The MPLS WG Archive

[<Santanu.Ganguly@swisscom.com>]

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:shilpa07@gmail.com]
Sent: Tuesday, December 12, 2006 5:03 AM
To: Roger Williams (rogwilli); mpls@UU.NET; mpls-ops@mplsrc.com
Subject: [mpls] Re: [MPLS-OPS]: label space configuration

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) <rogwilli@cisco.com> 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:shilpa07@gmail.com]
Sent: Monday, December 04, 2006 12:24 AM
To: mpls@uu.net; mpls-ops@mplsrc.com; mpls@lists.ietf.org
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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