JN0-364 Service Provider Routing and Switching - Specialist (JNCIS-SP) Questions and Answers

In the BGP finite state machine, theIdlestate is the "stop" or "start" point of the protocol. When a session is stuck in Idle, it means the BGP process is either administratively disabled or, more commonly, is unable to initiate the underlying TCP connection required for BGP.

According to Juniper Networks Service Provider documentation, the most common reason for a BGP session to remain in Idle is a lack ofrouting reachability. For BGP to move to theConnectstate, the Junos kernel must have a route to the IP address specified in the neighbor statement. If thepeer IP address is unreachable (Option A)—meaning there is no route in inet.0 (via OSPF, IS-IS, or static)—the router cannot initiate the TCP three-way handshake on port 179. Consequently, the state machine will never progress.

Analysis of incorrect options:

Option B:BGP does not care if the local AS is higher or lower than the peer's; it only cares if they match the configuration. AS numbers are identifiers, not priorities.

Option C:A mismatchedRouter IDdoes not prevent a session from leaving the Idle state. It would typically cause the session to reach theOpenConfirmstate, and then fail with a "Notification" message due to a collision or identification error.

Option D:While a mismatchedgroup type(internal vs. external) will cause the session to fail, it usually fails during theOpenmessage exchange (OpenSent state) because the AS numbers provided will not match the expected peer type (IBGP vs. EBGP).

Only the lack of a path to the neighbor (reachability) keeps the session at the very beginning of the process: theIdlestate.

The transition to IPv6 requires routing protocols that are capable of carrying 128-bit address information. Juniper Networks Junos OS supports several "IPv6-ready" protocols for dynamic routing.

1. IS-IS (Option B):

As discussed in previous questions,IS-ISis inherently extensible due to its use ofTLVs (Type, Length, Value). To support IPv6, the protocol did not need a major rewrite; instead, new TLVs (such as TLV 236 for IPv6 reachability and TLV 232 for IPv6 interface addresses) were added. A single IS-IS process in Junos can simultaneously carry both IPv4 and IPv6 routing information, making it a highly efficient choice for "dual-stack" service provider backbones.

2. BGP (Option D):

BGP was updated to support multiple protocols throughMultiprotocol Extensions (MP-BGP), defined in RFC 4760. By usingAddress Family Identifiers (AFI)andSubsequent Address Family Identifiers (SAFI), a single BGP session can exchange NLRI (Network Layer Reachability Information) for IPv4 unicast, IPv6 unicast, and even VPNv4/VPNv6 routes. In Junos, this is configured under the family inet6 unicast hierarchy within the BGP protocols configuration.

Why other options are incorrect:

IGMP (Option A):This is a management protocol for IPv4 multicast (Internet Group Management Protocol). Its IPv6 equivalent isMLD (Multicast Listener Discovery).

OSPFv2 (Option C):OSPF version 2 is strictly for IPv4. To run OSPF in an IPv6 environment,OSPFv3must be used, as it was specifically redesigned to handle the IPv6 address space and link-local communication.

In a service provider environment,Q-in-Q tunneling(also known as 802.1ad or double-tagging) is the standard solution for transporting multiple customer VLANs over a shared provider backbone while maintaining total separation.

According to Juniper Networks documentation, Q-in-Q works by adding a second 802.1Q tag (theService Provider tagor S-tag) to the customer’s already tagged frames (theCustomer tagor C-tag). This creates a "tunnel" at Layer 2. This solution specifically addresses all the customer's requirements:

Transparent Layer 2 Connectivity:Because the provider simply encapsulates the customer's frames, the customer's internal BPDU traffic (like Spanning Tree) and VLAN tags are preserved and delivered transparently to the remote site.

Separation of Internal VLANs:The customer can run their own internal VLAN IDs (1-4094) without the provider needing to know or manage them.

Conflict Avoidance:Different customers on the same provider infrastructure are assigned unique S-tags. Even if two different customers both use "VLAN 10" internally, they remain isolated because their traffic is encapsulated in different provider S-tags.

Why other options are incorrect:

Layer 3 VPN (Option B):While MPLS L3VPNs are common, they provide Layer 3 (IP) connectivity, not the "transparent Layer 2" connectivity requested.

GRE Tunnels (Option C):GRE is a Layer 3 encapsulation and does not natively provide the transparent VLAN bridging required for a multi-site Layer 2 service.

NAT/Firewall (Option D):These are security and address-translation services for internet access and do not facilitate site-to-site Layer 2 bridging.

In Juniper Networks Junos OS,Generic Routing Encapsulation (GRE)andIP-in-IP (IP-IP)are common tunneling mechanisms used to transport packets across a network by encapsulating them within another protocol. Understanding the header structure and the limitations of these protocols is essential for proper MTU (Maximum Transmission Unit) management and security design.

Overhead (Option A):

Both GRE and IP-IP tunnels operate by adding an additional IP header to the original packet. An IP-IP tunnel (Protocol 4) adds a20-byteIPv4 header. A GRE tunnel (Protocol 47) adds the same20-bytedelivery IP header plus a minimum4-byteGRE header (totaling 24 bytes, which can increase if keys or sequencing are used). Because these headers are added to the payload, the total size of the packet increases. This "overhead" means that if the original packet was already at the MTU limit (e.g., 1500 bytes), the encapsulated packet will exceed it, potentially leading to fragmentation or the need to adjust theTCP MSS (Maximum Segment Size).

Encryption (Option D):

Crucially, according to Juniper Service Provider documentation, neither GRE nor IP-IP provides nativeencryptionor data confidentiality. They are encapsulation protocols, not security protocols. The payload remains in cleartext and is visible to any device along the path. If security and encryption are required for data traversing these tunnels, they must be combined withIPsec (IP Security). While GRE is often used as the "carrier" for IPsec (to allow multicast or dynamic routing protocols which IPsec alone does not support), the GRE protocol itself remains an unencrypted delivery mechanism. Therefore, statements A and D accurately describe the architectural behavior of these tunnel types.

In the provided exhibit, the output of the command show spanning-tree interface for switch1 reveals critical details about the Spanning Tree Protocol (STP) operational state.

The first correct statement is thatthe switch1 device is the root bridge(Option B). This is determined by comparing the "Port ID" column with the "Designated port ID" column, as well as checking the "Designated bridge ID". In the exhibit, for every interface listed (from ge-0/0/6.0 to ge-0/0/13.0), the Port ID and the Designated port ID are identical. Furthermore, every port is in the "FWD" (Forwarding) state with the "DESG" (Designated) role. In a Spanning Tree topology, the root bridge is the only device where all active participating interfaces serve as designated ports, as it has no need for a "Root" port role (which points toward a root bridge).

The second correct statement is thatthe bridge priority for switch1 is 32k(Option D). Looking at the "Designated bridge ID" column, we see the value 32768.0019e2552481. In Junos and general networking standards, the Bridge ID is composed of a bridge priority and the device's MAC address. The default priority for most Spanning Tree variants (STP, RSTP, MSTP) is 32,768, which is commonly referred to in shorthand as "32k".

Regarding the incorrect options:

Option A:There is no evidence of VSTP (VLAN Spanning Tree Protocol); the output shows "instance 0," which is typical for IEEE standard RSTP or STP.

Option C:The Port IDs for ge-0/0/8, ge-0/0/9, and ge-0/0/11 all start with "32" (e.g., 32:521), whereas the default port priority is typically 128 (as seen in ge-0/0/6.0 with 128:519). This indicates that the interface priorities for these specific ports have been manually tuned to a non-default value.

In the context ofJuniper Networks Junos OSand theIS-IS (Intermediate System to Intermediate System)protocol, understanding the hierarchical relationship between router levels is critical for effective troubleshooting and design. IS-IS uses a two-level hierarchy to manage scalability:Level 1 (L1), which represents intra-area routing, andLevel 2 (L2), which represents inter-area backbone routing.

When a router is configured as aLevel 1/2 (L1/L2)device, it acts as a bridge between the two levels. According to Juniper technical documentation, an L1/L2 router maintains two completelyseparate Link-State Databases (LSDB)—one for Level 1 and one for Level 2. It does not merge these into a single topology. This separation ensures that local area topology changes (L1) do not necessarily flood into the backbone (L2) unless specific redistribution is configured, and vice versa. Therefore, statementCis correct because the L1/L2 router maintains distinct SPF (Shortest Path First) computations for each level.

Regarding the visibility of the Level 1 router, IS-IS is designed to keep L1 areas "stubby" by default. ALevel 1 routeronly possesses the topology information for its own area (the Level 1 LSDB). It does not receive specific L2 routes or the L2 topology. Instead, the L1/L2 router sets theAttached (ATT) bitin its L1 Link-State PDUs (LSPs) to signal to L1-only routers that it has a connection to the backbone. The L1 router then generates a default route pointing to the L1/L2 router to reach inter-area destinations. This confirms that statementDis correct: the L1 router's knowledge is limited to its local L1 topology.

Conversely, statements A and B are incorrect because merging topologies would violate the hierarchical scaling principles of IS-IS, and L1 routers never learn the full L2 topology without explicit, non-standard route leaking.

For OSPF routers to transition from the "Init" state to a full adjacency, they must agree on several parameters exchanged within theirHello packets. If these parameters do not match, the routers will refuse to form a neighbor relationship, a common point of failure in service provider networks.

According to Juniper Networks documentation, the following fields are mandatory matches:

Hello Interval (Option B):The frequency at which Hello packets are sent. Default is 10 seconds on broadcast networks.

Dead Interval (Option D):The time a router waits without receiving a Hello before declaring a neighbor down. Default is 4 times the Hello interval.

Network Mask (Option C):On broadcast and NBMA (Non-Broadcast Multi-Access) segments, the subnet masks must match because OSPF uses the mask to determine the network boundaries for the link-state advertisements.

Area ID:Routers must belong to the same logical OSPF area.

Authentication:If configured, the type and password/key must be identical.

Why other options are incorrect:

Router Priority (Option A):This is used to influence the election of the Designated Router (DR). It doesnotneed to match; in fact, different priorities are often used to ensure a specific router becomes the DR.

Designated Router (Option E):The DR is theresultof an election that happens after the initial Hello exchange. It is not a field that must match beforehand to start the process.

By ensuring the Hello/Dead timers and the Subnet Mask are synchronized, OSPF guarantees a stable and predictable environment for the subsequent exchange of Link-State Advertisements (LSAs).

When troubleshooting OSPF in a service provider environment, distinguishing between "stuck" adjacencies and "flapping" adjacencies is the first step. A session that transitions frequently betweenFullandDownindicates that the relationship can be established successfully (meaning parameters match), but it cannot be maintained.

According to Juniper Networks documentation, the most common cause for a session to drop from Full to Down is the expiration of theDead Interval. If a router does not receive a Hello packet within the Dead Interval (usually 40 seconds), it tears down the adjacency. Aphysical interface flapping (Option A)is the primary trigger for this. If the physical link or the underlying transport (like a leased line or a microwave link) goes down even momentarily, the OSPF process immediately detects the interface failure, flushes the neighbors, and moves the state to Down. As soon as the interface comes back up, the routers perform the Hello exchange and reach the Full state again, creating the flapping cycle.

Analysis of other options:

MTU Mismatch (Option D):This typically causes the adjacency to get "stuck" in theExchangeorExStartstate. The routers can exchange small Hello packets, but when they try to send larger Database Description (DBD) packets that exceed the MTU, the packets are dropped, preventing the session from ever reaching "Full."

Area ID Mismatch (Option C):This prevents the adjacency from even reaching theInitstate; the routers will never form a neighbor relationship.

Route Preference (Option B):This affects which route is chosen for the forwarding table but has no impact on the OSPF neighbor state machine itself.

In the Juniper Networks Junos operating system,aggregate routesare used to represent a group of more specific routes with a single, shorter prefix. This technique is essential for reducing the size of routing tables and minimizing the volume of routing updates sent to neighbors. According to Juniper technical documentation, for a destination IP address to "match" a specific route, it must fall within the range defined by the network address and its associated CIDR mask.

The provided exhibit shows a detailed lookup for the aggregate route$10.16.2.0/23$. To determine the range of IP addresses covered by a $/23$ mask, we examine the binary representation of the third octet. A $/23$ mask means the first 23 bits are fixed. For the address $10.16.2.0$:

The first two octets ($10.16$) are fixed.

The third octet ($2$) is $00000010$ in binary.

The 23rd bit is the second-to-last bit of this octet.

The $/23$ range allows the 24th bit (the last bit of the third octet) and all 8 bits of the fourth octet to vary.

This results in a range where the third octet can be either $2$ ($00000010$) or $3$ ($00000011$). Therefore, the aggregate route $10.16.2.0/23$ covers all IP addresses from$10.16.2.0$ to $10.16.3.255$. The exhibit further confirms this by listing the "Contributing Routes": $10.16.2.0/24$ and $10.16.3.0/24$.

Analyzing the provided options against this range:

10.16.3.79 (Option A):This address falls squarely within the $10.16.2.0$ to $10.16.3.255$ range.

10.16.0.4 (Option B):This address falls in the $10.16.0.0/23$ range ($0.0$ to $1.255$).

10.16.4.183 (Option C):This address falls in the $10.16.4.0/23$ range ($4.0$ to $5.255$).

10.16.1.214 (Option D):This address also falls in the $10.16.0.0/23$ range.

Consequently,10.16.3.79is the only destination listed that matches the aggregate route shown. It is also important to note theNext hop type: Rejectin the exhibit; this means that if a packet matches the aggregate but does not match any of the more specific contributing routes, the router will drop the packet and send an ICMP unreachable message to the source.

IS-IS is a link-state protocol that relies on the rapid and consistent flooding ofLink-State PDUs (LSPs)to ensure that every router in an area has an identical view of the topology. To manage the "freshness" of information, IS-IS uses aSequence Number—a 32-bit unsigned integer that increments every time the originating router makes a change to its LSP.

According to Juniper Networks technical documentation, when a router receives an LSP, it performs a comparison between the received LSP and the version it currently holds in itsLink-State Database (LSDB). If the received LSP has ahigher sequence number, the router concludes that this is "newer" and more accurate information. The router will then perform two immediate actions:

Update:It replaces the older LSP in its LSDB with the newly received version.

Flood:It propagates the new LSP to all other neighbors (except the one that sent it) to ensure the entire area converges on the new data.

If the sequence numbers were equal, the router would ignore the incoming PDU as it already has the information. If the received sequence number werelower, the router would conclude its own database is more recent and would actually send its own "newer" version back to the neighbor to bring them up to date (a process called "poisoning" or refreshing the neighbor).Complete Sequence Number PDUs (CSNPs)(Option C) are used during initial database synchronization or periodic checks on broadcast links, but the primary response to a "newer" LSP is immediate database update and flooding.

In Juniper Networks Junos OS, a "hidden" route in the BGP table typically signifies that the router has received the prefix but cannot install it into the active routing table because theBGP next hop is unreachable. This is a common occurrence in service provider environments when transitioning betweenExternal BGP (EBGP)andInternal BGP (IBGP).

According to Juniper technical documentation, when an EBGP speaker (R1) advertises a prefix to its peer (R2), it sets the next hop to its own interface IP address ($172.16.10.1$). By default, when R2 re-advertises that prefix to its IBGP peer (R3), itpreserves the original EBGP next-hop address. Unless R3 has a specific route in its Interior Gateway Protocol (IGP) or a static route to reach the $172.16.10.1$ subnet, it will mark the route as unusable (hidden).

In the exhibit, the show route output onR3explicitly shows the nexthop for $203.0.113.0/24$ as $172.16.10.1$. Since this route is marked "hidden," we can conclude R3 does not know how to reach R2's external peering link. To resolve this, the network administrator must modify the next-hop attribute before the route is sent to R3.

By adding the statementset policy-options policy-statement export-to-ibgp then next-hop self(Option B) on routerR2, R2 will replace the external next-hop ($172.16.10.1$) with its own internal peering address ($172.16.20.1$) before advertising the route to R3. Because R3 already has a direct or IGP connection to R2's internal address, it will successfully resolve the next hop, and the route will transition from "hidden" to "active."

Option A is unnecessary because the route is already being exported; Option C is redundant as the policy is already applied to the IBGP group; and Option D changes path preference but does not solve the underlying reachability problem.

On multi-access network segments, such asEthernet, OSPF could potentially face a scalability issue. If every router on a segment formed a full adjacency with every other router, the number of adjacencies would follow the formula $n(n-1)/2$. In a segment with 10 routers, this would result in 45 adjacencies, each generating redundant flooding of Link-State Advertisements (LSAs) and excessive Hello traffic.

To solve this, OSPF elects aDesignated Router (DR)and aBackup Designated Router (BDR). According to Juniper Networks documentation, the primary purpose of the DR is to act as a central point of contact for the segment, therebyreducing OSPF traffic (Option C).

Instead of every router syncing with every other router, they all form aFull adjacencyonly with the DR and BDR. When a router (a DR-Other) has an update, it sends it to the multicast address224.0.0.6(All DR Routers). The DR then acknowledges the update and floods it to all other routers on the segment using the multicast address224.0.0.5(All OSPF Routers). This "hub-and-spoke" signaling model within the local segment significantly minimizes the bandwidth consumed by protocol overhead and reduces the CPU load on the participating routers.

It is important to note that the DR's scope is limited to the local segment; it does not "assign IDs" (Option A) nor does it flood routes to the "entire domain" (Option D), as that is the responsibility of individual routers within their respective areas.

In the Junos OS, routing policy behavior is governed bydefault import and export rulesthat vary significantly between different protocols. ForIS-IS (Intermediate System to Intermediate System), the default export policy is "reject all." This means that, by default, an IS-IS process willexport nothingfrom the routing table into the IS-IS database.

According to Juniper Networks technical documentation, IS-IS automatically advertises its own direct interfaces that are configured under the [edit protocols isis] hierarchy. However, it does not automatically redistribute routes learned from other sources—such asStatic routes,OSPF, orBGP—into the IS-IS domain. This is a safety mechanism designed to prevent accidental routing loops or the flooding of unnecessary prefixes into the link-state database (LSDB), which could impact the stability of the SPF (Shortest Path First) algorithm.

To move routes from the routing table (inet.0) into IS-IS, an administrator must explicitly create arouting policyand apply it as anexport policywithin the IS-IS configuration. For example:

Code snippet

set policy-options policy-statement REDIST-STATIC term 1 from protocol static

set policy-options policy-statement REDIST-STATIC term 1 then accept

set protocols isis export REDIST-STATIC

Without such a policy, the IS-IS LSPs (Link-State PDUs) will only contain information about the IS-IS enabled interfaces and the reachability of other IS-IS neighbors. This behavior contrasts with protocols like BGP, which has different default rules for exporting active BGP routes to EBGP peers. In the context of IS-IS in a Juniper environment, "export nothing" is the standard operational baseline.

One of the most significant architectural improvements in IPv6 is the move from a complex, variable-length header (as seen in IPv4) to a streamlined, fixed-length base header of 40 bytes. Additional functionality that was previously handled by "Options" in IPv4 is now moved toExtension Headers, which are inserted between the IPv6 base header and the upper-layer protocol (TCP/UDP).

According to Juniper Networks technical documentation and RFC 8200, the following are valid IPv6 Extension Headers:

Hop-by-Hop Options (Option B):This header carries optional information that must be examined by every node along the delivery path. It is used for features like the Router Alert and Jumbo Payload options.

Fragment (Option E):Unlike IPv4, where any router can fragment a packet, in IPv6, fragmentation is performed only by the source node. The Fragment header contains the information necessary for the destination to reassemble the packet (Offset, Identification, and More Fragments flag).

Destination Options (Option A):This header carries information intended only for the destination node. It can appear twice: once before a routing header and once after.

Why other options are incorrect:

Protocol (Option C):In IPv4, this was a field in the header. In IPv6, this is replaced by theNext Headerfield, which identifies the type of the following header (whether it's an extension header or the upper-layer protocol).

Header Checksum (Option D):This field was entirely removed in IPv6. IPv6 relies on the data link layer (Ethernet) and the transport layer (TCP/UDP) to perform error detection, significantly reducing the processing overhead for routers in the core of a service provider network.

Multiple Spanning Tree Protocol (MSTP), as defined in IEEE 802.1s and implemented in Juniper Networks Junos OS, allows for the grouping of VLANs into specific spanning tree instances. This provides significant scalability and load-balancing advantages over traditional STP or RSTP. To achieve this, switches must be grouped into logical "Regions."

According to Juniper documentation, for two or more switches to be considered part of the sameMSTP Region, they must possess an identicalMSTP Configuration Identifier. This identifier consists of three specific attributes that must match exactly across all participating switches:

MSTI Name (Region Name):A descriptive string (up to 32 characters) that identifies the region.

MSTI Revision Level:A numerical value (0–65535) used to track configuration changes.

VLAN-to-Instance Mapping:The specific table that defines which VLAN IDs are associated with which Multiple Spanning Tree Instances (MSTIs).

If even one of these parameters—such as theRegion name(Option A)—differs, the switches will treat each other as being in separate regions. When switches are in different regions, they interact using theCommon Spanning Tree (CST), effectively seeing the other region as a single "virtual bridge," which limits the granularity of traffic engineering.

TheExtended system ID(Option B) is a component of the Bridge ID used to carry VLAN information in PVST+ but is not a region-matching requirement.Root bridge priority(Option C) andRoot bridge ID(Option D) are variables used during the STP election process to determine the topology's root, but they do not define the boundaries of an MSTP region itself.

In theIS-IS (Intermediate System to Intermediate System)protocol, communication between routers is performed usingProtocol Data Units (PDUs). To discover neighbors and maintain adjacencies, IS-IS relies on theHello PDU (IIH - IS-IS Hello).

According to Juniper Networks technical documentation, when IS-IS is enabled on an interface, the router begins transmitting Hello PDUs to a multi-destination address (multicast). These PDUs contain essential information such as the router'sSystem ID, its configuredArea Addresses, and itsLevel capability(Level 1, Level 2, or both). For two routers to become neighbors, they must exchange these Hello PDUs and agree on specific parameters, such as the MTU of the link and the hello/hold timers.

Once an adjacency is established, the Hello PDU serves as a "keepalive" mechanism. If a router stops receiving Hello PDUs from a neighbor for a duration exceeding theHolding Time, it assumes the neighbor is down and flushes the associated Link-State PDUs (LSPs) from its database.

To clarify the other options:

Link-State PDU (Option A):These are used to distribute actual topology and reachability information, not to form adjacencies.

Partial Sequence Number PDU (Option C):PSNPs are used on point-to-point links to acknowledge the receipt of LSPs or to request missing LSPs.

Update PDU (Option D):This is not a standard IS-IS term; in IS-IS, updates are handled via the flooding of LSPs.