PART 11: EIGRP Query Process

Enhanced Interior Gateway Routing Protocol

The EIGRP query process is a crucial mechanism for handling route failures and network convergence. When an EIGRP router loses a route, it initiates a structured process to either find an alternative path or confirm the route is truly unavailable.

The Query Process Workflow

When an EIGRP router loses a route (typically when the next-hop becomes unreachable), it follows these steps:

1. Active State Initiation:

   • The router marks the route as "Active" in its topology table

   • It sends Query messages (Type 3) to all EIGRP neighbors except through the interface where the route was lost

   • The Active Timer starts (default: 3 minutes)
     

2. Downstream Router Response Options:

   • If the downstream router has never learned about this route: It immediately sends a Reply (Type 4 EIGRP Packet) indicating it has no information

   • If the downstream router has the route in its routing table: It sends a Reply with itself as the next hop

   • If the downstream router previously knew the route but it is lost now: It queries its own neighbors and defers replying until it receives all responses
     

3. Reply Collection:

   • The originating router must collect replies from all neighbors it queried

   • Only after receiving all replies, it calculate a new successor or declare the route unreachable

   • All Reply packets must be acknowledged
     

     

Stuck-in-Active (SIA) Condition

The SIA condition occurs when a router fails to receive replies to all its queries within the Active Timer period. This creates a potentially disruptive situation.

Legacy Behavior (IOS 12.x)

In older IOS versions, the SIA handling was rather severe:

• The router would mark the unresponsive neighbor as "Stuck in Active"

• It would reset the neighbor relationship

• This triggered additional route recalculations for all routes learned from that neighbor

• The neighbor relationship would be re-established upon receiving the next Hello packet
  

Improved Behavior (IOS 15.x)

Newer IOS versions implement a more graceful approach:

• After half the Active Timer expires (1.5 minutes by default), the router sends an SIA-Query

• This gives the downstream router a chance to respond with an SIA-Reply

• The SIA-Reply indicates the downstream router is still processing the original query

• This prevents unnecessary neighbor relationship resets
  

Query Process Characteristics

Important technical aspects of the query process include:

• Query packets are sent as multicast on shared media like Ethernet

• All Query packets must be acknowledged

• If an acknowledgment isn't received, the sender retransmits as unicast

• Reply packets also require acknowledgment

• The querying router must collect all replies before calculating new paths
  

Optimizing Convergence

Several techniques can improve EIGRP convergence and minimize SIA conditions:

1. EIGRP Stub Routers:

   • Configure leaf/branch routers as stubs

   • Stub routers will not be queried for routes they don't have

   • This significantly reduces query scope in hub-and-spoke topologies

2. Route Summarization:

   • Implementing summarization at strategic points limits query propagation

   • If a component route fails, queries only propagate within the summarization domain

3. Adjusting Active Timer:

   • The Active Timer can be reduced (commonly to 1 minute) in stable networks:

# Numbered mode
Router(config-router)# timers active-time 60

# Named mode
Router(config-router-af-topology)# timers active-time 60

---