Subject: RE: Question about Labels

From: "Morgan, Richard" <[email-masked]>
Date: Mon, 22 Jan 2001 12:57:22 -0000

>   Why should be labels different in the devices that are near 
> and even why
>   are they different in all the devices involved in all the route ??

Labels only have local significance - ie a LSR need only know that to send a
packet to get a final destination it needs to send it in a direction with a
particalar label, also an LSR must know that a packet recieved with a label
on a interface must be sent out on another interface as it is destination is
already known (ie. where the label is popped).

There is no requirement for anything other than adjacent routers knowing the
association between a route and a label. Consider a ordinary ipv4 routing
table, it has no knowledge of what any other routing table contains, the IGP
that populates that table *does* have knowledge of the connections of the
adjacent routers. The same with MPLS - the protocols that populate the
tables know what they are doing and neogate the other MPLS tables so that
they work together. 

Consider an LSP across a series of LSR's as a physical chain, any link in
the middle of chain is connectioned to two other links. Local significances
means it only has to cooperate with these two links. The label swapping
ensures that the link between two routers is only know to the two connected
routers  - local significance is up held things are simple.

> Hi:
>   If we want do the things easier shouldnt be just be a 
> label, the same ,
>   for the path = service ??

There is an attempt to bring MPLS to optical switched paths, this is known
as GMPLS. In an optical path you want to keep the wave length of light for a
path constant across the entire network. In GMPLS the label used relates the
wavelength of light, hence you don't want to label swap - ie local
significances is broken and the orginal LSP setup gets a lot more
complicated.

> 2) What happend if the the tables, routes, in the Network 
> change, what is
> that related with the labels, how do they change ?? Does it 
> has to be with
> the LDP ??

The IGP routes in the table are kept separate from LDP / RSVP LSP routes.
Should the LSP break in response to a link being broken the information is
communicated back down the path to the source of the LSP and is be
recalculated using IGP information from there. There are a few extra
failovers a LSP can deploy to help the switch over - these are known as
secondary routes and fast reroute.

I hope that I understood your question and that this helps :)

Rich

---

From: "Yi Chu" <[email-masked]>
Date: Mon, 22 Jan 2001 09:15:13 -0500

-----Original Message-----
From:	[email-masked] [mailto:[email-masked]] On Behalf Of Morgan,
Richard
Sent:	Monday, January 22, 2001 7:57 AM
To:	'Arturo Nunez'
Cc:	[email-masked]
Subject:	RE: Question about Labels

<snip>

There is an attempt to bring MPLS to optical switched paths, this is known
as GMPLS. In an optical path you want to keep the wave length of light for a
path constant across the entire network. In GMPLS the label used relates the
wavelength of light, hence you don't want to label swap - ie local
significances is broken and the orginal LSP setup gets a lot more
complicated.
If we solve the technical problem of optically swapping lambda's, then
labels of GMPLS will only be locally significant too.  Any idea if and when
this will ever happen?

Yi
Winstar Communications