Avaya 8800 Planning And Engineering, Network Design page 67

MLT and spanning tree protocols
When you combine MLTs and STGs, the Spanning Tree Protocol treats multilink trunks as another
link, which can be blocked. If two MLT groups connect two devices and belong to the same STG,
the Spanning Tree Protocol blocks one of the MLT groups to prevent looping.
To calculate path cost defaults, the 8000 Series switch uses the following STP formulas (based on
the 802.1D standard):
• Bridge Path_Cost = 1000/Attached_LAN_speed_in_Mbit/s
• MLT Path_Cost = 1000/(Sum of LAN_speed_in_Mbit/s of all Active MLT ports)
The bridge port and MLT path cost defaults for both a single 1000 Mbit/s link and an aggregate 4000
Mbit/s link is 1. Because the root selection algorithm chooses the link with the lowest port ID as its
root port (ignoring the aggregate rate of the links), Avaya recommends that the following methods
be used when you define path costs:
• Use lower port numbers for multilink trunks so that the multilink trunks with the most active links
gets the lowest port ID.
• Modify the default path cost so that non-MLT ports, or the MLT with the least active links, has a
higher value than the MLT link with the most active ports.
Withthe implementation of 802.1w (Rapid Spanning Tree Protocol—RSTP) and802.1s (Multiple
Spanning Tree Protocol—MSTP), a new path cost calculationmethod is implemented. The following
table describes the new pathcosts associated with each interface type:
Table 20: New path cost for RSTP or MSTP mode
Link speed
Less than or equal 100 Kbit/s
1 Mbit/s
10 Mbit/s
100 Mbit/s
1 Gbit/s
10 Gbit/s
100 Gbit/s
1 Tbit/s
10 Tbit/s
MLT protection against split VLANs
When you create distributed VLANs, consider link redundancy. In a link failure, split subnets or
separated VLANs disrupt packet forwarding.
The split subnet problem can occur when a VLAN carrying traffic is extended across multiple
switches, and a link between the switches fails or is blocked by STP. The result is a broadcast
domain that is divided into two noncontiguous parts. This problem can cause failure modes that
higher level protocols cannot recover.
June 2016
Recommended path cost
200 000 000
20 000 000
2 000 000
200 000
20 000
2000
200
20
2
Planning and Engineering — Network Design
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Link redundancy
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