350-501 MPLS and Segment Routing Practice Test 5 — 15 Questions

Question 1

Which LDP label distribution mode is characterized by a router advertising labels for all prefixes in its routing table to all neighbors, regardless of whether they are needed?

Accepted Answer

Downstream unsolicited with liberal label retention. Downstream unsolicited means the router sends label bindings without being asked; liberal label retention means it keeps all bindings even if not from the next hop.

Answer

Downstream on demand with conservative label retention

Answer

Downstream on demand with liberal label retention

Answer

Downstream unsolicited with conservative label retention

Question 2

In Segment Routing over MPLS (SR-MPLS), how are Prefix-SIDs advertised in IS-IS?

Accepted Answer

Via IS-IS TLV 135 and TLV 149. In SR-MPLS, Prefix-SIDs are advertised in IS-IS using TLV 135 (Extended IP Reachability) to carry the prefix and its metric, and TLV 149 (IS-IS Router Capability) to carry the SID/Label Binding sub-TLV, which maps the prefix to a specific label (Prefix-SID). This allows IS-IS to distribute both the prefix reachability and its associated segment identifier in a single protocol update, enabling source routing without LDP or RSVP-TE.

Answer

Via LDP label mapping messages

Answer

Via RSVP-TE PATH messages

Answer

Via BGP VPN labels

Question 3

What is the default range of the Segment Routing Global Block (SRGB) on Cisco IOS XR?

Accepted Answer

16000-23999. On Cisco IOS XR, the default Segment Routing Global Block (SRGB) is the label range 16000-23999, which provides 8000 labels for global SID allocation. This range is consistent across IOS XR platforms and is the baseline from which prefix-SIDs and other global segments are carved. Administrators can shrink or shift this range, but 16000-23999 is the out-of-the-box value.

Answer

15000-25000

Answer

16000-24000

Answer

16-23999

Question 4

In EVPN with SRv6, which SID behavior is used for integrated routing and bridging (IRB) to forward packets between the IP fabric and the Ethernet segment?

Accepted Answer

End.DT4. End.DT2U is for L2VPN; for IRB, the router uses End.DT4/DT6 for IP forwarding and then bridges to the segment; however, the question asks for the SID behavior that enables IRB. Usually, IRB is achieved by combining a Layer 2 VPN SID and a Layer 3 VPN SID, but for the specific behavior that forwards between IP and MAC, it's often End.DT4 or End.DT6 plus a MAC lookup. But standard SRv6 IRB uses a dedicated SID like End.DT2U? Actually, IRB requires both routing and bridging; the common approach is to use a Layer 3 VPN SID for the routed part and an EVPN Ethernet segment for bridging. In SRv6, the endpoint behavior for IRB is typically End.DT4 or End.DT6 for the routing side, and for bridging, it's End.DT2U. But the question expects a single behavior? Perhaps End.DT2U is for L2VPN only. Let's think: Cisco documentation often describes 'End.DT2U' for L2VPN, and 'End.DT4' for L3VPN. For IRB, the service is provided by a combination. However, a specific SID behavior called 'End.DT2U' can be used for EVPN VPWS. To align with common SIDs, the answer is End.DT2U for L2VPN IRB? Actually, IRB typically uses a Layer 3 gateway SID. I'll choose End.DT4 as it routes IP, and then the router bridges locally. But the question says 'forward packets between the IP fabric and the Ethernet segment' - that is routing. So End.DT4 is correct.

Answer

End

Answer

End.DT2U

Answer

End.X

Question 5

Which RSVP reservation style is most commonly used in MPLS-TE to allow sharing of resources among multiple LSPs of the same session?

Accepted Answer

Shared Explicit (SE). Shared Explicit (SE) style allows multiple senders to share a reservation, which is typical for MPLS-TE where the headend may have multiple LSPs to the same destination.

Answer

Wildcard Filter (WF)

Answer

Distinct Filter (DF)

Answer

Fixed Filter (FF)

Question 6

Which THREE functions are provided by EVPN multi-homing? (Choose three.)

Accepted Answer

Aliasing allows other PEs to send traffic to any PE in the multi-homed group.. Option A is correct because EVPN multi-homing provides aliasing, which lets remote PEs treat all PEs in the same Ethernet Segment as reachable next hops, so traffic can be load-balanced and sent to any PE in the multi-homed group. Option B is correct because the Ethernet Segment Identifier (ESI) is the unique value that identifies the multi-homed site or Ethernet Segment, allowing PEs to recognize that they share the same customer edge link. Option E is correct because EVPN multi-homing uses Designated Forwarder (DF) election per Ethernet Segment and per VLAN/BUM service to choose which PE forwards broadcast, unknown unicast, and multicast traffic, preventing duplicate BUM delivery. Option C is not correct as a multi-homing function because split horizon is a loop-prevention mechanism used in EVPN for multi-homed access, but it is not one of the three functions asked for here and is not marked correct. Option D is not correct because MAC mobility is an EVPN capability for moving MAC addresses between PEs, not a function provided specifically by multi-homing.

Answer

Split horizon prevents loops by blocking traffic from the same ES.

Answer

MAC mobility is provided by multi-homing.

Question 7

Which TWO statements about SR-MPLS compared to LDP are correct? (Choose two.)

Accepted Answer

SR-MPLS uses a global label block (SRGB) that is consistent across the domain.. Option A is correct because SR-MPLS relies on the Segment Routing Global Block (SRGB), a reserved range of MPLS labels (e.g., 16000–23999) that is configured consistently across all routers in the SR domain, so that a given prefix SID maps to the same label on every node and no per-hop label signaling is needed. Option D is correct because in SR-MPLS each router advertises a Node SID (an index into the SRGB) for its loopback prefix via IGP extensions, and this per-prefix label identifies the node and is used to build source-routed label stacks. Option B is wrong because SR-MPLS distributes SIDs through IGP extensions (IS-IS/OSPF) and does not require LDP at all. Option C is wrong because SR-MPLS supports both dynamic and explicit (manual) SID/label assignment, not only dynamic allocation. Option E is wrong because SR-MPLS can create traffic-engineered paths using segment lists (SR-TE) without RSVP-TE.

Answer

SR-MPLS requires LDP for label distribution.

Answer

SR-MPLS supports only dynamic label allocation.

Answer

SR-MPLS requires RSVP-TE for TE tunnels.

Question 8

In EVPN with Integrated Routing and Bridging (IRB), which type of interface is used to connect the bridge domain to the IP-VRF for inter-subnet forwarding?

Accepted Answer

IRB interface. In EVPN with Integrated Routing and Bridging (IRB), the IRB interface is the logical interface that connects a bridge domain (Layer 2) to an IP-VRF (Layer 3) for inter-subnet forwarding. It acts as the gateway interface for hosts in the subnet, enabling routing between subnets within the same EVPN instance. This is the standard terminology in EVPN IRB deployments.

Answer

BD interface

Answer

L3VPN interface

Answer

SVI

Question 9

Which THREE are EVPN multi-homing functions? (Choose three.)

Accepted Answer

Ethernet Segment Identifier (ESI) uniquely identifies the multi-homed segment.. Option A is correct because the Ethernet Segment Identifier (ESI) is a 10-byte value that uniquely identifies the multi-homed Ethernet segment (the set of links connecting a CE to two or more PEs), and it is the key identifier used in EVPN Type-1/Type-4 routes to coordinate multi-homing. Option D is correct because Designated Forwarder (DF) election, driven by the DF election algorithm (e.g., RFC 7432 modulo-based or RFC 8584 HRW), ensures that only one PE per Ethernet segment forwards broadcast, unknown unicast, and multicast (BUM) traffic, preventing duplicate delivery to the multi-homed CE. Option E is correct because aliasing lets a PE that is not the DF, or has no locally learned MACs, advertise reachability for the segment's MAC addresses via the ESI so remote PEs can load-balance unicast traffic across all multi-homed PEs. Option B is not an EVPN multi-homing function per se; split horizon is a general loop-prevention mechanism, and in EVPN it is typically achieved via the ESI label and local-bias rules rather than being a named multi-homing function. Option C is incorrect in this context because MAC mobility is an EVPN feature for detecting and resolving MAC address moves (via the MAC Mobility extended community), which applies to host mobility, not specifically to multi-homing functions.

Answer

Split horizon prevents loops among PEs.

Answer

MAC mobility provides MAC address move detection.

Question 10

Which label operation is performed on a router when an MPLS packet arrives with a top label that matches a local label assigned to a directly connected prefix?

Accepted Answer

Pop. If the top label corresponds to a directly connected prefix, the router will pop the label (penultimate hop popping or explicit null) and forward the packet based on the inner header.

Answer

No operation, label is ignored

Answer

Push

Answer

Swap

Question 11

In SR-TE, a headend router is configured with an SR Policy using an explicit candidate path that includes a segment list [100, 200, 300]. If all routers have the same SRGB starting at 10000, what is the actual outgoing label stack for the first segment?

Accepted Answer

10100, 10200, 10300. In SR-MPLS, the SID values in the segment list are relative to the SRGB. If the SRGB starts at 10000, a SID value of 100 corresponds to label 10100 (10000 + 100). So the first segment label is 10100.

Answer

100, 200, 300

Answer

10000, 20000, 30000

Answer

Labels depend on LDP allocation

Question 12

An engineer is designing an EVPN IRB (Integrated Routing and Bridging) solution. Which two statements about EVPN IRB are correct? (Choose two.)

Accepted Answer

EVPN IRB uses an anycast default gateway IP and MAC that is shared across all PEs.. Option B is correct because EVPN IRB commonly uses a distributed anycast gateway, where the same default gateway IP and MAC address are configured on all participating PEs, allowing hosts to use a consistent gateway while traffic is routed locally at the closest PE. Option E is correct because in asymmetric IRB the ingress PE performs the Layer 3 routing from the source subnet and then bridges the packet over the EVPN to the egress PE, which only needs to bridge it to the destination host; routing is not repeated at the egress PE. Option A is incorrect because EVPN IRB is fully compatible with multi-homing mechanisms such as Ethernet Segment (ES) and all-active multihoming. Option C is incorrect because EVPN IRB supports both IPv4 and IPv6, not only IPv6. Option D is incorrect because a MAC-VRF is associated with a bridge domain and can be mapped to a VRF, but EVPN IRB does not require a separate VRF for every MAC-VRF.

Answer

EVPN IRB does not support multi-homing.

Answer

EVPN IRB only supports IPv6 traffic.

Answer

EVPN IRB requires a separate VRF for each MAC-VRF.

Question 13

Which four statements correctly describe the SRv6 micro-SID (uSID) concept? (Choose four.)

Accepted Answer

Micro-SID is incompatible with SR-MPLS.. Option B is correct because uSID packs several micro-SIDs into one 128-bit IPv6 address, using a shared locator block plus a sequence of 16-bit SID fields, so a single address can carry a whole segment list. Option D is correct because all nodes in the SRv6 domain share a common locator block (the uSID block, e.g., a /32 or /48 prefix), which is what allows the compressed SID sequence to be interpreted consistently across the domain. Option E is correct because the micro-SID function itself is typically 16 bits, which is the granularity used to pack multiple SIDs into the 128-bit address. Option A is correct because uSID is an SRv6 concept that operates on IPv6 addresses and is not compatible with SR-MPLS, which uses MPLS labels rather than IPv6 addressing. Option C is not correct because uSID is specifically designed to avoid requiring a dedicated SRH extension header, since the SID list is carried directly in the IPv6 destination address.

Answer

Micro-SID requires a dedicated SRH extension header.

Question 14

In an MPLS network using RSVP-TE for traffic engineering, which two mechanisms can be used for fast reroute (FRR) protection? (Choose two.)

Accepted Answer

Facility backup using a bypass tunnel. Option C (Facility backup using a bypass tunnel) is correct because RSVP-TE FRR facility backup creates a bypass tunnel that protects a whole facility (link or node), allowing many LSPs to share one backup path and locally redirect traffic around the failure. Option E (1:1 protection using a dedicated detour LSP) is correct because RSVP-TE FRR supports detour LSPs, where a dedicated backup path is pre-established for a protected LSP, providing local protection with a one-to-one relationship. Option A is not correct in this context because segment routing adjacency SIDs are used for SR-based TI-LFA, not RSVP-TE FRR. Option B is not correct because global repair via a secondary LSP from the headend is path protection, not local RSVP-TE fast reroute. Option D is not correct because HSRP is a first-hop redundancy protocol for IP gateways and has no role in MPLS RSVP-TE FRR.

Answer

Local Protection using segment routing adjacency SID

Answer

Global Repair using a secondary LSP from headend

Answer

Hot Standby Router Protocol (HSRP)

Question 15

In an MPLS network using LDP, which LDP mode allocates a label for every prefix in the routing table and advertises it to all LDP neighbors, regardless of whether the neighbor requests it?

Accepted Answer

Downstream unsolicited with liberal label retention. Downstream unsolicited (DU) mode means a router advertises labels for all its prefixes to all neighbors without waiting for a request.

Answer

Downstream on demand with liberal label retention

Answer

Downstream unsolicited with conservative label retention

Answer

Downstream on demand with conservative label retention