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:

MPLS LDP Filtering Example Topology

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:

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Forum Replies

  1. For r1 why is 3.3.3.3 untagged?
    Similar for r3 why is 1.1.1.1 untagged?

  2. 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.

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

  4. Rene,

    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?

     

    Thanks

     

    Hamoood

  5. 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.

    Rene

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