Avaya 8800 Planning And Engineering, Network Design page 78

Redundant network design
Aggregation switches always send traffic directly to an SMLT client switch. They only use the
interswitch trunk for traffic that they cannot forward in another, more direct way.
SMLT and VLACP
VLACP is recommended for all SMLT access links when the links are configured as MLT to ensure
both end devices are able to communicate. By using VLACP over SLT, enhanced failure detection is
extended beyond the limits of the number of SMLT or LACP instances that can be created on an
Avaya switch.
For more information about VLACP, see
SMLT and loop prevention
Split MultiLink Trunking (SMLT) based network designs form physical loops for redundancy that
logically do not function as a loop. Under certain adverse conditions, incorrect configurations or
cabling, loops can form.
The two solutions to detect loops are Loop Detect and Simple Loop Prevention Protocol (SLPP).
Loop Detect and SLPP detect a loop and automatically stop the loop. Both solutions determine on
which port the loop is occurring and shuts down that port.
For more information, see
Interswitch Trunking recommendations
Figure 21: SMLT configuration with switches as aggregation switches
requires only two SMLT-capable aggregation switches connected by an interswitch trunk. The
aggregation switches use the interswitch trunk to:
• Confirm that each switch is alive and to exchange MAC address information. Thus, the link
must be reliable and must not exhibit a single point of failure in itself.
• Forward flooded packets or packets destined for non-SMLT connected switches, or for servers
physically connected to the other aggregation switch.
The amount of traffic from a single SMLT wiring-closet switch that requires forwarding across the
interswitch trunk is usually small. However, if the aggregation switches terminate connections to
single-homed devices, or if uplink SMLT failures occur, the interswitch trunk traffic volume may be
significant. To ensure that no single point of failure exists in the interswitch trunk, Avaya
recommends that the interswitch trunk be a multigigabit multilink trunk with connections across
different modules on both aggregation switches.
The Interswitch Trunking (IST) session is established between the peering SMLT aggregation
switches. The basis for this connection is a common VLAN and the knowledge about the peer IP
addressing for the common VLAN. Avaya recommends that you use an independent VLAN for this
IST peer session. You can do so only by including the interswitch trunk ports in the VLAN because
only the interswitch trunk port is a member of the interswitch trunk VLAN.
Avaya recommends that you not enable any dynamic routing protocols on the IST VLAN. The
purpose of the IST VLAN is to support adjacent switches; do not use the IST as a next-hop route for
non-IST traffic or routing traffic. One exception to this rule is the case of multicast traffic with PIM-
SM. In this case, you must enable PIM-SM on the IST VLAN.
June 2016
End-to-end fault detection and VLACP
SLPP, Loop Detect, and Extended CP-Limit
Planning and Engineering — Network Design
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on page 57.
on page 102.
on page 77 shows that SMLT
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