HSRP/STP Help

Hi everyone,

I'm just labing up a HSRP scenario and I suppose this might be more an STP thing than anything else but I wonder if someone can help? I'll keep my explanation brief for now so it's a bit easier to follow. I have the below topology set up:

Now if I ping from PC A to the VLAN 20 default gateway (20.0.0.254) my pings are going up to SW1 and then up again to SW3 before being switched over to SW4.

Now I would expect the behavior to go ---->

A to SW1 to SW4 out fa1/2.

-The reason I say this is because SW4 is the root for VLAN20 and checking the 'sh spanning-tree' on SW1, Fa1/2 is the Root Port for this vlan and fa1/1 is BLK.

Could this be a GNS3 thing or am I missing something here?

SW1#

VLAN20
Spanning tree enabled protocol ieee
Root ID Priority 8191
Address c203.0da4.0002
Cost 19
Port 43 (FastEthernet1/2)
Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec

Bridge ID Priority 32768
Address c200.0da4.0002
Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec
Aging Time 300

Interface Designated
Name Port ID Prio Cost Sts Cost Bridge ID Port ID

----

---

FastEthernet1/1 128.42 128 19 BLK 19 8192 c201.0da4.0002 128.42
FastEthernet1/2 128.43 128 19 FWD 0 8191 c203.0da4.0002 128.43

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Comments

alliasneo

Ahh ok I just figured this out. I was so busy concentrating on HSRP I forgot the basic IP rules.

A is forwarding traffic to its default gateway of 10.0.0.254 and because Group 1 is set as the active for Vlan 10 it goes to SW3 over the FA1/1 link.

boobobobobob

Great question, these types of questions are the reason i lurk these forums!

According to the topology the active gateway for both VLANs would be SW4. Since both vlans have the default priority level, HSRP is going to choose the switch that has the higher IP address for that vlan. So we know all layer 2 packets have to reach SW4 before being routed. That means we need to know the STP tolopgy on SW1 for vlan 10 NOT vlan 20. PCA is on VLAN 10, but you show us the STP information for VLAN 20, which can be completely different (and is different for this example). Remember PCA is going to tranverse the network on VLAN 10 until it hits a router. Since SW1 is not routing it's going out on VLAN 10 still. This explains why the path is going from SW1 to SW3. Since S3 is the root for VLAN 10 f1/1 is in a forwarding state.