ldp sessions through IP or LSP ???

[g.rodegari]

Hi all,

suppose to have a basic MPLS with LDP configuration, between two routers A and B:

lo0-[ A ]-------Ethernet-OSPF-segment----[ B ]-lo1

The LDP session is established between the Router-ID addresses, probably between the 2 loopbacks, known through the OSPF protocol.

Now, from the router A perspective, before the LDP session establishment,

Lop1 is known like an OSPF route. After the LDP session go Up, this address is Known like an LDP route (because advertised as a FEC).

Isn'it better that the control plane about LDP goes through simple IP instead of to be nested into the LSP?

How can I force this behaviour?

Best Regards,

Graziano

[mheusinger]

Hello,

if you look at the LFIB (show mpls forwarding-table) you will see, that the outgoing label for the neighbor Loopback is imp-null, i.e. it will be an IP packet leaving the respective router (PHP - penultimate hop popping).

Thus LDP between directly connected neighbors will only involve IP traffic and not be labeled.

Hope this helps! please rate all posts.

Regards, Martin

[g.rodegari]

Hi Martin,

thank you very much.

Anyway can't I exclude those addresses from the FEC advertisement?

I suppose that if we have topology changes that

causes label withdraw, the LDP sessions should be recreated. Anymore, if I've problem on OSPF, also I've to wait the dead time of the tunnel...

Probably with this scenario nothing changes, but with more complicate networks?

Best Regards,

Graziano

[mheusinger]

Hello,

the LDP convergence is not affected by the existance or non-existance of imp-null label. The LDP session is not recreated if a label is withdrawn, but only if you lose the link between the neighboring routers.

To avoid even this use targeted LDP sessions, which are configured like this:

ip cef

mpls ldp neighbor 2.2.2.2 targeted ldp

interface Loopback0

ip address 1.1.1.1 255.255.255.255

interface Ethernet0

ip address 10.0.0.1 255.255.255.0

mpls mtu 1512

mpls ip

Or use MPLS LDP session protection:

mpls ldp session protection

There is no dead time of a tunnel involved in convergence. OSPF relies on link local multicast (224.0.0.5 and 224.0.0.6), which is not labeled. So your routing convergence has nothing to do with labels at all.

In case you want to reduce the number of labels announced you could use conditional label advertisement:

no mpls ldp advertise-tags

mpls ldp advertise-tags for 90 to 91

where 90 and 91 are access-lists (90 for which prefixes, 91 to which neighbors).

Be extremely careful though, because this could destroy LSPs necessary for your MPLS VPNs and the like.

So in brief: There is no drawback of the implementation between neighboring routers.

Hope this helps! Please rate all posts.

Regards, Martin

[g.rodegari]

Hi,

suppose that one of two routers does not support PHP.

Then the LDP control traffic should flow through LSP...

how can I exclude it?

thank you so much

Best Regards,

graz.

[mheusinger]

Hello,

exclude the loopback host route with acl 90 in my conditional label advertisement example. If no label is distributed, no label will ne used.

Hope this helps! Please rate all posts.

Regards, Martin

[g.rodegari]

thank you,

G.