JN0-664 Service Provider Routing and Switching, Professional Practice Questions

One of the key steps in the BGP best path selection algorithm is to prefer the path with the shortest AS_PATH length. The AS_PATH length is the number of AS numbers in the path. Path A has a length of 2, while Path B has a length of 3. Therefore, assuming all higher-priority attributes (like LOCAL_PREF) are equal, the router will select Path A as the best path.

This statement is false. Unlike the OSPF Designated Router (DR) election which is non-preemptive, the IS-IS DIS election is preemptive. If a new router comes online on a broadcast segment with a higher priority than the current DIS, it will take over the DIS role. This is a key operational difference between the two link-state protocols.

For BGP-signaled Layer 2 services like VPLS (and BGP L2VPNs), the specific address family used to carry the NLRI is l2vpn signaling. This address family was defined in RFC 4761 to advertise Layer 2 information, including site membership, label blocks, and encapsulation types, which is distinct from the inet-vpn unicast family used for Layer 3 VPNs or the more modern evpn family.

The Reverse Path Forwarding (RPF) check verifies that a multicast packet for a given source arrives on the same interface that the router would use to send unicast traffic back to that source. A common cause for failure is asymmetric routing, where the unicast path from the LHR to the source is different from the path the multicast traffic takes from the source to the LHR. If the packet arrives on an interface other than the RPF interface, the check fails and the packet is dropped.

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.