Service Provider Routing and Switching, Professional Free Sample Questions

The EVPN Type 4 route, the Ethernet Segment Route, is specifically used for the Designated Forwarder (DF) election process. Each PE router connected to the same Ethernet Segment (identified by a unique ESI) advertises a Type 4 route. These routes are then used in a default algorithm (or a preference-based one) to elect a single DF for handling Broadcast, Unknown Unicast, and Multicast (BUM) traffic for that segment, thereby preventing loops. Type 1 is for mass MAC withdrawal, Type 2 is for advertising MAC/IP bindings, and Type 3 is for BUM traffic forwarding paths.

Anycast RP provides redundancy by having multiple routers share the same IP address for the RP role. A Designated Router (DR) for a source will send PIM Register messages to the closest physical RP. For this design to work, all physical RPs must know about all active sources. The Multicast Source Discovery Protocol (MSDP) is used to create peering sessions between the physical RP devices to exchange Source Active (SA) messages, ensuring all RPs have a consistent view of active multicast sources across the domain.

In IS-IS, a Level 1/Level 2 router (an ABR) signals its connectivity to the Level 2 backbone by setting the Attached (ATT) bit in its Level 1 Link State PDU (LSP). When Level 1 routers receive an LSP from an ABR with the ATT bit set, they will automatically generate and install a default route pointing to that ABR for inter-area and external reachability. If the ATT bit is not set, the L1 routers will not create this default route.

The Site of Origin (SoO) is a BGP extended community used specifically for loop prevention in Layer 3 VPNs, particularly with multi-homed sites or when using a backdoor link. When the PE router at Site A receives routes from the CE, it attaches a unique SoO value to them before advertising them into the MPLS core via MP-BGP. If the same PE router receives a VPN route from the core that contains its own SoO value, it will not advertise that route back to its connected CE. This effectively prevents routes from being looped back to their originating site.

The Junos BGP route selection algorithm has a specific step that prefers eBGP-learned routes over iBGP-learned routes. This is a fundamental tie-breaking rule that occurs after evaluating LOCAL_PREF, AS_PATH length, and ORIGIN code. The logic is that an external path is generally considered more direct or authoritative than an internal one that may have been reflected multiple times.

Configuring a scheduler with priority strict-high ensures that the queue for the voice-traffic forwarding class will be serviced until it is empty before any lower-priority queues are serviced. This provides the lowest possible latency. Combining this with a transmit-rate (e.g., transmit-rate 5m) guarantees a minimum amount of bandwidth for this traffic. The strict-high priority handles the real-time requirement, while the transmit rate ensures its bandwidth needs are met. Other queues are not starved because once the strict-high queue is empty, the scheduler moves on to service the other queues.

A Not-So-Stubby Area (NSSA) is a special area type that allows the injection of external routes via an ASBR while still blocking external routes (Type 5 LSAs) from the rest of the OSPF domain. External routes redistributed into an NSSA are carried in Type 7 LSAs, which are only flooded within that NSSA. When these Type 7 LSAs reach the NSSA's Area Border Router (ABR), the ABR translates them into standard Type 5 LSAs and floods them into the backbone area (area 0) and subsequently to other non-stub areas.

This statement is true. LDP-signaled VPLS relies on a full mesh of pseudowires between all participating PE devices to create the multipoint-to-multipoint LAN service. Each pseudowire is signaled using a dedicated targeted-LDP session between the two PE loopback addresses. This requirement is a primary scaling limitation of LDP VPLS, which BGP-signaled VPLS and EVPN were designed to overcome.