cancel
Showing results for 
Search instead for 
Did you mean: 
cancel
Announcements

621
Views
0
Helpful
2
Replies
Beginner

Standby Flexlink not capable of discarding multicast?

I have doubts about Flexlink.

interface FastEthernet0/1
switchport trunk encapsulation dot1q
switchport mode trunk
switchport backup interface Fa0/2
switchport backup interface Fa0/2 multicast fast-convergence
switchport backup interface Fa0/2 preemption mode bandwidth
switchport backup interface Fa0/2 preemption delay 20
end

Recently I am replicating my customer's problem with flexlink. He told me whenever flexlink pairs are fully connected, the process cpu would shoot up to 80% and telneting to the 3750 switch would be very sluggish, however when he removed the flexlink standby interface cable, the sluggishness disappeared.

I did a lab just now to replicate my customers' problem using PRTG. I was surprised to see the standby interface fa0/2 has multicast traffic flowing in and out, I thought the standby flexlink was supposed to discard traffic? why did this happen?

1 ACCEPTED SOLUTION

Accepted Solutions
VIP Expert

Re: Standby Flexlink not capable of discarding multicast?

You see multicast traffic because you have configured this command

switchport backup interface Fa0/2 multicast fast-convergence

Leaking IGMP Reports

To achieve multicast traffic convergence with minimal loss, a redundant data path must be set up before the Flex Link active link goes down. This can be achieved by leaking only IGMP report packets on the Flex Link backup link. These leaked IGMP report messages are processed by upstream distribution routers, so multicast data traffic gets forwarded to the backup interface. Because all incoming traffic on the backup interface is dropped at the ingress of the access switch, no duplicate multicast traffic is received by the host. When the Flex Link active link fails, the access switch starts accepting traffic from the backup link immediately. The only disadvantage of this scheme is that it consumes bandwidth on the link between the distribution switches and on the backup link between the distribution and access switches. This feature is disabled by default and can be configured by using the switchport backup interface interface-id multicast fast-convergence command.

When this feature has been enabled at changeover, the switch does not  generate the proxy reports on the backup port, which became the  forwarding port.

Here is the link to it:

http://www.cisco.com/en/US/docs/switches/lan/catalyst2960/software/release/12.2_44_se/configuration/guide/swflink.html#wp1091960

HTH

Reza

View solution in original post

2 REPLIES 2
VIP Expert

Re: Standby Flexlink not capable of discarding multicast?

You see multicast traffic because you have configured this command

switchport backup interface Fa0/2 multicast fast-convergence

Leaking IGMP Reports

To achieve multicast traffic convergence with minimal loss, a redundant data path must be set up before the Flex Link active link goes down. This can be achieved by leaking only IGMP report packets on the Flex Link backup link. These leaked IGMP report messages are processed by upstream distribution routers, so multicast data traffic gets forwarded to the backup interface. Because all incoming traffic on the backup interface is dropped at the ingress of the access switch, no duplicate multicast traffic is received by the host. When the Flex Link active link fails, the access switch starts accepting traffic from the backup link immediately. The only disadvantage of this scheme is that it consumes bandwidth on the link between the distribution switches and on the backup link between the distribution and access switches. This feature is disabled by default and can be configured by using the switchport backup interface interface-id multicast fast-convergence command.

When this feature has been enabled at changeover, the switch does not  generate the proxy reports on the backup port, which became the  forwarding port.

Here is the link to it:

http://www.cisco.com/en/US/docs/switches/lan/catalyst2960/software/release/12.2_44_se/configuration/guide/swflink.html#wp1091960

HTH

Reza

View solution in original post

Highlighted
Beginner

Re: Standby Flexlink not capable of discarding multicast?

Thank you Reza, I have overlooked this section in cisco doc.... and misunderstood flexlink standby port.... it does not actually behave the same as RSTP alternate port. flexlink can cause multicast loop if a flexlink pair is connected from a switch to Cisco 871 and Cisco 881 switch ports, cpu uitilization will keep increasing, and utilization will never go down even the MC source is turned off.... Got to be cautious with flexlink, never configured and connect a pair to the upstream switch, should configure flexlink pair to connect to two different switch for each flexlink....

CreatePlease to create content
Content for Community-Ad
July's Community Spotlight Awards