Discard aggregate holes
04An aggregate may remain advertised while one of its component routes is unavailable. If the aggregating router sends that traffic back toward its upstream default, the packet can loop.
A new packet for .20 matches the still-reachable .0/26 component and forwards. One unreachable portion does not imply the entire aggregate is unreachable, nor that holes should loop.
Source for this drawing: RFC 4632 · Read RFC section 5.2 ↗
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At an aggregator, a matching aggregate with no more-specific route leads to discard, not an upstream default.
- §5.1 Rule 2 says a router generating an aggregate must discard packets that match it but match none of its more-specific routes. A null next hop for the aggregate implements the rule. This is the source’s lowercase “must”, not an invented RFC 2119 keyword convention.
- An upstream router can select a provider’s /24. Inside the provider, a matching reachable /26 component takes precedence over the /24 discard route by longest-prefix match. The aggregate does not discard traffic that has a valid more-specific forwarding route.
- If that /26 component disappears, the /24 discard route becomes the longest match at the aggregator. Do not use its shorter /0 to send the packet back upstream. §5.2 explains how this prevents a loop between the upstream and the aggregator.
- A different component can remain reachable, so the aggregate may stay advertised (§5.4). Discarding one missing component is consistent with retaining the aggregate. Do not read the aggregate as an acknowledgment from every destination.
- This drawing does not mandate a particular ICMP reply or routing timer. It isolates the aggregate-hole forwarding decision; learning failures, withdrawal timing and reachability tests belong to the routing system and local policy.
Example provider aggregate 192.0.2.0/24, component .64/26, packet destination .70. No real traffic or ICMP behavior claimed.
Read the RFC section ↗