Limitations And Pre-Requisites; Ospf Cost Calculation With Interface Bandwidth - Brocade Communications Systems FastIron SX 800 Configuration Manual

Bandwidth for IP interfaces
The bandwidth for IP interfaces feature can be used to:
• Query the bandwidth for an interface.
• Help OSPF avoid generating numerous LSAs while updating the cost value for a VE interface due to changes in associated
physical interfaces.
• Influence the cost on OSPF interfaces for specific tunnels, VE interfaces, and physical interfaces.
The bandwidth for IP interfaces feature enables OSPF to calculate its interface metric cost more precisely, based on the specified
interface bandwidth. If the interface bandwidth feature is disabled, OSPF calculates the cost as the reference-bandwidth divided by the
fixed port bandwidth, as outlined in the
the interface bandwidth feature is enabled, OSPF calculates the cost as the reference-bandwidth divided by the interface bandwidth. For
a physical interface, the interface bandwidth is assigned by default to the port speed.
The interface bandwidth feature also enables OSPF to use the configured interface bandwidth for a VE interface to calculate its routing
metric, without considering the bandwidth of the associated physical ports. When this feature is enabled, the bandwidth for a VE interface
is the interface bandwidth value if it is configured under the VE. Alternatively, it is the sum of the interface bandwidth for all associated
ports or all active ports when OSPF dynamic cost is enabled.
The bandwidth of a trunk port for OSPF is, by default, the sum of either all the associated ports or all active associated ports when OSPF
dynamic cost is enabled. The interface bandwidth of the primary port is used if the interface bandwidth is configured; otherwise it reverts
to the default behavior.
NOTE
If the interface bandwidth configuration of the primary port is different to any of the secondary ports, then the LAG is not
deployed. When the LAG is undeployed, the interface bandwidth value for all secondary ports is reset to the port speed.
The configured value is exposed in SNMP via ifSpeed (in ifTable) and ifHighSpeed (in ifXTable) objects.
NOTE
GRE or IPv6 tunnel bandwidth may limit routing protocol traffic propagating through the tunnel. For example, if the tunnel
defaults to 8kbps , OSPF uses 50% of the tunnel bandwidth for Hello and update traffic. Therefore, it is good practice to
increase the tunnel bandwidth when a routing protocol runs over it to eliminate flapping, and give the routing protocol more
capacity to send its update and Hello messages.
From FastIron Release 08.0.30, this feature is supported on all platforms.

Limitations and pre-requisites

• The bandwidth for IP interfaces feature does not support setting and adjusting GRE or IPv6 receiving and transmission
bandwidth.
• SNMP does not support any IP interface bandwidth related configurations.

OSPF cost calculation with interface bandwidth

OSPF uses a formula to calculate a path cost when interface bandwidth is available.
If the interface bandwidth feature is disabled, OSPF calculates the cost as the reference-bandwidth divided by the fixed port bandwidth,
as outlined in the Changing the reference bandwidth for the cost on OSPF interfaces
bandwidth feature is enabled, OSPF calculates the cost as the reference-bandwidth divided by the interface bandwidth.
OSPF uses the following formula to calculate the path cost when interface bandwidth is available:
• OSPF path cost = ((auto-cost × reference-bandwidth + interface bandwidth) -1) / interface bandwidth.
122
Changing the reference bandwidth for the cost on OSPF interfaces on page 246 section. When
on page 246 section. When the interface
FastIron Ethernet Switch Layer 3 Routing
53-1003627-04