Panasonic NN46240-710 - Manual (Page 152)

4 MPLS L2VPN troubleshooting
Nortel Secure Router 8000 Series
_________ Troubleshooting - VPN
Q: What is the function of the control word in Layer 2 VPN?
A: The control word on the PWE3 control plane is represented by a bit. The VC can be Up only
when the control word at both ends is the same.
If the forwarding plane supports the control word, a 32-bit field is added to the data packet to
show the packet order. Generally, disorder can occur in the case of load balancing.
When PEs connect through the Ethernet, and PE and CE connect in a PPP fashion, PPP
negotiation fails because the control packet of PPP is too small, less than the minimum MTU
supported by the Ethernet. In this case, the control word is used as padding to avoid this
problem.
Q: Can I specify multihop VCs with the same peer and different encapsulation
types when I configure switch-Layer 2 VC on SPE?
A: Yes, you can. A VC is determined by the combination of the VC ID and the VC type.
Q: Can I set the PW ID to 0 or ignore the configuration?
A: Yes, you can. You can set a PW ID for a static PW or choose not to set it. A static PW with a
PW ID of 0 cannot switch with other PWs.
Q: How can I configure Layer 2 VPN load balancing?
A: Load balancing uses multiple tunnels to carry a PW. Data payload is divided among these
tunnels and forwarded by them. The number of load balancing tunnels is determined by the
permit file.
When you configure load balancing, you need to specify a tunnel selection policy, that is,
specify a policy name when you configure VCs.
Q: What does the multihop mean in Layer 2 VPN? In what cases is multihop used?
How can I configure multihop?
A: Layer 2 VPN multihop is a type of technology used to implement interworking between CEs,
which connect multiple VCs.
When a VC must cross an AS, or no direct tunnel exists but multiple tunnel segments between
two UPEs exist, configure multihop. With multihop, the network operation has lower
requirement on the performance of the access device.
UPE cannot judge whether the remote is a UPE or SPE, so the configuration on UPE is the same
as that in single-hop case. Regard the SPE as the remote. Configuration on SPE involves three
scenarios:
• Dynamic to dynamic: Both sides are dynamic.
• Static to static: Both sides are static.
• Mixed: One side is dynamic and the other is static.
Q: Why can the same Layer 2 VC be configured with two types of encapsulation
when MPLS switch-Layer 2 VC is configured on SPE?
A: In the following example, the same Layer 2 VC is configured with two types of
encapsulation when MPLS switch-Layer 2 VC is configured on SPE:
4-42 Nortel Networks Inc. Issue 5.3 (30 March2009)