Traceroute and HSRP standby switch

[mahesh18]

Hi Everyone,

I have 2 switches running HSRP A is active and B is standby.

A and B both have vlan 10.

A and B have both trunk connection to Layer 2 switch which has vlan 10.

When i do traceroute from Standby B switch to layer 2 Switch Vlan10 IP 192.168.10.5

Here is result

traceroute 192.168.10.5

Type escape sequence to abort.

Tracing the route to 2950T (192.168.10.5)

  1 2950T (192.168.10.5) 4 msec *  0 msec

Need to confirm if traceroute traffic from standby switch goes via HSRP Active switch ?

When i sh sh ip route 192.168.10.5 on standby switch  and trace the mac it shows that traffic goes via HSRP Standby switch?

So does all traffic from Standby swithc to layer 2 switch goes by ACTIVE HSRP switch?

Thanks

mahesh

[mahmoodmkl]

Hi
u r tracing the management ip of the layer 2 switch it seems so as both the switced have svi for tht vlan they can reach it directly.
no not all traffic goes through the standby switch. it depends upon the vlans which are assigned in the access switch

Sent from Cisco Technical Support iPhone App

[mahesh18]

Hi,

Thanks for reply.

If i have just say vlan 20 on layer 2 switch not SVI Vlan 20 then also traffic goes directly from standby to layer 3 switch?

do you know how can i use command trace mac to find the layer to path that traffic takes from standby to layer 2 switch?

Thanks

MAhesh

[cadet alain]

Hi,

traffic from standby switch to L2 switch will not go through the active switch, why would it ?

Regards.

Alain

Don't forget to rate helpful posts.

[mahesh18]

Hi Alain,

When i do sh ip route on standby switch

3550SMIB#                  sh ip route 192.168.10.4

Routing entry for 192.168.10.0/24

  Known via "connected", distance 0, metric 0 (connected, via interface)

  Routing Descriptor Blocks:

  * directly connected, via Vlan10

      Route metric is 0, traffic share count is

where IP 192.168.10.4 is IP of PC  on layer 2 switch.

i was tracing the mac of this PC  from standby switch as below

3550SMIB#  sh ip arp 192.168.10.4

Protocol  Address          Age (min)  Hardware Addr   Type   Interface

Internet  192.168.10.4            1   f0bf.97de.4f48  ARPA   Vlan10

3550SMIB#sh mac ad

3550SMIB#sh mac address-table ad

3550SMIB#sh mac address-table address f0bf.97de.4f48

          Mac Address Table

-------------------------------------------

Vlan    Mac Address       Type        Ports

----    -----------       --------    -----

  10    f0bf.97de.4f48    DYNAMIC     Po10

Total Mac Addresses for this criterion: 1

3550SMIB#sh ether

3550SMIB#sh etherchannel su

3550SMIB#sh etherchannel summary

Flags:  D - down        P - bundled in port-channel

        I - stand-alone s - suspended

        H - Hot-standby (LACP only)

        R - Layer3      S - Layer2

        U - in use      f - failed to allocate aggregator

        M - not in use, minimum links not met

        u - unsuitable for bundling

        w - waiting to be aggregated

        d - default port

Number of channel-groups in use: 1

Number of aggregators:           1

Group  Port-channel  Protocol    Ports

------+-------------+-----------+-----------------------------------------------

10     Po10(SU)        LACP      Gi0/1(P)    Gi0/2(P)

3550SMIB#sh cdp nei gi

3550SMIB#sh cdp nei gigabitEthernet 0/1

Capability Codes: R - Router, T - Trans Bridge, B - Source Route Bridge

                  S - Switch, H - Host, I - IGMP, r - Repeater, P - Phone

Device ID        Local Intrfce     Holdtme    Capability  Platform  Port ID

3550SMIA.manveer.com

                 Gig 0/1           133          R S I  

from active switch

3550SMIA#sh mac address-table address f0bf.97de.4f48

          Mac Address Table

-------------------------------------------

Vlan    Mac Address       Type        Ports

----    -----------       --------    -----

  10    f0bf.97de.4f48    DYNAMIC     Fa0/24

Total Mac Addresses for this criterion: 1

3550SMIA#sh cdp nei fa0/24

Capability Codes: R - Router, T - Trans Bridge, B - Source Route Bridge

                  S - Switch, H - Host, I - IGMP, r - Repeater, P - Phone

Device ID        Local Intrfce     Holdtme    Capability  Platform  Port ID

2950A            Fas 0/24          136           S I      WS-C2950T Fas 0/24

So above shows that PC mac on Layer 2 switch is going via Active switch

Thanks

Mahesh

[Richard Burts · Accepted solution]

Mahesh

This is an interesting question and the issue that it raises is quite subtle. I believe that part of what makes it subtle is that it seems to be a question about layer 3 issues (traceroute, HSRP) but in reality it is a question about layer 2.

Since both switches are dealing with a locally connected subnet it is not a question about routing and layer 3. It really is about how the switch finds and communicates with devices which it considers to be locally connected. When you traceroute the switch checks its arp table and finds an entry for the IP address which gives MAC address f0bf.97de.4f48.  Your investigation then (quite correctly) looks in the mac address table (the layer 2 forwarding table) for the address and finds it associated with Po10.

So the question becomes why does the switch learn the mac address from the EtherChannel and not learn it directly. I wonder if the answer is based on the behavior of Spanning Tree. If you go to the downstream swtich 2950A and check its connections to its upstream switches, I wondre if you will find that its upstream connection to the standby switch is in blocking mode?

If the 2950A has a layer two connection to the active switch, another connection to the standby switch, and the upstream switches have an EtherChannel connection, then this would create a layer 2 loop and some interface should go into blocking mode to prevent the loop. Where is the  blocking mode interface? I would guess that the active switch is the root of the spanning tree and that the standby switch is not talking directly to the 2950A because of Spanning Tree issues.

HTH

Rick

[mahesh18]

Hi Rick,

Its always good to read you posts.

Layer 2 switch has 2 trunk connections one goes to Active other goes to standby switch.

Here is STP info

2950A#sh spanning-tree int fa0/24                   goes to active switch

Vlan             Role Sts Cost      Prio.Nbr Type

---------------- ---- --- --------- -------- --------------------------------

VLAN0001         Root FWD 19        128.24   P2p

VLAN0010         Root FWD 19        128.24   P2p

VLAN0020         Root FWD 19        128.24   P2p

VLAN0030         Root FWD 19        128.24   P2p

2950A#sh spanning-tree int fa0/20 ----------------goes to standby switch

Vlan             Role Sts Cost      Prio.Nbr Type

---------------- ---- --- --------- -------- --------------------------------

VLAN0001         Altn BLK 19        128.20   P2p

VLAN0010         Altn BLK 19        128.20   P2p

VLAN0020         Altn BLK 19        128.20   P2p

VLAN0030         Altn BLK 19        128.20   P2p

2950A#

So this shows you were spot on and this behaviour was indeed due to STP.

Upstream connection to standby switch is in blocking mode.

2 layer 3 switches have etherchannel connection between them.

yes you are right active switch is the root of the spanning tree.

One thing to confirm with you is that when we do the traceroute from any device and if any  device is only 1 hop away from us then if we do sh ip arp   it will show up there right?

If that device is few hops away it will not show up in sh ip arp right?

Thanks

Mahesh

[Richard Burts · Accepted solution]

Mahesh

I want to be sure that I understand your post correctly. When you talk about traceroute that is one hop away, then you are talking about a destination that is in the same subnet as the source? In that case then certainly show arp should show the destination address (and its associated mac address).

It is slightly ambiguous when the destination is more than one hop away. In particular if the source and the destination are in different subnets but are both connected on the same layer 3 device (router or layer 3 subnet) then it is quite likely that show arp will still show the destination IP and its associated mac address.  If the destination is more than 2 hops away then I would assume that the destination would not show up in the output of show arp.

HTH

Rick

[mahesh18]

Hi Rick,

Many thanks for answering all my queries.

After reading your posts i am understanding  the things much better.

One path was blocked by STP this concept did not stuck my mind.

But for future i will remember the STP role also.

Things are very clear to me now after getting  your reply backs

Best Regards

Mahesh