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directly connected or HSRP - on 6500

prakadeesh
Level 1
Level 1

Hello,

I have two 6500 say c1 and c2, intrerconnected via trunk. Now they both have HSRP and PVST running on them. The PVST is inline with the HSRP active state. Say for even vlans c2 is HSRP active and root, for odd valsn c1 is hsrp active and root. Now one of the machines with say IP address 10.4.4.4 ( vlan 4 , even vlan)is connected to c1. Now if a packet to 10.4.4.4 arrives on c1 may be from a different VLAN or say a WAN , will c1 use the IP ARP table in it and forward it using the directly connected link OR will is see c2 as the HSRP active for this even vlan and fwd to C2 to take the l3-l2 decision, now c2 will anyways fwd the packet back to c1 as the 10.4.4.4 is conencted to c1. Please help me get round this issue.

thanks,

1 Accepted Solution

Accepted Solutions

Hello Prakadeesh,

traffic from host 10.4.4.4 will use the c1-c2 link to reach VIP address.

return traffic may come back via c1 or via c2.

if via c2 the c1-c2 is used also on return path else c1 will resolve 10.4.4.4 and forwards the frame to host directly

Hope to help

Giuseppe

View solution in original post

15 Replies 15

davy.timmermans
Level 4
Level 4

as the link is blocked by spanning-tree it will go via the interconnected trunk to C2

Davy

I think we are reading the question differently :-)

The client is directly connected to c1 so that link can't be blocked as you don't block client machine links with STP ?

Or have i misunderstood ?

Jon

Jon Marshall
Hall of Fame
Hall of Fame

Prakadeesh

c1 would use the arp table and send it back to the client direct. HSRP is used by the client but the 6500 switches do not care about HSRP. If c1 has a L3 vlan interface in vlan 4 it will send the packet direct to the client.

Jon

Hi Jon

I think you forget that the link between C1 and the access switch is blocked by STP for vlan 4

What access switch ?

Jon

Giuseppe Larosa
Hall of Fame
Hall of Fame

Hello Prakadeesh,

>> will c1 use the IP ARP table in it and forward it using the directly connected link

from a routing point of view c1 is connected to subnet 10.4.4.0 so the packet is sent using L2 forwarding.

However, c1 will reach host 10.4.4.4 going through the c1-c2 trunk because the link c1-accessswitch is blocked by STP on the access switch side.

the HSRP VIP for Vlan4 has no use for c1 for forwarding packets to 10.4.4.4

Hope to help

Giuseppe

Giuseppe

Perhaps it's me :-), but what access switch ?

Jon

It's an assumption but as C1 and c2 are configured as STP root,backup and hsrp active, standby it's pretty clear it's about an access-distribution/core situation. The poster was wondering how the return traffic flows.

From the standpoint the traffic for a certain vlan always arrive on the same switch hsrp active. but that's not true for the return traffic.

The situation would be different if C1 and C2 are connected via a L3 link (routed ports).

If that's the case, both links towards the access switch are forwarding and then it would do a lookup in the mac table and not the arp.

Davy

Guess that's what i meant about both of us reading the question differently :-)

From original post -

"Now one of the machines with say IP address 10.4.4.4 ( vlan 4 , even vlan)is connected to c1"

I agree if client is connected to an access-layer switch which is then dual connected to 6500's then yes the traffic will flow across the trunk and be forwarded back but that's not the way i read the question.

Jon

you mean host device is directly connected?

Then he should talk about servers with dual uplinks because of the HSRP in the story and STP

Davy

Maybe, maybe not. I have seen setups where clients and servers are connected directly into a pair of 6500's and so even clients singly honed are connected to switches running both HSRP & STP. Not good design though.

I'm not disagreeing with anything you say, just think we both interpreted the question differently that's all.

Jon

Hello Jon,

you're right I'm sleeping in front of my pc...

from OP post

>> Now one of the machines with say IP address 10.4.4.4 ( vlan 4 , even vlan)is connected to c1

However, to be noted that in this case it is the traffic from host 10.4.4.4 that could make a nice trip to reach c2 that is the HSRP active router on the c1-c2 trunk.

So the c1-c2 trunk is used in one direction (at least)

Hope to help

Giuseppe

Hello folks,

Sorry I should have mentioned that there is no access layer involved. The machine/server 10.4.4.4 is directly/physically connected to c1, not via any intermediate switch. So incase c1 recieves a packet for 10.4.4.4 will it follow the HSRP rule to send to c2 and get the same packet back from c2 or just use its MAC table and give to 10.4.4.4 directly. I understand that the traffic from the 10.4.4.4 will traverse c1 and to c2 for any L3 stuff. Hope I am clear this time :).

Thanks,

prakadeesh

Prakadeesh

Traffic from client will go to c1 then across link to c2 and be routed from there.

Traffic coming back to client will go the most direct way so if the packet goes to c1 first, c1 will just forward it to the client.

Note - the above applies to routed traffic.

Jon

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