CCIE за год. Цикл 7. Тема 37. Вопросы.

1. У вас на коммутаторе настроено 30 транков и через каждый из них проходит трафик 60 вланов. Сколько stp инстансов будет работать на коммутаторе?

Если VLANs одинаковые на транках то будет 60 STP instance. (кол-во VLAN = кол-во STP instance)
Если VLANs разные, то количество STP Instance зависит от платформы коммутатора:
2960 — 64 STP instance
3560, 3750 — 128 STP instance

2. Приведи примеры неудобства использования STP

При большом количестве VLAN, будет большое количество STP instance(что загружает коммутатор), но в реальности будет огриниченное количество физических топологий STP

3. Назовите ключевое отличие MST от STP

Уменьшение количества STP instance

4. Как ведет себя MSTP на access портах?

MSTP runs on these ports and have logical topologies either blocking or forwarding on the ports. Depending on VLANs to MSTI mapping, a given VLAN could be blocked on the access ports due to MSTP decision – even though the access VLANs are different, they use the same STP. To avoid this problem, do not run MSTP on accessports and use them for connecting “stub” devices only e.g. hosts and leaf switches.

5. Какие условия должны выполняться для того, чтобы кадр мог быть отправлен через порт?


6. Какие значения должны совпадать, для того, чтобы коммутаторы были в одном регионе?

— name of the region
— revision number
— MST VLAN-to-instance assignment map

7. Что использует MST для доставки информации о коммутаторах и как это работает?

The IST is the only spanning-tree instance that sends and receives BPDUs. All of the other spanning-tree instance information is contained in M-records, which are encapsulated within MSTP BPDUs. Because the MSTP BPDU carries information for all instances, the number of BPDUs that need to be processed to support multiple spanning-tree instances is significantly reduced.
All MST instances within the same region share the same protocol timers, but each MST instance has its own topology parameters, such as root switch ID, root path cost, and so forth. By default, all VLANs are assigned to the IST.

8. Дайте описание CIST Root и CIST Regional Root?

CIST Root is the bridge that has the lowest Bridge ID among ALL regions. This could be a bridge inside a region or a boundary switch in a region.

CIST Regional Root is a boundary switch elected for every region based on the shortest external path cost to reach the CIST Root. Path cost is calculated based on costs of the links connecting the regions, excluding the internal regional paths. CIST Regional Root becomes the root of the IST for the given region as well.

9. Опишите процесс выбора CIST Root.

— When a switch boots up, it declares itself as CIST Root and CIST Regional Root and announces this fact in outgoing BPDUs. The switch will adjust its decision upon reception of better information and continue advertising the best known CIST Root and CIST Regional Root on all internal ports. On the boundary ports, the switch advertises only the CIST Root Bridge ID and CIST External Root Path Cost thus hiding the details of the regions internal topology.
— CIST External Root Path Cost is the cost to reach the CIST Root across the links connecting the boundary ports i.e. the interregion links. When a BPDU is received on an internal port, this cost is not changed. When a BPDU is received on a boundary port, this cost is adjusted based on the receiving boundary port cost. In result, the CIST External Root Path Cost is propagated unmodified inside any region.
— Only a boundary switch could be elected as the CIST Regional Root, and this is the switch with the lowest cost to reach the CIST Root. If a boundary switch hears better CIST External Root Path cost received on its internal link, it will relinquish its role of CIST Regional Root and start announcing the new metric out of its boundary ports.
— Every boundary switch needs to properly block its boundary ports. If the switch is a CIST Regional Root, it elects one of the boundary ports as the CIST Root port and blocks all other boundary ports. If a boundary switch is not the CIST Regional Root, it will mark the boundary ports as CIST Designated or Alternate. The boundary port on a non regionalroot bridge becomes designated only if it has superior information for the CIST Root: better External Root Path cost or if the costs are equal better CIST Regional Root Bridge ID. This follows the normal rules of STP process.
— As a result of CIST construction, every region will have one switch having single port unblocked in the direction of the CIST Root. This switch is the CIST Regional Root. All boundary switches will advertise the regions CIST Regional Root Bridge ID out of their nonblocking boundary ports. From the outside perspective, the whole region will look like a single virtual bridge with the Bridge ID = CIST Regional Root ID and single root port elected on the CIST Regional Root switch.
— The region that contains the CIST Root will have all boundary ports unblocked and marked as CIST designated ports. Effectively the region would look like a virtual root bridge with the Bridge ID equal to CIST Root and all ports being designated. Notice that the region with CIST Root has CIST Regional Root equal to CIST Root as they share the same lowest bridge priority value across all regions.

10. Для чего используется параметр diametr в разрезе MST ?

Use the diameter keyword, which is available only for MST instance 0, to specify the Layer 2 network diameter (that is, the maximum number of switch hops between any two end stations in the Layer 2 network). When you specify the network diameter, the switch automatically sets an optimal hello time, forward-delay time, and maximum-age time for a network of that diameter, which can significantly reduce the convergence time. You can use the hello keyword to override the automatically calculated hello time.

11. Каким образом изменение топологии в CST влияет на другие регионы? Дайте полное объяснение.

Every MSTP region builds IST instance using the internal path costs and following the optimal internal topology, using the CIST Regional Root as the IST Root. The changes to CST may affect the IST in every region, as those changes may result in reelecting of the new CIST Regional Roots. Changes to the regions internal topologies normally do not affect the CST, unless those changes partition the region.