Active Process Enhancement
Before
Router A resets relationship to router B when the normal active timer expires. However, the problem is the link between router B and C.
After
Router A sends an SIA-Query at half of the normal active timer. Router B acknowledges the query there by keeping the relationship up.
Router A resets relationship to router B when the normal active timer expires. However, the problem is the link between router B and C.
Router A sends an SIA-Query at half of the normal active timer. Router B acknowledges the query there by keeping the relationship up.
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SIA-Query and SIA-Reply are two new additions to the Type, Length, Value (TLV) triplets in the EIGRP packet header. These packets are generated automatically with no configuration required, from Cisco IOS Software Release 12.1(5) and later with the active process enhancement feature. This feature enables an EIGRP router to monitor the progression of the search for a successor route and ensure that the neighbor is still reachable. Improved network reliability results from reducing the unintended termination of the neighbor adjacency.
The figure on the left illustrates what would happen before this feature was introduced. Router A sends a query for network 10.1.1.0/24 to router B. Router B has no entry for this network, so it queries router C. If problems exist between router B and C, the reply packet from router C to router B may be delayed or lost. Router A has no visibility of downstream progress and assumes that the lack of response indicates problems with router B. After the router A 3-minute active timer expires, the neighbor relationship with router B is reset, along with all known routes from router B.
By contrast, with the active process enhancement feature, router A queries downstream router B (with an SIA-Query) at the midway point of the active timer (1.5 minutes by default) about the status of the route. Router B responds (with an SIA-Reply) that it is searching for a replacement route. Upon receiving this SIA-Reply response packet, router A validates the status of router B and does not terminate the neighbor relationship.
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Meanwhile router B sends up to three SIA-Queries to router C. If they go unanswered, router B terminates the neighbor relationship with router C. Router B then updates router A with an SIA-Reply indicating that the network 10.1.1.0/24 is unreachable. Routers A and B remove the active route from their topology tables. The neighbor relationship between routers A and B remains intact.
© 2006 Cisco Systems, Inc. Configuring EIGRP 2-93
Graceful Shutdown
This topic describes how graceful shutdown prevents loss of packets when routers go down.
Graceful shutdown, implemented with the goodbye message feature, is designed to improve EIGRP network convergence.
In the figure, router A is using router B as the successor for a number of routes; router C is the feasible successor for the same routes. Router B normally would not tell router A if the EIGRP process on router B was going down; for example, if router B was being reconfigured. Router A would have to wait for its hold timer to expire before it would discover the change and react to it. Packets sent during this time would be lost.
With graceful shutdown, the goodbye message is broadcast when an EIGRP routing process is shut down to inform adjacent peers about the impending topology change. This feature allows supporting EIGRP peers to synchronize and recalculate neighbor relationships more efficiently than would occur if the peers discovered the topology change after the hold timer expired.
The goodbye message is supported in Cisco IOS Software Release 12.3(2), 12.3(3)B, and 12.3(2)T and later. Goodbye messages are sent in hello packets. EIGRP sends an interface goodbye message with all K values set to 255 when taking down all peers on an interface. The following message is displayed by routers that support goodbye messages when one is received:
*Apr 26 13:48:42.523: %DUAL-5-NBRCHANGE: IP-EIGRP(0) 1: Neighbor 10.1.1.1 (Ethernet0/0) is down: Interface Goodbye received
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A Cisco router that runs a software release that does not support the goodbye message will misinterpret the message as a K-value mismatch and therefore display the following message:
*Apr 26 13:48:41.811: %DUAL-5-NBRCHANGE: IP-EIGRP(0) 1: Neighbor 10.1.1.1 (Ethernet0/0) is down: K-value mismatch
Note The receipt of a goodbye message by a peer that does not support this feature does not disrupt normal network operation. The peer will terminate the session when the hold timer expires. The sending and receiving routers will reconverge normally after the sender reloads.
© 2006 Cisco Systems, Inc. Configuring EIGRP 2-95
Continue reading here: OSPF Packet Header Format
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