Exam 642-885 topic 1 question 6 discussion

Refer to the topology diagram shown in the exhibit and the partial configurations shown below.


Once the attack from 209.165.201.144/28 to 209.165.202.128/28 has been detected, which additional configurations are required on the P1 IOS-XR router to implement source-based remote-triggered black hole filtering?

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router bgp 123
address-family ipv4 unicast
redistribute static route-policy test
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Suggested Answer: C 🗳️
Source-Based RTBH Filtering -
With destination-based black holing, all traffic to a specific destination is dropped after the black hole has been activated, regardless of where it is coming from.
Obviously, this could include legitimate traffic destined for the target. Source-based black holes provide the ability to drop traffic at the network edge based on a specific source address or range of source addresses.
If the source address (or range of addresses) of the attack can be identified (spoofed or not), it would be better to drop all traffic at the edge based on the source address, regardless of the destination address. This would permit legitimate traffic from other sources to reach the target. Implementation of source-based black hole filtering depends on Unicast Reverse Path Forwarding (uRPF), most often loose mode uRPF. Loose mode uRPF checks the packet and forwards it if there is a route entry for the source IP of the incoming packet in the router forwarding information base (FIB). If the router does not have an FIB entry for the source
IP address, or if the entry points to a null interface, the Reverse Path Forwarding (RPF) check fails and the packet is dropped, as shown in Figure 2. Because uRPF validates a source IP address against its FIB entry, dropping traffic from specific source addresses is accomplished by configuring loose mode uRPF on the external interface and ensuring the RPF check fails by inserting a route to the source with a next hop of Null0. This can be done by using a trigger device to send
IBGP updates. These updates set the next hop for the source IP to an unused IP address that has a static entry at the edge, setting it to null as shown in Figure 2.