LDP Implicit Withdraw

Load isis.cfg

#IOS-XE
config replace flash:isis.cfg

#IOS-XR
configure
load bootflash:isis.cfg
commit replace
y

• Configure LDP between R1 and R2 using interface commands

• Configure R2 so that any existing FEC bindings with a new label are advertised and withdrawn using a single label mapping message

Answer

#R1
int giw.12
 mpls ip

#R2
int giw.12
 mpls ip
!
mpls ldp neighbor 1.1.1.1 implicit-withdraw

Explanation

In IOS, the old method for advertising a new label for an existing FEC was to implicitly withdraw the label by simply advertising a new label for that FEC. This is similar to BGP’s behavior, in which a new Update for the same prefix from a peer is considered as “replacing” the existing route. This is why BGP AddPath functionality requires a path ID - to keep paths separate and not implement implicit withdraw.

In IOS-XE, the new style for LDP was to first withdraw the label explicitly, and then advertise a new label binding. To restore the old method we can use the following command. This functionality does not appear to be supported on IOS-XR in version 6.x.x.

#IOS-XE
mpls ldp neighbor 1.1.1.1 implicit-withdraw

To test this out, we can create a static label binding on R2. This allows us to easily change the label associated with the FEC. First we create a static route:

#R2
ip route 10.10.10.10 255.255.255.255 20.2.3.3

R2 will advertise this FEC binding with a dynamic label to R1. Next we create a static label range on R2.

#R2
mpls label range 200 299 static 2000 2999

We can now allocate a static local label for this FEC on R2.

#R2
mpls static binding ipv4 10.10.10.10 255.255.255.255 2001

R2 has not yet configured implicit-withdraw for R1. On R1, we can see that R2 sends two messages: a withdraw and then a label mapping:

Next, we set R2 for implicit-withdraw, and change the local static label for the FEC:

#R2
mpls ldp neighbor 1.1.1.1 implicit-withdraw
!
mpls static binding ipv4 10.10.10.10 255.255.255.255 2002

On R1 we now see only the new mapping message. This is an implicit withdraw for the old binding, instead of an explicit withdraw as we saw before.