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# Ethernet Link Redundancy Overview

You can use 802.3ad Link Aggregation (LAG) to configure Ethernet link redundancy for Fast Ethernet and Gigabit Ethernet interfaces. Ethernet link redundancy enables you to protect against physical link failure and account for network topology changes that redirect network traffic to redundant ports.

The following configurations are available:

- LAG to LAG—Provides redundancy capabilities for two or more ports that are assigned to a LAG. One member link is configured as the backup interface for all other ports in the LAG bundle (1:N). Traffic is not forwarded over the backup member interface; it is disabled until it takes over for an active member interface.
- LAG to non-LAG—Provides redundancy capabilities when redundant ports are connected to a bridged network that has Rapid Spanning Tree Protocol (RSTP) controlling the topology. This configuration supports only two links in the LAG.

For information about the modules that support link aggregation, see ERX Module Guide, Appendix A, Module Protocol Support and E120 and E320 Module Guide, Appendix A, IOA Protocol Support.

## Ethernet Link Redundancy Configuration Models

The link connections determine the configuration model for link redundancy. The following connection types are available:

- Single-homed—Connections are between the local Ethernet interface and a single remote device. When the peer is also configured with LAG, LACP can be used to control link access.
- Dual-homed—Connections are between two separate, uncoordinated remote devices. The remote interfaces can be on the same module or on separate hardware. If LAG is not configured on the peers, LACP cannot be used to select ports; other protocols such as RSTP can be used.

The type of hardware used for connections further characterizes the single-homed and dual-homed configuration models. The following hardware types are available:

- Homogeneous—Remote interface is on another Fast Ethernet or Gigabit Ethernet port in a back-to-back router configuration of identical hardware and JUNOSe software versions. Both interfaces support the same redundant cabling and algorithm. The interfaces can be cabled on the same ports (port 0–port 0, port 1–port 1) or cross-cabled (port 0–port 1, port 1–port 0).
- Heterogeneous—Remote interface is on a different type of hardware that might or might not support redundant cabling, or on the same type of equipment with different software versions. For example, a heterogeneous configuration can include an ES2-S1 GE-4 IOA and an ES2-S1 GE-8 IOA on the E320 router, or an E-series router operating JUNOSe software connected to another vendor’s router and software.

  | Note: You cannot configure link redundancy across different types of line modules in a router. You also cannot configure link redundancy across two GE-4 IOAs on the E120 router or the E320 router.

Figure 21 illustrates the configuration models for Ethernet link redundancy.

Figure 21: Ethernet Link Redundancy Configuration Models

### Ethernet Link Redundancy Configuration Diagrams

The diagrams in this section illustrate examples of Ethernet link redundancy configurations. The diagrams display adjacent ports bundled in a LAG.

GE-2 Line Module Configurations

These diagrams compare physical port redundancy and link redundancy on a GE-2 line module.

Figure 22 displays a GE-2 line module with physical port redundancy on both ports.

Figure 22: GE-2 Line Module Using Physical Port Redundancy

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Figure 23 displays a single-homed configuration with port 0 backing up port 1 on a GE-2 line module.

Figure 23: Single-Homed GE-2 Line Module Configuration

FE-8 Line Module Configurations

Figure 24 displays an FE-8 line module with a link failure in a 1:N single-homed configuration.

Figure 24: Single-Homed FE-8 Line Module Configuration (1:N)

Figure 25 displays an FE-8 line module with four redundant Ethernet links in a 1:1 configuration.

Figure 25: FE-8 Line Module with 4 Redundant Ethernet Links (1:1)

E120 and E320 Router Configurations

Figure 26 and Figure 27 display link redundancy configurations on the E120 and E320 routers.

Figure 26 displays a single-homed 1:4 configuration on an E120 router.

Figure 26: Single-Homed GE-4 IOA Configuration (1:4)

Figure 27 displays an E320 router with 1:N configuration across IOAs.

Figure 27: GE-8 IOA Configuration Across IOAs (1:N)

Dual-Homed Configurations with LAG Disabled

Figure 28 displays how you can configure Ethernet link redundancy with LACP disabled locally using a dual-homed configuration. LACP is disabled because there is no LAG at the peer.

Figure 28: Dual-Homed Configuration (1:1)

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