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 0We’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 ipR2(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 ipLet’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 22.22.22.22:0So 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 Fa0/0 192.168.23.2For 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:
For r1 why is 3.3.3.3 untagged?
Similar for r3 why is 1.1.1.1 untagged?
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.
Hello Ahmed
When the filtering mechanism is enabled on each router, it is applied for all networks whether those networks exist on the local router or are received and transmitted by intervening routers. What this means is that R2 will see 1.1.1.1 and 3.3.3.3 labelled while 11.11.11.11 and 33.33.33.33 will not have labels, as expected.
Similarly, R1 will see that 2.2.2.2 is labelled while 22.22.22.22 will not have labels, again as expected. However, because R3 sends its information to R1 via R2, the
... Continue reading in our forummpls ldp advertise-labels for 1command applied to R2 will aHello,
What is use-case to filter the label for the certain prefixes, does that means if prefixes doesn’t have a label and traffic to un-labeled prefixes will be drop or blackhole in transit.
in this case R1 doesn’t have a label for R3 (3.3.3.3) by virtue if access-list defined on R1 does that means traffic to destination 3.3.3.3 from R1 will be drop?
Loved this explanation, thanks Laz!