Glossary

Prefix lists are used in route maps and route filtering operations. You can use prefix lists as an alternative to existing access lists
(ACLs). A prefix is a portion of the IP address. Prefix lists constitute any number of bits in an IP address starting from the far
left bit of the far left octet. By specifying the exactly number of bits in an IP address that belong to a prefix list, the prefix list
can be used to aggregate addresses and perform some functions; for example, redistribution.
You can use the following options to enable or disable the BFD session:
● Permit – The permit option enables creation of a BFD session on the specified prefix list or prefix list range. The no permit
option enables tear down of the BFD session if and only if the ACL has no permit entry that shares the same neighbor.
● Deny – The deny option prevents BFD sessions from getting created for the specified prefix list or prefix list range.
For more information on prefix lists, see IP Prefix Lists.
To enable BFD sessions on specific neighbors, perform the following steps:
Enter the following command to enable BFD session on specific next-hop neighbors:
CONFIGURATION
ip route bfd prefix-list prefix-list-name
The BFD session is established for the next-hop neighbors that are specified in the prefix-list.
● The absence of a prefix-list causes BFD sessions to be enabled on all the eligible next-hop neighbors.
● You can use only valid unicast address prefixes in the BFD prefix list. An erroneous prefix in a prefix-list causes the entire
prefix-list to be rejected.
● A BFD session is enabled for the directly connected next-hop neighbor specified in the configured destination prefix list.
● If you attach an empty prefix-list, all the existing established BFD sessions are teared down. If a destination prefix or prefix
range is not present in the prefix-list, then it is considered as an implicit deny.
● When a destination prefix is deleted from the prefix-list using the no permit option, the corresponding BFD session is torn
down immediately. In this scenario, the BFD session tear down occurs only if the other destination prefixes in the prefix-list
are not pointing to the same neighbor.
● The permit option enables creation of a BFD session for the specified static destination prefix or prefix range. The system
prevents creation of BFD sessions for all other destination prefixes that are explicitly specified as Deny in the prefix list.
● If other destination prefixes in the prefix-list are pointing to the same neighbor, then the no permit or the deny option
on a particular destination prefix neither creates a BFD session on a neighbor nor removes the static routes from the unicast
database.
● BFD sessions created using any one IP prefix list are active at any given point in time. If a new prefix list is assigned, then
BFD sessions corresponding to the older (existing) prefix list are replaced with the newer ones.
● Each time a prefix list is modified, only addition or deletion of new entries in that prefix list are processed for BFD session
establishment or tear down.
Changing Static Route Session Parameters
BFD sessions are configured with default intervals and a default role.
The parameters you can configure are: Desired TX Interval, Required Min RX Interval, Detection Multiplier, and system role.
These parameters are configured for all static routes. If you change a parameter, the change affects all sessions for static
routes.
To change parameters for static route sessions, use the following command .
● Change parameters for all static route sessions.
CONFIGURATION mode
ip route bfd [prefix-list prefix-list-name] interval milliseconds min_rx milliseconds
multiplier value role [active | passive]
To view session parameters, use the show bfd neighbors detail command.
Disabling BFD for Static Routes
If you disable BFD, all static route BFD sessions are torn down.
A final Admin Down packet is sent to all neighbors on the remote systems, and those neighbors change to the Down state.
To disable BFD for static routes, use the following command.
● Disable BFD for static routes.
CONFIGURATION mode
Bidirectional Forwarding Detection (BFD)
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