multicast routing on single cisco layer 3 switch

[Herman2018]

hi , we have only one Cisco layer3 switch (C3850) in our office ,and want to enable multicast routing between two vlan 2 & 3. How should configure pim on C3850? can use spare-mode or dense-spare-mode? Can anyone pls advise? thanks in advance!

[Elliot Dierksen]

It depends on the multicast application. I usually lean towards sparse mode unless there is a compelling reason to do otherwise. Sparse mode means multicast is not sent unless a join is received. Dense mode means multicast is sent unless a prune is received. Here is a link to another article that discusses sparse-dense mode.

https://community.cisco.com/t5/switching/ip-pim-sparse-dense-mode/td-p/786776 

[Herman2018]

thanks @Elliot Dierksen for your kind reply. As we only have one layer 3 switch, and the vlans  for multicast source and receivers  are connected to this layer 3. switch. If using sparse mode, need to configure a RP or not? There is no other layer 3 device. Please advise, thanks. 

[Herman2018]

hi @Elliot Dierksen , fyi, our multicast app is voice system. 

[MHM Cisco World]

since you dont have RP so no need spare mode 

use dense mode if receiver and sender connect to same SW

MHM

[Herman2018]

Thanks @MHM Cisco World for your advice. a further question: if we configure like below, will it work or not? 

1. ip multicast-routing

2. inter vlan 2

  ip pim spare

3. inter-vlan 3 

ip pim sparse

[MHM Cisco World]

ip pim dense not spare 
spare need RP 

MHM

[christy2951hernandez]

Hello nystateofhealth ny gov@Herman2018,

To enable multicast routing between two VLANs (2 & 3) on a single Cisco Layer 3 switch (C3850), you’ll need to configure Protocol Independent Multicast (PIM). You can use either sparse mode or dense mode, depending on your network’s requirements.

Here’s a simplified guide on how to configure PIM on your Cisco C3850.
Enable IP Multicast Routing.

conf t
ip multicast-routing

Configure PIM on VLAN Interfaces. For Sparse Mode.

interface Vlan2
ip pim sparse-mode
exit
interface Vlan3
ip pim sparse-mode
exit

Define a Rendezvous Point (RP) (if using Sparse Mode)

ip pim rp-address <RP-IP-Address>

Verify Configuration

show ip pim interface
show ip mroute

Sparse mode is generally more scalable and efficient for networks with many multicast groups and dispersed receivers. Dense mode is simpler and may be more suitable for smaller networks with dense receiver populations.

Remember, if you’re using Sparse Mode, you’ll need to configure a Rendezvous Point (RP). This can be done statically or dynamically using Auto-RP or Bootstrap Router (BSR).

For detailed steps and considerations, refer to the official Cisco documentation for configuring PIM on Catalyst 3850 switches. If you encounter any issues or need further assistance, the Cisco Community forums are a great place to seek advice.

Please ensure you have the necessary permissions and understand the impact of these changes on your network before proceeding with the configuration.

Best Regards,
nystateofhealth ny gov

[Herman2018]

Thanks @christy2951hernandez for your advice! as there is only one layer 3 device (core switch) in the small office network, it is not suitable to configure a RP on the same switch, right? RP is only applicable when there are more than 1 layer 3 devices (layer 3 switch or router), right? pls advise, thanks.

[Elliot Dierksen]

I don't think you will need an RP to make it work for your use case, but it is easy to test that out. I would start with sparse mode, and only switch to dense if it proves to be needed.

[Herman2018]

thanks @Elliot Dierksen for your advice. However, it is production environment, already configure "ip pim sparse-dense " . Maybe I can try to test in EVE-NG first. 

[Elliot Dierksen]

So you already have multicast working? What is it you are trying to change? I think I am confused.

[Herman2018]

Sorry @Elliot Dierksen , last week was too busy and didn't come there. Currently we enabled pim sparse-dense mode in production, but the vendor saw the packet loss, then they want to try pim sparse mode. But it sounds it is not workable.. Thanks. 

[MHM Cisco World]

What is relate of packet loss with mode of multicast you use?

The multicast mode use to build S,G only 

And again no need sparse-mode mode use dense mode there is no RP

For traffic drop check your QoS and multicast threshold level you config in interface 

MHM

[paul driver]

Hello

---

@Herman2018 wrote:
 If using sparse mode, need to configure a RP or not? 

---

Yes & No,
ASM = sparse mode (in BDIR) requires RP
SSM  =sparse mode does not require RP

Based on what you have described in your OP ASM sparse-mode would be applicable using static RP on the L3 switch, and igmp snooping on all access layer switches for vlans 2/3 and as suggested already by @Elliot Dierksen refrain from using dense mode.

Note:
The example that @christy2951hernandez  suggested looks applicable