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Question 136
In MPLS networks, what is the primary function of the Label Distribution Protocol (LDP) in establishing Label Switched Paths (LSPs)?
A) To create traffic-engineered paths with specific bandwidth requirements
B) To distribute labels for destination prefixes following the IGP shortest path
C) To establish point-to-multipoint LSPs for multicast traffic
D) To provide fast reroute protection for primary LSPs
Answer: B
Explanation:
Label Distribution Protocol is the primary protocol used in MPLS networks for distributing labels between label switching routers. LDP operates by following the IGP shortest path to destination prefixes and automatically distributing labels for these destinations. Unlike RSVP-TE, LDP does not support traffic engineering capabilities and simply follows the underlying IGP routing decisions. When a router learns about a destination prefix through an IGP like OSPF or IS-IS, LDP automatically creates a label binding for that prefix and advertises it to its LDP peers.
The protocol works in a downstream unsolicited mode by default, where routers advertise label bindings to their neighbors without being explicitly requested. LDP establishes TCP sessions between neighboring routers on port 646 and uses UDP for discovery messages. The label distribution follows the IGP topology, making it ideal for basic MPLS forwarding without complex traffic engineering requirements.
Traffic engineering with specific bandwidth requirements is handled by RSVP-TE, not LDP. RSVP-TE allows operators to explicitly define paths with bandwidth reservations and constraints. Point-to-multipoint LSPs for multicast traffic are typically established using protocols like mLDP or P2MP RSVP-TE extensions. Fast reroute protection is a feature provided by RSVP-TE through facility backup or one-to-one backup methods, not by basic LDP, although LDP does have an IGP synchronization feature to prevent traffic blackholing during convergence.
Question 137
Which BGP attribute is used to prevent routing loops in BGP confederations?
A) AS_PATH
B) ORIGINATOR_ID
C) CLUSTER_LIST
D) LOCAL_PREF
Answer: B
Explanation:
The ORIGINATOR_ID attribute is specifically designed to prevent routing loops within BGP confederations and route reflector environments. When a route reflector reflects a route, it adds the ORIGINATOR_ID attribute containing the router ID of the originating router. If a router receives a BGP update with an ORIGINATOR_ID that matches its own router ID, it discards the update to prevent a routing loop. This mechanism is essential in complex BGP topologies where routes might be reflected multiple times through different route reflectors.
BGP confederations divide a large autonomous system into smaller sub-autonomous systems to reduce the iBGP mesh requirement. Within each sub-AS, iBGP rules apply, but between sub-AS members, a modified eBGP peering is used. The ORIGINATOR_ID ensures that a route originated by a specific router does not get readvertised back to that same router through confederation or route reflection paths.
The AS_PATH attribute is used to prevent loops between different autonomous systems in regular eBGP, not specifically within confederations. The CLUSTER_LIST attribute works alongside ORIGINATOR_ID in route reflector scenarios by tracking which route reflector clusters a route has passed through, providing additional loop prevention for route reflector topologies. LOCAL_PREF is used for path selection within an autonomous system but does not provide loop prevention functionality. Together, ORIGINATOR_ID and CLUSTER_LIST provide comprehensive loop prevention in hierarchical BGP designs.
Question 138
In IS-IS protocol, what is the purpose of the Designated Intermediate System (DIS) on a broadcast network?
A) To act as the only router that can forward traffic on the segment
B) To create and flood pseudonode LSPs representing the broadcast network
C) To prevent routing loops by blocking redundant paths
D) To provide backup routing in case of primary path failure
Answer: B
Explanation:
The Designated Intermediate System in IS-IS serves the critical function of representing a broadcast network as a pseudonode in the link-state database. On broadcast networks like Ethernet, having every router advertise adjacencies to every other router would create excessive LSP overhead and complexity. Instead, the DIS creates a pseudonode LSP that represents the entire broadcast network, and all routers on that segment simply advertise their connection to the pseudonode. This dramatically reduces the number of adjacencies in the link-state database.
The DIS election process in IS-IS differs from OSPF’s DR/BDR election in several important ways. IS-IS elects the DIS based on the highest priority value, and if priorities are equal, the highest MAC address breaks the tie. Unlike OSPF, there is no backup DIS, and the election is preemptive, meaning a router with higher priority can immediately become DIS even if another DIS already exists. The DIS continues to form adjacencies with all routers on the segment and maintains full synchronization.
The DIS does not control traffic forwarding on the segment; all routers continue to forward traffic independently based on their routing tables. It is not responsible for preventing routing loops, as IS-IS uses sequence numbers and other mechanisms in the link-state protocol for loop prevention. The DIS role is purely a control-plane function for LSP generation and flooding optimization, not a data-plane or redundancy mechanism.
Question 139
Which RSVP-TE feature allows LSPs to automatically re-optimize their paths based on changing network conditions while maintaining existing traffic flow?
A) Fast Reroute
B) Make-before-break
C) Graceful Restart
D) Administrative groups
Answer: B
Explanation:
Make-before-break is a critical RSVP-TE feature that enables LSPs to re-optimize their paths without disrupting existing traffic. When network conditions change or better paths become available, make-before-break establishes a new LSP path before tearing down the existing one. This ensures zero packet loss during the transition. The process involves signaling a new LSP with the same tunnel parameters but a different LSP ID, waiting for the new path to be fully established, switching traffic to the new path, and then gracefully removing the old path.
This feature is particularly valuable for service provider networks where traffic interruption is unacceptable. Re-optimization can be triggered manually, on a timer basis, or when specific events occur such as a better path becoming available. The make-before-break mechanism uses the SESSION_ATTRIBUTE object in RSVP messages with a specific flag to indicate that shared explicit style reservation should be used, allowing both the old and new LSPs to share bandwidth during the transition period.
Fast Reroute provides protection against link or node failures through backup paths but does not provide path re-optimization capabilities. Graceful Restart allows RSVP state to be preserved across control plane restarts but is not related to path optimization. Administrative groups are constraint-based routing attributes used during initial path computation to include or exclude certain links but do not handle the dynamic re-optimization process. Make-before-break is specifically designed for hitless path optimization in operational networks.
Question 140
In a BGP network, what is the primary purpose of Route Target (RT) extended communities in Layer 3 VPNs?
A) To assign unique identifiers to VPN routes for loop prevention
B) To control the import and export of routes between VRFs
C) To determine the best path among multiple VPN routes
D) To encrypt VPN traffic between PE routers
Answer: B
Explanation:
Route Target extended communities are fundamental to the operation of BGP MPLS Layer 3 VPNs. They control which routes are imported into and exported from Virtual Routing and Forwarding instances on Provider Edge routers. When a PE router exports routes from a VRF, it attaches one or more Route Target export values as extended community attributes. Other PE routers examine these Route Target values and import routes into their VRFs only if the Route Target matches their configured import policy. This mechanism enables flexible VPN topology designs including hub-and-spoke, full mesh, and extranet configurations.
The Route Target is a BGP extended community attribute that follows the format of either a two-octet AS number followed by a four-octet number, or a four-octet IP address followed by a two-octet number. Multiple Route Targets can be associated with a single VRF, allowing complex routing policies. For example, a hub site might export routes with one RT and import routes with multiple RTs from various spoke sites. This flexibility makes Route Targets the key mechanism for controlling VPN membership and route distribution.
Route Distinguishers, not Route Targets, assign unique identifiers to VPN routes to make them globally unique in the BGP table. Path selection in BGP uses standard attributes like LOCAL_PREF, AS_PATH, and MED, not Route Targets. VPN traffic encryption is handled by IPsec or other security protocols, not by Route Target communities. Route Targets purely control the control-plane distribution of routing information between VRFs.
Question 141
What is the main advantage of using Segment Routing over traditional MPLS with RSVP-TE for traffic engineering?
A) Segment Routing provides better encryption for data in transit
B) Segment Routing eliminates the need for per-LSP state on transit routers
C) Segment Routing offers faster convergence during link failures
D) Segment Routing uses less bandwidth for control plane signaling
Answer: B
Explanation:
Segment Routing represents a significant architectural shift from traditional MPLS by eliminating the requirement for per-LSP state maintenance on transit routers. In traditional RSVP-TE networks, every LSP requires state information to be stored and maintained on each router along the path, creating scalability challenges as the number of LSPs grows. Segment Routing solves this by encoding the entire path as a stack of segment identifiers in the packet header at the ingress router. Transit routers simply forward based on the top segment identifier without maintaining any per-LSP state.
This stateless approach dramatically improves network scalability and simplifies operations. The source router determines the path and encodes it using a combination of prefix segments, adjacency segments, and node segments. These segments are represented by MPLS labels in SR-MPLS implementations or as IPv6 extension headers in SRv6 implementations. The elimination of signaling protocols like RSVP-TE reduces protocol complexity, speeds up network convergence, and makes troubleshooting simpler.
Segment Routing does not inherently provide encryption capabilities; encryption must be implemented through separate mechanisms like IPsec regardless of the forwarding paradigm. While Segment Routing can contribute to faster convergence through its integration with IGP and the ability to pre-compute backup paths using TI-LFA, this is not its primary advantage over RSVP-TE. Control plane bandwidth usage is reduced since there is no need for RSVP signaling messages, but the primary benefit remains the elimination of per-LSP state on transit nodes, which fundamentally changes network scalability.
Question 142
In OSPF, what is the purpose of the Link State Advertisement (LSA) Type 5?
A) To describe router links within a single area
B) To advertise external routes imported from other routing protocols
C) To summarize routes between OSPF areas
D) To represent multi-access network segments
Answer: B
Explanation:
LSA Type 5, also known as AS-External-LSA, is used specifically to advertise external routes that have been redistributed into OSPF from other routing protocols or sources. These LSAs are generated by Autonomous System Boundary Routers when they import routes from protocols like BGP, RIP, static routes, or other routing domains. Type 5 LSAs are flooded throughout the entire OSPF routing domain, except into stub areas, totally stubby areas, and not-so-stubby areas by default. This allows all OSPF routers to learn about destinations outside the OSPF domain.
Type 5 LSAs contain important information including the external network prefix, subnet mask, metric, metric type (E1 or E2), and the forwarding address. The metric type is particularly significant: E2 routes maintain only the external cost and do not add internal OSPF costs, while E1 routes include both the external cost and the internal cost to reach the ASBR. By default, redistributed routes appear as E2, making path selection simpler but potentially suboptimal in complex topologies.
LSA Type 1 describes router links and is flooded only within a single area, not for external routes. LSA Type 3 is used by Area Border Routers to advertise inter-area routes and summarize networks between areas, not for external routes. LSA Type 2 represents multi-access network segments and is generated by the Designated Router on broadcast networks. Understanding the specific purpose of each LSA type is crucial for OSPF design and troubleshooting in complex service provider networks.
Question 143
Which BGP feature allows service providers to influence inbound traffic from neighboring autonomous systems?
A) LOCAL_PREF attribute
B) MED attribute
C) WEIGHT attribute
D) ORIGIN attribute
Answer: B
Explanation:
The Multi-Exit Discriminator attribute is the primary BGP mechanism for influencing how neighboring autonomous systems send traffic into your network when multiple entry points exist. MED is advertised to external BGP peers to indicate the preferred path for inbound traffic. A lower MED value indicates a more preferred path. When a neighboring AS receives multiple routes to the same destination with different MED values, it will prefer the route with the lowest MED, assuming all other attributes are equal. This allows service providers to implement traffic engineering policies for inbound traffic flows.
MED is a non-transitive attribute, meaning it is typically exchanged only between directly connected autonomous systems and is not propagated beyond the receiving AS. This property makes MED suitable for local traffic engineering between peering partners. Service providers commonly use MED in scenarios with multiple interconnection points to the same neighbor, such as when connecting to an upstream provider through multiple locations. By setting different MED values at each interconnection point, the provider can influence which path the neighbor uses to send traffic.
LOCAL_PREF is used to influence outbound traffic within your own autonomous system, not inbound traffic from neighbors. WEIGHT is a Cisco-proprietary attribute that affects local path selection only on the router where it is configured and is not advertised to any BGP peers. The ORIGIN attribute indicates how a route was introduced into BGP and has minimal impact on traffic engineering. For controlling inbound traffic from external autonomous systems, MED remains the most effective standard BGP mechanism available.
Question 144
In MPLS VPN implementations, what is the function of the Route Distinguisher (RD)?
A) To control which routes are imported into specific VRFs
B) To make overlapping IPv4 addresses unique in the BGP table
C) To determine the MPLS label assigned to VPN routes
D) To encrypt traffic between PE routers
Answer: B
Explanation:
The Route Distinguisher serves a critical function in MPLS Layer 3 VPNs by making potentially overlapping IPv4 addresses globally unique within the service provider’s BGP infrastructure. Different VPN customers may use the same private IP address space, such as RFC 1918 addresses, which would create conflicts in the BGP routing table without a mechanism to distinguish them. The RD is prepended to the IPv4 prefix to create a unique VPN-IPv4 address format, allowing the provider’s BGP infrastructure to carry multiple instances of the same IP prefix for different customers.
The RD is an eight-octet value typically formatted as either ASN:nn or IP-address:nn, where ASN is a two-octet autonomous system number and IP-address is a four-octet IP address, with nn being an administrator-assigned number. For example, an RD of 65000:1 prepended to the prefix 10.1.1.0/24 creates a unique VPN-IPv4 route that can coexist with another VPN using 65000:2:10.1.1.0/24. The RD must be configured on each VRF and is automatically prepended to routes when they are exported from the VRF into BGP.
Route Targets, not Route Distinguishers, control the import and export of routes between VRFs. MPLS label assignment is handled by the label distribution protocol or the PE router’s VRF label allocation mechanism, not by the RD. Encryption is provided by separate security protocols like IPsec and is unrelated to the RD. The sole purpose of the RD is to ensure uniqueness of overlapping address spaces in the provider’s BGP routing infrastructure.
Question 145
What is the primary difference between IS-IS Level 1 and Level 2 routing?
A) Level 1 routes within areas while Level 2 routes between areas
B) Level 1 uses link-state updates while Level 2 uses distance-vector
C) Level 1 supports IPv4 only while Level 2 supports both IPv4 and IPv6
D) Level 1 is used for internal routes while Level 2 is for external routes
Answer: A
Explanation:
IS-IS implements a two-level hierarchical routing architecture where Level 1 routing operates within individual areas and Level 2 routing operates between areas, forming the backbone of the IS-IS domain. Level 1 routers maintain detailed topology information only about their own area and use a default route to reach destinations in other areas. Level 2 routers maintain topology information about the inter-area backbone and can route between different areas. Routers can be configured as Level 1 only, Level 2 only, or Level 1-2, which participates in both levels of routing.
This hierarchical design provides scalability by limiting the scope of link-state flooding and reducing the size of routing tables on Level 1 routers. Level 1 routers form adjacencies only with other Level 1 and Level 1-2 routers in the same area. Level 2 routers form adjacencies with other Level 2 and Level 1-2 routers regardless of area. Level 1-2 routers act as area border routers, maintaining separate link-state databases for Level 1 and Level 2 and providing connectivity between the levels.
Both Level 1 and Level 2 use link-state routing protocols based on the same fundamental IS-IS mechanisms; they are not different routing protocol types. Both levels support IPv4 and IPv6 equally through the use of TLVs in LSPs. The distinction between internal and external routes in IS-IS is handled through route types and redistribution, not through the Level 1 and Level 2 hierarchy. The primary architectural purpose of the two levels is to create a scalable hierarchical routing domain with area-based topology containment.
Question 146
In BGP, what is the purpose of the NEXT_HOP attribute and how does it differ in eBGP versus iBGP scenarios?
A) NEXT_HOP identifies the AS path; it changes in eBGP but not in iBGP
B) NEXT_HOP specifies the next router to reach a destination; it changes in eBGP but is preserved in iBGP
C) NEXT_HOP indicates route preference; it is set locally in both eBGP and iBGP
D) NEXT_HOP determines the BGP router ID; it remains constant in both scenarios
Answer: B
Explanation:
The NEXT_HOP attribute is a well-known mandatory BGP attribute that specifies the IP address of the router that should be used as the next hop to reach the advertised destination prefix. The behavior of NEXT_HOP differs significantly between eBGP and iBGP sessions due to the different trust and connectivity assumptions in these scenarios. In eBGP sessions, when a router advertises a prefix to an external peer, it changes the NEXT_HOP to its own interface address on the link connecting to that peer. This ensures the receiving router can directly reach the next hop.
In iBGP sessions, the default behavior is to preserve the NEXT_HOP attribute when advertising routes learned from eBGP peers to iBGP peers. This means that when a route is learned via eBGP and then redistributed via iBGP, the NEXT_HOP remains set to the eBGP peer’s address, not the advertising iBGP router’s address. This behavior assumes that all iBGP routers have reachability to the original next hop through the IGP. However, this can cause issues in certain topologies, which is why route reflectors often use next-hop-self to change the NEXT_HOP to their own address when reflecting routes.
The AS_PATH attribute, not NEXT_HOP, identifies the autonomous systems a route has traversed. NEXT_HOP does not indicate route preference; attributes like LOCAL_PREF and MED handle preference. The BGP router ID is a separate identifier used to uniquely identify BGP speakers and is not related to the NEXT_HOP forwarding attribute. Understanding NEXT_HOP behavior is essential for troubleshooting BGP connectivity issues.
Question 147
Which MPLS traffic engineering feature provides sub-50ms convergence by pre-computing backup paths around network failures?
A) Link Protection
B) Fast Reroute with facility backup
C) Graceful Restart
D) Bidirectional Forwarding Detection
Answer: B
Explanation:
Fast Reroute with facility backup is the MPLS traffic engineering mechanism that provides extremely rapid convergence, typically achieving sub-50 millisecond failover times by pre-computing and pre-signaling backup paths around potential failure points. Facility backup creates a bypass LSP that protects multiple primary LSPs simultaneously. When a protected link or node fails, traffic is immediately switched to the pre-established bypass LSP without waiting for IGP convergence or new LSP signaling. This mechanism is crucial for service providers who need to meet stringent service level agreements requiring carrier-grade reliability.
The facility backup method works by creating a single bypass LSP that can protect many primary LSPs traversing the same protected resource. A Point of Local Repair router detects the failure through mechanisms like BFD and immediately reroutes all affected traffic onto the bypass LSP. This approach is more scalable than one-to-one backup, which creates a separate backup LSP for each protected LSP. The bypass LSP is pre-computed to avoid the protected link or node and is already established with MPLS labels allocated before any failure occurs.
Link Protection and node protection are types of Fast Reroute protection but facility backup specifically describes the efficient method of using one bypass for multiple LSPs. Graceful Restart helps preserve forwarding state during control plane restarts but does not provide rapid failover around network failures. Bidirectional Forwarding Detection is a failure detection protocol that can trigger Fast Reroute but is not itself the protection mechanism. The combination of BFD for rapid failure detection and facility backup for pre-computed protection delivers the sub-50ms convergence requirement.
Question 148
What is the purpose of BGP communities and how are they typically used in service provider networks?
A) To encrypt BGP updates between autonomous systems
B) To tag routes with attributes for implementing routing policies
C) To establish BGP peering sessions automatically
D) To synchronize routing tables between BGP speakers
Answer: B
Explanation:
BGP communities are optional transitive attributes that allow network operators to group destinations and apply common routing policies to them. Communities are essentially tags attached to BGP routes that can be used to signal information between BGP routers without affecting other BGP attributes. They are expressed as 32-bit values typically written in the format ASN:Value, such as 65000:100. Service providers extensively use communities to implement sophisticated traffic engineering policies, control route advertisement, and coordinate routing decisions across administrative boundaries.
Common use cases include marking routes learned from specific customers or peers, controlling route propagation through no-export or no-advertise well-known communities, influencing path selection through community-based LOCAL_PREF assignments, and implementing blackhole routing for DDoS mitigation. For example, a service provider might assign community 65000:200 to all customer routes that should not be advertised to certain peers, then use this community in export policies to filter appropriately. Communities provide a flexible, scalable mechanism for policy implementation without requiring complex AS_PATH or prefix-based matching.
BGP communities do not provide encryption; security is handled by TCP MD5 authentication or other mechanisms. BGP peering requires explicit configuration and cannot be automated through communities. Route synchronization in BGP occurs through the standard BGP update mechanism and is not controlled by communities. Extended communities, a related concept, provide additional functionality for MPLS VPNs through Route Targets and Site of Origin attributes. Understanding community usage is essential for operating complex service provider networks with sophisticated routing policies.
Question 149
In a Layer 2 VPN using VPLS, what is the function of a Virtual Switching Instance (VSI)?
A) To perform Layer 3 routing between different VPN sites
B) To emulate a bridge or switch for a specific VPLS instance
C) To establish MPLS labels for unicast traffic only
D) To provide encryption for Layer 2 frames
Answer: B
Explanation:
A Virtual Switching Instance is the fundamental component in VPLS that emulates a bridge or Ethernet switch for a specific VPLS instance on a Provider Edge router. The VSI performs all the functions of a traditional Ethernet bridge, including MAC address learning, forwarding, and flooding of broadcast, unknown unicast, and multicast traffic. Each VPLS instance has its own VSI, which maintains a MAC address table learned from both local attachment circuits and remote pseudowires. This allows VPLS to provide transparent LAN services across a wide area network.
The VSI interconnects local customer-facing interfaces with pseudowires to remote PE routers participating in the same VPLS instance. When a frame arrives on a local attachment circuit, the VSI learns the source MAC address and associates it with that circuit. When forwarding frames, the VSI looks up the destination MAC address in its forwarding table and sends the frame out the appropriate local interface or remote pseudowire. If the destination is unknown, the VSI floods the frame to all interfaces and pseudowires in the VPLS instance, exactly as a physical Ethernet switch would.
Layer 3 routing is not performed by the VSI; VPLS operates at Layer 2, providing transparent Ethernet connectivity. The VSI handles both unicast and multicast traffic, not just unicast. MPLS labels are used to transport traffic across the provider network, but the VSI itself is responsible for the Layer 2 switching function, not label assignment. Encryption is provided by separate mechanisms like IPsec and is not a function of the VSI. The VSI is purely a Layer 2 forwarding construct.
Question 150
What is the primary benefit of using BGP add-path feature in a service provider network?
A) To reduce BGP convergence time by advertising multiple paths
B) To allow BGP to advertise multiple paths for the same prefix to peers
C) To increase the maximum number of BGP prefixes that can be stored
D) To provide load balancing across unequal cost paths automatically
Answer: B
Explanation:
BGP add-path is an extension to the BGP protocol that allows a BGP speaker to advertise multiple paths for the same network prefix to its peers, overcoming the traditional BGP limitation of advertising only the best path. In standard BGP, a router selects a single best path from all available paths and advertises only that path to its neighbors. This can lead to suboptimal routing decisions and path hiding, especially in route reflector environments where diverse paths available at the route reflector are not visible to route reflector clients. Add-path solves this by allowing the advertisement of multiple paths with unique path identifiers.
The implementation uses a path identifier field added to BGP updates that allows multiple NLRI entries for the same prefix to coexist. This capability is particularly valuable in service provider networks with route reflectors, where add-path can prevent the loss of path diversity. For example, if a route reflector has multiple equally good paths from different clients, it can advertise all of them to other clients rather than selecting just one. This gives receiving routers better visibility into available paths and enables better traffic engineering and faster convergence.
While add-path can indirectly improve convergence by maintaining knowledge of backup paths, its primary purpose is enabling the advertisement of multiple paths, not reducing convergence time per se. Add-path does not increase the maximum prefix storage capacity, which is determined by router memory and BGP implementation limits. Load balancing across multiple paths is a potential application of add-path, but the feature itself only enables the advertisement of multiple paths; actual load balancing requires additional configuration and support.