ECP-206 Exam Dumps and Practice Questions

To differentiate traffic types for the purpose of prioritization in their Ethernet LAN and IP/MPLS backbone, an operator can use the following three fields:

Traffic Class (TC): This is a 3-bit field in the MPLS label that can be used to indicate the priority or class of service (CoS) of the packet. The TC field can be mapped to the DSCP field in the IP header or the PCP field in the Ethernet header.The TC field can be used by MPLS routers to apply different per-hop behaviors (PHBs) such as queuing, scheduling, and dropping policies to different traffic classes12.

DSCP: This is a 6-bit field in the IP header that can be used to mark packets according to the differentiated services (DiffServ) model. The DSCP field can indicate the per-hop behavior (PHB) that the packet should receive at each router along its path. The DSCP field can be mapped to the TC field in the MPLS label or the PCP field in the Ethernet header.The DSCP field can be used by IP routers to provide QoS for different traffic types34.

PCP: This is a 3-bit field in the Ethernet header that can be used to mark packets according to the IEEE 802.1Q standard. The PCP field can indicate the priority or class of service (CoS) of the packet on an Ethernet LAN. The PCP field can be mapped to the TC field in the MPLS label or the DSCP field in the IP header.The PCP field can be used by Ethernet switches to provide QoS for different traffic types56.

P routers forward packets based on the LSP label. The LSP label is the label that corresponds to the label-switched path (LSP) that is established between two PE routers in an MPLS network. The LSP label is also called the outer label or the transport label, because it is used to transport packets across the MPLS core network. P routers are also called LSRs (label switch routers) or transit routers, because they switch packets based on their labels or remove the labels. P routers do not need to look at the IP header or any other information in the packet, except for the top label in the label stack. P routers perform one of three possible operations on labels: swap, pop, or PHP (penultimate hop popping). In a swap operation, the label is swapped with a new label, and the packet is forwarded along the path associated with the new label. In a pop operation, the label is removed from the packet, and the packet is forwarded based on its IP header or another label in the stack. In a PHP operation, the label is removed from the packet at the last P router before reaching the egress PE router, and the packet is forwarded without any label to the egress PE router.

The LSP label is different from other types of labels that may be used in MPLS networks, such as VPN labels or service labels. These labels are used to identify different VPNs or services that run over MPLS networks, such as Layer 2 VPNs, Layer 3 VPNs, traffic engineering, or QoS. These labels are also called inner labels or payload labels, because they are related to the payload of the packet. These labels are not used by P routers for forwarding decisions, but only by PE routers for delivering packets to their destinations.

Therefore, the answer is C.

A P router is a provider router that is part of the service provider's core network in an MPLS environment. A P router does not have any customer routes or VPN information, but only has information about how to reach other P routers and PE routers in the same MPLS domain. A P router performs label switching, which means that it forwards labeled packets based on their top label in the label stack. A P router can perform two possible label actions:

Swap: The P router replaces the incoming label with a new label that corresponds to the next hop along the label-switched path (LSP). The new label is determined by looking up the label forwarding information base (LFIB) based on the incoming label and interface.

PHP: The P router removes the top label from the packet at the penultimate hop before reaching the egress PE router. This is done to avoid an extra lookup on the egress PE router, which can forward the packet based on its IP header or another label in the stack.

A P router does not perform push or drop actions on labels. A push action means adding one or more labels to the packet, which is done by an ingress PE router when initiating an LSP. A drop action means discarding a packet, which is done by any router when there is no matching entry in its LFIB or routing table.Reference:Provider (P) Router in IP MPLS Network - Cisco Community,MPLS Fundamentals: Forwarding Labeled Packets - Cisco Press,MPLS Label Switching | MPLS Operation | Push, Swap,Push IPCisco

LDP relies on IGP for all routing-related decisions. LDP is a protocol that distributes labels in an MPLS environment, but it does not perform any routing functions. LDP uses the underlying routing information provided by an IGP, such as OSPF or IS-IS, to forward label packets. LDP and IGP do not exchange their databases at regular intervals, but rather use hello messages to maintain adjacencies and sessions.Reference:Ericsson IP Networking - Routing Protocols,Label Distribution Protocol - Wikipedia

The code that is commonly used to assign higher priority to VoIP traffic is EF, which stands for Expedited Forwarding. EF is a per-hop behavior (PHB) defined by the Differentiated Services (DiffServ) model for QoS. EF provides low delay, low jitter, and low loss for real-time applications such as VoIP.EF is marked by setting the DSCP value to 101110, which corresponds to decimal 4678. AF31, which stands for Assured Forwarding class 3 low drop probability, is another PHB defined by DiffServ, but it provides lower priority than EF.AF31 is marked by setting the DSCP value to 011010, which corresponds to decimal 2678. DF, which stands for Default Forwarding, is a PHB defined by DiffServ that provides best-effort service for unmarked traffic.DF is marked by setting the DSCP value to 000000, which corresponds to decimal 078. AF11, which stands for Assured Forwarding class 1 low drop probability, is another PHB defined by DiffServ, but it provides lower priority than EF and AF31.AF11 is marked by setting the DSCP value to 001010, which corresponds to decimal 1078.