Exam 300-410 topic 1 question 26 discussion

D The set ip default next-hop command verifies the existence of the destination IP address in the routing table, and… if the destination IP address exists, the command does not policy route the packet, but forwards the packet based on the routing table. if the destination IP address does not exist, the command policy routes the packet by sending it to the specified next hop. The set ip next-hop command verifies the existence of the next hop specified, and… if the next hop exists in the routing table, then the command policy routes the packet to the next hop. if the next hop does not exist in the routing table, the command uses the normal routing table to forward the packet.

it´s D

the language of the quiz not clear .. I think ..

If a packet's destination IP address isn't in the routing table but meets the criteria of the PBR route-map with the set ip default next-hop command, the router sends the packet to the next hop specified by that command. If the destination IP address is already in the routing table, the router ignores the set ip default next-hop command and forwards the packet according to the routing table's existing entry.

ChatGPT:When a packet matches a policy-based routing (PBR) rule configured with the "set ip default next-hop" command for a destination IP address that does not exist in the routing table, the packet will be forwarded according to the specified next-hop IP address. In this scenario, since the destination IP address does not exist in the routing table, the router would normally use the default route (if configured) to forward the packet. However, with PBR configured and the "set ip default next-hop" command applied to the packet, the router will ignore the routing table lookup for this packet and forward it directly to the next-hop IP address specified in the PBR rule. In essence, PBR allows you to override the normal routing behavior based on criteria other than the destination address, such as source address, packet size, or protocol type, and forward packets according to policies configured by the network administrator.

A is correct

when the destination route is not in the routing table, the packet is policy routed (to the specified next hop) https://www.cisco.com/c/en/us/support/docs/ip/ip-routed-protocols/47121-pbr-cmds-ce.html#anc12

At first glance D would seem to be correct but if you think about it, to get to a path it must be known and remember there is the RIB and FIB. So if it is not known then it is not in the RIB and the default action is drop or send to the Default Gateway

In my opinion is "B". This document provides a sample configuration for policy-based routing (PBR) with the set ip default next-hop and set ip next-hop commands. The set ip default next-hop command verifies the existence of the destination IP address in the routing table, and: if the destination IP address exists, the command does not policy route the packet, but forwards the packet based on the routing table. if the destination IP address does not exist, the command policy routes the packet and sends it to the specified next hop. The set ip next-hop command verifies the existence of the next hop specified, and: if the next hop exists in the routing table, then the command policy routes the packet to the next hop. if the next hop does not exist in the routing table, the command uses the normal routing table to forward the packet. Source: https://www.cisco.com/c/en/us/support/docs/ip/ip-routed-protocols/47121-pbr-cmds-ce.html

ChatGPT: Yes, if the router has been configured with a policy-based routing (PBR) rule using the "set ip default next-hop" command and a packet arrives at the router with a destination IP address that is not present in the router's routing information base (RIB), the router will forward the packet to the next-hop address specified in the PBR rule. This is because PBR allows the router to apply forwarding policies that are independent of the routing table lookup process. In other words, the router will use the PBR policy to determine where to forward the packet, regardless of whether the destination IP address is present in the RIB or not. However, it's important to note that forwarding packets using PBR rules that reference non-existent destinations can result in unexpected behavior and can lead to packet loss if the next-hop address specified in the PBR rule is not reachable. It's generally recommended to ensure that all destination IP addresses referenced in PBR rules are present in the RIB to avoid any unexpected packet drops.

D is corrected, with "set ip default next-hop x.x.x.x" if there is no specific route on the routing table, it will use PBR.

D. The set ip default next-hop command verifies the existence of the destination IP address in the routing table, and… if the destination IP address exists, the command does not policy route the packet, but forwards the packet based on the routing table. if the destination IP address does not exist, the command policy routes the packet by sending it to the specified next hop. The set ip next-hop command verifies the existence of the next hop specified, and… if the next hop exists in the routing table, then the command policy routes the packet to the next hop. if the next hop does not exist in the routing table, the command uses the normal routing table to forward the packet.

d is the ans, policy base routing overrides the routing table

I Tested it on GNS3 and the packet was forwarded to the specific next hop, and the route wasn't in the routing table and not default gateway for the network

The destination IP/Subnet is not in the routing table, NOT the next hop IP address.

The correct answer is: D

D is correct