EIGRP Convergence Flashcards
Hello packet
A secondary function for the EIGRP hello packets is to ensure that EIGRP neighbors are still healthy and available.
Hello and Hold Timers
EIGRP hello packets are sent out in intervals determined by the hello timer.
The default EIGRP hello timer is 5 seconds, but it is 60 seconds on slow-speed interfaces (T1 or lower).
The hold time value defaults to 3 times the hello interval. The default value is 15 seconds, and it is 180 seconds for slow-speed interfaces.
The hold time decrements, and upon receipt of a hello packet, the hold time resets and restarts the countdown.
If the hold time reaches 0, EIGRP declares the neighbor unreachable and notifies DUAL of a topology change.
Convergence
When a link fails, and the interface protocol moves to a down state, any neighbor attached to that interface moves to a down state, too. When an EIGRP neighbor moves to a down state, path recomputation must occur for any prefix where that EIGRP neighbor was a successor (upstream router).
When EIGRP detects that it has lost its successor for a path, the feasible successor instantly becomes the successor route, providing a backup route.
The router sends out an update packet for that path because of the new EIGRP path metrics. Downstream routers run their own DUAL for any impacted prefixes to account for the new EIGRP metrics.
Convergence
Topology Link Failure Example
Link between R1 and R3 fails.
R3 installs the feasible successor path advertised from R2 as the successor route.
R3 sends an update packet with a new RD of 19 for the 10.1.1.0/24 prefix.
R5 receives the update packet from R3 and calculates an FD of 29 for the R1–R2–R3 path to 10.1.1.0/24.
R5 compares that path to the one received from R4, which has a path metric of 25.
R5 chooses the path via R4 as the successor route.
No feasible successor available
If a feasible successor is not available for a prefix, DUAL must perform a new route calculation. The route state changes from passive (P) to active (A) in the EIGRP topology table.
The router detecting the topology change sends out query packets to EIGRP neighbors for the route. The query packet includes the network prefix with the delay set to infinity so that other routers are aware that it has gone active.
What does the router upon receipt of a query packet?
It might reply to the query that the router does not have a route to the prefix.
If the query did not come from the successor for that route, it detects the delay set for infinity but ignores it because it did not come from the successor. The receiving router replies with the EIGRP attributes for that route.
If the query came from the successor for the route, the receiving router detects the delay set for infinity, sets the prefix as active in the EIGRP topology, and sends out a query packet to all downstream EIGRP neighbors for that route.
The query process continues from router to router until a router establishes the query boundary.
How a query boundary is established?
A query boundary is established when a router does not mark the prefix as active, meaning that it responds to a query as follows:
- It says it does not have a route to the prefix.
- It replies with EIGRP attributes because the query did not come from the successor.
When a router receives a reply for every downstream query that was sent out, it completes the DUAL, changes the route to passive, and sends a reply packet to any upstream routers that sent a query packet to it.
Query Process
Convergence Topology Example
R2’s perspective for calculating a new route to the 10.1.1.0/24 network:
- R2 detects the link failure. R2 does not have a feasible successor for the route, sets the 10.1.1.0/24 prefix as active, and sends queries to R3 and R4.
- R3 receives the query from R2, processes the Delay field that is set to infinity. R3 does not have any other EIGRP neighbors, sends a reply to R2 saying that a route does not exists.
- R4 receives the query from R2, processes the Delay field that is set to infinity. Because the query was received by the successor, and a feasible successor for the prefix does not exist, R4 marks the route as active and sends a query to R5.
- R5 receives the query from R4 and detects that the Delay field is set to infinity. Because the query was received by a nonsuccessor and a successor exists on a different interface, a reply for the 10.4.4.0/24 network is sent back to R2 with the appropriate EIGRP attributes.
- R4 and R2 receives the replies, acknowledges the packet and compute a new path. Both routers set the prefix as passive.
What is Stuck in Active?
If a successor path is lost and no feasible successor is identified, the router goes active for that route. It sends out queries on all interfaces in an attempt to identify an alternative path. No successor can be chosen until the router receives a reply to all queries. If a neighbor does not reply within 3 minutes, the router becomes stuck in active (SIA). In that case, it resets the neighbor relationship with the neighbor that did not reply.
Newer versions of EIGRP use SIA-Queries and SIA-Replies to prevent the loss of a neighbor unnecessarily during SIA conditions.
A router sends its neighbor an SIA-Query after no reply to a normal query. If the neighbor responds with an SIA-Reply, the router does not terminate the neighbor relationship after 3 minutes because it knows the neighbor is available, just not able to answer the query.
Stuck in Active Process
DUAL is very efficient at finding loop-free paths quickly, and it normally finds backup paths in seconds.
Occasionally, an EIGRP query is delayed because of packet loss, slow neighbors, or a large hop count.
EIGRP maintains a timer, known as the active timer, which has a default value of 3 minutes (180 seconds). EIGRP waits half of the active timer value (90 seconds) for a reply.
If the router does not receive a response within 90 seconds, the originating router sends a stuck in active (SIA) query to EIGRP neighbors that did not respond.Upon receipt of an SIA query, the router should respond within 90 seconds with an SIA reply.
An SIA reply contains the route information or provides information on the query process itself.
If a router fails to respond to an SIA query by the time the active timer expires, EIGRP deems the router SIA. If the SIA state is declared for a neighbor, DUAL deletes all routes from that neighbor, treating the situation as if the neighbor responded with unreachable message for all routes.
Common causes for SIA routes
- CPU or memory usage is so high on the neighbor that it cannot process the query or reply.
- The link between the routers drops packets. Enough packets get through to maintain the neighbor relationship, but some queries or replies are dropped.
- Unidirectional link, so the router never receives packets from its neighbor.
Outputs for SIA
The active timer is set to 3 minutes by default. The active timer can be disabled or modified with the command timers active-time {disabled | 1-65535-minutes} under the EIGRP process.
