EIGRP SIA condition occurs when queries are not responded to within the 3 minute default timer. EIGRP queries use a reliable multicast communication, so a reply is necessary.
Here is an example of a topology where R1 begins a query process which cascades through the network. R3 does not receive a reply from R5 within 3 minutes:
When the timer expires, R3 goes SIA and resets the neighbor relationship with R5, which removes all routes learned through that neighbor until routing information is exchanged between the two routers again in a formal update process.
The error message generated is EIGRP DUAL-3-SIA .
There are three common reasons for SIA behavior:
- High CPU or Memory issues may prohibit a device from actively processing a reply. Software caveats have been known to cause this issue too.
- A bad link between two routers, exhibiting errors which cause reply packets to get lost.
- A unidirectional link or cable that is functioning suboptimally, resulting in lost query or reply packets.
There is a mechanism to solve the problem of EIGRP neighbor SIA resets called Active Process Enhancement. It has the following characteristics.
- Monitors the search for a successor
- Adds new SIA-Query and SIA-Reply messages
- Reduces the need for a neighbor reset
The SIA-Query packet is sent at half the normal Active timer, by default, at 1.5 minutes. Consider the following diagram, where the following communications would take place.
- At 1.5 minutes (half of the active timer), R1 sends SIA-Query to R2.
- R2 responds with SIA-Reply, indicating it is still searching. The neighborship is not reset between R1 and R2, and R1 notes the health of R2.
- R2 sends up to three SIA-Queries to R3. If they go unanswered, R2 will terminate the neighbor relationship with R3, then update R1 with an SIA-Reply, telling it to remove the destination prefix.
- R1 and R2 both remove the route from their tables.
Stuck-in-Active: Journal of an IT-Network Administrator 