19 posts were merged into an existing topic: EIGRP Queries and Stuck in Active
If a route/neighbour fails, how can you recover from SIA? Recycle the interface facing the neighbour thatâs down?
Iâm also curious how you can simulate this in GNS3. Surely the main cause of SIA is congestion? How did you replicate the replys from R2 not reaching R1? Use an ACL?
Hello Chris
If a route or neighbour fails, and there is no feasible successor, an EIGRP enabled router will go from Passive to Active. This is normal behaviour. This is also the expected behaviour because it must attempt to find a new route to the destination. This does not mean that the router goes into SIA.
The router will only go into SIA if several things go wrong on the network as described in the lesson.
Now if you do end up in SIA, there is no âquick fixâ solution like the bouncing of an interface. You must look more deeply into the problem. What is the reason for SIA? It is usually due to congestion on the network, a malfunctioning link or a unidirectional link.
Another reason may be the use of an IOS before 12.1 that doesnât use the SIA query and reply packets (which reduce the probability of an SIA situation).
Another way to resolve SIA problems is to configure EIGRP summarization and/or the EIGRP stub feature. All of these must be implemented using proper troubleshooting procedures to evaluate the results of each change.
To simulate lost EIGRP packets in GNS3 you can use a traffic generator to create network congestion. There are several options for this, one of which you can find here. You can make your interfaces 10Mbps if they are Ethernet or you can create serial links and modify the data rate accordingly to make the data congestion reach 100% of bandwidth. When you do this however, make sure that you configure the ip bandwidth-percent eigrp command on the interface in question at a very low percentage such as 1 or 2 percent. This should have the result of losing some EIGRP packets.
Try it out and let us know your results!
I hope this has been helpful!
Laz
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Hello
Is this feature enable by default in EIGRP?
Yes it is 
Thanks for such a good brief .Can we have a lab where we can test it.
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as Router send SIA query to router after 1.5 Second but as link is of fiber and there is unidirectional link failure then SIA query of neighbor router will not reach towards query sender (i.e. Router)
then what will happen?
Hello Kunal
SIA queries and replies are used in order to minimize situations where neighbor adjacencies that are a result of lost replies to regular EIGRP queries. These lost replies are usually due to intermittent problems that may prevent some replies from reaching their destinations, such as high CPU, flapping links, network congestion etc. If you have a situation where there is a permanent unidirectional link failure, SIA will not resolve this issue. The SIA response will never reach the sender.
Take a look at the lesson below that further describes the EIGRP Queries and SIA:
I hope this has been helpful!
Laz
I am probably missing something, but wouldnât a router already know about every possible route to a destination? I guess a router could know about a route and not be advertising it for some reason?
Hello Justin
Iâm not quite sure to what your statement is referring, but just to clarify, EIGRP is the method by which routers learn about every possible route to the destination. If the routing protocol mechanism fails, then the learning of the routes also fails.
If this does not address your question fully, please clarify.
I hope this has been helpful!
Laz
Who Actually start the Query Timer, The original initiator or every router manage and count their timer itself.??
Once the route will get stuck in SIA then how that route will be removed from topology table ??
lets a route fails , and after how many sec the router will generate the query ??, because by default till 15 sec (hold time )the route will be there in topology table , so after hold down time expire it will generate the query or all of sudden it will generate the query ?.if all of sudden it sends the query then how it detects the failure ,is it because of incremental update ??
Hello Narad
Take a look at this post:
This happens by definition. If a route is stuck in active, the neighbor adjacency will be dropped. When you drop a neighbor adjacency, you essentially lose all routes you learned via that neighbor.
The hello and hold-down timers are used to maintain neighbor adjacencies, and these use different values. The SIA timer is used not only to resolve EIGRP failure but also to deal with the propagation delay of learning EIGRP routes in a large topology. Take a look at the following post for more information:
I hope this has been helpful!
Laz
What i understand is that âŚ
if within 180 sec the router will get the Query reply of the loosing route and even teh neighbor says that we dont know that route then the neighborship will not be broken âŚBut if the neighbor router will not respond the query within 3 min then the router will be called as SIA state and then the neighborship will be resetted âŚ
so conclusively , most often in production the router gets the reply bcz its a huge time as 3min âŚ
kindly correct me , if a neighbor router does not know about the loosing route and neighbor router replied to our query saying i donât have any alternate path to this loosing route then the neghborship will not be broken âŚrightâŚ!!!
Hello Narad
Under normal circumstances, a query will be responded to and there will be no loss of EIGRP neighbors. The value of 3 minutes is more than enough time for the EIGRP queries to reach the farthest EIGRP routers on the network and to get a response. But the primary problem is not simply due to a large network, but due to lost query replies. Remember, on a large network, many things can go wrong, including congestion, corruption of packets, and link failures. Query responses are not resent if they donât reach their destination, so any lost query response will result in a stuck in active.
If a query response is successfully received for all queries that a router sends out when a link fails, then yes, the neighbor adjacency will not be torn down.
I hope this has been helpful!
Laz
woot finally made it past the EIGRP stuff for my initial read and study anyway. Great lesson!
On to OSPF!
Hi Brian
Good job! Keep the momentum going, and weâll be here to answer any questions you have!
Laz
Hi,
Doesnât eigrp send hello messages periodically? Shouldnât it consider the hello messages and not drop the neighborhood even if there is no query response?
Hello Funda
Yes, EIGRP routers do send hello messages periodically. However, the reason for dropping the neighbor adjacency in this case is not because the neighbors lost connectivity or because of a lack of hellos, but because, through the process described in the lesson, the R1 router was put into a Stuck in Active (SIA) state. When this happens, the adjacency is dropped. Thatâs simply how EIGRP is designed.
The purpose of dropping the adjacency is to remove all the routes learned from that neighbor from the local router to get out of the SIA state. Once this is done, relearning the routes begins from scratch.
To help improve the operation of EIGRP and reduce the number of dropped adjacencies, Cisco introduced the use of SIA query and SIA reply, as described in the lesson.
I hope this has been helpful!
Laz
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Thanks Lazaros, its been helpful
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I would like to add the following information for reference. Letâs say R1 queries R2 for a missing router. R2 will only query its neighbors if it previously knew about the requested route.
This is very important, because it doesnât do so if e.g. R2 received a summary route from R1 and R1 queries for a specific route contained in this summary route. This gets important when we talk about EIRP traffic engineering etc.
I hope this helps.