40 VLANs, Spanning Tree, and Static Link AggregationExample 1-Traffic flow from a to b1 or b2 Assuming a and b1/b2are communicating using Layer 2, traffic flows from A to switch E and isforwarded over the direct link to B. Traffic coming from b1 or b2 to a is sentby B on one of its MLT ports.B sends traffic from b1 to a on the link to switch E, and traffic from b2 to aon the link to F. In the case of traffic from b1, switch E forwards the trafficdirectly to switch A, while traffic from b2, which arrived at F, is forwardedacross the IST to E and then on to A.Example 2-Traffic flow from b1/b2 to c1/c2 Traffic from b1/b2 to c1/c2is always sent by switch B through the MLT to the core. No matter whichswitch (E or F) it arrives at, traffic is sent directly to C through the local link.Example 3-Traffic flow from a to d Traffic from a to d (and the reverse)is forwarded across the IST because it is the shortest path. This link istreated purely as a standard link with no account taken of SMLT and the factthat it is also an IST.Example 4-Traffic flow from f to c1/c2 Traffic from f to c1/c2 is sentdirectly from F. With return traffic from c1/c2, you can have one active VRRPMaster for each IP subnet. The traffic is passed across the IST if switch Csends it through the link to E.Inter-Switch TrunksSMLT aggregation switches must be connected with an Inter-Switch Trunk(IST). For example, in Figure 10 "8300 switches as SMLT aggregationswitches" (page 39), edge switches B and C are connected to theaggregation switches using multilink trunks split between the twoaggregation switches. The implementation of SMLT requires only twoSMLT-capable aggregation switches.Aggregation switches use the IST to:• Confirm that they are alive and exchange MAC address forwardingtables.• Carry the SMLT control packets.• Send traffic between single switches attached to the aggregationswitches.• Serve as a backup if one SMLT link fails.Because the IST is required for the SMLT, Nortel recommends that you usemultiple links on the IST to ensure reliability and high availability. Nortelrecommends using Gigabit Ethernet links for IST connectivity to provideenough bandwidth for potential cross traffic.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.
