Dell Brocade G620 Brocade 8.0.1 Fabric OS Administratiors Guide - Page 108

Principal ISLs

Page 108 highlights

Routing Traffic also keeps track of the state of the links on all switches in the fabric and associates a cost with each link. The protocol computes paths from a switch to all the other switches in the fabric by adding the cost of all links traversed by the path, and chooses the path that minimizes the costs. This collection of the link states, including costs, of all the switches in the fabric constitutes the topology database or link state database. Once established, FSPF programs the hardware routing tables for all active ports on the switch. FSPF is not involved in frame switching. FSPF uses several frames to perform its functions. Because it may run before fabric routing is set up, FSPF does not use the routing tables to propagate the frames, but floods the frames throughout the fabric hop-by-hop. Frames are first flooded on all the ISLs; as the protocol progresses, it builds a spanning tree rooted on the principal switch. Frames are only sent on the principal ISLs that belong to the spanning tree. When there are multiple ISLs between switches, the first ISL to respond to connection requests becomes the principal ISL. Only one ISL from each switch is used as the principal ISL. The following figure shows the thick red lines as principal ISLs, and thin green lines as regular ISLs. FIGURE 5 Principal ISLs NOTE FSPF only supports 16 routes in a zone, including Traffic Isolation Zones. FSPF makes minimal use of the ISL bandwidth, leaving virtually all of it available for traffic. In a stable fabric, a switch transmits 64 bytes every 20 seconds in each direction. FSPF frames have the highest priority in the fabric. This guarantees that a control frame is not delayed by user data and that FSPF routing decisions occur very quickly during convergence. FSPF guarantees a routing loop-free topology at all times. It is essential for a fabric to include many physical loops because, without loops, there would not be multiple paths between switches, and consequently no redundancy. Without redundancy, if a link goes down, part of the fabric is isolated. FSPF ensures both that the topology is loop-free and that a frame is never forwarded over the same ISL more than once. FSPF calculates paths based on the destination domain ID. The fabric protocol must complete domain ID assignments before routing can begin. ISLs provide the physical pathway when the Source ID (SID) address has a frame destined to a port on a remote switch Brocade Fabric OS Administration Guide, 8.0.1 108 53-1004111-02

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also keeps track of the state of the links on all switches in the fabric and associates a cost with each link. The protocol computes paths
from a switch to all the other switches in the fabric by adding the cost of all links traversed by the path, and chooses the path that
minimizes the costs. This collection of the link states, including costs, of all the switches in the fabric constitutes the topology database or
link state database.
Once established, FSPF programs the hardware routing tables for all active ports on the switch. FSPF is not involved in frame switching.
FSPF uses several frames to perform its functions. Because it may run before fabric routing is set up, FSPF does not use the routing
tables to propagate the frames, but floods the frames throughout the fabric hop-by-hop. Frames are first flooded on all the ISLs; as the
protocol progresses, it builds a spanning tree rooted on the principal switch. Frames are only sent on the principal ISLs that belong to the
spanning tree. When there are multiple ISLs between switches, the first ISL to respond to connection requests becomes the principal
ISL. Only one ISL from each switch is used as the principal ISL.
The following figure shows the thick red lines as principal ISLs, and thin green lines as regular ISLs.
FIGURE 5
Principal ISLs
NOTE
FSPF only supports 16 routes in a zone, including Traffic Isolation Zones.
FSPF makes minimal use of the ISL bandwidth, leaving virtually all of it available for traffic. In a stable fabric, a switch transmits 64 bytes
every 20 seconds in each direction. FSPF frames have the highest priority in the fabric. This guarantees that a control frame is not
delayed by user data and that FSPF routing decisions occur very quickly during convergence.
FSPF guarantees a routing loop-free topology at all times. It is essential for a fabric to include many physical loops because, without
loops, there would not be multiple paths between switches, and consequently no redundancy. Without redundancy, if a link goes down,
part of the fabric is isolated. FSPF ensures both that the topology is loop-free and that a frame is never forwarded over the same ISL
more than once.
FSPF calculates paths based on the destination domain ID. The fabric protocol must complete domain ID assignments before routing
can begin. ISLs provide the physical pathway when the Source ID (SID) address has a frame destined to a port on a remote switch
Routing Traffic
Brocade Fabric OS Administration Guide, 8.0.1
108
53-1004111-02