42 VLANs, Spanning Tree, and Static Link AggregationTraffic flow in an SMLT environmentTraffic flow in an SMLT environment follows these rules:• If a packet is received from an interswitch trunk port, it is not forwardedto any active SMLT groups, which is key in preventing network loops.• When a packet is received, a look-up is performed on the forwardingdatabase. If an entry exists, and if the entry was learned locally fromthe split multilink trunk or through the interswitch trunk as a remotesplit multilink trunk, it is forwarded out the local port (the packet cannotbe sent to the interswitch trunk for forwarding unless there is no localconnection). Unknown and Broadcast packets are flooded out all portsthat are members of this VLAN.• For loadsharing purposes in an SMLT scenario, the Ethernet RoutingSwitch 8300 obeys the trunk distribution algorithm. See Nortel EthernetRouting Switch 8300 Planning and Engineering—Network DesignGuidelines (NN46200-200) for more details about the algorithms.Traffic flow exampleIn an SMLT environment, the two aggregation switches share the sameforwarding database by exchanging forwarding entries using the IST. Inthe following figure, Figure 11 "show vlan info fdb-entry 10 sample output"(page 43), the forwarding databases are shown for a pair of IST nodes(B and C). Note that the entry for 00:E0:7B:B3:04:00 is shown on nodeC as being learned on MLT-1, but because SMLT REMOTE is true, thisentry was actually learned from node B. On B, that same entry is shownas being directly learned through MLT-1 because SMLT REMOTE is false.Figure 12 "Network topology for traffic flow example" (page 43) shows thenetwork topology.When a packet arrives at node C destined for 00:E0:7B:B3:04:00, if theSMLT REMOTE status is true, the switch tries to send the packet out MLT-1first, rather than through the interswitch trunk. Traffic rarely traverses theinterswitch trunk unless there is a failure. If this same packet arrives at B, itis forwarded to MLT-1 on the local ports.Nortel Ethernet Routing Switch 8300Configuration — VLANs, Spanning Tree, and Static Link Aggregation using Device ManagerNN46200-510 03.01 Standard4.0 27 August 2007Copyright © 2005-2007, Nortel Networks.
