What is global MPLS?

[hfakoor222]

I'm doing an online class and it mentions global MPLS labels (it's in a foreign language but it translates roughly to global MPLS label).

Can someone explain to me what's the difference between global/local labeling. 

[Harold Ritter · Accepted solution]

Hi @hfakoor222 ,

The global label space mode (RFC 5036, section 2.2.1 refers to it as per platform label space) is one of the two label space modes defined for LDP. The other mode is per interface label space. Most vendors and devices use the per platform label space mode these days. Per interface was more prevalent in the ATM days.

Please refer to RFC 5036 section 2.2.1 for more information.

https://datatracker.ietf.org/doc/rfc5036/

Regards,

[hfakoor222]

Thank you Mr. Ritter

[Harold Ritter]

You are very welcome @hfakoor222 and thanks for the feedback

[M02@rt37]

Hello @hfakoor222,

To go further, Global MPLS labels and are unique across the entire network. They are used to identify the label-switched paths (LSPs) between different routers in the network. The global MPLS label is typically used to forward packets between different autonomous systems, or between different MPLS networks.

On the other hand, local MPLS labels are assigned by each individual router and are only used within a specific segment of the network. These labels are used to identify the next-hop router in the path towards the destination. Local MPLS labels are not unique across the network and may be reused in different segments of the network.

[Harold Ritter]

Hi M02@rt37 ,

This is a different concept than the one I was referring to and it is specific to segment routing (SR). I am not sure whether @hfakoor222 was referring to global label in the context of LDP or SR.

With SR it is generally recommended to use the same global block range across the network, which causes the node segment ID (SID) for a given router is the same across the networks. Every router in the network only advertise an index to other routers. This index is unique across the network and is used against the global block to derive an absolute label value.

Let's say that the global block is 16000 to 23999 and that the index for R1 is 1, every router in the network will use 16001 to reach R1. This is make it simpler to troubleshoot as you always need to look for the same label on any router in the network.

Regards,

[M02@rt37]

I'm agree with you @Harold Ritter, the global MPLS label is derived from the SID [SR] and is used for forwarding packets along the path.

Thanks for pointing that.

[MHM Cisco World]

This online course talking about mpls segment routing? 

[hfakoor222]

I don't think the course mentioned which. I think it was describing both in their own context, separate from eachother, which caused me a little confusion. One of the questions on the assignments was :

"What are the consequences of global label space?"

The treatments of the nodes are reduced: no longer needed to change labels (Correct)

This requires more memory space to stock the LSR (Correct)

This requires less memory space to stock the LSR 

We can't say there is a significative impact on memory space in storing the LSR 

We significatively augment the time in label switching We significatively reduce the time in label switching 

We can't really say ther's a significative impact on label switching time (Correct)

We significatively increase the quantity of traffic due to label distribution (referring to distributing the LSR's to neighbors) 

We significatively reduce the quantity of traffic due to label distribution 

We can't really say there's a significative impact on quantity of traffic due to label distributiution (referring to LSR packets) (Correct)

as Mo2@rt37  mentioned

The global MPLS label is typically used to forward packets between different autonomous systems, or between different MPLS networks.

On the other hand, local MPLS labels are assigned by each individual router and are only used within a specific segment of the network. These labels are used to identify the next-hop router in the path towards the destination.

Which basically the class was describing as global LSR's were used to communicate traffic accross service providers, and internal LSR's were used by the internal routers. Also global LSR's are able to have administrator policies applied to them for backup paths and different costs, whereas internal LSR's are used locally between routers as labels are swapped out. The class mentioned using inteServ as the labels were decided coming backwards from the last router, and also deciding between DiffServ and administartor policies. 

Here is a powerpoint on DiffServ, intServ, and MPLS.

https://www.kiv.zcu.cz/~ledvina/DHT/tugraz/DiffServ%20IntServ%20MPLS.pdf

[Harold Ritter]

Hi @hfakoor222 ,

> The treatments of the nodes are reduced: no longer needed to change labels (Correct)

This is a clear indication that it refers to Segment Routing (SR), because one of the effect of the SR global space is that the label does not need to be swapped as the packets are being forwarded between the different nodes along the LSP, as the label remains the same across the entire network.

Regards,

[hfakoor222]

Did you mean this is a clear indication of global LSR?

[Harold Ritter]

Hi @hfakoor222 ,

The fact that the LSR does not need to swap the label is a characteristic of SR, since the label is global and the packet can just be forwarded to the next hop in the LSP without the label being swapped.

Regards, 

[MHM Cisco World]

First thanks for sharing the link of course 

Second each line of text you post need from me hour/s to understand. that how I study new topics'

Note:- dont depend one website but instead read book. Book is more more helpful for studying especially ciscopress

[MHM Cisco World]

Believe me, I also read about segment routing and and don't get until now what different between global label and local label. 

[hfakoor222]

Here's the main takeaway from the link I provided:

Label distribution:
MPLS does not mandate a single method of signaling for label
distribution
 BGP has been enhanced to piggyback the label information
within the contents of the protocol
 RSVP has also been extended to support piggybacked
exchange of labels.
 IETF has also defined a new protocol known as the label
distribution protocol (LDP) for explicit signaling and
management
 Extensions to the base LDP protocol have also been defined
to support explicit routing based on QoS requirements.

Label-Switched Paths (LSPs):
 A collection of MPLS—enabled devices represents an MPLS
domain. Within this domain, a path is set up for a given
packet to travel on an FEC. The LSP is set up prior to data
transmission. MPLS provides the following two options to set
up an LSP:
 hop-by-hop routing: Each LSR independently selects the next
hop for a given FEC. The LSR uses any available routing
protocols, such as OSPF, ATM private network-to-network
interface (PNNI), etc.
 explicit routing: is similar to source routing. The ingress LSR
(i.e., the LSR where the data flow to the network first starts)
specifies the list of nodes through which the ER–LSP traverses.

It seems like the explicit routing refers to global LSR's, which can be defined through administrative routing set ahead in time, which we can have backup and alternative paths in case of a failure or to maximize using available bandwidth in a system, as opposed to using an internal routing protocol to choose our labels, which chooses the best path and may not use all avalable bandwidth.

and finally:

Label Merging:
 Incoming traffic from different interfaces can be merged
together and switched using a common label if they are
traversing the network toward the same destination (stream
merging or aggregation of flows )
 Signaling Mechanisms
 label request: A LSR requests a label from its downstream
neighbor so that it can bind to a specific FEC. This
mechanism can be employed down the chain of LSRs up to
the egress LER
 label mapping: In response to a label request, a
downstream LSR will send a label to the upstream initiator
using the label mapping mechanism

MPLS performs the following functions:
 specifies mechanisms to manage traffic flows of various
granularities, such as flows between different hardware,
machines.
 remains independent of the Layer-2 and Layer-3 protocols
 provides a mechanism to map IP addresses to simple, fixedlength labels
 interfaces to existing routing protocols (RSVP, OSPF)

RSVP as in choosing the label from the end to the beginning.

 The labels are distributed using label distribution protocol
(LDP) or RSVP or piggybacked on routing protocols like
border gateway protocol (BGP) and OSPF.
 Each data packet encapsulates and carries the labels
during their journey from source to destination
 Hardware can be used to switch packets quickly
between links.

Labels and Label Bindings
 A label is carried or encapsulated in a Layer-2 header along with
the packet
 The label values are derived from the underlying data link layer.
 The receiving router examines the packet for its label content to
determine the next hop
 Once a packet has been labelled, the rest of the journey of the
packet through the backbone is based on label switching.
 The label values are of local significance

My key takeaway is global LSR's gives us more control over the network, however we have to make sure the policies we apply make sense ahead of time.