mac flap

[jay_7301]

Hello,

I came across an issue with and cisco 1841 fa0/1 plug into a 8 port cisco switch  (G0/1)

there are 3 sub ifs on the router

10

20

30

I keep getting mac flap between GIG0/1 the uplink to the router and f0/1 which is in access mode and faces the customer.

The mac in question was the BIA of fa0/1 I don't understand how this can happen spanning tree is enabled portfast is not enabled

any idea's?

[acampbell]

Hi,

May be I missed something here in your description.

Your router fas0/1 has sub interfaces
Your switch gig0/1 is in access mode ???

If you are using sub interfaces on your router then to me the switch port should be in trunk mode
something like this:-

ROUTER

!
int fas0/1
desc ** TRUNK TO SWITCH **
duplex auto
speed auto
!
int fas 0/1.10
desc ** VLAN 10 ***
 encapsulation dot1Q 10
 ip address 10.0.10.254 255.255.255.0
 no ip redirects
 no ip unreachables
!
int fas 0/1.20
desc ** VLAN 20 ***
 encapsulation dot1Q 20
 ip address 10.0.20.254 255.255.255.0
 no ip redirects
 no ip unreachables
!
int fas 0/1.30
desc ** VLAN 30 ***
 encapsulation dot1Q 30
 ip address 10.0.30.254 255.255.255.0
 no ip redirects
 no ip unreachables
!

SWITCH

!
int gig0/1
desc ** TRUNK TO SWITCH **
speed auto
duplex auto
 switchport trunk encapsulation dot1q
 switchport trunk allowed vlan 10,20,30
 switchport mode trunk
!

Regards
Alex

[Aaron Harrison]

Do you have bridging enabled on the 1841?

Can't think why else the MAC of one interface would be seen on the other, unless you have a link from the 8-port switch to something on the other side of the router... Or the customer has connected a cable from the switch to whatever is on the 'outside' fa0/1 side of the router?

Aaron