How does Spectrum create links when the "Source Address Tables" parameter is selected in the Discovery Protocols Option
Article ID: 97330
Updated On:
Products
Spectrum
Issue/Introduction
How does Spectrum create links when the "Source Address Tables" parameter is selected in the Discovery Protocols Option
Environment
Spectrum all versions
Resolution
Here is an extract from SE documentations regarding the Spectrum use of Source Address Tables
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Layer 2 Mapping
Mapping devices that connect network layers below layer 3 can be a difficult task because there may not be an abundance of complete and accurate information available in device MIBs. Many devices store connectivity information in a proprietary format while others store no information at all. In these situations, connections need to be implied from other devices or determined heuristically through complex analysis.
The basic mechanism for determining Layer 2 connectivity is a device’s Source Address Tables (SAT). This table contains information about MAC addresses that are downstream from a specific port. But one single device’s SAT can’t provide a clear picture of the device neighbor topology. Only after considering all potential neighbor device SAT tables, is it possible to draw conclusions about port-level connectivity by realizing that two devices may mutually hear one another.
On the market today, virtually all Layer 2 devices implement a standard MIB called Spanning Tree. This technology explores a bridge’s neighbor topology, and allows a bridge to enable or disable ports to ensure that there is only one active path for data to flow between any two end-points in the network. This neighbor topology information is well suited to be leveraged for connectivity mapping.
While Spanning Tree provides very accurate connectivity information, it does not give the complete picture, as intermediate non-bridging devices will be skipped, because they don’t participate in the Spanning Tree algorithm and therefore they don’t present a Spanning Tree MIB. For example, bridges that connect non-ethernet technologies may appear to be connected when in fact there are many intermediate devices. Furthermore, because Spanning Tree only deals with bridges, layer 1 devices are not considered.
Spectrum currently supports some proprietary protocol MIBs, such as Cabletron Switch Discovery Protocol, Cisco Discovery Protocol and Extreme Protocol for Layer 2 mapping. These proprietary protocols often provide a fast and efficient means for mapping its topology in vendor homogenous environment. However, they all share the same problem that they don't work for mapping between two different protocol devices. Therefore each proprietary MIB table on a device may not provide the complete topology picture, much like Spanning Tree.
In “Layer 2 Options”, by default, SAT, Spanning Tree, and all the proprietary MIB tables are selected for Layer 2 mapping, which is perhaps overkill in some customer environments (particularly those with layer 2 devices from a single vendor), however this provides Layer 2 mapping with the best chances of resolving all connections.
Layer 1 devices, such as repeaters and end-stations, do not often provide connectivity information. Although some intelligent repeaters will give proprietary access to a MIB object that contains port-level forwarding information, they are not currently supported, except Cabletron repeaters. Fortunately, Layer 2 mapping can map the connections between Layer 2 devices and end-stations because the only MAC address the Layer 2 interface heard must be of the end-station.
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SOURCE ADDRESS TABLES (SAT)
Application model: Transparant_App/0x04a0004
MIB information used in Spectrum: dot1dBridge (1.3.6.1.2.1.17)
dot1dTpFdbTable (1.3.6.1.2.1.17.4.3)
Description:
In Spectrum, connections are resolved for SAT by checking for dot1dTpFdbAddress, dot1dTpFdbPort, dot1dTpFdbStatus and dot1dBasePortIfIndex.
Neighbor device port MAC Address is learned from dot1dTpFdbAddress table.
Neighbor port MAC Address to Local port IfIndex mapper value is learned from dot1dTpFdbPort table.
Once the Local port IfIndex mapper value is learned (dot1dTpFdbPort). Spectrum reads the dot1dBasePortIfIndex table to map local & neighbor device ports. (This step will be processed for each entry in the table)
Example:
dot1dTpFdbAddress Table: - (Value is Neighbor Port MAC Address)
Name Instance Value
dot1dTpFdbAddress 0.15.35.32.70.175 0.f.23.20.46.af
dot1dTpFdbPort Table: - (Value is Local port to Neighbor port mapper value)
Name Instance Value
dot1dTpFdbPort 0.15.35.32.70.175 2
dot1dBasePortIfIndex Table: - (Value is Local Port IfIndex)
Name Instance Value
dot1dBasePortIfIndex 2 20
Output: -
Local port IfIndex Neighbor port MAC Address
20 0.f.23.20.46.af
Using the Neighbor port Mac Address Spectrum checks if the interface is already discovered.
If the matching port model is found Spectrum discovers connections b/w the Local port If Index and the Neighbor port MAC Address models.
..........
Layer 2 Mapping
Mapping devices that connect network layers below layer 3 can be a difficult task because there may not be an abundance of complete and accurate information available in device MIBs. Many devices store connectivity information in a proprietary format while others store no information at all. In these situations, connections need to be implied from other devices or determined heuristically through complex analysis.
The basic mechanism for determining Layer 2 connectivity is a device’s Source Address Tables (SAT). This table contains information about MAC addresses that are downstream from a specific port. But one single device’s SAT can’t provide a clear picture of the device neighbor topology. Only after considering all potential neighbor device SAT tables, is it possible to draw conclusions about port-level connectivity by realizing that two devices may mutually hear one another.
On the market today, virtually all Layer 2 devices implement a standard MIB called Spanning Tree. This technology explores a bridge’s neighbor topology, and allows a bridge to enable or disable ports to ensure that there is only one active path for data to flow between any two end-points in the network. This neighbor topology information is well suited to be leveraged for connectivity mapping.
While Spanning Tree provides very accurate connectivity information, it does not give the complete picture, as intermediate non-bridging devices will be skipped, because they don’t participate in the Spanning Tree algorithm and therefore they don’t present a Spanning Tree MIB. For example, bridges that connect non-ethernet technologies may appear to be connected when in fact there are many intermediate devices. Furthermore, because Spanning Tree only deals with bridges, layer 1 devices are not considered.
Spectrum currently supports some proprietary protocol MIBs, such as Cabletron Switch Discovery Protocol, Cisco Discovery Protocol and Extreme Protocol for Layer 2 mapping. These proprietary protocols often provide a fast and efficient means for mapping its topology in vendor homogenous environment. However, they all share the same problem that they don't work for mapping between two different protocol devices. Therefore each proprietary MIB table on a device may not provide the complete topology picture, much like Spanning Tree.
In “Layer 2 Options”, by default, SAT, Spanning Tree, and all the proprietary MIB tables are selected for Layer 2 mapping, which is perhaps overkill in some customer environments (particularly those with layer 2 devices from a single vendor), however this provides Layer 2 mapping with the best chances of resolving all connections.
Layer 1 devices, such as repeaters and end-stations, do not often provide connectivity information. Although some intelligent repeaters will give proprietary access to a MIB object that contains port-level forwarding information, they are not currently supported, except Cabletron repeaters. Fortunately, Layer 2 mapping can map the connections between Layer 2 devices and end-stations because the only MAC address the Layer 2 interface heard must be of the end-station.
......
SOURCE ADDRESS TABLES (SAT)
Application model: Transparant_App/0x04a0004
MIB information used in Spectrum: dot1dBridge (1.3.6.1.2.1.17)
dot1dTpFdbTable (1.3.6.1.2.1.17.4.3)
- dot1dTpFdbAddress – Neighbor Device MAC Address list -> Instance
- dot1dTpFdbPort - Neighbor Device Port list
- dot1dTpFdbStatus - Neighbor Port Status list
- dot1dBasePortIfIndex – Local & Neighbor device port mapping list
Description:
In Spectrum, connections are resolved for SAT by checking for dot1dTpFdbAddress, dot1dTpFdbPort, dot1dTpFdbStatus and dot1dBasePortIfIndex.
Neighbor device port MAC Address is learned from dot1dTpFdbAddress table.
Neighbor port MAC Address to Local port IfIndex mapper value is learned from dot1dTpFdbPort table.
Once the Local port IfIndex mapper value is learned (dot1dTpFdbPort). Spectrum reads the dot1dBasePortIfIndex table to map local & neighbor device ports. (This step will be processed for each entry in the table)
Example:
dot1dTpFdbAddress Table: - (Value is Neighbor Port MAC Address)
Name Instance Value
dot1dTpFdbAddress 0.15.35.32.70.175 0.f.23.20.46.af
dot1dTpFdbPort Table: - (Value is Local port to Neighbor port mapper value)
Name Instance Value
dot1dTpFdbPort 0.15.35.32.70.175 2
dot1dBasePortIfIndex Table: - (Value is Local Port IfIndex)
Name Instance Value
dot1dBasePortIfIndex 2 20
Output: -
Local port IfIndex Neighbor port MAC Address
20 0.f.23.20.46.af
Using the Neighbor port Mac Address Spectrum checks if the interface is already discovered.
If the matching port model is found Spectrum discovers connections b/w the Local port If Index and the Neighbor port MAC Address models.
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