07-16-2009 02:43 PM - edited 03-06-2019 06:49 AM
Hi every body!
RStp has following port roles:
root port
designated port
alternate port
back up port
Let focus on alternate port role.
My book says a switch receiving the 2nd best bpdu on a port, will consider that port as alternate port.
So we have a switch sw1 with three ports connected to switched networks in s uch a manner that f0/1 becomes a root port because it receives the best bpdu. The other two ports f0/2,f0/3connected to same neighboring switch, are also receiving bpdu but f0/2 receives the 2nd best bpdu( f0/2 has lower port id), so based on the excerpt from my book, f0/2 will be considered as alternate port. What about f0/3 ? what role it will be assigned? it can not be alternate port as f0/2 has already been choosen as alternate port?
=========================
Back up port:
That is what the book says"Backup ports are ports that received more useful bpdu from the same switch they are on."
let say sw1 and sw2 has already determined their root port and alternate ports. These two switches are not connected directly.
Let say we connect these two switches via hub;
sw1's f0/1.f0/2 are connected to hub
sw2 f0/3 is connected to same hub as well.
Let say f0/3 is designated port. What role will be assigned to f0/1 and f0/2 of sw1? they cannot be back up . Based on the defination above in the execrpt, f0/2 and f0/3 of sw1 can not be back up . So if they are not back up then what are they?
Thanks a lot!
Solved! Go to Solution.
07-16-2009 02:56 PM
Hi,
Assuming f0/3 will be configured the same as the other two (2) port. It will be assigned as alternate port also.
Just a quick test from my lab:
SW1 - STP ROOT -
!
spanning-tree mode rapid-pvst
spanning-tree vlan 1-1004 priority 24576
!
vlan 10
!
interface GigabitEthernet0/13
switchport trunk encapsulation dot1q
switchport mode trunk
!
interface GigabitEthernet0/14
switchport trunk encapsulation dot1q
switchport mode trunk
!
interface GigabitEthernet0/15
switchport trunk encapsulation dot1q
switchport mode trunk
!
SW2 -
!
vlan 10
!
spanning-tree mode rapid-pvst
!
interface GigabitEthernet0/13
switchport trunk encapsulation dot1q
switchport mode trunk
!
interface GigabitEthernet0/14
switchport trunk encapsulation dot1q
switchport mode trunk
!
interface GigabitEthernet0/15
switchport trunk encapsulation dot1q
switchport mode trunk
!
show spanning-tree vlan 10
SW1 -
VLAN0010
Spanning tree enabled protocol rstp
Root ID Priority 24586
Address 001b.d418.eb80
This bridge is the root
Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec
Bridge ID Priority 24586 (priority 24576 sys-id-ext 10)
Address 001b.d418.eb80
Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec
Aging Time 300
Interface Role Sts Cost Prio.Nbr Type
------------------- ---- --- --------- -------- --------------------------------
Gi0/13 Desg FWD 4 128.13 P2p
Gi0/14 Desg FWD 4 128.14 P2p
Gi0/15 Desg FWD 4 128.15 P2p
SW2 -
VLAN0010
Spanning tree enabled protocol rstp
Root ID Priority 24586
Address 001b.d418.eb80
Cost 4
Port 13 (GigabitEthernet0/13)
Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec
Bridge ID Priority 32778 (priority 32768 sys-id-ext 10)
Address 001b.d418.f200
Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec
Aging Time 300
Interface Role Sts Cost Prio.Nbr Type
------------------- ---- --- --------- -------- --------------------------------
Gi0/13 Root FWD 4 128.13 P2p
Gi0/14 Altn BLK 4 128.14 P2p
Gi0/15 Altn BLK 4 128.15 P2p
HTH,
jerry
07-16-2009 03:13 PM
A bridge X puts a redundant port in backup role when it receives a BPDU from itself (i.e. the designated port for this LAN is on bridge X).
An alternate port on bridge X is blocked because it receives BPDUs from a designated port on bridge Y.
Regards,
Francois
07-16-2009 05:08 PM
Hi,
The following link has very good explaination of the RSTP port role.
http://www.cisco.com/en/US/tech/tk389/tk621/technologies_white_paper09186a0080094cfa.shtml#roles
Regards,
jerry
07-16-2009 10:45 PM
Yes, they won't be backup. If the information in the BPDU received does not beat the one of the current root port, they'll both be alternate.
Only if f0/1 (resp f0/2) is designated for this LAN will f0/2 (resp f0/1) be backup.
Regards,
Francois
07-16-2009 02:56 PM
Hi,
Assuming f0/3 will be configured the same as the other two (2) port. It will be assigned as alternate port also.
Just a quick test from my lab:
SW1 - STP ROOT -
!
spanning-tree mode rapid-pvst
spanning-tree vlan 1-1004 priority 24576
!
vlan 10
!
interface GigabitEthernet0/13
switchport trunk encapsulation dot1q
switchport mode trunk
!
interface GigabitEthernet0/14
switchport trunk encapsulation dot1q
switchport mode trunk
!
interface GigabitEthernet0/15
switchport trunk encapsulation dot1q
switchport mode trunk
!
SW2 -
!
vlan 10
!
spanning-tree mode rapid-pvst
!
interface GigabitEthernet0/13
switchport trunk encapsulation dot1q
switchport mode trunk
!
interface GigabitEthernet0/14
switchport trunk encapsulation dot1q
switchport mode trunk
!
interface GigabitEthernet0/15
switchport trunk encapsulation dot1q
switchport mode trunk
!
show spanning-tree vlan 10
SW1 -
VLAN0010
Spanning tree enabled protocol rstp
Root ID Priority 24586
Address 001b.d418.eb80
This bridge is the root
Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec
Bridge ID Priority 24586 (priority 24576 sys-id-ext 10)
Address 001b.d418.eb80
Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec
Aging Time 300
Interface Role Sts Cost Prio.Nbr Type
------------------- ---- --- --------- -------- --------------------------------
Gi0/13 Desg FWD 4 128.13 P2p
Gi0/14 Desg FWD 4 128.14 P2p
Gi0/15 Desg FWD 4 128.15 P2p
SW2 -
VLAN0010
Spanning tree enabled protocol rstp
Root ID Priority 24586
Address 001b.d418.eb80
Cost 4
Port 13 (GigabitEthernet0/13)
Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec
Bridge ID Priority 32778 (priority 32768 sys-id-ext 10)
Address 001b.d418.f200
Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec
Aging Time 300
Interface Role Sts Cost Prio.Nbr Type
------------------- ---- --- --------- -------- --------------------------------
Gi0/13 Root FWD 4 128.13 P2p
Gi0/14 Altn BLK 4 128.14 P2p
Gi0/15 Altn BLK 4 128.15 P2p
HTH,
jerry
07-16-2009 03:06 PM
Thanks Jerry!
Could you please explain back up port question that is if you have time ?
07-16-2009 03:13 PM
A bridge X puts a redundant port in backup role when it receives a BPDU from itself (i.e. the designated port for this LAN is on bridge X).
An alternate port on bridge X is blocked because it receives BPDUs from a designated port on bridge Y.
Regards,
Francois
07-16-2009 04:00 PM
Thanks Francois!
Assume the switch has already determined its root port f0/4, and alternate port f0/5.
How about if the same switch receives bpdu on its two ports f0/1,f0/2 connected to same segment from designated port on a different switch?
they(f0/1,f0/2) can not be assigned back up role as these ports are receiving bpdu from different switch?
Thanks a lot!
Thanks a lot!
07-16-2009 05:08 PM
Hi,
The following link has very good explaination of the RSTP port role.
http://www.cisco.com/en/US/tech/tk389/tk621/technologies_white_paper09186a0080094cfa.shtml#roles
Regards,
jerry
07-16-2009 10:45 PM
Yes, they won't be backup. If the information in the BPDU received does not beat the one of the current root port, they'll both be alternate.
Only if f0/1 (resp f0/2) is designated for this LAN will f0/2 (resp f0/1) be backup.
Regards,
Francois
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