CCNA Spcor Mpls Sr Questions

LDP uses downstream unsolicited mode by default in Cisco IOS; liberal label retention means it keeps all labels even if not directly reachable. Options B and C are correct.

End.DT4 is the SRv6 endpoint behavior for decapsulation and table lookup in the VPN IPv4 table.

EVPN IRB requires an IP-VRF to represent the tenant, a bridge domain (EVI) for Layer 2, and IRB interfaces (Layer 3 gateway) that connect the bridge domain to the VRF. BGP EVPN control plane is also needed.

An SR Policy requires a headend, endpoint, and a color to distinguish multiple policies to the same endpoint. Segment lists and candidate paths are part of the policy but not all are strictly required (at least one candidate path with segment list is needed). However, the question asks for essential components of the policy definition: headend, endpoint, and color are the key identifiers.

CSPF considers bandwidth, affinity (color), and explicit constraints, but not administrative distance, which is a routing protocol metric.

The Locator portion of an SRv6 SID identifies the node and is routable in the IGP.

EVPN-VPWS provides multi-homing with fast convergence and simplifies provisioning using BGP.

Cisco's SR/LDP interworking allows coexistence. Routers can run both LDP and SR, and a mapping server advertises SR labels for LDP-learned prefixes. This enables seamless migration.

LDP uses downstream unsolicited mode by default, where each LSR advertises labels for all FECs to all neighbors without being asked, and liberal label retention stores all received labels even if not currently used.

In RSVP-TE, the PATH message includes the ERO to define the explicit route the traffic should take. The RESV message is sent back to confirm the reservation.

CSPF computes a path that meets constraints (bandwidth, affinity, etc.) and avoids links/ nodes not meeting them.

DF election determines which PE forwards traffic for a given VLAN to avoid duplication.

Each SID in the list is a node SID (Prefix-SID) of intermediate routers, forming a strict hop-by-hop path.

SRGB defines a range of labels reserved for global segments (e.g., Node SIDs). It ensures that a given Node SID maps to the same label across all routers in the SR domain, simplifying operations.

ESI identifies the segment, DF election avoids duplicates, and aliasing allows traffic to be forwarded to any PE in the multi-homing group.

An explicit path with strict hops requires each hop to be directly connected. If a hop is not directly connected, the path fails. Also, if there is no RSVP capability along the path, the tunnel cannot be signaled.

SR-MPLS eliminates LDP and RSVP state, using IGP for label distribution (e.g., via IS-IS TLVs).

The Ethernet Segment Identifier (ESI) is a 10-byte value that identifies a specific multihomed site.

SR-TE policies are stateful at the headend, use segment lists for path steering, and can be associated with a color for traffic engineering.

In RSVP-TE, the PATH message includes the ERO to specify the explicit route for the tunnel.

In MPLS, the swap operation replaces the incoming top label with a new outgoing label as per the LFIB entry.

End.DT4 is the SRv6 endpoint behavior for decapsulation and table lookup for IPv4 L3VPN. End.DT6 is for IPv6 L3VPN, End.DT2U is for L2VPN.

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.

IS-IS uses the IS-IS Router CAPABILITY TLV (135) with sub-TLV 19 to carry the SR-capable information and prefix-SID. Specifically, the Prefix-SID is advertised in the IS-IS Extended IP Reachability TLV (135) with sub-TLV 3.

The Node-SID (or Prefix-SID) is a globally unique label from the SRGB assigned to the router's loopback.

1:1 detour creates a one-to-one backup LSP for each protected LSP. The detour LSP is pre-signaled and protects against link failure by re-routing traffic around the failed link.

When the top label is the router's own label (often an implicit null or explicit null), the router pops the label and forwards the packet based on the next label or IP header.

SR-TE uses a segment list consisting of Node SIDs and/or Adjacency SIDs to define the explicit path, avoiding RSVP state.

The RESV message is sent upstream from the destination to the source to reserve resources and confirm the path; the ERO is in the PATH message, not RESV.

Option B is correct because downstream unsolicited (DU) label distribution is the default mode in LDP, where each LSR advertises labels for all its directly connected prefixes to its neighbors without waiting for a request. Liberal label retention means the LSR stores all received label bindings, even if they are not currently used for forwarding, which enables rapid convergence upon topology changes.

End.DT4 is the correct SRv6 endpoint behavior for L3VPN decapsulation and IP forwarding to the destination in a VRF. It removes the outer IPv6 header and SRH, then performs a lookup in the specified VRF IPv4 routing table to forward the inner IPv4 packet. This behavior is defined in RFC 8986 and is the direct SRv6 equivalent of the MPLS-based per-VRF label allocation for L3VPN.

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

The Locator is the portion of the SRv6 SID that identifies the node (or the segment endpoint) and is used for routing to that node.

Shared Explicit (SE) allows sharing among senders with different selectors, and Fixed Filter (FF) creates distinct reservations per sender.

The SRv6 SID is structured as Locator:Function:Argument. The Function part identifies the specific behavior to be executed on the node, such as End or End.X.

The preference value (or priority) is used to select the best candidate path; higher preference wins.

In MPLS-TE Fast Reroute (FRR), a 1:1 protection scheme means a dedicated detour LSP is created for each protected LSP. This provides per-LSP backup, ensuring that if a link fails, traffic is immediately switched to the detour tunnel. Option C correctly describes this exact mechanism, where each protected LSP has its own pre-established detour path.

Node protection uses a bypass tunnel that bypasses the protected node, while link protection only bypasses the link.

Liberal label retention stores all label bindings, allowing quick convergence when a path changes, while conservative retention stores only bindings from the next-hop.

The PATH message is sent downstream from the headend and contains the ERO that specifies the explicit route.

An Adjacency SID is a local label for a specific adjacency, distributed only to neighbors.

Shared Explicit (SE) style allows multiple senders to share a single reservation, with explicit sender list.

EVPN IRB provides bridging at Layer 2 and routing at Layer 3 with an anycast default gateway. It uses asymmetric IRB (bridging locally, routing via VRF) and symmetric IRB (both directions route). The anycast gateway MAC is shared across all PEs.

Adjacency SIDs force the packet to traverse a specific link. For a strict hop, the adjacency SID ensures the packet goes through that exact link. A Node SID would allow any path to the node.

Option A is correct because SR-MPLS uses a Segment Routing Global Block (SRGB), which is a reserved label range (e.g., 16000–23999 per RFC 8660) that is configured consistently across all routers in the SR domain. This global consistency allows any router to interpret a label from the SRGB as the same topological or service instruction, eliminating the need for per-hop label negotiation.

SRv6 micro-SID uses a compressed SID format where multiple SIDs are encoded in a single 128-bit IPv6 address. It reduces overhead by combining SIDs into a carrier SID. The uSID instruction is usually 16 bits, and the locator block is shared.

Swap is the operation where the incoming label is replaced with a new outgoing label, as determined by the LFIB.

CSPF (Constrained Shortest Path First) is the algorithm used by RSVP-TE to compute a path that satisfies constraints such as bandwidth, explicit path, and link affinities.

Verifying LDP sessions with 'show mpls ldp neighbor' and checking interfaces with 'show mpls ldp interface' are standard first steps. Checking MTU or OSPF neighbors are less direct.

EVPN multi-homing uses ESI, DF election for BUM traffic, and aliasing for load balancing.

The Ethernet Segment Identifier (ESI) is a unique identifier for a multihomed segment. It is used for DF election, aliasing, and split-horizon.

SRGB is a reserved label range used for globally significant prefix SIDs. All routers in the domain use the same SRGB to ensure consistent label mapping.

LDP liberal retention keeps all received labels even if not used, which speeds convergence but uses more memory.

Downstream unsolicited means the downstream router sends label bindings to all neighbors without being asked. Liberal label retention means the router keeps all received label bindings, even if the next hop is not reachable.

In PHP, the penultimate hop router pops the label before forwarding the packet to the egress router, reducing the egress's workload.

SRv6 SIDs are 128-bit and structured as Locator:Function:Arguments. End.DT4 is a function that decapsulates the outer IPv6 header and forwards the inner IPv4 packet based on destination.

A bypass tunnel is a single tunnel protecting a link, carrying traffic for all LSPs that traverse that link. Detour tunnels are per-LSP.

IRB uses an Integrated Routing and Bridging interface (IRB interface) or BVI (Bridge Virtual Interface) to route between VLANs within the EVPN.

LDP uses downstream unsolicited mode by default, where a router advertises labels to all neighbors without being asked.

End.DT4 for IPv4 L3VPN, End.DT6 for IPv6 L3VPN, and End.DT2U for L2VPN are defined SRv6 behaviors.

Default SRGB is 16000-23999, but can be configured. The label range is global across SR domain.

The LFIB (Label Forwarding Information Base) is the MPLS forwarding table that maps incoming labels to outgoing labels and interfaces.

In PHP, the penultimate router pops the label before forwarding to the egress, reducing the egress router's work.

In SR-MPLS deployments, segment routing replaces LDP as the label distribution protocol. LDP traditionally distributes labels for each prefix in the IGP, while segment routing uses the IGP itself to advertise prefix-SIDs and adjacency-SIDs, eliminating the need for a separate label distribution protocol. This simplifies the control plane and removes the dependency on LDP.

At the ingress, a label is pushed; at the egress, the label is popped. Swap occurs at intermediate routers.

The IS-IS SR-Capabilities sub-TLV (TLV 242) advertises the SRGB range, while the SR-Algorithm sub-TLV (TLV 242) and Prefix-SID sub-TLV (TLV 135) advertise Node-SIDs.

Segment Routing replaces LDP by using IGP extensions (IS-IS or OSPF) to advertise Prefix-SIDs and Adjacency-SIDs, eliminating the need for a separate label distribution protocol.

ERO specifies the strict or loose hops that the TE LSP must traverse. It allows the headend to enforce a specific path through the network.

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.

The micro-SID block consists of a locator followed by compressed function and argument, reducing the SID list size.

IS-IS uses the SR-Capabilities sub-TLV and the SR-Algorithm sub-TLV, but the Node SID is advertised in the SR-MPLS SID/Label Binding TLV (TLV 149) with a sub-TLV. The exact TLV is 149 for SR-MPLS.