Which two statements are true regarding the CSPF algorithm? (Choose two.)
A.
The selected path for a given LSP is passed to the TED in the form of an ERO.
B.
LSPs with lower numerical setup priorities are computed before LSPs with higher setup priority values.
C.
The selected path for a given LSP is passed to RSVP in the form of an ERO.
D.
LSPs with higher numerical setup priorities are computed before LSPs with lower setup priority values.
Explanation:
The CSPF algorithm first calculates the highest-priority LSP (the one with the lowest setup priority value).
Do I need to enable CSPF to control where to send my traffic? The answer is no. You can manually configure
an Explicit Route Objects (ERO) list, and let RSVP doing the job. In the presence of the ERO object, the RSVP
Path messages will follow the path specified, thus the Resv messages carrying the labels in the opposite
direction.
https://www.netflask.net/jnpr-constrained-shortest-path-first/
B&C correct
To select a path, CSPF follows certain rules. The rules are as follows:
1- Computes LSPs one at a time, beginning with the highest priority LSP (the one with the lowest setup priority value). Among LSPs of equal priority, CSPF services the LSPs in alphabetical order of the LSP names.
https://www.juniper.net/documentation/en_US/junos/topics/concept/mpls-cspf-path-selection-method.html
RSVP-signaling: The router passes the computed ERO list to RSVP for LSP signaling. Note that because the TED contains a relatively up-to-date view of the entire network’s current state, a high probability exists that the RSVP-signaled LSP will succeed.
Put another way, if no path in the network meets a provided constraint, CSPF does not compute an ERO list, and RSVP does not even attempt to signal an LSP that would be doomed to failure anyway. Last minute changes in the state of the network might result in the TED being slightly out of date, and this can lead to RSVP path signaling failures until the TED is again synchronized with the true state of the network.
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