MPLS supports nesting of multiple labels. After receiving an MPLS packet, a device first processes the label next to the Layer 2 header, that is, the top MPLS label.
Answer(s): A
In MPLS (Multiprotocol Label Switching), packets can have multiple labels stacked (label stacking), and this is commonly referred to as MPLS label nesting. The device receiving an MPLS packet processes the top label first, which is the label closest to the Layer 2 header. After processing the top label, the device may pop the label (remove it), swap it for another label, or push additional labels depending on the MPLS forwarding requirements.
The figure shows the LSP information of R1 and R3 on an MPLS network. An administrator runs the ping -a 1.1.1.1 3.3.3.3 command on R1. Given this, which of the following statements is true?
Answer(s): B
R1 to R2 to R3: The LSP from R1 to R3 for FEC 3.3.3.3/32 shows that R1 uses label 1025 on the outgoing interface GE0/0/1 toward R2. On R3, the incoming label from R2 for the same FEC is 1025, and R3 pops the label (with a NULL label for outgoing).Return Path (R3 to R1): The LSP from R3 to R1 for FEC 1.1.1.1/32 shows that R3 uses label 1024 on the outgoing interface GE0/0/0 toward R2. R2 swaps the label 1024 for label 3, and R1 sees the label 3 for the incoming packet.The Echo Request packet from R1 to R3 will carry label 1025 as it reaches R2. The Echo Reply packet from R3 to R1 will carry label 1024 when it reaches R2 before label swapping occurs.
An administrator runs the display mpls lsp command to view LSPs on a device. The command output is shown in the figure. Which of the following statements are true? (Choose all that apply.)
Answer(s): B,C
For the traffic destined for 3.3.3.3, the device is the penultimate transit device. The "Out Label" value of 3 for traffic destined to 3.3.3.3 indicates the use of the implicit NULL label. This suggests that the device is the penultimate hop, and the final hop will pop the label.For the traffic destined for 1.1.1.1, the device is the last hop, that is, the egress device. The table shows an "In Label" of NULL for traffic destined to 1.1.1.1, indicating that the device is the egress for this traffic (as no label is needed for forwarding).
Service routes of an enterprise are transmitted on the network. Some configurations of PE1 and PE2 are shown in the figure. Which of the following LSAs may CE2 receive? (Choose all that apply.)
Answer(s): A,B
Type 1 LSA (Router LSA): Type 1 LSAs are used to advertise the router's directly connected interfaces within the same OSPF area. Since CE2 is part of an OSPF area, it will receive Type 1 LSAs from routers within its area.Type 3 LSA (Summary LSA): Type 3 LSAs are used to advertise inter-area routes. PE1 and PE2 will generate Type 3 LSAs when importing VPN routes into OSPF. CE2 can receive these LSAs for routes learned from other OSPF areas.
In CE multi-homing scenarios, if BGP AS number substitution is enabled on PEs, you can run the peer soo command on the EBGP peers of the PEs and CE to configure the Site of Origin (SoO) attribute to prevent loops.
In CE multi-homing scenarios, where a customer edge (CE) router is connected to multiple provider edge (PE) routers, BGP AS number substitution (also known as AS override) is often used to prevent AS path loops. However, this does not fully prevent routing loops when the same prefixes are learned via different PEs. To further prevent loops, the Site of Origin (SoO) attribute can be configured on PE-CE links using the peer soo command. The SoO attribute helps identify routes that originated from the same site and prevents the re-advertisement of these routes back into the same site, thereby avoiding routing loops.
For high reliability purposes, the following LDP messages are transmitted through TCP: discovery messages, session messages, advertisement messages, and notification messages.
LDP (Label Distribution Protocol) uses both UDP and TCP for different types of messages: Discovery messages are sent over UDP to establish initial communication between LDP neighbors. Session, advertisement, and notification messages are transmitted over TCP to ensure reliable delivery.Therefore, discovery messages are not transmitted via TCP but via UDP, while the other messages (session, advertisement, notification) are transmitted over TCP.
The path that IP packets pass through on an MPLS network is called a label switched path (LSP). An LSP is a bidirectional path that specifies the transmission direction of data flows.
An LSP (Label Switched Path) in MPLS is typically unidirectional, meaning it only defines the path for traffic flowing in one direction. For traffic flowing in the opposite direction, a separate LSP must be established. LSPs are used to forward packets based on their labels, and each LSP specifies the forwarding path for packets in a specific direction.
If end-to-end QoS is configured on an MPLS network, the penultimate hop device on the MPLS network can pop MPLS labels from packets based on penultimate hop popping (PHP), so that the egress node can process packets based on the QoS priority in inner packets.
In an MPLS network, Penultimate Hop Popping (PHP) allows the penultimate hop (the router just before the egress node) to remove (pop) the outermost MPLS label before forwarding the packet to the egress node. This reduces the processing overhead on the egress router by eliminating the need for it to process the MPLS label. When end-to-end QoS is configured, PHP allows the egress node to process the packet based on the QoS priority found in the inner packet headers (such as the IP packet’s DSCP value). By popping the MPLS label at the penultimate hop, the egress node can focus on handling QoS policies without needing to deal with MPLS labels.
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