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MPLS LDP Label Filtering Example

Once you enable MPLS on the interfaces between the routers and LDP neighbor adjacencies have been formed, a label will be advertised for each network. With LDP however we can configure filters to decide what networks should get a label and which ones shouldn’t be tagged. I’ll use the following topology to demonstrate this:

Above we have 3 routers and each router has 2 loopback interfaces so that we have plenty of networks to play with. Before we enable MPLS we’ll configure OSPF so that all networks are advertised:

R1,R2,R3:
(config)#router ospf 1
(config-router)#network 0.0.0.0 255.255.255.255 area 0

We’ll do this the easy way and activate OSPF on all interfaces. Now let’s enable MPLS on the FastEthernet interfaces:

R1(config)#interface fastEthernet 0/0
R1(config-if)#mpls ip

R2(config)#interface fastEthernet 0/0
R2(config-if)#mpls ip
R2(config-if)#exit
R2(config)#interface fastEthernet 0/1
R2(config-if)#mpls ip

R3(config)#interface fastEthernet 0/0
R3(config-if)#mpls ip

Let’s check if we have LDP neighbors:

R2#show mpls ldp neighbor | include Peer
    Peer LDP Ident: 11.11.11.11:0; Local LDP Ident 22.22.22.22:0
    Peer LDP Ident: 33.33.33.33:0; Local LDP Ident 22.22.22.22:0

So far so good, now let’s take a look at the LDP labels that have been generated:

R1#show mpls forwarding-table 
Local  Outgoing    Prefix            Bytes tag  Outgoing   Next Hop    
tag    tag or VC   or Tunnel Id      switched   interface              
16     Pop tag     2.2.2.2/32        0          Fa0/0      192.168.12.2 
17     17          33.33.33.33/32    0          Fa0/0      192.168.12.2 
18     18          3.3.3.3/32        0          Fa0/0      192.168.12.2 
19     Pop tag     22.22.22.22/32    0          Fa0/0      192.168.12.2 
20     Pop tag     192.168.23.0/24   0          Fa0/0      192.168.12.2

R2#show mpls forwarding-table 
Local  Outgoing    Prefix            Bytes tag  Outgoing   Next Hop    
tag    tag or VC   or Tunnel Id      switched   interface              
16     Pop tag     1.1.1.1/32        0          Fa0/0      192.168.12.1 
17     Pop tag     33.33.33.33/32    0          Fa0/1      192.168.23.3 
18     Pop tag     3.3.3.3/32        0          Fa0/1      192.168.23.3 
19     Pop tag     11.11.11.11/32    0          Fa0/0      192.168.12.1

R3#show mpls forwarding-table 
Local  Outgoing    Prefix            Bytes tag  Outgoing   Next Hop    
tag    tag or VC   or Tunnel Id      switched   interface              
16     Pop tag     192.168.12.0/24   0          Fa0/0      192.168.23.2 
17     16          1.1.1.1/32        0          Fa0/0      192.168.23.2 
18     Pop tag     2.2.2.2/32        0          Fa0/0      192.168.23.2 
19     Pop tag     22.22.22.22/32    0          Fa0/0      192.168.23.2 
20     19          11.11.11.11/32    0          Fa0/0      192.168.23.2

For all networks a label has been generated by LDP. Now let’s configure filtering so that we only generate labels for the loopback 0 interfaces. This is how you do it:
[teaser]

R1(config)#access-list 1 permit 1.1.1.1 0.0.0.0
R1(config)#no mpls ldp advertise-labels
R1(config)#mpls ldp advertise-labels for 1

R2(config)#access-list 1 permit 2.2.2.2 0.0.0.0
R2(config)#no mpls ldp advertise-labels
R2(config)#mpls ldp advertise-labels for 1

R3(config)#access-list 1 permit 3.3.3.3 0.0.0.0
R3(config)#no mpls ldp advertise-labels
R3(config)#mpls ldp advertise-labels for 1

First use no mpls ldp advertise-labels to disable the advertisement of all labels. Secondly use the mpls ldp advertise-labels for command and refer to an access-list or prefix-list to choose what networks should have a label.

Be careful, if you forget to use the no mpls ldp advertise-labels command you will discover that LDP is STILL advertising a label for each network…

Let’s verify our work:

R1#show mpls forwarding-table 
Local  Outgoing    Prefix            Bytes tag  Outgoing   Next Hop    
tag    tag or VC   or Tunnel Id      switched   interface              
16     Pop tag     2.2.2.2/32        0          Fa0/0      192.168.12.2 
17     Untagged    33.33.33.33/32    0          Fa0/0      192.168.12.2 
18     Untagged    3.3.3.3/32        0          Fa0/0      192.168.12.2 
19     Untagged    22.22.22.22/32    0          Fa0/0      192.168.12.2 
20     Untagged    192.168.23.0/24   0          Fa0/0      192.168.12.2

R2#show mpls forwarding-table 
Local  Outgoing    Prefix            Bytes tag  Outgoing   Next Hop    
tag    tag or VC   or Tunnel Id      switched   interface              
16     Pop tag     1.1.1.1/32        0          Fa0/0      192.168.12.1 
17     Untagged    33.33.33.33/32    0          Fa0/1      192.168.23.3 
18     Pop tag     3.3.3.3/32        0          Fa0/1      192.168.23.3 
19     Untagged    11.11.11.11/32    0          Fa0/0      192.168.12.1

R3#show mpls forwarding-table 
Local  Outgoing    Prefix            Bytes tag  Outgoing   Next Hop    
tag    tag or VC   or Tunnel Id      switched   interface              
16     Untagged    192.168.12.0/24   0          Fa0/0      192.168.23.2 
17     Untagged    1.1.1.1/32        0          Fa0/0      192.168.23.2 
18     Pop tag     2.2.2.2/32        0          Fa0/0      192.168.23.2 
19     Untagged    22.22.22.22/32    0          Fa0/0      192.168.23.2 
20     Untagged    11.11.11.11/32    0          Fa0/0      192.168.23.2

Above you can see that only network 1.1.1.1/32, 2.2.2.2/32 and 3.3.3.3/32 now have a label when advertised to a LDP neighbor.

Configurations

Want to take a look for yourself? Here you will find the configuration of each device.

R1

hostname R1
!
ip cef
!
no mpls ldp advertise-labels
mpls ldp advertise-labels for 1
!
interface Loopback0
 ip address 1.1.1.1 255.255.255.255
!
interface Loopback1
 ip address 11.11.11.11 255.255.255.255
!
interface FastEthernet0/0
 ip address 192.168.12.1 255.255.255.0
 mpls ip
!
router ospf 1
 network 0.0.0.0 255.255.255.255 area 0
!
access-list 1 permit 1.1.1.1
!
end

R2

hostname R2
!
ip cef
!
no mpls ldp advertise-labels
mpls ldp advertise-labels for 1
!
interface Loopback0
 ip address 2.2.2.2 255.255.255.255
!
interface Loopback1
 ip address 22.22.22.22 255.255.255.255
!
interface FastEthernet0/0
 ip address 192.168.12.2 255.255.255.0
 mpls ip
!
interface FastEthernet0/1
 ip address 192.168.23.2 255.255.255.0
 mpls ip
!
router ospf 1
 network 0.0.0.0 255.255.255.255 area 0
!
access-list 1 permit 2.2.2.2
!
end

R3

hostname R3
!
ip cef
!
no mpls ldp advertise-labels
mpls ldp advertise-labels for 1
!
interface Loopback0
 ip address 3.3.3.3 255.255.255.255
!
interface Loopback1
 ip address 33.33.33.33 255.255.255.255
!
interface FastEthernet0/0
 ip address 192.168.23.3 255.255.255.0
 mpls ip
!
router ospf 1
 network 0.0.0.0 255.255.255.255 area 0
!
access-list 1 permit 3.3.3.3
!
end

That’s all I wanted to show you, if you have any questions feel free to leave a comment!

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MPLS LDP (Label Distribution Protocol)

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Tags: LDP

Notable Replies

system says:

For r1 why is 3.3.3.3 untagged?
Similar for r3 why is 1.1.1.1 untagged?
ReneMolenaar says:

That’s what this tutorial is about…filtering tags so not all prefixes get a tag. R2 only sends a tag for 2.2.2.2 /32 to R1 and R3, not for 1.1.1.1/32 or 3.3.3.3/32.
ReneMolenaar says:
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
```

R3(config)#access-list 1 permit 3.3.3.3 0.0.0.0
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

lagapides says:

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!

laz

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MPLS LDP Label Filtering Example

Configurations

R1

R2

R3

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