoiding path computation of sub LSPs which creates a re-merge condition.

  • Saves the network bandwidth by rejecting the P2MP sub LSP re-merge at the transit node.

What is P2MP LSP Re-merge?

In a P2MP MPLS LSP network, the term re-merge refers to the case of an ingress (headend) or transit node (re-merge node) that creates a re-merge branch intersecting the P2MP LSP at another node down the tree. This may occur due to events such as an error in path calculation, an error in manual configuration, or network topology changes during the establishment of the P2MP LSP.

RFC 4875 defines the following two approaches for handling the P2MP LSP re-merge:

  • First, the node detecting the re-merge allows the re-merge case to persist, but data from all but one incoming interface is dropped at the re-merge node. This works by default without any configuration.

  • Second, the re-merge node initiates the pruning of the re-merge sub LSPs through signaling.

On Juniper Networks MX Series routers, the first approach (as defined by RFC 4875) works by default. The second approach can be implemented by one of the following CLI configuration statements depending upon where the Juniper Networks MX Series routers are placed (ingress node or transit node) in the P2MP RSVP MPLS network:

  • no-re-merge—This CLI configuration statement when enabled at the ingress (headend) router avoids the path computation of P2MP sub LSPs which creates a re-merge condition. When this CLI configuration statement is configured at the ingress, then configuring the no-p2mp-re-merge CLI configuration statement at the transit router is not required.

  • no-p2mp-re-merge—This CLI configuration statement when enabled at the transit router changes the default behavior of allowing the P2MP sub LSP sessions re-merge to rejecting the re-merge. This CLI configuration statement is primarily required when the ingress (headend router) is not a Juniper Networks MX Series router.

  • single-abr—This command when enabled reduces re-merge condition beyond the inter-area, or inter-domain, or inter-AS RSVP P2MP LSPs.

The following topology explains the re-merge behavior in a P2MP LSP network:

What is P2MP LSP Re-merge?

In this topology, R1 acts as an ingress (headend) router and R2 as the transit (re-merge node) router. There are two sub LSP sessions created in this network, LSP 1 and LSP 2. LSP 1 is a session established among R1, R2, and R3 devices. LSP 2 is a session established between R1, R4, R2, R3, and R5 devices. By default, the transit router allows the re-merge to happen from both the sub LSPs and drops one of the sub LSP branch traffic at the re-merge node. You can control this re-merge behavior by enabling the no-re-merge CLI configuration statement at the ingress router, or the no-p2mp-re-merge CLI configuration statement at the transit router.

If you enable the no-re-merge CLI configuration statement at the ingress router (R1), only one of the two sub LSP sessions is established. For example, if LSP 1 (R1-R2-R3) session is established first, then the other sub LSP session (LSP 2) will not be established.

If you enable the no-p2mp-re-merge CLI configuration statement at the transit router (R2), the transit router rejects the re-merge of one of the sub LSPs and sends a path error message to the ingress router (R1) preventing the ingress router from creating the second P2MP LSP re-merge branch. You can use the show rsvp statistics CLI command to view the path error message.

Modify the Default P2MP LSP Re-merge Behavior

You can modify the default re-merge behavior either at the ingress (headend) node, or at the transit node in a P2MP RSVP MPLS network.

On the ingress (headend router), disable the default re-merge behavior so that the ingress router does not do the path computation of sub LSPs which creates the re-merge condition. The default behavior allows the path computation of sub LSPs.

On the transit router, disable the default re-merge behavior so that the transit router rejects the re-merge of sub LSPs.

For inter-area, or inter-domain, or inter-AS RSVP P2MP LSPs, use the single-abr CLI configuration statement at the ingress (headend router) so that all the P2MP sub LSPs prefer to select the same exit router (ABR or ASBR), thereby reducing the re-merge condition.