Which Router Needs IP Directed Broadcast for Wake-on-LAN?

Design multicampus Layer 3 infrastructures Design campus Layer 2 infrastructures

Answer Correct answer: D — enable IP directed-broadcast on R2, the last-hop router facing the sleeping PCs, so the Wake-on-LAN magic packet is broadcast in that VLAN.

[sampleXML/xmlfile-138_1.jpg] Refer to the exhibit. An architect is designing a network for a customer supporting a Wake-on-LAN application. Which solution must the architect choose?

IP directed-broadcasts on R1
spanning-tree uplinkfast on SW1
spanning-tree uplinkfast on SW2
IP directed-broadcasts on R2 Correct Answer

Community Insight

The exam tests knowledge that an inter-subnet Wake-on-LAN design requires ip directed-broadcast on the last-hop router facing the target VLAN; the trap is enabling it on the source-side router (R1) or reaching for a Layer 2 STP feature (uplinkfast), which does nothing for broadcast delivery.

Wake-on-LAN magic packets originate as UDP traffic from an application server and must be delivered as a Layer 2 broadcast to powered-down PCs on a different subnet. Because a directed broadcast is only converted to a real broadcast on the final hop, the architect must enable ip directed-broadcast on R2 — the router directly connected to the target PC LAN — not on R1 and not with spanning-tree uplinkfast.

Option A (IP directed-broadcasts on R1) is the most tempting wrong choice: learners quote the textbook definition of a directed broadcast without realizing the command only takes effect on the router whose interface is directly connected to the destination subnet — in this exhibit, R2.

Community Discussion (13 comments)

e14efdb 👍 1 Selected: D

For inter-subnet Wake-on-LAN, enable IP directed-broadcast on the last-hop router toward the target VLAN. In this topology, that’s R2.

NoHombre 👍 1 Selected: D

For inter-subnet Wake-on-LAN, enable IP directed-broadcast on the last-hop router toward the target VLAN. In this topology, that’s R2. On R1 there needs to be some kind of IP helper / UDP forwarder

ajinkya_gooner 👍 1

I got similar question as MCQ. Where should be ip directed brodcast enabled and ip helper address enabled.

mgiuseppe86 👍 1 Selected: D

Scenario, SCCM uses WoL against machines that are turned off to patch them. This is your application server. The direction in this case is very similar. So i am going with D. The workstations need to be woken.

J2J2J2J 👍 1 Selected: A

A device that is not directly connected to its destination subnet forwards an IP directed broadcast in the same way it would forward unicast IP packets destined to a host on that subnet. When a directed broadcast packet reaches a device that is directly connected to its destination subnet, that packet is broadcast on the destination subnet. The destination address in the IP header of the packet is rewritten to the configured IP broadcast address for the subnet, and the packet is sent as a link-layer broadcast.

jahax 👍 2

"supporting a Wake-on-LAN application". WoL is mostly for endpoints, and we have application server on drawing. It seems to be that we have applicaiton server that hosts WoL App and awake devices in LAN > thats why we need directed broadcast on R2 (D)

dgonzalezexamtopics 👍 4

From my point of view, it depends on which has to be the awake device/s. If Server, then should be A, because of being R1 on same subnet and then can redirect the IP broadcast directly. If are the PCs, as Reinier says, then should be D. I think that could make more sense to wake up the server, but with Cisco...who knows.

Emily23 👍 1

Someone who thinks when posting. Finally. I agree to above. Would make sense to be A.

Eards 👍 3 Selected: D

Looking this up IP helper addresses are configured on R1 and Directed broadcasts on R2. So I think D is correct

Eards 👍 1

https://community.cisco.com/t5/routing/wake-on-lan-across-a-corporate-environment/td-p/2096194

Reinier_veen 👍 3 Selected: D

As I understand "IP directed broadcast" must be supported on the last router to the destination subnet. Since the sleeping PC's dont have IP adresses, the machines must be calles awake by broadcast that behaves like an unicast untill they reach the destination network. There the directed broadcast is handled like a proper broadcast to wake all WOL machines. https://www.cisco.com/c/en/us/support/docs/switches/catalyst-3750-series-switches/91672-catl3-wol-vlans.html

johnu329 👍 2 Selected: A

It's A. "An IP directed broadcast is an IP packet whose destination address is a valid broadcast address for some IP subnet but which originates from a node that is not itself part of that destination subnet." (https://content.cisco.com/chapter.sjs?uri=/searchable/chapter/content/en/us/td/docs/routers/sdwan/configuration/System-Interface/systems-interfaces-book/ip-directed-broadcast.html.xml)

Hope66 👍 1

I think that ip direct-broadcast should be configured on R1 interface Fa0/0 Answer is A

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Expert Analysis

Why the Answer Is Correct

A directed broadcast is forwarded hop-by-hop exactly like a unicast until it reaches the router directly connected to the destination subnet; only that last router rewrites it into a true Layer 2 broadcast for the VLAN. Wake-on-LAN targets powered-down PCs that hold no IP session, so the magic packet must land as a broadcast in the PC LAN, which sits behind R2 in this exhibit. Therefore ip directed-broadcast must be enabled on R2's interface facing the target subnet, otherwise R2 silently discards the packet — Cisco routers drop directed broadcasts by default for security (smurf) reasons. As Reinier noted, "IP directed broadcast" must be supported on the last router to the destination subnet.

Why the Other Options Are Wrong

Option A fails because R1 is the source-side router (the WoL application server's router, where an IP helper/UDP forwarder would live); enabling directed broadcast there has no effect on delivery into the remote PC subnet. Options B and C (spanning-tree uplinkfast) address rapid STP reconvergence when an uplink fails — they change nothing about how a broadcast frame is generated or routed, and sleeping PCs generate no STP events. Uplinkfast is in any case a per-VLAN, non-default legacy feature irrelevant to magic-packet delivery. As e14efdb put it, "For inter-subnet Wake-on-LAN, enable IP directed-broadcast on the last-hop router toward the target VLAN."

Community Comment Notes

The comments converge on D, with the reasoning that the workstations, not the server, are the devices being awakened. Reinier and jahax both explain that the sleeping PCs on the R2 LAN are the WoL targets, so the broadcast must terminate there. Eards adds the complementary point that "IP helper addresses are configured on R1 and Directed broadcasts on R2." A minority (johnu329, J2J2J2J) argue for A by quoting the generic definition of a directed broadcast, and dgonzalezexam-topics notes it would only be A if the server were the device to wake — but a powered-down server would not be hosting the WoL application, which makes R2/D the only internally consistent design.

Official Reference

https://content.cisco.com/chapter.sjs?uri=/searchable/chapter/content/en/us/td/docs/routers/sdwan/configuration/System-Interface/systems-interfaces-book/ip-directed-broadcast.html

Exam Strategy

For any broadcast-delivery question (Wake-on-LAN, DHCP relay, NetBIOS), identify the router whose interface is directly connected to the target subnet — that is where ip directed-broadcast goes; the source side gets the ip helper-address instead. Then verify that no Layer 2 STP tuning option (uplinkfast, backbonefast, PortFast) actually delivers a routed broadcast, and eliminate it.

Frequently Asked Questions

Why isn't IP directed-broadcast on R1 correct for this Wake-on-LAN design?

R1 is the source-side router for the WoL application server; a directed broadcast is only converted into a real broadcast by the router directly connected to the destination subnet, so R2 is the router that must permit it.

Does spanning-tree uplinkfast help Wake-on-LAN packet delivery?

No. Uplinkfast only speeds up STP reconvergence after an uplink failure; it has no effect on how a routed broadcast reaches powered-down PCs in a remote VLAN.

What else is typically required alongside ip directed-broadcast for Wake-on-LAN?

The source-side router (R1) normally needs an IP helper-address or UDP forwarder to relay the WoL UDP port toward the target network, and R2 must explicitly permit the directed broadcast on its target-facing interface.

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