R3303-HP HSR6800 Routers MPLS Configuration Guide

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• Label-block Offset—Offset of the label block. When CEs increase in a VPN and the existing label
block size is not enough, you do not need to withdraw the label block on the PEs. Instead, you can
assign a new label block in addition to the existing label block to enlarge the label range. A PE uses
LO to identify a label block among all label blocks, and to determine from which label block it
assigns labels. The LO value of a label block is the sum of LRs of all previously assigned label blocks.
For example, if the LR and LO of the first label block is 10 and 0, the LO of the second label block
is 10. If the LR of the second label block is 20, the LO of the third label block is 30.
The following describes a label block in the format of LB/LO/LR. For example, a label block whose LB,
LO, and LR are 1000, 10, and 5 is represented as 1000/10/5.
With label blocks, you can reserve some labels for the VPN for future use. This wastes some label
resources in a short term, but can reduce the VPN deployment and configuration workload in the case
of expansion.
Assume that an enterprise VPN contains 10 CEs and the number of CEs might increase to 20 in future.
In this case, set the LR to 20. When you add a CE to the VPN, you only need to modify the configurations
of the PE to which the new CE is connected. No change is required for the other PEs, which simplifies
VPN expansion.
Figure 41 VC label calculation
As shown in Figure 41, PEs calculates VC labels for a VC as follows (take the VC between CE 1 and CE
12 as an example):
• PE 1 calculates the VC label it assigns to the VC:
PE 1 compares the ID (12) of the peer CE (CE 12) with the label blocks assigned by PE 1. If a label
block satisfies LO<=CE ID<LO+LR, PE 1 assigns a label from the label block. In this example, label
block 2 (1055/5/10) satisfies LO<=CE ID<LO+LR (5<=12<5+10). PE 1 assigns a label to the VC
from label block 2. The assigned label value = LB+CE ID-LO, namely 1062 (1055+12-5).
• PE 1 calculates the VC label that PE 2 assigns to the VC:
PE 1 compares the ID (1) of the local CE (CE 1) with the label blocks assigned by PE 2. A label
block that satisfies LO<=CE ID<LO+LR is used by PE 2 to assign a label value to the VC. In this
example, label block (3000/0/15) satisfies LO<=CE ID<LO+LR (0<=1<0+15). PE 2 assigns a
label value to the VC from the label block. The assigned label value = LB+CE ID-LO, namely 3001
(3000+1-0).
• PE 2 performs the same calculations and gets the same results: the VC label that PE 2 assigns to the
VC is 3001 and that PE 1 assigns to the VC is 1062.
VPN 1
CE 1
CE 2
PE 1 PE 2
P
VPN 1 VPN 1
Label block 1: 1000/0/5
Label block 2: 1055/5/10
CE 12
Label block for CE 2: 2000/0/15
Label block for CE 12: 3000/0/15
VC connected CE 1 and CE 2:
local VC label: 1002
remote VC label: 2001
VC connected CE 1 and CE 12:
local VC label: 1062
remote VC label: 3001
VC connected CE 1 and CE 2:
local VC label: 2001
remote VC label: 1002
VC connected CE 1 and CE 12:
local VC label: 3001
remote VC label: 1062
Label block allocated Label block allocated
VC labels calculated