While P-CSCF is resolving a DNS request for I-CSCF, and Site 1 is down, how will the traffic be distributed?

Interview
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Q. While P-CSCF is resolving a DNS request for I-CSCF, and Site 1 is down, how will the traffic be distributed?

Received DNS priorities and weights:

  • Site 1: priority 10, weight 100
  • Site 2: priority 20, weight 50
  • Site 3: priority 20, weight 50
  • Site 4: priority 30, weight 100

If Site 1 is down, how will the traffic be distributed?

a) The traffic will go to Site 4
b) The traffic will be distributed equally on all sites
c) 100 percent of the traffic will go to Site 2
d) 50 percent of the traffic will go to Site 2 and other 50 percent will go to Site 3

Check 🔎

:arrow_right: Correct answer: Letter d.

In DNS-based service discovery (such as SIP/IMS), lower priority is preferred. Among records with the same priority, traffic is distributed based on weight.

After Site 1 (priority 10) is removed, the remaining sites are:

  • Site 2: priority 20, weight 50
  • Site 3: priority 20, weight 50
  • Site 4: priority 30, weight 100

The resolver will choose only among the lowest remaining priority, which is priority 20 (Site 2 and 3). Their weights are equal (50 each), so traffic is evenly distributed between them.

Q. While P-CSCF is resolving a DNS request for I-CSCF, and Site 1 is down, how will the traffic be distributed?

a) :x: The traffic will go to Site 4
→ Incorrect. Site 4 has higher priority (30) than Sites 2 and 3, so it won’t be selected unless all lower-priority sites are down.

b) :x: The traffic will be distributed equally on all sites
→ Incorrect. DNS prioritization restricts selection to the lowest available priority group (priority 20 in this case), not all sites.

c) :x: 100 percent of the traffic will go to Site 2
→ Incorrect. Site 2 and 3 have equal priority and weight, so traffic will be load balanced.

d) :white_check_mark: 50 percent of the traffic will go to Site 2 and other 50 percent will go to Site 3
→ Correct. With equal priority and weight, RFC 2782 dictates the load is split based on weight within the same priority group.