show internal vlan usage

[Akash R. Bhame]

Hi All,

This is Akash.

Have query regarding internal vlan, we have Cisco 6500 switch with IOS version

s72033-adventerprisek9_wan-mz.122-33.SXI9, we have not configured ipv6 multicast still when we run the showw internal vlan usage we can see vlan 34 is used for IPv6 Multicast Egress multicast want to know how i can free this vlan for normal use and why it is showing used as internal vlan even tough we have not enable the ipv6 multicast.

CoreSwitch#sh vlan internal usage

VLAN Usage

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

34   IPv6 Multicast Egress multicast

35   GigabitEthernet1/9

quick response on this will realy appriciate.

Thanks in advance for your kind support.

Regards

Akash

[daniel.dib]

Seems this internal VLAN is reserved for optimization of replicating multicast packets:

"Egress replication mode uses a special internal egress VLAN to distribute the replicated multicast packets to all egress (destination) modules. Membership to this internal VLAN is determined simply by including all egress-capable modules in a distribution list. Furthermore, there is only a single egress VLAN (per VRF). Hence, there is no guarantee that all programming messages will arrive (once transmitted), and all groups and associated egress modules share the same context."

http://www.cisco.com/en/US/prod/collateral/switches/ps5718/ps708/white_paper_c11-652029.html#wp9000359

The only thing you can choose is if to allocate internal VLANs from the bottom or from the top:

vlan internal allocation policy ascending or descending

Daniel Dib
CCIE #37149

[Akash R. Bhame]

Thanks Daniel for your response, want to know following  things.

1) is there any way i can free this vlan and use as L2 vlans if yes then what i have do to release this vlan from internalusage.

2) how vlan 34 got reserved for internal use.

Regards

[daniel.dib]

The only thing I know of is to start assigning it from the top instead with the descending command. But I think that requires a reboot.

Not sure how the internal VLANs are assigned but probably that was the first available one? Do you use other VLANs up to 33?

Daniel Dib
CCIE #37149

[Akash R. Bhame]

Thanks for your quick response Daniel.

yes up to 33 vlans we have created, 34 vlan is not created may be that is the reason system has taken this vlan.

my question is can we disable the optimization of replicating multicast packets and what would be the impact of same.

to free vlan 34.

if no then other option would be

vlan internal allocation policy descending

this will require reload?

regards

Akash

[daniel.dib]

I'm not sure if it can be disabled. Do you run IPv6 multicast routing? It seems the allocation is done on bootup:

"

During system bootup, the internal VLANs that are required for features in the startup-config file are allocated first. The user-configured VLANs in the startup-config file are configured next. If you configure a VLAN that conflicts with an existing internal VLAN, the VLAN that you configured is put into a nonoperational status until the internal VLAN is freed and becomes available.

After you enter the write mem command and the system reloads, the reconfigured allocation scheme is used by the port manager.

"

https://supportforums.cisco.com/thread/2102857

So the last time you booted VLAN 34 was allocated. Seems there is no easy solution unless it can be disabled or to reboot.

Daniel Dib
CCIE #37149

[Akash R. Bhame]

Hi Daniel,

Multicast is disabled on switch.

The url which you have shared says extended vlan used for internal vlan usage and that range is 1006 to 4094 then how come in this case it is used vlan 34?

Regards

Akash