>Many routers deprioritize or drop ICMP to save CPU.
Not exactly.
Most big routers have ASICs (custom silicon) that can handle the bulk of routing decisions, like an interface card will have a chip that can directly determine where a packet needs to go and forwards it there. These are extremely fast, but limited, and are called "fast path".
Aside: Too many ACLs is a common way that packets fall off the fast path, and is why routers on the public Internet will happily forward along bogon traffic that by it's very nature is just wasting bits on the pipes.
There are some things that the fast path cannot handle, and generating ICMP TTL exceeded messages is one of them. Those go over to the router CPU, which historically has been insanely underpowered. Back when I was doing more routing it was common to have host CPUs in the multi-GHz range with multiple cores, but routers of a similar class would have a 100MHz MIPS CPU.
That's why, as the article goes on to explain, "*"s in the traceroute may not indicate a problem. It's not necessarily a literal deprioritization of ICMP.
If you ever see packet loss in a trace at one step but the steps after it aren't showing it, you can ignore that packet loss, it's likely a CPU limitation on a busy router.
linsomniac•16m ago
Funny timing of this, just last night I experimented with AI building an "ip intelligence" tool that includes a fast traceroute function and a bunch of IP information (ASN, organization, geolocation, CIDR info, DNSBL, reverse IP... mtr is my go-to for network traces, but (given root permissions) the trace it'll run in less than a second.
linsomniac•23m ago
Not exactly.
Most big routers have ASICs (custom silicon) that can handle the bulk of routing decisions, like an interface card will have a chip that can directly determine where a packet needs to go and forwards it there. These are extremely fast, but limited, and are called "fast path".
Aside: Too many ACLs is a common way that packets fall off the fast path, and is why routers on the public Internet will happily forward along bogon traffic that by it's very nature is just wasting bits on the pipes.
There are some things that the fast path cannot handle, and generating ICMP TTL exceeded messages is one of them. Those go over to the router CPU, which historically has been insanely underpowered. Back when I was doing more routing it was common to have host CPUs in the multi-GHz range with multiple cores, but routers of a similar class would have a 100MHz MIPS CPU.
That's why, as the article goes on to explain, "*"s in the traceroute may not indicate a problem. It's not necessarily a literal deprioritization of ICMP.
If you ever see packet loss in a trace at one step but the steps after it aren't showing it, you can ignore that packet loss, it's likely a CPU limitation on a busy router.