MPLS and VPN Technologies Flashcards

What is the primary function of a Label Edge Router (LER) in an MPLS network?

Answer: Push or pop MPLS labels at the boundary of the MPLS domain

An LER (also called PE router) sits at the edge of the MPLS domain and is responsible for pushing labels onto incoming IP packets or popping labels from outgoing packets.

In MPLS LDP, what is the default label distribution mode on Cisco routers?

Answer: Independent control with downstream unsolicited

Cisco IOS uses independent control mode with downstream unsolicited distribution by default, meaning each LSR independently assigns and advertises labels without waiting for downstream peers.

Which MPLS label value is reserved to indicate the Explicit Null label for IPv4?

Answer: 0

Label value 0 is the IPv4 Explicit Null label, which signals that the label must be popped but the packet should still be forwarded based on its IP header.

What mechanism does Penultimate Hop Popping (PHP) use to reduce processing on the egress LSR?

Answer: The second-to-last LSR pops the outermost label before the final hop

PHP allows the penultimate (second-to-last) LSR to remove the top MPLS label so the egress LSR receives a plain IP packet or inner label, reducing its lookup overhead.

In an MPLS L3VPN, which protocol is used to exchange VPN routing information between PE routers?

Answer: MP-BGP with VPNv4 address family

Multiprotocol BGP (MP-BGP) with the VPNv4 address family carries customer VPN prefixes between PE routers using Route Distinguishers and Route Targets to separate and import/export routes.

What is the purpose of a Route Distinguisher (RD) in MPLS L3VPN?

Answer: It makes overlapping customer IP prefixes unique in the BGP table

An RD is prepended to customer prefixes to create a unique 96-bit VPNv4 address, allowing two customers with overlapping IP address spaces to coexist in the same MP-BGP table.

Which command verifies the MPLS label forwarding table on a Cisco router?

Answer: show mpls forwarding-table

The 'show mpls forwarding-table' command displays the Label Forwarding Information Base (LFIB), showing incoming labels, outgoing labels, and next-hop interfaces.