Stuck in Active (SIA) in EIGRP is a state a route enters when a router fails to receive replies to all outstanding queries for that route from its neighbors within a specific timeframe.
EIGRP is known for its reliable transport protocol, which requires acknowledgment for certain packet types, including queries and replies used in its Diffusing Update Algorithm (DUAL) for route computation. When a router loses a route or needs to find a new successor for a route that is in the "active" state (meaning it's currently searching for a new path), it sends queries to its neighbors. These neighbors are expected to reply, either confirming they don't have a path or providing their best path.
According to the provided reference:
EIGRP is a reliable protocol and for each query, a router sends to its neighbors, it must get a reply in response within 3 minutes. If the router does not receive a reply to ALL its outstanding queries it will put the route in SIA (Stuck in Active) state and will kill the neighbor adjacency.
This 3-minute timer is crucial. If the querying router does not receive a reply from every neighbor it sent a query to within this time limit, the route transitions from the 'Active' state to the 'Stuck in Active' (SIA) state.
Understanding the SIA State
The SIA state indicates a problem in the EIGRP network's ability to converge on a stable route. It typically points to issues preventing replies from reaching the querying router.
Why SIA Occurs
Several reasons can lead to a route becoming Stuck in Active:
- Packet Loss: Queries or replies are dropped somewhere along the path between neighbors.
- Congestion: Network congestion can cause replies to be significantly delayed, exceeding the 3-minute timer.
- Neighbor Router Issues: The neighbor router might be too busy, experiencing high CPU utilization, or encountering internal DUAL processing delays preventing it from generating or sending the reply in time.
- One-Way Communication: A neighbor might receive the query but be unable to send a reply back due to a one-way link or routing issue.
- Insufficient Resources: The neighbor router may have insufficient memory or CPU to handle the queries, especially in large or unstable networks.
Consequences of SIA
As highlighted in the reference, the most severe consequence of a route entering the SIA state is:
- Neighbor Adjacency Loss: The router that declared the route SIA will immediately terminate the EIGRP neighbor adjacency with the neighbor(s) from which it did not receive a timely reply.
- Route Instability: The route causing the SIA is removed from the routing table, potentially causing routing black holes or suboptimal path selection.
- Network-Wide Instability: The loss of neighbor adjacencies can trigger further queries and recalculations, potentially causing cascading failures or widespread network instability until the underlying issue is resolved.
Diagnosing and Resolving SIA
Diagnosing SIA often involves identifying which routes are SIA and which neighbors are failing to reply.
- Checking SIA Routes: Use the
show ip eigrp topology activecommand. Routes in the SIA state will often be marked as "SIA". - Checking Missing Replies: The output of
show ip eigrp topology activeoften indicates which neighbors are missing replies for a specific active route. - Troubleshooting Connectivity: Verify Layer 3 reachability and Layer 2 stability between the affected neighbors.
- Checking Neighbor Health: Examine the CPU and memory utilization of the neighbor router(s) not sending replies.
- Analyzing Packet Flow: Use packet capture tools to see if queries are being sent and if replies are being received (or sent by the neighbor).
| Problem Indicator | Potential Cause(s) | Action |
|---|---|---|
show ip eigrp topology shows route as "SIA" |
Packet loss, Congestion, Busy Neighbor, One-way comm | Identify neighbor, check connectivity & neighbor resources |
show ip eigrp topology active shows missing replies from a neighbor |
Neighbor unable to process query or send reply | Troubleshoot neighbor router (CPU, memory, DUAL process) |
| Repeated SIA issues | Network instability, resource exhaustion, design issue | Analyze network design, traffic patterns, increase timers (caution advised) |
Resolving SIA requires fixing the underlying issue preventing the queries or replies from being successfully exchanged within the timer. This could involve resolving congestion, fixing connectivity problems, or addressing performance issues on the neighbor router. In some rare cases, increasing the EIGRP active timer might be considered, but this only masks the problem and delays detection, which is generally not recommended without addressing the root cause.
In summary, Stuck in Active is a critical state in EIGRP that signifies a failure in the reliable delivery of query replies within the allotted time, leading to route removal and the termination of neighbor adjacencies.
