diff --git a/specs/altair/p2p-interface.md b/specs/altair/p2p-interface.md
index ccc31a280..8de85188d 100644
--- a/specs/altair/p2p-interface.md
+++ b/specs/altair/p2p-interface.md
@@ -85,6 +85,12 @@ The `message-id` MUST be the following 20 byte value computed from the message:
   the topic byte string, and the raw message data:
   i.e. `SHA256(MESSAGE_DOMAIN_INVALID_SNAPPY + uint_to_bytes(uint64(len(message.topic))) + message.topic + message.data)[:20]`.
 
+Implementations may need to carefully handle the function that computes the `message-id`. In particular, messages on topics with the Phase 0
+fork digest should use the `message-id` procedure specified in the Phase 0 document.
+Messages on topics with the Altair fork digest should use the `message-id` procedure defined here.
+If an implementation only supports a single `message-id` function, it can define a switch inline;
+for example, `if topic in phase0_topics: return phase0_msg_id_fn(message) else return altair_msg_id_fn(message)`.
+
 The new topics along with the type of the `data` field of a gossipsub message are given in this table:
 
 | Name | Message Type |