CCIE за год. Цикл 7. Тема 39. Вопросы.

1. Дайте объяснение понятию Feasibility Condition.

The feasibility condition is a sufficient condition for loop freedom in EIGRP-routed network. It is used to select the successors and feasible successors that are guaranteed to be on a loop-free route to a destination.
If, for a destination, a neighbor router advertises a distance that is strictly lower than our feasible distance, then this neighbor lies on a loop-free route to this destination.

2. Дайте объяснение понятию Feasible Distance.

Feasible distance is the best metric along a path to a destination network, including the metric to the neighbor advertising that path.

3. Дайте объяснение понятию Successor.

The best path learned through EIGRP to a destination network is known as the Successor route.

A feasible successor is a path whose reported (advertised) distance is less than the feasible distance (current best path).

4. Дайте объяснение понятию Advertised Distance.

Reported (Advertised) distance is the total metric along a path to a destination network as advertised by an upstream neighbor.

5. Объясните выражение: “The Feasibility Condition cannot guarantee that all loop-free paths will be accepted”

It should be pointed out, however, that the EIGRP Feasibility Condition is not perfect:
— The Feasibility Condition can guarantee that any path with a loop will not be accepted
— The Feasibility Condition cannot guarantee that all loop-free paths will be accepted.

Because of how varied the EIGRP metric can be, there is no perfect formula that would guarantee denying all looped-paths and accepting all loop-free paths. Between the choices of accidentally denying a loop-free path or accidentally accepting a looped-path, the latter is far more dangerous. As such, EIGRP (correctly) took the more conservative approach, and the Feasibility Condition’s main purpose is to guarantee that a looped-path will not be accepted.

6. Дайте описание формуле композитной метрики EIGRP.

EIGRP composite cost metric = 256*((K1*Bw) + (K2*Bw)/(256 – Load) + (K3*Delay)*(K5/(Reliability + K4)))

bandwidth — The minimum bandwidth of the route, in kilobits per second. It can be 0 or any positive integer. The bandwidth for the formula is scaled and inverted by the following formula:(10^7/minimum bandwidth (Bw) in kilobits per second)
delay — Route delay, in tens of microseconds.
reliability — The likelihood of successful packet transmission, expressed as a number between 0 and 255, where 255 means 100 percent reliability and 0 means no reliability.
load — The effective load of the route, expressed as a number from 0 to 255 (255 is 100 percent loading).
mtu — The minimum maximum transmission unit (MTU) size of the route, in bytes. It can be 0 or any positive integer.

7. Дайте объяснение тому, что cisco не рекомендует менять значения EIGRP LOAD и RELIABILITY

Надёжность (reliability) и загрузка линка (load) являются динамическими параметрами, поэтому эти значения пересчитываются только при изменении в сети.

8. Почему не рекомендуется менять метрику Bandwidth?

Для изменения метрики обычно меняют delay, так как bandwidth влияет на QoS, кроме этого, изменение bandwidth не всегда меняет метрику (если наихудший линк не изменился).

9. Дайте объяснение применению Wide Metric.

The EIGRP composite cost metric (calculated using the bandwidth, delay, reliability, load, and K values) is not scaled correctly for high-bandwidth interfaces or Ethernet channels, resulting in incorrect or inconsistent routing behavior. The lowest delay that can be configured for an interface is 10 microseconds. As a result, high-speed interfaces, such as 10 Gigabit Ethernet (GE) interfaces, or high-speed interfaces channeled together (GE ether channel) will appear to EIGRP as a single GE interface. This may cause undesirable equal-cost load balancing. To resolve this issue, the EIGRP Wide Metrics feature supports 64-bit metric calculations and Routing Information Base (RIB) scaling that provide the ability to support interfaces (either directly or via channeling techniques like port channels or ether channels) up to approximately 4.2 terabits.

10. Дайте объяснение equal load balancing.

The routes have to have same metric to be put in routing table as equal cost routes.
Default number of equal cost routes for EIGRP is 4.
Can increase them up to 16 using command below.
conf t
router eigrp 5
maximum-paths 16

11. Дайте объяснение unequal load balancing.

This is where you can have several routes in a routing table which are not of equal cost.
Use command below to specify unequal cost load balancing.

conf t
router eigrp 5
variance 2

The value 2 in variance means that the metric ratio of second route divided by lowest metric (first route) is less than or equal to 2.

The routes have to pass the feasibility condition too.

12. Дайте объяснение RIB_Scale

Because the Wide Metrics composite value can well result in a number wider than 32 bits while the routing table (Routing Information Base, RIB) is capable of handling only 32-bit metrics, the Wide Metrics composite value has to be downscaled before the route can be passed down to the RIB in IOS. This is done by dividing the Wide Metrics composite value by a factor configured in the metric rib-scale EIGRP command. The
default value is 128 and can be configured in the range 1–255. Note that this downscaled value is not used by EIGRP in any way. EIGRP makes all its path selections based on the Wide Metrics composite value; only after a best path toward a destination is selected, its composite metric value is downscaled as the route is installed to the RIB.

  • Kirill Pletnev

    Дмитрий Филогь в http://linkmeup.ru/blog/137.html пишет:
    7. Надёжность (reliability) и загрузка линка (load) являются динамическими параметрами, поэтому эти значения пересчитываются только при изменении в сети.
    8. Для изменения метрики обычно меняют delay, так как bandwidth влияет на QoS, кроме этого, изменение bandwidth не всегда меняет метрику (если наихудший линк не изменился).

    • nmstech

      Спасибо.