Iso is a read-only Counter32 giving the number of International Standards Organization (ISO) protocol frames counted for this MAC-to-MAC conversation; per the MIB a value of 4294967294 means the count is unknown rather than genuinely zero. Because it is broken out on its own instead of folded into a generic IP total, an admin can watch this counter to see how much of the MAC-to-MAC conversation's traffic is legacy OSI-stack traffic, which is unusual to see on a modern IP network and worth investigating if it appears. If this Iso counter climbs sharply between 00:1A:2B:3C:4D:01 and 00:1A:2B:3C:4D:02 while total traffic stays flat, that shift shows legacy OSI-stack traffic, which is unusual to see on a modern IP network and worth investigating if it appears is displacing whatever other protocol mix was there before.