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RSTP and Trunk links

Piwo
Beginner
Beginner

I am mixing the concepts between P2P links and trunking:

 

"RSTP can only achieve rapid transition to the forwarding state on edge ports and on point-to-point links. The link type is automatically derived from the duplex mode of a port. A port that operates in full-duplex is assumed to be point-to-point, while a half-duplex port is considered as a shared port by default."

 

1 - What is the diference between a trunk and a P2P link ?

2 - Is necessary any config to have a P2P link?

3 - RSTP does not work on trunk links ? So what is the advantage of RSTP as most the links formed between switches are trunks ?

 

Thank you a lot for your support guys !

 

4 Replies 4

acampbell
VIP Alumni
VIP Alumni

Hi,

You can think of a trunk as a point to point link that is carrying multiple VLANS.

Here are a couple of links that is configured to trunk

myswitch#show int trunk:-

Port        Mode             Encapsulation  Status        Native vlan
Fa0/1       on               802.1q         trunking      1
Gi0/1       on               802.1q         trunking      1

Port        Vlans allowed on trunk
Fa0/1       1-4094
Gi0/1       1-4094

Port        Vlans allowed and active in management domain
Fa0/1       1,6,10,14,20-22,40,93,101-105,130-131,150-158,199,201,500,802
Gi0/1       1,6,10,14,20-22,40,93,101-105,130-131,150-158,199,201,500,802

Port        Vlans in spanning tree forwarding state and not pruned
Fa0/1       1,6,10,14,20-22,40,101-105,130-131,150-158,199,500,802
Gi0/1       1


Picking on my interface fast0/1, below shows how RTSP is behaving for each VLAN
running accross the trunk setup:-


myswitch#show spanning-tree int fast0/1

Vlan                Role Sts Cost      Prio.Nbr Type
------------------- ---- --- --------- -------- --------------------------------
VLAN0001            Root FWD 19        128.3    P2p
VLAN0006            Root FWD 19        128.3    P2p
VLAN0010            Root FWD 19        128.3    P2p
VLAN0014            Root FWD 19        128.3    P2p
VLAN0020            Root FWD 19        128.3    P2p
VLAN0021            Root FWD 19        128.3    P2p
VLAN0022            Root FWD 19        128.3    P2p
<SNIP>

As you can see, in this instance the role is Root and frwarding for each VLAN and the type
of link is P2P (point-to-point).


Below shows another trunk with the roles as designated, Altn and in states of forwarding and blocking on a per VLAN basis.
Note again the type is P2P ver Vlan:-

myswitch#show spanning-tree int gig0/1

Vlan                Role Sts Cost      Prio.Nbr Type
------------------- ---- --- --------- -------- --------------------------------
VLAN0001            Desg FWD 4         128.1    P2p
VLAN0006            Desg FWD 4         128.1    P2p
VLAN0010            Altn BLK 4         128.1    P2p
VLAN0014            Altn BLK 4         128.1    P2p
VLAN0020            Altn BLK 4         128.1    P2p
VLAN0021            Desg FWD 4         128.1    P2p
VLAN0022            Altn BLK 4         128.1    P2p
VLAN0040            Altn BLK 4         128.1    P2p
<SNIP>

 

Below is an excellent link from Cisco TAC engineers (really worth a read):-
https://www.cisco.com/c/en/us/support/docs/lan-switching/spanning-tree-protocol/24062-146.html


I hope this helps.
Regards
Alex

Regards, Alex. Please rate useful posts.

Hi Alex,

 

Thanks a lot for your quick feedback and explanation.

However, I have still have doubts regarding this topic when I saw this sentence in the cisco theory.

Does RSTP not work with trunk links ?

I know the link types are  Link-Type point-to-point, Edge-Type point-to-point,Link-Type Shared...

 

"RSTP can only achieve rapid transition to the forwarding state on edge ports and on point-to-point links, not on trunk links."

 

Thank you

 

 

Hi,

 

As per my post showing the link types across my Vlans the trunk is built of many Point-to-Point links.

So RSTP functions as expected across trunks- remember it is on a per Vlan basis.

 

Regards, Alex. Please rate useful posts.

Hello

1 - What is the difference between a trunk and a P2P link ? --- Trunk is a layer interconnect carrying multiple vlans tags, STP P2P trunks define a non portfast link which means they can negotiate to a stp forwarding quicker but they still need to goes through the stp process to get there.

The P2P part of it is defined by the link duplex mode or by manual intervention

P2P edge port ( portfast) the same as above but the STP process is bypassed and is transitioned straight into a forwarding state as these port are deemed to have end hosts attach to them.



2 - Is necessary any config to have a P2P link? - No but its recommended

 

3 - RSTP does not work on trunk links ? So what is the advantage of RSTP as most the links formed between switches are trunks ?

STP does work on Trunks, it needs to, negate loops due to trunks forwarding the same multiple vlan tags from multiple interconnects, Again P2P has to do with the negociation of the link and how fast stp can transision into a forwarding state.

res
Paul






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Kind Regards
Paul
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