Virtual Private LAN Services
7750 SR OS Services Guide Page 491
Pseudowire Signaling Details
The pseudowire is setup using the following NLRI fields:
• VE Block offset (VBO): used to define for each VE-ID set the NLRI is targeted:
→ VBO = n*VBS+1 ; for VBS=8 this results in 1, 9, 17, 25, …
→ Targeted Remote VE-IDs are from VBO to (VBO + VBS - 1)
• VE Block size (VBS): defines how many contiguous pseudowire labels are reserved
starting with the Label Base
→ Alcatel-Lucent implementation uses always a value of eight (8).
• Label Base (LB): local allocated label base
→ The next eight (8) labels allocated for remote PEs
This BGP update is telling the other PE(s) that accept the RT: “in order to reach me (VE-ID = x)
use a pseudowire label of LB + VE-ID – VBO using the BGP NLRI for which VBO =< local VE-
ID < VBO + VBS.
Here is an example of how this algorithm works assuming PE1 has VE-ID 7 configured:
• PE1 founds a Label Block of eight (8) consecutive labels available, starting with LB =
1000
• PE1 then starts sending BGP Update with pseudowire information of (VBO = 1, VBS=8,
LB=1000) in the NLRI.
• This pseudowire information will be accepted by all participating PEs with VE-IDs from
one (1) to eight (8).
• Each of the receiving PEs will use the pseudowire label = LB +VE-ID-VBO to send
traffic back to the originator PE. For example VE-ID 2 will use pseudowire label 1001.
Assuming that VE-ID = 10 is configured in another PE4 the following procedure applies:
• PE4 sends BGP Update with the new VE-ID in the network that will be received by all the
other participating PEs, including PE1
• PE1 upon reception will generate another label block of 8 labels for the VBO = 9. For
example the initial PE will create now new pseudowire signaling information of (VBO =
9, VBS = 8, LB = 3000) and insert it in a new NLRI and BGP Update that is sent in the
network.
• This new NLRI will be used by the VE-ID from 9 to 16 to establish pseudowires back to
the originator PE1. For example PE4 with VE-ID 10 will use pseudowire label 3001 to
send VPLS traffic back to PE1
• The PEs owning the set of VE-IDs from 1 to 8 will ignore this NLRI.
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