Networking

An SP engineer on an IOS XR ASBR must advertise a single summary prefix 203.0.113.0/22 to an eBGP peer instead of the four contributing /24 routes currently in the BGP table. However, the more-specific 203.0.113.0/24 must still be advertised to the same peer for a downstream customer failover requirement, while the other three /24s remain hidden. Which configuration approach achieves this?

An SP operator peers with a customer over EBGP. The customer wants certain prefixes learned by the SP's PE to be usable within the SP's IBGP mesh but never advertised to any of the SP's upstream or peer EBGP neighbors. The customer sets a BGP community on these prefixes. Which well-known community achieves exactly this behavior?

A service provider running IOS XR has a large IBGP domain with over 200 routers. To reduce the full-mesh requirement without deploying route reflectors, the design team implements BGP confederations, dividing AS 65000 into sub-ASes 65001, 65002, and 65003. After configuration, an operator notices that when routes are advertised between the sub-ASes, the AS_PATH shows sub-AS numbers in parentheses but external neighbors in AS 100 never see these sub-AS numbers. When these routes reach the external EBGP peer, how does the confederation affect BGP path selection and loop prevention?

A service provider establishes an EBGP session between two routers, R1 (AS 65001) and R2 (AS 65002), that are directly connected via a single physical link. The engineer configures the peering using each router's loopback interface address as the neighbor and update source. Static routes to reach the remote loopbacks are in place, and the loopbacks are reachable via ping. However, the EBGP session remains stuck in Active/Idle. What must be configured to bring this session up?

A service provider runs a single AS with 12 IBGP-speaking PE routers configured in a full mesh. The network team plans to grow to 40 PEs and wants to reduce IBGP peering overhead without splitting the AS or introducing loops. A senior engineer proposes deploying two route reflectors in the core. When a route reflector receives an IBGP prefix from one of its clients, how will it advertise that prefix, and what loop-prevention attribute is used?

An ISP running IOS XR peers with a customer over an EBGP IPv6 session. The operator wants all IPv6 prefixes received from this customer to be tagged with community 65001:100 so downstream route-policies can match them, while leaving all other attributes unchanged. Which routing policy configuration applied inbound on the neighbor achieves this?

A service provider is migrating a customer's eBGP peering from legacy AS 65010 to a new AS 65020 after a network merger. The customer's CPE is still configured to peer with AS 65010 and cannot be reconfigured during the maintenance window. On the PE router (now in AS 65020), which BGP feature allows the PE to present itself to the customer as if it still belongs to AS 65010, so the existing eBGP session comes up without customer-side changes?

A service provider runs IBGP between PE1 and PE2 (both in AS 65100) over an internal /30 link. PE1 has an EBGP session to a customer router in AS 65200 over the subnet 203.0.113.0/30. After the customer advertises 198.51.100.0/24, PE2 receives the prefix in its BGP table but marks it as inaccessible, and the route never enters the RIB. The IGP on the provider core does not carry the 203.0.113.0/30 subnet. Which configuration on PE1 resolves the issue?

An SP network engineer is troubleshooting outbound path selection on an IOS XR PE router. The router receives two eBGP advertisements for prefix 203.0.113.0/24. Both paths have equal weight (0), equal local preference (100), neither is locally originated, both are non-aggregate routes, and MED is not being compared because the paths come from different neighboring autonomous systems. Path 1 has an AS_PATH of '65010 65020 65030' and Path 2 has an AS_PATH of '65040 65030'. All other attributes up to this decision point are equal. Based on the BGP best-path algorithm, which path will the router select and why?

An IBGP-enabled PE router in AS 65000 receives two paths to the same external prefix 203.0.113.0/24 from two different route reflectors. Both paths have identical weight, local preference, AS-path length, origin code, and MED. Both are learned via IBGP (neither is locally originated or EBGP). The BGP next-hop of path 1 (10.0.0.5) is reachable via an IGP metric of 30, while the BGP next-hop of path 2 (10.0.0.9) is reachable via an IGP metric of 20. Assuming 'bgp bestpath compare-routerid' is NOT configured and both paths are still valid, which path will BGP select as best and why?

An SP operates AS 65000 and receives the same prefix 203.0.113.0/24 from a single neighboring AS 65100 over two separate EBGP sessions. Both received paths have identical weight, local preference, AS-path length, and origin type. The path via session A carries MED 50, and the path via session B carries MED 100. All other attributes up to MED are equal. Assuming default BGP behavior, which path does the router install as best, and why?

An SP network engineer is troubleshooting inconsistent path selection for prefix 203.0.113.0/24 on an IOS-XR router. Two eBGP paths are received. Both paths have identical weight, local preference, AS_PATH length, and are not locally originated (no AS_PATH via aggregation). The MED values are also equal. Path 1 has an origin code of 'incomplete' (learned via redistribution), while Path 2 has an origin code of 'IGP' (learned via the network statement). Assuming all preceding BGP best-path attributes are tied, which path will the router select and why?

An ISP peers with two upstream providers, ISP-A and ISP-B, and receives the prefix 203.0.113.0/24 from both. On router R1 (IOS XR), the operator wants all traffic to this prefix to prefer ISP-A but needs the preference to remain local to R1 only, without influencing any other IBGP router in the AS. Both received routes are otherwise identical (same AS-path length, origin, and MED). Which attribute should the operator set inbound from ISP-A to accomplish this?

A service provider is running out of public IPv4 addresses as its residential subscriber base grows. Management wants to continue offering IPv4 connectivity to subscribers while conserving the provider's dwindling public IPv4 pool, without requiring changes to customer premises equipment (CPE). Which service provider architecture element should the network architect deploy to meet this requirement?

An ISP is deploying DS-Lite to allow residential subscribers with IPv6-only access networks to reach the legacy IPv4 Internet. The subscriber CPE receives only an IPv6 prefix from the access network. During design review, an engineer must explain how a subscriber's IPv4 traffic actually traverses the network to reach IPv4 destinations. Which statement correctly describes the DS-Lite data plane behavior?

A service provider's PE router receives the same prefix 10.50.0.0/16 from two different route reflectors, each advertising a distinct next-hop. The network engineer wants the PE to install both paths into the RIB for load balancing. All BGP attributes for both paths are identical up to and including the IGP metric to the next-hop, but by default only one path is being used. Which configuration change enables the PE to install both IBGP paths?

A service provider runs IBGP with two route reflectors (RR1 and RR2) serving the same set of clients for redundancy. To avoid unnecessary route storage and ensure loop prevention within the redundant reflector design, both RRs are configured with the same cluster-id. A network engineer troubleshooting missing paths observes that when an RR receives a reflected route, it silently discards updates that contain its own cluster identifier. Which attribute is responsible for this loop-prevention behavior among route reflectors sharing a cluster-id?

Two IOS-XR routers, PE1 and PE2, are directly connected over a point-to-point link and configured for IS-IS Level-2. The adjacency repeatedly cycles between INIT and DOWN, never reaching UP. Logs on PE1 show IIH packets received from PE2, but PE2 never sees PE1's hellos acknowledged. PE1's interface MTU is 1500; PE2's interface MTU was recently changed to 1400 during a maintenance window. What is the most likely cause of the failure?

A service provider runs a two-level IS-IS domain. Branch routers in a Level 1 area only receive a default route toward the nearest L1/L2 router via the ATT bit, which causes suboptimal exit selection when a specific prefix in another area is actually best reached through a different L1/L2 router. The network engineer wants the L1 routers to make optimal forwarding decisions for a set of important L2 prefixes without converting the entire area to Level 1-2. Which action achieves this?

You are migrating a large IS-IS backbone from narrow metrics to wide metrics on IOS XR routers. To avoid routing loops and suboptimal paths during the transition, you configure the routers to advertise and accept both TLV formats simultaneously before completing the migration. Which metric-style configuration should you apply during this transition window?