Beta Draft Confidential
7-581/19/05 ATM Services Configuration Guide for CBX 3500, CBX 500, GX 550, and B-STDX 9000
Configuring Trunks
Configuring VNN OSPF
VNN OSPF optimized flooding — When you enable VNN OSPF optimized
flooding on a Lucent switch connected to another switch in the same area by multiple
parallel trunks, the switch establishes a single neighbor-switch adjacency in that area.
The neighbor-switch adjacency identifies a neighbor at the switch level, and not at the
trunk interface level (as is the case with standard OSPF). Each additional trunk that
interconnects the two switches in the same area becomes part of the trunk interface
list.
The switch then selects the first interface in the trunk interface list that has a fully
adjacent neighbor relationship to serve as the flooding interface for LSAs. Once the
flooding interface (also known as the flooding trunk) is selected, all subsequent LSAs
are flooded, received, processed, and retransmitted only on this trunk interface and not
on all parallel trunk interfaces between the two switches, as occurs with standard
OSPF flooding. These LSAs are added to the neighbor-switch retransmission list, and
not to the OSPF neighbor retransmission list on that trunk.
If the flooding trunk fails, the switch selects a new flooding trunk that inherits the
neighbor-switch retransmission list from the original flooding trunk. This ensures that
LSAs will be retransmitted on the new flooding trunk if the original flooding trunk
fails before receiving acknowledgement for the LSAs.
Enabling this optimized VNN OSPF flooding mechanism has the following benefits in
a Lucent switch network:
• Reduces the volume of network traffic caused by repeated flooding of identical
LSAs on multiple parallel trunks
• Reduces CPU utilization and bandwidth consumption on the switch processor
modules of both switches in a neighbor-switch adjacency; one switch floods fewer
LSAs, and the other switch receives fewer LSAs to process
• Improves performance by reducing the sizes of the VNN and OSPF databases in
the network
Note – To ensure that these benefits apply to both switches in the neighbor-switch
adjacency, you should enable VNN OSPF optimized flooding on both switches
interconnected by multiple parallel trunks.
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