2. Consider this switched network (from CNA Scaling Networks 2.1.2.8). Please ig

Question

2. Consider this switched network (from CNA Scaling Networks 2.1.2.8). Please ignore the boxes drawn on top of each inter-switch link. Assume that Spanning Tree Protocol (STP) is running on this network. The table below shows the Bridge ID (BID) value for each switch, split into Priority and MAC Address parts. All ports are on VLAN 1 G1/1 S1 S2 Fa0/1 G1 G1/3 G1/1 Fa0/1 S3 S4 Priority 32769 24577 32769 32769 MAC Address S1 S2 S3 S4 000A00222222 000A00333333 000A00444444 (5 points) When STP protocol is run, which switch (S1, S2, S3 or S4) will become the Root Switch for this network? Why? a.

Explanation / Answer

A) Switch with lowest bridge Id is selected as the Root Bridge. Bridge Id is the combination of Mac address and Priority of the Switch. Default priority is 32768 (and it can be multiples of 4096 if configured manually). In this senario switch S2 has the lowest Bridge Id making it a Root Bridge.

B) Port Roles:

**Root Port: Root Port is a port that is directly connected to root bridge or port with lowest cost to root bridge.If there is a tie in cost they go throw below tie breakers.

Tie Breakers:

a)Cost

b)Forwarding BID or upstream BID

c)Forwarding Port number.

Note: There can be only one Root port on Non-Root Bridges.

**Designated Port:All other ports of non-root bridges again go through election to decide which should be in forwarding state and elected ports are called Desingnated Ports.Port which is having the lowest path cost is elected as Designated Ports. These ports forward the frames.If there is a tie in cost they go throw below tie breakers.

Tie Breakers:

a)Cost

b) Local switch BID

c)Port Number.

**Blocked Ports: All other ports which are neither root nor desingnated are called Blocked or Alternate Ports.

i)S1 Fa01 is elected as "Desingnated Port" because when election take place between S3 Fa0/1 and S1 Fa0/1 (Non Root-Ports) there will be a tie in cost as S1 has lower BID due to Lower mac thereby making S3 Fa0/1 blocked port and S1 Fa0/1 as Desingnated Port.

ii)S1 G1/1 is "Root port" because this is directly connected to root bridge or This port have Lower cost to reach Bridge Id.

iii,iv,v)All the ports of Root Bridge are Root Ports. So all three ports of Switch S2 are Root Ports.

vi)S3 Fa/01 will be a "Alternate Port" because 'Refer to (i)

vii) S3 G1/1 is "Root port" because this is directly connected to root bridge or This port have Lower cost to reach Bridge Id.

viii)S4 G1/1 is "Root port" because this is directly connected to root bridge or This port have Lower cost to reach Bridge Id.

C) Disabled, Blocking, and Listening states are said to be in Discarding state in RSTP.

D)In STP all the traffic flow should happen centrally only through Root Bridge.So,

(i)In S2 Forwarding table Port G1/2 is associated with 11.22.33.44.55.66 as S3 send broadcast only through G1/1.

(ii)In S1 Forwarding table Port G1/1 is associated with 11.22.33.44.55.66 as S1 learns from G1/1 of S2

E) Typing below command in CLI of S3 will make it root bridge:

spanning-tree vlan <vlan_id> root primary.

F) If S4 want to reach S3 path cost will be sum of path cost form S4 to S2 and S2 to S3. So, total path cost will be 4+4.

Refer to below table to get cost based upon type of connection used(The greater the bandwidth the lower the cost is)

Bandwidth   Cost

10 Mbit 100 100 Mbit 19 1000 Mbit 04

Related Questions

Navigate

• Browse (All)
• Browse 2
• Subjects

---

---

Integrity-first tutoring: explanations and feedback only — we do not complete graded work. Learn more.