When running MPLS in the frame mode over an Ethernet, how does the receiving device identify
that the frame contains MPLS information?
A.
MPLS frames are sent over a reserved session.
B.
MPLS is identified in the protocol port of the Layer 3 header.
C.
MPLS frames are sent in a special frame with a multicast address similar to CDP.
D.
The Ether Type of PID in the Layer 2 header identifies the frame as an MPLS frame.
Explanation:
1) According to the “Implementing Cisco MPLS” Student Guide (Text Part Number: 97-1153-01)
Volume 1, version 1.0, page 2-31. A label does not contain any information about the layer 3
protocol being carried in a packet. A new protocol identifier is used for MPLS-enabled Layer 3
protocol. The following list shows the ethertype values used to identify Layer 3 protocols with most
Layer 2 encapsulations.:
– Unlabeled IP unicast:PID=0x0800 identifies that the frame payload is an IP packet
– Labeled IP unicast: PID=0x8847 identifies that the frame payload is a unicast IP packet with a
least one label preceding the IP header. The Bottom-of-stack bit indicates when the IP header
actually starts.
– Labeled IP multicast: PID-0x8848 identifies that the frame payload is a multiicast IP packet with
at least one label preceding the IP header. The Bottom-of-stack bit indicates when the IP header
actually starts.
Summary: Protocol identifier in a layer 2 header specifies that the payload starts with a label
(labels) and is followed by an IP header.
2) According to the RFC 3032 – MPLS lable stack encoding <http://www.rfceditor.org/rfc/rfc3032.txt
Transporting Labeled Packets over LAN Media:
Exactly one labeled packet is carried in each frame. The label stack entries immediately precede
the network layer header, and follow any data link layer headers, including, e.g., any 802.1Q
headers that may exist. The ethertype value 8847 hex is used to indicate that a frame is carrying
an MPLS unicast packet. The ethertype value 8848 hex is used to indicate that a frame is carrying
an MPLS multicast packet. Not B: Option B cannot be correct as the
QUESTION is asking
frames on ethernet, this obviously means identification should be done at layer 2.