Eigrp and Spanning tree

Routing & Switching
1

Hi,
I’m just wondering how does rapid spanning tree affect routing protocols ?
I never really thought about before.
What I am specifically wondering is how the blocked ports affect EIRGP (or OSPF) neighbourships and routing.
For example I have one network with two exit points to a core switch. I can see spanning tree is blocking the vlans on one of those ports to the core but even though its blocking I can still see there are Eigrp neighbourships up and connected accross the link that spanning tree has blocked.
I’m not sure if this is normal or not ?

But the main think I need to ask is I currently have a dedicated Layer two Point to multipoint link with my service provider.
This p2mp link has 3 sites link a triangle.
Lets say site A, B and C.
One site is in the far end of the county (Site A) and the other two sites (Site B and C) are close by each other in the same city.
The two sites close by each other in the same city share a core switch.
That core switch has a connection to the Site B P2mp circuit and Site C P2mp circuit.
I have lets say vlan 500 running trough the 3 sites over the L2mp circuit and also on the Core switch.
To stop any loops I have fine tuned spanning tree to block the link for vlan 500 where it enters the core switch at site B from the service provider p2mp link and all works fine.
The issue is now I want to route Eigrp over this L2P2MP circuit.
So if I rolled out and SVI on the core for Vlan 500 and an SVI on the switch at site A and ran eigrp would the routing traverse site B and C to Site A (given that it is equal cost so by default it would) or would it only route between site C and A and not utilize site B as its blocked by spanning tree ?
Until such a time as the link in site C fails and then it would fail over to site B ?
Or does spanning tree have any effect on Layer 3 neighbourships and rounting and possibly would site A route with site B and C to the Core behind them load balancing given the path is equal ?
Normally I could roll out a separate vlan for each site to site with a corrosponding SVI for each and I could utilize all links that way but Given that this is a “triangle” topology with three connection points and the networks behind two of the sites have to connect to each other I have to block whatever Vlans I use somewhere between the two sites that connect to the same switch.
Basically I’m wondering how Eigrp (or even OSPF) behaves with its neighbours and routes over blocked spanning tree links ?

Thanks

2

Hello Sean

RSTP and routing protocols like EIGRP and OSPF operate at different layers of the OSI model. RSTP operates at Layer 2 and is responsible for preventing loops in the network by blocking redundant paths. On the other hand, EIGRP and OSPF are Layer 3 protocols and they are responsible for finding the best path for data packets to reach their destination. But you know that already :wink: I’m just stating the fundamentals before I go on…

When it comes to how these protocols interact, it’s important to remember that a blocked port in RSTP means that packets are not being forwarded through that port. That includes routing protocol messages. Therefore, routing protocol adjacencies will not form over blocked links.

Routing protocols like EIGRP will form neighbor relationships and exchange routing information over the unblocked or designated ports in the topology. If a port is blocked by RSTP, EIGRP won’t be able to form a neighbor relationship over that port because they won’t forward or receive hello packets.

EIGRP messages will be exchanged between routing devices or Layer 3 ports. When you have Layer 3 switches involved, it is the SVIs that send and receive those routing protocol messages. RSTP will block certain ports at Layer 2 in order to perform its function, but it will always have some path that forwards packets. The routing devices exchanging EIGRP messages will always use that available path to exchange messages and form adjacencies.

So if you see an adjacency, you can be sure that it has formed over the forwarding path, and not over the link that has been blocked by RSTP. Does that make sense?

Router devices will always communicate over the underlying Layer 2 forwarding path provided by the STP protocol being used.

I hope this has been helpful!

Laz

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3

Thanks Laz,

I think I also now know why I was mistaken when I saw the Eigrp neighbourship over the blocked link.
You are correct, it probably wasn’t forming the adjacency over the blocked spanning tree link but it was forming the adjacency over the link which was forwarding.
So when I looked at Eigrp neighbours and saw the neighbour up on the blocked link that neighbourship was actually forming over the forwarding link which makes sense as its on the same vlan also.
Thanks again for your answer. It was very helpful.

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