Re: CAM table not updating after STP TCN

Hey

In STP, when u receive a TCN, rom a non root bridge, the other non root
bridge will send the TCN towards the root port to the Root bridge and when
the root bridge sets the TC bit to 1 and send the BPDU to all other non
root bridges,
When Non root bridges receive this BPDU, they all flush out their MAC
address on all the ports .

So its now a pretty empty MAC table. Unlike IP ROUTE Table, there is no
separate MAC exchange protocols ( Yes BGP can do waterer u say ) , But
generally ur MAC table will be populated only with the user traffic. So
initially the unknown unicast flooding will happen for all unknown
destination MAC address and all the MAC will be learnt from various ports
and STP reconvergence happens and everything will be set.

There will be initial flooding within the broadcast domain. So STP TCN has
nothing to do with CAM table learning, it can help to inform Root bridge
about a topology change ( ie Port going from blocking to forwarding or
Forwarding to blocking )
and root bridge informs the other non root bridges to flush the MAC address
learned on all the ports.

So the user generated traffic will really help u in building the MAC table.

Speaking about ARP in switches, u wont find arp table in a pure L2 switch.
Switch doesn't generate any ARP Packets, it just switches from one port to
another.

U can see ARP table in hosts , routers and L3 Switches . ARP will be
generated by hosts inorder to get to the gateway or to another machine in
the same domain.
Routers / L3 switches generate ARP inorder to reach the hosts , when a
packet comes to the router/L3 switch which is destined to that particular
host.

So ARP table and CAM table has nothing to do with each other. But there is
a scenario where ur L3 Switch will be doing asysmetric forwarding bcoz of
Aging timer mismatch in ARP and CAM Table

For example, when a frame enter the switch, it checks the CAM table, if
there is any entry it will forward it,
If the packet is destined to routing engine, it goes to ROute table and
checks that route is installed in the table and we can send this packet out
this interface. So to send this outside, we need L2 address to fill up. In
that case it will check the ARP table and then fill up the L2 address .

When there is no traffic sent for a while, ur CAM table will first age out
in the Switch and it will do a unknown unicast flood and ur ARP table still
holds the MAC info. Now inorder to avoid this discrepancy, when designing
MAC and ARP table aging timers should be set higher for CAM and bit lower
for ARP. So ARP will poll always and fill up the MAC table in order to
avoid unknown unicast flooding

Hope this helps

On Mon, Jan 14, 2013 at 4:20 PM, ramesh Kumar
<rameshkumar123321_at_yahoo.com>wrote:

> I have 2 switches connected back to back on port-channel 1. As per my
> understanding after any STP change, I should see CAM table update and
> shortly
> after that ARP table update. I see the ARP being refreshed but it looks
> like
> the CAM table on SW2 didnt update.
>
>
> SW1# sh ip arp | i 34a8.4eb6.0aa2
> 172.20.210.38 00:01:10 34a8.4eb6.0aa2 Vlan2010
>
>
> SW1# sh mac add
> dy | i 34a8.4eb6.0aa2
> * 2010 34a8.4eb6.0aa2 dynamic 0 F
> F Eth1/9---> aging time 0 shows that we are getting traffic hitting this
> entry continuously. Right?
>
>
> SW2# sh ip arp | i 34a8.4eb6.0aa2
> 172.20.210.38
> 00:01:14 34a8.4eb6.0aa2 Vlan2010
>
>
> SW2# sh mac add dy | i
> 34a8.4eb6.0aa2
> * 2010 34a8.4eb6.0aa2 dynamic 120 F F
> Po1----> it seems the CAM timer didn't reset after TCN but the ARP
> refreshed
> The link between the switches are configured like this and these switches
> are
> running HSRP for all the vlans with SW1 being STP root and HSRP primary for
> all the vlans
>
>
> interface port-channel1
> switchport
> switchport mode trunk
>
> switchport trunk allowed vlan 2,650,660,720,730,790,1920,1930
> switchport
> trunk allowed vlan add 1640,1960,1910,1930,1490,2000,2010
> spanning-tree
> link-type point-to-point
> mtu 9216
> load-interval counter 1 60
>
>
> For arp
> there is a command "show ip arp internal event-history" which can show if
> the
> arp was refreshed due to a TCN or not. which command can help in checking
> if
> mac table was refreshed due to TCN or not apart from show mac address
> table?
>
>
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-- 
Regards
Routing Freak CCIE#35889 (SPv3)
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