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Example: Configuring Ingress PE Redundancy

 

Understanding Ingress PE Redundancy

In many network topologies, point-to-multipoint label-switched paths (LSPs) are used to distribute multicast traffic over a virtual private network (VPN). When traffic engineering is added to the provider edge (PE) routers, a popular deployment option has been to use traffic-engineered point-to-multipoint LSPs at the origin PE. In these network deployments, the PE is a single point of failure. Network operators have previously provided redundancy by broadcasting duplicate streams of multicast traffic from multiple PEs, a practice which at least doubles the bandwidth required for each stream.

Ingress PE redundancy eliminates the bandwidth duplication requirement by configuring one or more ingress PEs as a group. Within a group, one PE is designated as the primary PE and one or more others become backup PEs for the configured traffic stream. The solution depends on a full mesh of point-to-point (P2P) LSPs among the primary and backup PEs. Also, you must configure a full set of point-to-multipoint LSPs at the backup PEs, even though these point-to-multipoint LSPs at the backup PEs are not sending any traffic or using any bandwidth. The P2P LSPs are configured with bidirectional forwarding detection (BFD). When BFD detects a failure on the primary PE, a new designated forwarder is elected for the stream.

Example: Configuring Ingress PE Redundancy

This example shows how to configure one PE as part of a backup PE group to enable ingress PE redundancy for multicast traffic streams.



Requirements

Before you begin:

  • Configure the router interfaces.

  • Configure a full mesh of P2P LSPs between the PEs in the backup group.

Overview

Ingress PE redundancy provides a backup resource when point-to-multipoint LSPs are configured for multicast distribution. When point-to-multipoint LSPs are used for multicast traffic, the PE device can become a single point of failure. One way to provide redundancy is by broadcasting duplicate streams from multiple PEs, thus doubling the bandwidth requirements for each stream. This feature implements redundancy between two or more PEs by designating a primary and one or more backup PEs for each configured stream. The solution depends on the configuration of a full mesh of P2P LSPs between the primary and backup PEs. These LSPs are configured with Bidirectional Forwarding Detection (BFD) running on top of them. BFD is used on the backup PEs to detect failure on the primary PE routing device and to elect a new designated forwarder for the stream.

A full mesh is required so that each member of the group can make an independent decision about the health of the other PEs and determine the designated forwarder for the group. The key concept in a backup PE group is that of a designated PE. A designated PE is a PE that forwards data on the static route. All other PEs in the backup PE group do not forward any data on the static route. This allows you to have one designated forwarder. If the designated forwarder fails, another PE takes over as the designated forwarder, thus allowing the traffic flow to continue uninterrupted.

Each PE in the backup PE group makes its own local decision regarding the designated forwarder. Thus, there is no inter-PE communication regarding designated forwarder. A PE computes the designated forwarder based on the IP address of all PEs and the connectivity status of other PEs. Connectivity status is determined based on the state of the BFD session on the P2P LSP to a PE.

A PE chosen is as the designated forwarder if it satisfies the following conditions:

  • The PE is in the UP state. Either it is the local PE, or the BFD session on the P2P LSP to that PE is in the UP state.

  • The PE has the lowest IP address among all PEs that are in the UP state.

Because all PEs have P2P LSPs to each other, each PE can determine the UP state of each other PE, and all PEs converge to the same designated forwarder.

If the designated forwarder PE fails, then all other PEs lose connectivity with the designated forwarder, and their BFD session ends. Consequently, other PEs then choose another designated forwarder. The new forwarder starts forwarding traffic. Thus, the traffic loss is limited to the failure detection time, which is the BFD session detection time.

When a PE that was the designated forwarder fails and then resumes operating, all other PEs recognize this fact, rerun the designated forwarder algorithm, and choose the PE as the designated forwarder. Consequently, the backup designated forwarder stops forwarding traffic. Thus, traffic switches back to the most eligible designated forwarder.

This example includes the following statements:

  • associate-backup-pe-groups—Monitors the health of the routing device at the other end of the LSP. You can configure multiple backup PE groups that contain the same routing device’s address. Failure of this LSP indicates to all of these groups that the destination PE routing device is down. So, the associate-backup-pe-groups statement is not tied to any specific group but applies to all groups that are monitoring the health of the LSP to the remote address.

    If there are multiple LSPs with the associate-backup-pe-groups statement to the same destination PE, then the local routing device picks the first LSP to that PE for detection purposes.

    We do not recommend configuring multiple LSPs to the same destination. If you do, make sure that the LSP parameters (for example, liveliness detection) are similar to avoid false failure notification even when the remote PE is up.

  • backup-pe-group—Configures ingress PE redundancy for multicast traffic streams.

  • bfd-liveness-detection—Enables BFD for each LSP.

  • label-switched-path—Configures an LSP. You must configure a full mesh of P2P LSPs between the primary and backup PEs.

    Note

    We recommend that you configure the P2P LSPs with fast reroute and node link protection so that link failures do not result in the LSP failure. For the purpose of PE redundancy, a failure in the P2P LSP is treated as a PE failure. Redundancy in the inter-PE path is also encouraged.

  • p2mp-lsp-next-hop—Enables you to associate a backup PE group with a static route.

  • static—Applies the backup group to a static route on the PE. This ensures that the static route is active (installed in the forwarding table) when the local PE is the designated forwarder for the configured backup PE group.

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, copy and paste the commands into the CLI at the [edit] hierarchy level, and then enter commit from configuration mode.

Step-by-Step Procedure

The following example requires you to navigate various levels in the configuration hierarchy. For information about navigating the CLI, see Using the CLI Editor in Configuration Mode in the CLI User Guide.

To configure ingress PE redundancy:

  1. Configure the multicast settings.

  2. Configure the RPF policy.

  3. Configure the backup PE group.

  4. Configure the static routes for the point-to-multipoint LSPs backup PE group.

  5. Configure the MPLS interfaces.

  6. Configure the LSP to the redundant router.

  7. Configure LSPs to two traffic destinations.

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

Results

Confirm your configuration by entering the show policy, show protocols, and show routing-options commands.

Verification

To verify the configuration, run the following commands:

  • show mpls lsp

  • show multicast backup-pe-groups

  • show multicast rpf