Nov 26 2008
Controlling Label distribution
Unlike label filtering, controlling label distribution actually stops label bindings being advertised.
Consider the toplogy below.
Without controlling label distribution, PE2 will generate a label for all IGP and connected prefixes. PE2s LIB can be seen below.
PE2#show mpls ldp bindings
lib entry: 1.1.1.1/32, rev 19
local binding: label: 17
remote binding: lsr: 2.2.2.2:0, label: 17
lib entry: 2.2.2.2/32, rev 20
local binding: label: 16
remote binding: lsr: 2.2.2.2:0, label: imp-null
lib entry: 3.3.3.3/32, rev 21
local binding: label: imp-null
remote binding: lsr: 2.2.2.2:0, label: 16
lib entry: 10.0.0.0/30, rev 22
local binding: label: 18
remote binding: lsr: 2.2.2.2:0, label: imp-null
lib entry: 10.0.0.4/30, rev 23
local binding: label: imp-null
remote binding: lsr: 2.2.2.2:0, label: imp-null
PE2#
These labels are also propogated to P1 as you can see below.
P1#show mpls ldp bindings neighbor 3.3.3.3
lib entry: 1.1.1.1/32, rev 12
remote binding: lsr: 3.3.3.3:0, label: 17
lib entry: 2.2.2.2/32, rev 5
remote binding: lsr: 3.3.3.3:0, label: 16
lib entry: 3.3.3.3/32, rev 10
remote binding: lsr: 3.3.3.3:0, label: imp-null
lib entry: 10.0.0.0/30, rev 7
remote binding: lsr: 3.3.3.3:0, label: 18
lib entry: 10.0.0.4/30, rev 8
remote binding: lsr: 3.3.3.3:0, label: imp-null
If we now add the following command to PE2 you will so no label binding being advertised to P1.
no mpls ldp advertise-labels
Lets take another looks at P1s LIB.
P1#show mpls ldp bindings neighbor 3.3.3.3
P1#
As you can see there are no labels learnt from PE2
In most cases however you will want to advertise at least the loopback address of PE2. This can be done in 3 steps.
Step 1 – on PE2 configure the following ACL.
ip access-list standard LOOPBACK0
permit 3.3.3.3 0.0.0.0
Step 2 – next attach the ACL to the MPLS label distribution configuration as below.
mpls ldp advertise-labels for LOOPBACK0
Step 3 – the third and final step is to stop advertising all the remaining labels using the command below.
no mpls ldp advertise-labels
Verify that you configuration has worked on P1. We should only see a binding for the loopback.
P1#show mpls ldp bindings neighbor 3.3.3.3
lib entry: 3.3.3.3/32, rev 10
remote binding: lsr: 3.3.3.3:0, label: imp-null
