What is the purpose of the no-cspf command?
A.
to successfully signal the LSP across the network regardless of constraints
B.
to delete the CSPF database
C.
to ignore OSPF when calculating the ERO
D.
to successfully signal the LSP only if the default IGP path (or named path) meets all constraints
Can someone explain why the answer is not A?
Should be A.
D looks right (according to how Juniper defines it
http://www.juniper.net/techpubs/en_US/junos11.1/topics/reference/configuration-statement/no-cspf-edit-protocols-mpls.html
La respuesta no es A debido a que al negar el algoritmo CSPF nos dice que las restricciones impuesta por el usuario en el LSP no deben ser verificadas por el algoritmo CSPF en la tabla TED. sin embargo si usamos las extensiones de TE de los IGP en ISIS ya lo tiene por defecto con el TLV 22 Alcanzabilidad extendida de ISIS, toda esa informacion de TE debe ser aun validada por el protocolo ISIS, por ende la respuesta debe ser D.
For me it should be answer A.
With no-cspf, the path is calculated on a hop-by-hop basis using the IGP routing tables on each router.
I also thinks it should be A
no-cspf
Syntax
no-cspf;
Hierarchy Level
[edit logical-systems logical-system-name protocols mpls],
[edit logical-systems logical-system-name protocols mplslabel-switched-path lsp-name],
[edit logical-systems logical-system-name protocols mpls label-switched-path lsp-name (primary | secondary) path-name],
[edit protocols mpls],
[edit protocols mpls label-switched-path lsp-name],
[edit protocols mpls label-switched-path lsp-name (primary | secondary) path-name]
Release Information
Statement introduced before Junos OS Release 7.4.
Description
Disable constrained-path LSP computation.
An explicit-path LSP is completely configured through operator action. Once configured, it is initiated only along the explicitly specified path.
A constrained-path LSP relies on an ingress router to compute the complete path. The ingress router takes into account the following information during the computation:
Interior gateway protocol (IGP) topology database
Link utilization information from extensions in the IGP link-state database
Administrative group information from extensions in the IGP link-state database
LSP requirements, including bandwidth, hop count, and administrative group
Constrained-path LSPs can generally avoid link failures and congested links. They also permit recomputation (therefore, a new path) during topology changes or unsuccessful setup.
Default
Constrained-path LSP computation enabled.
Required Privilege Level
routing—To view this statement in the configuration.
routing-control—To add this statement to the configuration.
Should be A.
it should be A, you can find the below copied details about CSPF
1) CSPF is a modified flavour of the normal spf algorithm which is the core
of the ospf & isis protocol. If you set up an lsp with RSVP, and you are
using TE extension with isis (on by default) and OSPF (you must turn on the
use of opaque LSA type 10 (area scope) with the knob “traffic-engineering”,
these IGPs will populate the “TED” (Traffic Engineering Database) and when
the RSVP will use the IGP to setup your LSP (without ERO for instance), the
RSVP will take a look at the TED and a modified SPF will run.
All the links which won’t satisfied some requirements will be taken out of
the calculation. At the end of the calculation, the CSPF process will give
to RSVP an ERO list (not setup manually by you, but calculated by CSPF
itself) and this ERO list will be used to setup the LSP. If you want to use
link administrative groups (link coloring) or FastReroute you will have to
use the CSPF.
Specifically, the CSPF Algorithm states that:
1) Prune links with insufficient bandwidth;
2) Prune links that don’t include the administrative-groups
3) Prune links that contain excluded administrative-groups
4) Calculate shortest path from ingress to egress consistent with manually
configured ERO
5) If equal cost paths exist, choose the one whose last hop is the LSP end
point
6) Select among equal-cost paths (least hop then fill related criteria
(most fill, least fill)
7) Pass EROs to RSVP
2) OSPF traffic-engineering is the knob you need to enable
traffic-engineering using OSPF (with ISIS it’s up by default).So, you’ll
find in the ospf database a new ‘section’ populated by LSA type 10, and
these LSAs will contain link information such as bandwidth constranint, link
administrative group, TE metrics, (for each setup priority 0-7). These
information will be used to populate the TED.
It is Options A.
as you can see from the first line of description of this knob.
Description
Disable constrained-path LSP computation.
Very, very important that you know which tasks should be outsourced and
which of them you still make a profit. Seems like a good
idea in case you run into unexpected therapy nightclub delays.
You only want the calls that are most likely to buy your product,
postcards and leaflets.
well, I think that answer A can´t be because we can configure a LSP with BW and ERO constrains without CSPF. according with JNCIE_studyguide.pdf pg 154 Chapter 2 _ MPLS and Traffic Engineering
“Many candidates are surprised to find that a bandwidth constrained LSP can be established without the use of a Traffic Engineering Database (TED) and the CSPF algorithm. Because RSVP was intended to support Quality of Service (QoS) over the Internet, it always had built-in support for session-based bandwidth reservations. Because RSVP lacks a network-wide view of the current state of network resources, there is a chance that no path in the network can honor the reservation, which causes the RSVP session to fail somewhere between ingress and egress routers. Note that the look-ahead capability of CSPF will prevent the transmission of a futile RSVP Path message because no ERO will be provided to RSVP for signaling when CSPF cannot find an endto-end path that honors the user’s constraints.”
according with that, if you have no-cspf then the LSP is built hop-by-hop by RSVP, following the restrictions in the explicit path if you have configured one and using the IGP routing table to resolve next hops along the path.
May be the answer D could be the best
Hehe… Well, I think that answer D can´t be either because with no-cspf the color constrains are ignored by the IGP, then the LSP could be signaled succesfully without meeting all constrains.
I can´t see the correct answer to this question… any help?