MPLS LDP Label Filtering Example

(Rene Molenaar) #1

This topic is to discuss the following lesson:

(system) #2

For r1 why is untagged?
Similar for r3 why is untagged?

(Rene Molenaar) #3

That’s what this tutorial is about…filtering tags so not all prefixes get a tag. R2 only sends a tag for /32 to R1 and R3, not for or

(system) #4

My bad. Just realized that r2 is between r1 and r3

(Hamood R) #5


Great lesson as usual. However, I have question loop backs which are not label it means they can not be reached and can not be in the routing table?





(Rene Molenaar) #6

Hi Hamood,

If there is no label for a prefix then we can still reach it through normal routing with the routing table, the only difference is that it won’t be label switched.


(Priti B) #7

Good.! I had the same question what cla had. Thanks for answering Rene.!!

(SUMIT B) #8

why i am seeing TDP instead of LDP.

R1#show mpls ldp neighbor
    Peer TDP Ident:; Local TDP Ident
        TCP connection: -
        State: Oper; PIEs sent/rcvd: 5/5; Downstream
        Up time: 00:01:01
        TDP discovery sources:
          FastEthernet0/0, Src IP addr:
        Addresses bound to peer TDP Ident:

(Rene Molenaar) #9

TDP is the old Cisco equivalent protocol for LDP. LDP is an IETF standard. On older IOS versions, TDP is the default. Nowadays LDP is the default. There is a command to change the protocol:

R1(config)#mpls label protocol ?
  ldp  Use LDP (default)
  tdp  Use TDP
1 Like
(SUMIT B) #10

Thank you so much Rene…

(Mohammad Hasanuz Zaman) #11

Hi Rene,

I am little bit confused that where we can use the label filtering in Production network .I am looking for scenario where we deploy it . Thx


(Rene Molenaar) #12

Hi Zaman,

Think of an MPLS VPN network where we have PE and CE routers. The PE routers only require an LSP (Label Switched Path) between their loopback interfaces and the /32 IP address that is configured on these loopback interfaces. These are the IP addresses that are used for the next hops in BGP.

By default, LDP creates and advertises a label for each and every prefix that is learned. This increases memory usage but also the number of advertisements between peers. If you only advertise labels for the LSPs that you need, you reduce convergence time and memory usage.

(bahri a) #13


R3(config)#access-list 1 permit
R3(config)#no mpls ldp advertise-labels
R3(config)#mpls ldp advertise-labels for 1

(mpls ldp advertise-labels for ) is a command ok but ( 1) what for i dont understand

(Lazaros Agapides) #14

Hello Bahri

The mpls ldp advertise-labels for 1 command is used to control which labels are advertised to which LDP neighbors. The “1” at the end of the command indicates that it is the access list “1” which is configured above that will be used to filter the labels such that labels will be generated only for loopback 0. On R3, is the IP address of loopback 0.

I hope this has been helpful!


(Staut S) #15


I can see that “tag / pop tag” is used in the output. In the previous lesson “label / pop label” was used. Is there any difference?

Many thanks,

(Lazaros Agapides) #16

Hello Staut

When speaking about MPLS, the term label and tag may be used interchangeably. The term label is the officially correct one, but keep in mind that in the context of MPLS, the term tag may be used as well.

I hope this has been helpful!


1 Like