Quick Nortel MLT (Link Aggregation) Reference

These excerpts and images were taken from Nortel Reference Manual for Passport 8600. These notes are very helpful for those who are configuring a LAG (MLT) on a Nortel Switch for the first time.

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IEEE 802.3ad-based link aggregation, through the Link Aggregation Control Protocol (LACP), supports a dynamic link aggregation function as they become available to a trunk group. LACP dynamically detects when links can be aggregated into a link aggregation group (LAG) and does so as links become available. LACP also provides link integrity checking at Layer 2 for all links within the LAG.




Supported Link Aggregation Types:
- MLT with LACP
- MLT : statically configured link bundling
- SMLT

Virtual LACP (VLACP) is a Nortel modification that provides end-to-end
failure detection. VLACP is not a link aggregation protocol; You can run VLACP on single ports or on ports that are part of a MLT.

MLT provides module redundancy via Distributed MultiLink Trunking (DMLT). DMLT allows you to aggregate similar ports from different modules.
- Nortel recommends that LACP not be configured on the IST MLT.
- Nortel recommends that you do not configure VLACP on LACP-enabled ports. VLACP does not operate properly with LACP. You can configure VLACP with any SMLT configuration.
- Nortel recommends always using DMLT when possible.

MLT and MLT with LACP configuration rules:
• MLT is supported on 10BASE-T, 100BASE-TX, 100Base-FX, Gigabit Ethernet, and 10 Gigabit Ethernet module ports.
• All MultiLink trunk ports must have the same speed and duplex settings, even when auto-negotiation is set.
• The media type of MLT ports can be different; a mix of copper and fiber are allowed.
• All MultiLink trunk ports must be in the same STG unless the port is tagged. Tagging allows ports to belong to multiple STGs, as well as multiple VLANs.
• MLT is compatible with Spanning Tree Protocol (STP), Multiple Spanning Tree Protocol (MSTP) (IEEE 802.1s), and Rapid Spanning Tree Protocol (RSTP) (IEEE 802.1w).
• Tagging (IEEE 802.1Q) is supported on a MultiLink trunk.
• MLT ports can span modules, providing module redundancy.
• Apply filters individually to each port in a MultiLink trunk.
If identical BPDUs are received on all ports, the MultiLink trunk mode is forwarding. You can disable the Nortel STP (ntstg ) if you do not want to receive BPDUs on all ports.

LAG rules:
• All LAG ports operate in full-duplex mode.
• All LAG ports operate at the same data rate.
• Assign all LAG ports in the same VLANs.
• Link aggregation is compatible with the STP, MSTP, and RSTP.
• Assign all ports in an LAG to the same STP groups.
• Ports in an LAG can exist on different modules.
• For Gigabit and 10 Gigabit ports, you can use link aggregation groups 1 to 31.
• For Fast Ethernet ports, you can use link aggregation groups 1 to 7 only.
• Each LAG supports a maximum of eight active links.
• Each LAG supports a maximum of eight standby links.
• After a MultiLink trunk is configured with LACP, you cannot add or delete ports or VLANs manually without first disabling LACP.

SMLT is an option that improves Layer 2 and Layer 3 resiliency. These SMLT switches form a Switch Cluster and are referred to as an IST Core Switch pair
Switch Clusters are always formed as a pair, but pairs of clusters can be combined in either a square of full-mesh fashion to increase the size and port density of the Switch Cluster.
When configured in a Layer 3 or routed topology, the configuration is referenced as Routed SMLT (RSMLT).

• Before you reboot a switch that is the LACP master, you must configure the LACP system ID globally to prevent an RSMLT failure.
• A properly designed SMLT network inherently does not have any logical loops.
• SMLT solves the spanning tree problem by combining two aggregation switches into one logical MLT entity, thus making it transparent to any type of edge switch. In the process, it provides quick convergence, while load sharing across all available trunks.

Single Port SMLT rules:
• The dual-homed device that connects to the aggregation switches must support MLT.
• Single-port SMLT is supported on Ethernet ports.
• Each single-port SMLT is assigned an SMLT ID from 1 to 512.
• You can designate Single Port SMLT ports as Access or Trunk (IEEE 802.1Q tagged or not); changing the type does not affect behavior.
• You cannot change a Single Port split MultiLink trunk to an MLT-based split MultiLink trunk by adding additional ports. You must delete the single port split MultiLink trunk and reconfigure the port as SMLT/MLT.
• You cannot change an MLT-based split MultiLink trunk into a single port split MultiLink trunk by deleting all ports except one. You must remove the SMLT/MLT and reconfigure the port as Single Port SMLT.
• You cannot configure a port as an MLT-based SMLT and as single-port SMLT at the same time.
• Two or more aggregation switches can have single port Split MultiLink trunk with the same IDs. You can have as many single port Split MultiLink trunk as there are a available ports on the switch.
• LACP is supported on single port SMLT.

Simple Loop Prevention Protocol (SLPP) is used to prevent loops in a SMLT network. SLPP is focused on SMLT networks but works with other configurations. Nortel recommends that you always use SLPP in any SMLT environment. SLPP requires the use of 4.0.x code or higher.

MLT with LACP configuration considerations
When you configure standard-based link aggregation, you must enable the aggregation parameter. After you enable the aggregation parameter, the LACP aggregator is mapped one-to-one to the specified MultiLink trunk.

Perform the following steps to configure an LAG:
1. Assign a numeric key to the ports you want to include in the LAG.
2. Configure port aggregation to true.
3. Enable LACP on the port.
4. Create an MultiLink trunk and assign the same key as in step 1 to it. The MultiLink trunk/LAG only aggregates ports whose key matches its own. The newly created MultiLink trunk or LAG adopts the VLAN membership of its member ports when the first port is attached to the aggregator associated with this LAG. When a port detaches from an aggregator, the associated LAG port deletes the member from its list. After a MultiLink trunk is configured with LACP, you cannot add or delete ports or VLANs manually without first disabling LACP. To enable tagging on ports belonging to a LAG, disable LACP on the port and then enable tagging and LACP on the port.

MLT with LACP and SMLT configuration considerations

Split MultiLinkTrunks (SMLT) can be configured with MLT or MLT with LACP. Follow these guidelines when you configure SMLT with LACP:
• When you set the LACP system ID for SMLT, configure the same LACP SMLT system ID on both aggregation switches to avoid the loss of data. Nortel recommends that you configure the LACP SMLT system ID to be the base MAC address of one of the aggregate switches, and that you include the SMLT-ID. Ensure that the same System ID is configured on both of the SMLT core aggregation switches.
• If you use LACP in an SMLT square configuration, the LACP ports must have the same keys for that SMLT LAG; otherwise, the aggregation can fail if a switch fails.
• If an SMLT aggregation switch has LACP enabled on some of its MultiLink trunks, do not change the LACP system priority. If some ports do not enter the desired MultiLink trunk after a dynamic configuration change, enter the following CLI command:
config mlt lacp clear-link-aggrgate
• When you configure SMLT links, Nortel recommends that you set the multicast packets-per-second value to 6000 pps.
• Nortel recommends that you do not enable LACP on interswitch trunks to avoid unnecessary processing. Use VLACP if a failure detection mechanism is required when there is an optical network between the SMLT core switches.



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