PE1 and PE2 provide a BGP-signaled VPLS service named VPN Blue. PE1 has site ID 2, label base 2500, andlabel offset 1. PE2 has site ID 4, label base 3000, and label offset 1.
Which label does PE2 expect to receive on VPLS VPN Blue traffic received from PE1 site ID 2?
A.
2505
B.
5499
C.
2503
D.
3001
Explanation:
The formula used is VPN label = label-base-remote + local-site-id – label-offset-remote, which here is 2500 + 4
-1 = 2503.
https://inetzero.com/verifying-a-bgp-signaled-vpls/
I think the answer is D
TX=remote base label+local site ID-remote offest
RX= local base labe+remote site ID-local offset
So, PE2 should be expecting to receive 3001 (3000+2-1)
and it should be transmitting 2503 (2500+4-1)
2
0
remote label (or transmit label) = remote base + local site id – remote offset
local label (or receive label) = local base + remote site id – local offset
0
0
correct is D
1
0
The correct answer is D.
Explanation:
To set up a pseudowire to PE2, PE1 must select a label to use to send traffic to PE2 and also select a label that it expects PE2 to use to send traffic to itself. In this case, the site ID contained in the VPLS NLRI from PE2 is 4, and that if PE1 is 2.
PE1 learns of the existence of site ID 4 in the same VPLS domain. Using the equation VBO <= Local Site ID < (VBO +VBS), PE1 checks if the route advertised by site ID 4 fits in the label block and block offset that it previously advertised to PE2. In this example it does fit, so the site ID 4 is mapped by the VPLS NLRI advertised by PE1, and PE1 is ready to set up a pseudowire to PE2.
To select the label to reach PE2, PE1 looks at the label block advertised by PE2 and performs a calculation. The calculation a PE router uses to check if its site ID is mapped in the label block from the remote peer is VBO <= Local Site ID < (VBO +VBS). So, PE1 selects label (3000 + (2 -1)) = 3001 to send traffic to PE2. Using the same equation, PE1 looks at its own label block that it advertised and selects label (2500 + (4 – 1)) = 2503 to receive traffic from PE2. PE1 programs its forwarding state such that any traffic destined to PE2 carries the pseudowire label 3000 and any traffic coming from PE2 is expected to have the pseudowire label 2501. This completes the operations on the VPLS NLRI received from PE2. PE1 now has a pseudowire set up to PE2.
PE2 operation is very similar to the PE1 operation. Since the PE2 site ID of 2 fits in the label block and block offset advertised by PE1, PE2 selects label (2500 + (4 – 1)) = 2503 to send traffic to PE1. PE2 looks at its own label block that it advertised and selects label (3000 + (2 – 1)) = 3001 to receive traffic from PE1. This completes the creation of a pseudowire to PE1.
0
0