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What is SIA (Stuck in Active) in EIGRP?
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Answer Posted / dins
The SIA state means that an EIGRP router has not received a reply to a query from one or more neighbors within the time allotted (approximately 3 minutes). When this happens, EIGRP clears the neighbors that did not send a reply and logs a DUAL-3-SIA error message for the route that went active.
Consider the following topology as an example:
10.1.2.0/24-----R1----R2-----R3
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R5 R4
R2 learns about network 10.1.2.0/24 via R1.
The link between R1 and R2 goes down. R2 looses its successor (R1) for 10.1.2.0/24.
R2 checks the EIGRP topology table for a feasible successor (another neighbor with a route to 10.1.2.0/24 that meets the feasibility condition); it has none.
R2 transitions from passive to active for 10.1.2.0/24.
R2 sends queries to R3 and R5, asking if they have another path to 10.1.2.0/24. The SIA timer starts.
R5 checks the EIGRP topology table for a feasible successor; it has none.
R5 transitions from passive to active for 10.1.2.0/24.
R5 checks its EIGRP neighbor table and only finds EIGRP neighbors out the interface facing R2 (its former successor for 10.1.2.0/24).
R5 replies with an unreachable message because it has no alternative path and has no other neighbors to query.
R5 transitions from active to passive for 10.1.2.0/24.
R3 checks the EIGRP topology table for a feasible successor; it has none.
R3 transitions from passive to active for 10.1.2.0/24.
R3 checks its EIGRP neighbor table and finds R4.
R3 sends a query to R4 for network 10.1.2.0/24. The SIA timer starts.
R4 never receives the query either due to problems with the link between R3 and R4 or congestion. You can see this problem by issuing either the show ip eigrp neighbor command or the show ip eigrp topology active command on R3; the queue count for R4 should be higher than usual.
The SIA timer on R2 reaches approximately 3 minutes.
R3 can not reply to R2’s query until it hears a reply from R4.
R2 logs a DUAL-3-SIA error for network 10.1.2.0/24 and clears the neighbor adjacency with R3.
DEC 20 12:12:06: %DUAL-5-NBRCHANGE: IP-EIGRP 1:
Neighbor 10.1.4.3 (Serial0) is down: stuck in active
DEC 20 12:15:23: %DUAL-3-SIA:
Route 10.1.2.0/24 stuck-in-active state in IP-EIGRP 1.
Cleaning up
R3’s retry timer for R4 expires.
Note: This event prevents R3 from also reporting a DUAL-3-SIA error because R3’s SIA timer may also be about to reach 3 minutes.
R3 clears its neighbor adjacency with R4.
R3 reports the following error to its log:
DEC 20 12:12:01: %DUAL-5-NBRCHANGE: IP-EIGRP 1:
Neighbor 10.1.5.4 (Serial1) is down: retry limit exceeded
R3 now replies to R2’s query with an unreachable message.
R4 reports the following error to its log:
DEC 20 12:12:06: %DUAL-5-NBRCHANGE: IP-EIGRP 1:
Neighbor 10.1.5.3 (Serial0) is down: peer restarted
Note: The DUAL-5-NBRCHANGE messages will only be displayed if you have configured the eigrp log-neighbor-changes command under the EIGRP process. Configuring this command on all EIGRP routers is recommended for troubleshooting EIGRP SIA problems. Without it, there is no way to tell why EIGRP neighbors are being reset or which router reset the adjacency.
As you can see above, the DUAL-3-SIA error is caused by the following concurrent, yet unrelated, problems:
An interface problem between R1 and R2, which causes the 10.1.2.0/24 route to disappear from R2. The route flap may have been caused by something other than an actual link failure (for instance, a remote user disconnected and the PPP-derived host route is then removed).
An interface, congestion, or delay problem between R3 and R4.
When the SIA error message occurs, it indicates that the EIGRP routing protocol failed to converge for the specified route. Usually, this failure is caused by a flapping interface, a configuration change, or dialup clients (the route loss is normal). The routing to other destinations is not affected while the EIGRP process is in active state for the specified route. When the SIA timer for the neighbor that did not reply expires, the neighbor is cleared (EIGRP does not trust the state of a neighbor that exceeds the timer). As a consequence, routes in the topology table beyond that neighbor are cleared and must then re-converge. This means that the forwarding table can be effected by an SIA, and that packets can be dropped while the network is converging.
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