Subject: [mpls] Re: [MPLS-OPS]: label space configuration

From: shilpa goel <[email-masked]>
Date: Tue, 12 Dec 2006 09:33:03 +0530

Hi,

 Thanks for the clarification. I still have 2 doubts in this.

Firstly, what is meant by the line  'as the router knows about all of
them'?  (2nd para)

Also, this is clear that for ATM, interface label space should be used but
why should platform wide labelling be used in an Ethernet configuration i.e.
why can't we use interface label space for ethernet also thereby seeking the
same advantage as we are getting for ATM?

thanks,
Shilpa

On 12/4/06, Roger Williams (rogwilli) <[email-masked]> wrote:
>
>  Shilpa, platform-wide labeling is used in an Ethernet configuration,
> whereas port or interface lablespace is used for ATM.
>
> Basically what that means is that for Ethernet a given router advertises
> its whole self with one label upstream. All ports would be covered under
> that single label advertisement, as the router knows about all of them. And
> it is done as soon as the underlying IGP routing protocol knows there are
> routes to be labeled, not waiting for the demands of traffic to build label
> paths.
>
> With ATM the doling out of labels is much more conservative, and done only
> when requested by traffic demand. The reason for this is the limited number
> of VCs that an ATM switch can support (I think 4096). The labels point to a
> VC underneath it all. Since there is no routing done at the Layer 2 of an
> ATM switch, each label is used as a per-interface advertisement, which
> effectively is a per-VC advertisement.
>
> I hope that helps.
>
> Roger.
>
>  ------------------------------
> *From:* shilpa goel [mailto:[email-masked]]
> *Sent:* Monday, December 04, 2006 12:24 AM
> *To:* [email-masked]; [email-masked]; [email-masked]
> *Subject:* [MPLS-OPS]: label space configuration
>
> Hi..
>
>   Can you please give me an insight as to how label space (per interface
> or per platform) is configured in a LSR i.e. what all considerations need
> to be taken into account:
>
> for 2 cases-
>     -LSR consisting of only ethernet ports
>     -LSR consisting of both ATM and ethernet interfaces
>
> thanks and regards,
> Shilpa
>
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---

From: <[email-masked]>
Date: Thu, 14 Dec 2006 10:09:37 +0100

Hi Shilpa,
 
1) I suspect what Roger means by 'as the router knows about all of them'
is the following:
 
Generally speaking ther are 2 types of label spaces, as we all know:
Per-Interface & Per-Platform. Per-platform assigns labels 
from a platform-wide pool of labels and  uses resources that are shared
across the platform. Hop-by-hop best-effort IP/MPLS 
forwarding is an example of using the per-platform label space....
 
Frame based MPLS interfaces ( POS, Ethernet, fast Ethernet etc...:) use
a per-platform label space , which basically means that the label space
is router-wide and assigned from a unique pool of labels in the LSR; the
label can used on ANY interface. The LFIB has no information about the
incoming interfaces ( i think...:-))
 
However, the nature of the labels dolled out on LC-ATM interfaces stops
us from using per-paltform label space. 2 LC-ATM interfaces can use the
same VPI/VCI pair as a label. As such , LC-ATM interfaces use
per-interface labels which has non-zero label space ID.
 
Note : In some router platforms it is possible to manually configure
something like a "mpls ldp label-space-id < ID number> " on frame based
interfaces if indeed this becomes an issue...in case a labe-space is not
defined it takes the deafult value ( 0 for ethernet)...
 
2) Consider the scenario of 2 routers connected back to back via 2 links
of Etherent. Without the present definition of Etherenet 
label space, they will open up 2 different LDP sessions, for which they
may require to open up 2 TCP sessions..., which really 
would be unnecessary....maybe :-)
 
3) As far as I know, historically speaking, IETF developed a series of
solutions for various Layer 2 VPN applications including 
pseudowire emulations. I think Cisco Systems also has developed an
alternate pseudowire emulation ( L2TPv3 ?)...I am sure there 
are enough "cisco.com " email addresses here in this forum to correct me
if I am wrong or to elaborate on this if i am not :-)
 
Cheers :-)
 
santanu
 
Santanu Ganguly
Swisscom Fixnet Wholesale 
Binz Ring 17
8045 Zurich
Switzerland

________________________________

From: shilpa goel [mailto:[email-masked]] 
Sent: Tuesday, December 12, 2006 5:03 AM
To: Roger Williams (rogwilli); [email-masked]; [email-masked]
Subject: [mpls] Re: [MPLS-OPS]: label space configuration

Hi,

Also, this is clear that for ATM, interface label space should be used
but why should platform wide labelling be used in an Ethernet
configuration i.e. why can't we use interface label space for ethernet
also thereby seeking the same advantage as we are getting for ATM?

thanks,
Shilpa

	Shilpa, platform-wide labeling is used in an Ethernet
configuration, whereas port or interface lablespace is used for ATM. 
	 
	Basically what that means is that for Ethernet a given router
advertises its whole self with one label upstream. All ports would be
covered under that single label advertisement, as the router knows about
all of them. And it is done as soon as the underlying IGP routing
protocol knows there are routes to be labeled, not waiting for the
demands of traffic to build label paths. 
	 
	With ATM the doling out of labels is much more conservative, and
done only when requested by traffic demand. The reason for this is the
limited number of VCs that an ATM switch can support (I think 4096). The
labels point to a VC underneath it all. Since there is no routing done
at the Layer 2 of an ATM switch, each label is used as a per-interface
advertisement, which effectively is a per-VC advertisement.
	 
	I hope that helps.
	 
	Roger.

________________________________

	From: shilpa goel [mailto:[email-masked]] 
	Sent: Monday, December 04, 2006 12:24 AM
	To: [email-masked]; [email-masked]; [email-masked]
	Subject: [MPLS-OPS]: label space configuration

	Hi..
	
	  Can you please give me an insight as to how label space (per
interface or per platform) is configured in a LSR i.e. what all
considerations need to be taken into account:
	
	for 2 cases- 
	
	thanks and regards,
	Shilpa

---

From: David Charlap <[email-masked]>
Date: Thu, 14 Dec 2006 09:01:20 -0500

[email-masked] wrote:
> Hi Shilpa,
>  
> 1) I suspect what Roger means by 'as the router knows about all of 
> them' is the following:
>  
> from a platform-wide pool of labels and  uses resources that are 
> shared across the platform. Hop-by-hop best-effort IP/MPLS
> Frame based MPLS interfaces ( POS, Ethernet, fast Ethernet etc...:) 
> use a per-platform label space , which basically means that the label 
> space is router-wide and assigned from a unique pool of labels in the 
> LSR; the label can used on ANY interface. The LFIB has no information 
> about the incoming interfaces ( i think...:-))
>  
> *However*, the nature of the labels dolled out on LC-ATM 
> interfaces stops us from using per-paltform label space. 2 LC-ATM 
> interfaces can use the same VPI/VCI pair as a label. As such , LC-ATM 
> interfaces use per-interface labels which has non-zero label space ID.

Note, however, that there is nothing in any standard mandating a 
platform label space for Ethernet/POS or a per-interface label space for 
LC-ATM.

Depending on the forwarding architecture (a centralized forwarding 
engine vs. one distributed across all port-cards), it may make more 
sense for a switch to use per-interface labels for Ethernet/POS, or 
platform labels for LC-ATM.  This is perfectly legal.

While it is important to be aware of the different type of label spaces, 
and to know what is commonly used, you should not assume that "common" 
means "universally mandated".

-- David