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Redundant network L2 - L3

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Redundant network L2 - L3

Any help to clarify a setup for me would be greatly appreciated.

I want to implement a similar setup (see drawing) only with more VLANs but connected in a identical way as VLAN10.

I want XRRP L3 redundancy and RSTP. And what I understand is that XRRP does a total failover even if only 1 VLAN
is affected.

Ok, now let's say link A fails, link C would become active. XRRP would not failover to switch 2 because in VLAN10
switch 2 can still see switch 1 on Layer 3 via link C-E, right? And thus, Host A would still be able to access his
default gateway via switch 3-> C-> E.

Do I have this correct? Is there a better setup?

Thanks in advance, guys...


Kell van Daal
Respected Contributor

Re: Redundant network L2 - L3

Hi Marvin,

The drawing itself looks good, but I have a few notes about the numbers in it.

Why do you define port costs in this drawing? And even more values like 100, 200 and 300? For reference: 100 is a 10 Mbps link with normal spanningtree, for RSTP even 10 GbE would have a cost of 2000.

If the costs are based on normal STP (802.1D), and the port costs are actually path costs, then link E is defined wrong, and should be 100 (assuming it has the same speed as the other links).
In this case I would just use the default values of RSTP. Assuming all links are 1 Gbit, the path costs would be:
A: 20000
B: 20000
C: 40000
D: 40000
E: 20000
placing link C in blocking on switch 3 and placing link D in blocking on switch 4.
This is assuming switch 1 is root bridge, which would very likely be the best place for the root bridge in your situation.

About your "let's say" situation. If link A fails, link C would indeed become active. XRRP wouldn't fail over since the XRRP multicast advertisements from switch 1 would still be seen by switch 2.
So host A can still reach his default gateway via 3>C>E.
Above is with the assumption that VLAN 10 is active on all links.
And I assume that switch 2 in "switch 2 can still see switch 1 on layer 3 via link C-E" should be switch 3.

And you are right, XRRP does a total failover even if one VLAN cannot be served. This shouldn't be a problem in your situation.

So my "pointers" in your story:
- Use default costs
- Make switch 1 root bridge
- Tag VLAN 10 (and all other VLAN's you want to use) on all links

As you see I used some assumptions. If any of them are wrong, let me know and maybe we can adjust a little.

Re: Redundant network L2 - L3

Thanks again, Kel. Your assumptions were right, a bit sloppy on my side while typing my 'story'.