EIGRP Administrative Distance, EIGRP Route Summarization, EIGRP Stub

The default administrative distance - AD of EIGRP internal route is 90 which is marked by the "D" symbol. The AD of an external route or considered as an external route using route distribution is 170 which is marked by the "D EX" symbol. There is also an eigrp summary route with the AD of 5. All of this can be shown with the #show ip route eigrp command.

To change the AD of EIGRP use the command:
(config)#router eigrp
(config-router)#distance eigrp

Auto summarization is a default feature at EIGRP. The purpose of this feature is used to automatically summarize the routes that are advertised through classfull network boundaries (EIGRP and RIPv2). It is therefore almost always necessary to turn this feature off in order make the router available to advertise classless networks routes by using the no auto-summary command. It is important to configure this route on the router sending the route. Remember that when applying the summarization, the adjecency goes down for a moment before it goes up again.

There are several advantages when applying a route summarization. A crucial advantage is that the route tables are getting smaller, because several routes are summarized. Another advantage is that CPU processing will be lighter, as the routes in the table are smaller.

To configure auto summarization use the following command:
(config)#int
(config-if)#ip summary-address eigrp <>

There are several ways to summarize routes where each route needs to have a specific bandwidth:

1. If there are multiple routes with the same CIRs, then add the amount of the CIRs together and use that as the minimum bandwidth needed. To set this use the command:
(config-if)#bandwidth

2. If there are multiple routes with different CIRs, then configure the route using a point to point (sub-)interface mode and assign each VC with the required CIR bandwidth value. To set this use the command on each sub-interface:
(config)#int serial point-to-point
(config-subif)#ip address
(config-subif)#bandwidth

3. There is also a possibility to configure a multipoint sub-interfaces. This can be done by adding all the CIRs from the routers. To set this use the command on the main router:
(config)#int serial multipoint
(config-subif)#bandwidth

The EIGRP can also be configured as a stub. However the stub has to be set only in the hub router. The spoke router does not need to be configured for stub (although possible) because the spoke router does not need to keep the routing table. It simply forwards all the routers to the hub router. This feature can also combat the SIA (stuck in active) problem because in stub networks, spoke routers will not be queried for routes when the hub does not have a feasible successor. To configure stub use the command:
(config)#router eigrp 100
(config-router)#eigrp stub

-- 10 February 2011 --

Variance, Feasible Distance & Advertising Distance

Variance is used when we want to apply unequal cost load sharing. Remember that EIGRP can support load sharing up to 6 links. In order for this load sharing to apply, the result of variance value (X) multiplied by the FD of the successor must be bigger than the FD of the feasible successor. Nevertheless, make sure that the variance value is as small as possible, since big variance value can bring some problems. When the variance command is applied, load sharing will be applied, but there is no change in the FD of the successor nor the feasible successor. This can be verified by the command #show ip eigrp topology.

Applying variance is easy using the command:
(config)#router eigrp (eigrp AS number)
(config-router)#variance

On the EIGRP topology table there are 2 values which is the FD (feasible distance) and the AD (advertised distance) or also known as reported distance.
- The FD (first value) is the full metric from the router to the destination network.
- The AD (Second value) is the metric from the next hop of the router to the destination network. Therefore, in most times the AD is lower than the FD.

If the AD has a bigger value than the FD, then there is a possibility of an routing loop. This rule is checked before the successor and feasible successor is applied to the routing table. If this condition is found, the router will not let that route take place into the election of a route or feasible route. Another condition that must be met for the potential feasible successor is that the AD of the feasible successor must be smaller than the FD of the successor. In a condition that where there are no feasible successors, the route on the router will go to the active state and in the meanwhile send DUAL Query Packets.

-- 1 February 2011 --

EIGRP topology states & SIA

There are a few Codes in the EIGRP Topology Table. This table can be shown with the command #show ip eigrp topology. There are 3 important codes to understand which are Active (A), Passive (P) and replay Status (r).

P : This code shows that the route is a Passive route which means that the route is NOT being calculated by the DUAL algorithm (not in process of calculating). Therefore, a route in this state is usable.

A : This code shows that the route is in an Active state which means that the route is currently being calculated by the DUAL algorithm. While being in this process, the route is not available for use. The DUAL calculation is usually short, so this Active state is also short and soon changes to the Passive Route (P). However, there are certain conditions when the route stays in this condition which is the SIA (Stuck In Active) condition. SIA happens when the router loses its successor and does not have a feasible successor. The router will send queries to the neighboring routers for the feasible successor and if there is no answer for a very long time from them, the neighbor relationship will be reset and SIA state will occur.

r : This code means that the query has been sent, but there is no response from the neighbor.

There are 4 possibilities why a route goes to SIA:

1. The link is a unidirectional link. The main router can send a packet, but it is not able to receive a packet back.

2. The destination router resource is not available. This can happen when the router's CPU processing processor is very high.

3. The destination router memory is corrupted, so that it cannot reply the query.

4. The link is a low quality link so that the reply cannot be received by the original router.


-- 1 February 2011 --