[j-nsp] P2MP LSPs :: TailEnd/Bud nodes behavior

[Egor Zimin]

Hello, guys

Today I noticed very interesting difference in implementation of P2MP
LSPs by Cisco and Juniper.
The difference is related to explicit/implicit-null behavior of S2L
Sub-LSP tailend routers:
Cisco implementation:
(http://www.cisco.com/en/US/docs/ios/mpls/configuration/guide/mp_te_p2mp_ps6922_TSD_Products_Configuration_Guide_Chapter.html)
---
The tailend routers allocate unreserved labels, which are greater than
15 and do not include implicit or explicit null labels.
---

In Juniper's implementation tailend allocates implicit/explicit null
label as a usual.
As a consequence of this, (it looks like) we can have unnecessary
replication before "Bud" nodes.

For example:
Let's consider this configuration:
###
label-switched-path LSP-P2MP-16--19 {
    to 10.245.87.19;
    p2mp TREE1;
}
label-switched-path LSP-P2MP-16--18 {
    to 10.245.87.18;
    p2mp TREE1;
}
label-switched-path LSP-P2MP-16--15 {
    to 10.245.87.15;
    p2mp TREE1;
}
label-switched-path LSP-P2MP-16--17 {
    to 10.245.87.17;
    p2mp TREE1;
}
###
> show mpls lsp p2mp ingress
Ingress LSP: 1 sessions
P2MP name: TREE1, P2MP branch count: 4
To              From            State Rt P     ActivePath       LSPname
10.245.87.17    10.245.87.16    Up     0 *                      LSP-P2MP-16--17
10.245.87.15    10.245.87.16    Up     0 *                      LSP-P2MP-16--15
10.245.87.18    10.245.87.16    Up     0 *                      LSP-P2MP-16--18
10.245.87.19    10.245.87.16    Up     0 *                      LSP-P2MP-16--19
Total 4 displayed, Up 4, Down 0
###
As you can see, there are four leaves. Three bottom leaves use the
same downstream interface:
###
> show rsvp session p2mp detail | match "PATH sentto"
  PATH sentto: 10.245.87.146 (xe-0/0/2.0) 4 pkts
  PATH sentto: 10.245.87.149 (xe-0/0/1.0) 2 pkts
  PATH sentto: 10.245.87.149 (xe-0/0/1.0) 2 pkts
  PATH sentto: 10.245.87.149 (xe-0/0/1.0) 3 pkts
###
> show rsvp session p2mp
Ingress RSVP: 18 sessions
P2MP name: TREE1, P2MP branch count: 4
To              From            State   Rt Style Labelin Labelout LSPname
10.245.87.17    10.245.87.16    Up       0  1 SE       -        3
LSP-P2MP-16--17
10.245.87.15    10.245.87.16    Up       0  1 SE       -        3
LSP-P2MP-16--15
10.245.87.18    10.245.87.16    Up       0  1 SE       -   309200
LSP-P2MP-16--18
10.245.87.19    10.245.87.16    Up       0  1 SE       -   309200
LSP-P2MP-16--19
Total 4 displayed, Up 4, Down 0
###

As you can see, we have two different out labels (3 and 309200) for
the same P2MP LSP. Label 3 is allocated by node 10.245.87.15 because
of PHP.

Can anybody explain, what IETF speaks about this case ? Must tailend
routers allocate unreserved label or not ? I can't find any mention of
this case in RFCs (4875, 4461).

-- 
Best regards,
Egor Zimin