156-915.71 · Question #110
In a R70 ClusterXL Load Sharing configuration, which type of ARP related problem sometimes forces the use of Unicast Mode (Pivot) configuration due to incompatibility on some adjacent routers and…
The correct answer is D. Multicast MAC address response to a Unicast IP request. See the full explanation below for the reasoning.
Question
In a R70 ClusterXL Load Sharing configuration, which type of ARP related problem sometimes forces the use of Unicast Mode (Pivot) configuration due to incompatibility on some adjacent routers and switches?
Options
- AMulticast MAC address response to a RARP request
- BMGCP MAC address response to a Multicast IP request
- CUnicast MAC address response to a Multicast IP request
- DMulticast MAC address response to a Unicast IP request
How the community answered
(25 responses)- A4% (1)
- B16% (4)
- C4% (1)
- D76% (19)
Community Discussion
8
Imani T.Jun 26, 2026The answer is D, Multicast MAC address response to a Unicast IP request. In ClusterXL Load Sharing Multicast mode, all cluster members share a single virtual IP, and when a neighboring router or switch ARPs for that unicast IP, the cluster responds with a multicast MAC address. That is where the incompatibility hits, because a lot of routers and switches are built to expect a unicast MAC back when they ARP for a unicast IP, and they will either reject the entry or refuse to cache it at all. When you run into those devices, Unicast Pivot mode is the workaround because now one member handles the forwarding and the ARP behavior looks normal to the adjacent hardware. If you see a question on this exam asking why you would choose Unicast over Multicast for Load Sharing, adjacent device ARP incompatibility with multicast MACs is almost always the reason they are looking for.
Mateus R.Jun 29, 2026Solid explanation, and the one thing worth adding is that the ARP table entry itself is not always the failure point, since some devices will cache it fine but then drop traffic at the forwarding stage because their hardware ASIC simply will not forward a frame whose destination MAC sits in the multicast range.
Mateus R.Jun 26, 2026I almost went with C because I was thinking about the IP side of things being multicast, but then I remembered the real headache here is like asking "who owns this one specific house address?" and getting back a group mailbox key as the answer, which is exactly what happens when adjacent routers do an ARP for the cluster's unicast virtual IP and the cluster hands back a multicast MAC, and that mismatch is what breaks compatibility on picky routers and switches, making D the right pick.
Eitan E.Jun 29, 2026Solid reasoning, and the exam also likes to test the standard workaround for exactly that mismatch, which is adding a static ARP entry on the upstream router to manually map the cluster virtual IP to the multicast MAC, bypassing the router's refusal to accept the ARP reply on its own.
Hiroshi T.Jun 21, 2026I initially leaned toward C because the pairing of "Unicast MAC" and "Multicast IP" sounded like the kind of mismatch that would confuse a switch, but the Check Point R70 ClusterXL Administration Guide is specific that in Multicast Load Sharing mode the cluster replies to a standard Unicast ARP request with a Multicast MAC address, and certain routers and switches simply reject or mishandle that response because their ARP implementation does not allow a Multicast MAC to be mapped to a Unicast IP, which is exactly what forces the fallback to Unicast (Pivot) mode, making D the right pick.
Eitan E.Jun 26, 2026Almost bubbled C, then remembered ARP resolves unicast IPs, not multicast.
Anjali D.Jun 26, 2026Saw this exact question on my exam last year and I almost talked myself into C because "multicast IP" sounded more technically dramatic, but when I slowed down I remembered that in Load Sharing Multicast mode the cluster IP is still a regular unicast IP and the ARP reply maps it to a multicast MAC, and that pairing is what trips up certain Cisco routers and older switches that refuse to accept a multicast MAC in their ARP table for a unicast address. The answer is D, and once that clicked I never mixed it up again. Has anyone else seen adjacent router vendor matter here, or did your exam just leave it generic like mine did?
Imani T.Jun 29, 2026The Cisco specifically is the classic example but worth knowing that some older Juniper MX boxes in certain software revisions have the same grudge against that unicast/multicast pairing, so if your exam throws in a Juniper or "third party router" distractor, the same logic holds.