Tag Archives: 3.5h

3.5.h Implement, troubleshoot and optimize EIGRP convergence and scalability

3.5.h [i] Describe fast convergence requirements

EIGRP has been designed and used to achieve sub-second convergence for years. Lab testing has shown that the key factor for EIGRP convergence is the presence or absence of a feasible successor. When there is no feasible successor, EIGRP uses queries to EIGRP peers and has to wait for responses. This slows convergence.

Proper network design is required for EIGRP to achieve fast convergence. Summarization helps limit the scope of EIGRP queries, indirectly speeding convergence. Summarization also shrinks the number of entries in the routing table, which speeds up various CPU operations. The effect of CPU operation on convergence is much less significant than the presence or absence of a feasible successor. A recommended way to ensure that a feasible successor is present is to use equal-cost routing.

EIGRP metrics can be tuned using the delay parameter. However, adjusting the delay on links consistently and tuning variance are next to impossible to do well at any scale. In general, it is unwise to have a large number of EIGRP peers. Under worst-case conditions, router CPU or other limiting factors might delay routing protocol convergence.

Adam, Paul (2014-07-12). All-in-One CCIE V5 Written Exam Guide (Kindle Locations 3248-3252).  . Kindle Edition.

http://blog.ine.com/tag/eigrp/

3.5.h Implement, troubleshoot and optimize EIGRP convergence and scalability

3.5.h [ii] Control query boundaries

EIGRP is an advanced distance-vector protocol; it doesn’t have LSA flooding like OSPF. EIGRP relies only on its neighbors for information on network reachability and availability. EIGRP keeps a list of backup routes called feasible successors. When the primary route is not available, EIGRP immediately uses the feasible successor as the backup route. This shortens convergence time. Now, if the primary route is gone and no feasible successor is available, the route is in active state. The only way for EIGRP to converge quickly is to query its neighbors about the unavailable route. If the neighbor doesn’t know the status of the route, the neighbor asks its neighbors, and so on, until the edge of the network is reached.

The query stops if one of the following occurs:

● All queries are answered from all the neighbors.

● The end of network is reached.

● The lost route is unknown to the neighbors.

The problem is that, if there are no query boundaries, EIGRP potentially can ask every router in the network for a lost route. When EIGRP first queries its neighbor, a stuck in active timer starts. By default, the timer is 180 seconds. If, in three minutes, EIGRP doesn’t receive the query response from all its neighbors, EIGRP declares that the route is stuck in active state and resets the neighbor that has not responded to the query.

Adam, Paul (2014-07-12). All-in-One CCIE V5 Written Exam Guide (Kindle Locations 3259-3271).  . Kindle Edition.

http://blog.ine.com/2009/08/02/eigrp-query-scoping/

3.5.h Implement, troubleshoot and optimize EIGRP convergence and scalability

3.5.h [iii] IP FRR/fast reroute [single hop]

The EIGRP Loop-Free Alternate Fast Reroute feature allows the Enhanced Interior Gateway Routing Protocol (EIGRP) to reduce the routing transition time to less than 50 ms by pre-computing repair paths or backup routes and installing these paths or routes in the Routing Information Base (RIB). Fast Reroute (FRR) is the mechanism that enables traffic that traverses a failed link to be rerouted around the failure. In EIGRP networks, pre-computed backup routes or repair paths are known as feasible successors or loop-free alternates (LFAs).

Adam, Paul (2014-07-12). All-in-One CCIE V5 Written Exam Guide (Kindle Locations 3274-3277).  . Kindle Edition.

http://www.cisco.com/en/US/docs/ios-xml/ios/iproute_eigrp/configuration/15-2s/ire-ipfrr.html

 

3.5.h Implement, troubleshoot and optimize EIGRP convergence and scalability

3.5.h [v] Summary metric

When EIGRP creates a summary route, it includes a metric with the route in order to advertise it. EIGRP searches for components of the summary to be suppressed and represented by the summary. EIGRP finds the component with the best metric and copies the metric from it into the summary. Components of the summary may come and go, which means that every time the best component changes, the summary needs to be re-advertised to all of its peers. Even if the best component is not the one that changed, EIGRP still has to search every topology entry to make sure the summary is not affected.

This can add significant processing overhead. Use the summary-metric command to mitigate this metric churn and processing overhead. Rather than searching for the best component metric, EIGRP uses the values configured with the summary-metric command.

Adam, Paul (2014-07-12). All-in-One CCIE V5 Written Exam Guide (Kindle Locations 3279-3286).  . Kindle Edition.

http://www.cisco.com/c/en/us/td/docs/ios/iproute_eigrp/command/reference/ire_book/ire_s1.html#wp1059226