Re: RSVP and LDP

Hi Mustafa

LDP has been designed for distributing labels exclusively, But RSVP has
been a Host based protocol to inform the transit routers that to reserve
the bandwidth what the application needs.
So here in MPLS TE , RSVP is used as a signalling protocol to establish a
BANDWIDTH DEFINED LSP from head end router to your Tail End Router

In ur normal LDP , For each prefix u build a particular LSP . These
prefixes are distributed by ur CORE IGP. SO basically ur LDP is relying on
ur IGP info for setting up a LSP for that prefix

But in MPLS TE, RSVP will build a LSP with the Tail End Router with some
constraints. So basically it is a Tunnel where ur Head End Router is trying
to form with the Tail End and to switch packets on the Tunnel. It is a
unidirectional Tunnel.

TE also uses ur CORE IGP to check whether the requirements to form a
tunnel are met. For example , If the TE Tunnel requires 500Kb of bandwidth
ie ur Head End's routers requirement is 500Kbs of B.W end to end ( Assume
that there are some users sitting behind Head End Router which requires a
500kb feed from the server which is present behind ur Tail End Router ) So
now ur Head End has to know the end to end B.W capacity of each transit
router . This information is fed by our CORE IGP which can be ISIS or OSPF
. ( These r the two protocol in use currently which has the capability to
carry MPLS TE extended attributes . OSPF uses Opaque LSA and ISIS uses
extended TLVS for it )

So TE Tunnel will be up only when it is met with these requirements and
after that it signals the RSVP Path and Tail End will send Reserve Messages
to assign Labels ..
This is also same as ur LDP where ur Downstream router starts assigning
labels to the prefixes.

So basically LDP is a prefix driven label assignment protocol but RSVP is
all about how ur Head End wants the Constraint driven LSP with ur Tail End
router and ( so u must know ur network and u know there is greedy hosts
which wants a feed and u configure the TE Tunnel accordingly )

Without LDP also, RSVP will work .For RSVP u just need ur CORE IGP to be
converged and has the capabilities to carry TE extended information ( So no
RIP and EIGRP as of now ) RSVP Head End just sends Path message to ur Tail
End router to signal each and every transit router that this is the
requirement

Do not confuse between LDP and RSVP. LDP is always Prefix Driven From IGP
.. RSVP is basically manually configured Constraint Driven LSP ( knowing ur
network end to end )

Hope this helps :)

On Thu, Feb 2, 2012 at 5:43 PM, GAURAV MADAN <gauravmadan1177_at_gmail.com>wrote:

> I think things are being mixed up here .
>
> When RSVP is used with TE .. RSVP will do the label distribution .
> Hence role of LDP as a protocol for label distibution is eliminated as you
> are doing same thing using RSVP .
>
> And also ; i believe that in MPLS L3 VPN scenario as well ; running only
> RSVP and no LDP anywhere in core can achieve your goal .
>
> Regards
> Gaurav Madan
> CCIE
>
> On Thu, Feb 2, 2012 at 5:14 PM, Mustafa Yadav <mustafa.yadav_at_gmail.com
> >wrote:
>
> > so when ldp is going down all te tunnles are going to be down?
> >
> > but in one of my case ldp goes down but te tunnels did not.
> >
> > Also I seee ldp neigbor is up for 4 hours but mpls te tunnel up for 8
> > hours.and 4 hours ago ldp went down.
> >
> > On Thu, Feb 2, 2012 at 1:18 PM, sundeep sadhwani <
> > sundeep.sadhwani_at_gmail.com
> > > wrote:
> >
> > > Hi Mustafa,
> > >
> > > RSVP is used for reservation of bandwidth in TE environment( i am
> giving
> > u
> > > a very high level view). However plain MPLS-TE is not used in
> production
> > > scenario as SPs have MPLS-VPN as well. for that purpose we require LDP
> as
> > > well to distribute IGP labels that will be required for PE to PE
> > > communication in case of MPLS-VPN.
> > >
> > > Regards
> > > Sundeep
> > >
> > > On Thu, Feb 2, 2012 at 4:41 PM, Mustafa Yadav <mustafa.yadav_at_gmail.com
> > >wrote:
> > >
> > >> HD0,
> > >>
> > >>
> > >> In a cisco network I have seen mpls traffic engineneering tunnels are
> > >> being
> > >> used .So as far as I know rsvp must be configured .But I alos see that
> > ldp
> > >> is configured in all boxes.why is that?Is not rsvp enough for
> > distrbuting
> > >> labels?
> > >>
> > >>
> > >> Blogs and organic groups at http://www.ccie.net
> > >>
> > >>
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