CCNA 200-105 Designated Forwarding Ports

Uriah7

Hello,

I have found a discrepancy between Lammle's 200-105 Study Guide and the Boson practice exam literature regarding STP. After the root-bridge and root ports have been determined for a network, any segments that are left without port designations should have one end placed into designated forwarding state and the other end blocked. Lammle claims that the end of the segment with the lowest Bridge-ID is to be designated forwarding and the other end blocking. Boson claims that the end of the segment with the lowest port-priority is to be designated forwarding and the other end blocking. Kevin Wallace's videos don't even explain it and just assume I already know. Any advice on this topic is appreciated.

Thanks

clarson

Hard to say which is right without knowing what the topology is.

Lowest sender bridge ID - Serves as a tie breaker if multiple upstream switches have equal cost to root
such as you have 3 switches connected in a triangle. the lowest mac address is the root bridge. The second lowest is the designated port and the highest mac address is going to be blocked


Lowest sender port ID - Serves as a tie breaker if a switch has multiple (non-Etherchannel) links to a single upstream switch
such as in a two switch topology, the lowest mac address will be the root bridge. But, everything else is going to be equal till you get to the port priorities. Then, the port connected to the lowest port address on the root bridge will be a designated port and the rest will be blocked.

WastedHat

Uriah7 wrote: »

Lammle claims that the end of the segment with the lowest Bridge-ID is to be designated forwarding and the other end blocking. Boson claims that the end of the segment with the lowest port-priority is to be designated forwarding and the other end blocking.

The cost is used first, if cost ties then its lowest BID, if BIDs tie then its senders port ID.

I would say the Boson wording is not accurate: "the end of the segment with the lowest port-priority is to be designated forwarding", you can try this in a lab with 2 switches with multiple redundant links. The BID will tie, and changing the local port priority will make no difference to the local switchs STP port states, it effects the switch on the other end of the link. Pretty sure I learned this from Kevin Wallance's ICND2 videos.

Uriah7

Okay, I figured out where my confusion lies. The idea of a "designated port" has been confusing me to no end. Since the port is considered "forwarding," I assumed that the port forwarded information OUT. Now that I look at topologies, I have no idea why I thought that a designated port forwarded traffic out toward the neighbor switch which has a blocked port. Designated forwarding ports forward traffic off of the segment in which they belong to, in which case, the cost of the segment that they belong to has no bearing on which side of the segment becomes Designated Forwarding. The paths away from the segment, and toward the root bridge, are the only path costs that matter. Now I can see why ports are elected as designated forwarding ports, as you have both kindly explained above. Wow, I feel like I shouldn't have a degree.... or a computer.... I am so deep into studying that common sense has taken a vacation. Thanks again for helping me on this.

WastedHat

Man theres so much logic and terminology with STP its easy to get confused. Sounds like you've got it, knowing the forward path cost really helps.

Uriah7 wrote: »

I have no idea why I thought that a designated port forwarded traffic out toward the neighbor switch which has a blocked port.

Depends what type of traffic, if you're thinking about unicast frames then you're right, there will be no unicast frames arriving on the blocked segement so there will be no dynamic MAC table entries, and therefore no unicast traffic going out. However I believe a DP to non-DP segement would still send broadcast traffic out of the DP to be dropped on the other end. It will also send BPDUs which are the only packets a blocked port will accept.

Uriah7

WastedHat wrote: »

Man theres so much logic and terminology with STP its easy to get confused. Sounds like you've got it, knowing the forward path cost really helps.



Depends what type of traffic, if you're thinking about unicast frames then you're right, there will be no unicast frames arriving on the blocked segement so there will be no dynamic MAC table entries, and therefore no unicast traffic going out. However I believe a DP to non-DP segement would still send broadcast traffic out of the DP to be dropped on the other end. It will also send BPDUs which are the only packets a blocked port will accept.

Thanks for bringing that up. I forgot that the BPDUs are still being sent to the blocked port. I think that loop guard keeps that port from becoming a forwarding port due to BPDUs not being received due to various reasons. I think that this is beyond to scope of the 200-105, but I found it interesting. Are you working in the field?

WastedHat

Nope, I got my degree 5 years ago but spent a lot of time travelling, now I'm playing catch up. I managed to get an interview for a graduate job so fingers crossed! What about yourself?

dontstop

WastedHat wrote: »

The cost is used first, if cost ties then its lowest BID, if BIDs tie then its senders port ID.

This logic only applies to root ports, In the instance where you have multiple equal cost links to the root bridge. With a designated port you have one per segment not per switch. The calculation will never require a port priority because the MAC Address of the Switches will never tie.

With designated ports you see which end has of that segment/link has the lowest cost to the root, if they tie you just pick the bridge with the lowest BID (Priority + MAC Address) and make it the designated port side. If there are two links between those switches then you end up with 2 designated ports on the same end of each link.

So in summary: For determining the designated port: End of link with lowest cost to root, otherwise Lowest BID.

I'm not sure why but a lot of blogs and sites mention the priority for Designated ports. But here is an artcile from ExtremeNetwork: https://community.extremenetworks.com/extreme/topics/802_1d_spanning_tree_election_rules

WastedHat

Thats a very good point that I totally overlooked, thanks.