Free Juniper Service Provider Routing and Switching, Professional Exam JN0-664 Exam Questions

Understanding the Problem in MPLS CoS Classification

How EXP Bits Are Used for CoS in MPLS

Traffic is sent from VPN-A Site-1 CE-1 PE-1 P-1 PE-2 CE-2.

The MPLS LSP (Label Switched Path) from PE-1 to PE-2 is expected to carry MPLS EXP bits, which are used for Class of Service (CoS) classification.

PE-2 should classify traffic based on EXP bits received in the MPLS label.

What Happens with PHP (Penultimate Hop Popping)?

By default, the penultimate router (P-1) pops the top MPLS label before sending the packet to PE-2.

Since the EXP bits are in the top MPLS label, they get removed along with the label.

This means that PE-2 no longer sees the correct EXP bits, leading to incorrect traffic classification.

Solution: Configure Explicit-Null on PE-2

Explicit Null (explicit-null) must be configured on PE-2 to ensure that P-1 does NOT remove the MPLS label.

Instead of removing the label, P-1 will send a label of 0 (for IPv4) or 2 (for IPv6) to PE-2.

This preserves the MPLS EXP bits, allowing PE-2 to classify the traffic correctly.

Evaluating the Answer Choices Again

B. Configure the explicit-null statement on PE-2.

Correct, because:

PE-2 is the egress LSR, where Ultimate Hop Popping (UHP) must be enabled.

Configuring explicit-null ensures that P-1 does not remove the label, preserving the EXP bits for CoS classification at PE-2.

Configuration on PE-2:

set protocols mpls explicit-null

Juniper Documentation Reference:

'Explicit-null must be configured on the egress LSR to prevent PHP from removing the top MPLS label, thereby preserving the EXP bits.'

A. Configure the explicit-null statement on PE-1.

Incorrect, because:

Explicit-null must be configured on the egress LSR (PE-2), not the ingress LSR (PE-1).

PE-1 only labels the traffic but does not control PHP behavior on P-1.

C. Configure VPN prefix mapping for the PE-1_to_PE-2 LSP.

Incorrect, because:

VPN prefix mapping is used for mapping VPN routes to LSPs but does not solve the EXP bit issue.

The problem here is label removal (PHP), not route mapping.

D. Set a static CoS value for the PE-1_to-PE-2 LSP.

Incorrect, because:

This does not preserve the original EXP bits, it only applies a static CoS value.

It's a workaround, not a fix.

Final Answer: B. Configure the explicit-null statement on PE-2.

Key Takeaways

Penultimate Hop Popping (PHP) removes the outer MPLS label at P-1, which also removes the EXP bits used for CoS classification.

To keep EXP bits intact, configure explicit-null on the egress PE (PE-2).

This forces P-1 to send a label (0 for IPv4, 2 for IPv6) to PE-2, preserving the EXP bits for CoS classification.

Official Juniper Documentation Reference

Juniper MPLS CoS and PHP Behavior Guide

'To retain CoS EXP bits at the egress LSR, configure explicit-null on the egress PE. This prevents PHP from stripping the MPLS label before reaching the final PE router.'

BGP add-path is a feature that allows the advertisement of multiple paths through the same peering session for the same prefix without the new paths implicitly replacing any previous paths. This behavior promotes path diversity and reduces multi-exit discriminator (MED) oscillations. BGP add-path is implemented by adding a path identifier to each path in the NLRI. The path identifier can be considered as something similar to a route distinguisher in VPNs, except that a path ID can apply to any address family.Path IDs are unique to a peering session and are generated for each network3. In this question, we have a route reflector (RR) that receives two routes for the same prefix (192.168.0.0/16) from an EBGP neighbor. By default, the RR will only advertise its best path to its clients (R1 and R2). However, we want R1 and R2 to receive both routes from the RR. To achieve this, we need to configure BGP add-path on the RR and enable it to send multiple paths for the same prefix to its clients.

PIM sparse mode (PIM-SM) is a multicast routing protocol that uses a pull model to deliver multicast traffic. In PIM-SM, routers with receivers send join messages to their upstream neighbors toward a rendezvous point (RP) or a source-specific tree (SPT). The RP or SPT acts as the root of a shared distribution tree for a multicast group. Traffic is only forwarded to routers that request to join the distribution tree by sending join messages. PIM-SM does not flood traffic to all routers or prune routers without receivers, as PIM dense mode does.

BGP route target filtering can be configured on PE devices or on route reflectors (RRs). Configuring BGP route target filtering on RRs is more efficient and scalable, as it reduces the number of BGP sessions and updates between PE devices. To configure BGP route target filtering on RRs, the following steps are required:

Configure the family route-target statement under the BGP group or neighbor configuration on the RRs. This enables the exchange of the route-target address family between the RRs and their clients (PE devices). Configure the resolution rib bgp.l3vpn.0 resolution-ribs inet.0 statement under the routing-options configuration on the RRs. This enables the RRs to resolve next hops for VPN routes using the inet.0 routing table.

class-of-service (CoS) is a feature that allows you to prioritize and manage network traffic based on various criteria, such as application type, user group, or packet loss priority. CoS uses different components to classify, mark, queue, schedule, shape, and drop traffic according to the configured policies.

One of the components of CoS is drop profiles, which define how packets are dropped when a queue is congested. Drop profiles use random early detection (RED) algorithm to drop packets randomly before the queue is full, which helps to avoid global synchronization and improve network performance. Drop profiles can be discrete or interpolated. A discrete drop profile maps a specific fill level of a queue to a specific drop probability. An interpolated drop profile maps a range of fill levels of a queue to a range of drop probabilities and interpolates the values in between.

In the exhibit, we can see that the class-of-service configuration shows an interpolated drop profile with two fill levels (50 and 75) and two drop probabilities (20 and 60). Based on this configuration, we can infer the following statements:

The drop probability jumps immediately from 20% to 60% when the queue level reaches 75% full. This is not correct because the drop profile is interpolated, not discrete. This means that the drop probability gradually increases from 20% to 60% as the queue level increases from 50% full to 75% full. The drop probability for any fill level between 50% and 75% can be calculated by using linear interpolation formula.

The drop probability gradually increases from 20% to 60% as the queue level increases from 50% full to 75% full. This is correct because the drop profile is interpolated and uses linear interpolation formula to calculate the drop probability for any fill level between 50% and 75%. For example, if the fill level is 60%, the drop probability is 28%, which is calculated by using the formula: (60 - 50) / (75 - 50) * (60 - 20) + 20 = 28.

To use this drop profile, you reference it in a scheduler. This is correct because a scheduler is a component of CoS that determines how packets are dequeued from different queues and transmitted on an interface. A scheduler can reference a drop profile by using the random-detect statement under the [edit class-of-service schedulers] hierarchy level. For example: scheduler test { transmit-rate percent 10; buffer-size percent 10; random-detect test-profile; }

To use this drop profile, you apply it directly to an interface. This is not correct because a drop profile cannot be applied directly to an interface. A drop profile can only be referenced by a scheduler, which can be applied to an interface by using the scheduler-map statement under the [edit class-of-service interfaces] hierarchy level. For example: interfaces ge-0/0/0 { unit 0 { scheduler-map test-map; } }

The output is from the show l2vpn connections command on a Juniper router. This command is used to verify the status of Layer 2 VPN (L2VPN) pseudowires between Provider Edge (PE) routers.

Breakdown of Key Information:

Instance: vpn-A

This is the L2VPN instance being monitored.

Connection Status (St)

The connection status is 'Up', meaning the pseudowire is operational.

Local Site: CE1-2 (2)

The PE router is attached to a single local site (CE1-2).

Uptime & Connection Flaps

The output shows the last time the connection was up:

Time last up: Apr 11 14:35:27 2020

The '# Up trans' value is 1, meaning this connection has been established once and has not flapped since it was initiated.

VLAN ID Mismatch Check

The legend includes 'VM -- VLAN ID mismatch', but this status is not present in the connection output.

This means there is NO VLAN ID mismatch.

Flow Labels

The Flow Label Transmit is No, and the Flow Label Receive is No.

This means the PE router does NOT have the capability to pop flow labels.

The resolve-vpn parameter is a BGP option that allows a router to resolve labeled VPN-IPv4 routes using unlabeled IPv4 routes received from another BGP peer with family inet labeled-unicast configured. This option enables interprovider VPNs without requiring MPLS labels between ASBRs or using VRF tables on ASBRs. In this scenario, you need to configure the resolve-vpn parameter under [edit protocols bgp group external family inet labeled-unicast] hierarchy level on both ASBRs.

OSPF is an interior gateway protocol that uses link-state routing to exchange routing information among routers within a single autonomous system. OSPF prevents routing loops in multi-area networks by using two methods: area hierarchy and SPF algorithm. Area hierarchy is the concept of dividing a large OSPF network into smaller areas that are connected to a backbone area (area 0). This reduces the amount of routing information that each router has to store and process, and also limits the scope of link-state updates within each area.All areas are required to connect to area 0 either directly or through virtual links2. SPF algorithm is the method that OSPF uses to calculate the shortest path to each destination in the network based on link-state information. The SPF algorithm runs on each router and builds a shortest-path tree that represents the topology of the network from the router's perspective.The SPF algorithm prunes looped paths within an area by choosing only one best path for each destination3.