149
•
Export target attribute
—When a PE sends L2VPN information (such as CE ID and RD) to the
peer PE through a BGP update message, it sets the route target attribute carried in the update
message to export target.
•
Import target attribute
—When a PE receives an update message from the peer PE, it checks
the route target attribute in the update message. If the route target value matches an import
target on the PE, the PE accepts the L2VPN information in the update message.
In a word, route target attributes define which PEs can receive L2VPN information, and from which
PEs that a PE can receive L2VPN information.
Different from Martini mode, the Kompella mode does not distribute the VC label assigned by the
local PE directly to the peer PE through the signaling protocol. Instead, it uses label blocks to assign
labels to multiple connections at a time. A PE advertises label blocks to all PEs in the same VPN.
Each PE calculates the VC labels according to the label blocks from other PEs.
A label block includes the following parameters:
•
Label
Base
—Initial label value of the label block. A PE automatically selects the LB value that
cannot be manually modified.
•
Label
Range
—Number of labels that the label block contains. LB and LR determine the labels
contained in the label block. For example, if the LB is 1000 and LR is 5, the label block contains
labels 1000 through 1004.
•
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
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