Skip to contentSkip to conversation

GRE devices and MULTICAST flag.

DISCUSSION Ben Greear 2 participants 3 messages Sep 23, 2011

Subsystems (2)
Networking Network drivers
lore
Share & tools
lore

Ben Greear

author
#0 lore raw

I was under the impression that GRE tunnels could support MULTICAST traffic, but the MULTICAST flag is not set. This is making the xorp router crap out because it checks the MULTICAST flag and will not start multicast on the interface if it's not set....

Is it supposed to be set?

7: gre0: <NOARP,UP,LOWER_UP> mtu 1476 qdisc noqueue state UNKNOWN

    link/gre 0.0.0.0 brd 0.0.0.0
8: netb: <POINTOPOINT,NOARP,UP,LOWER_UP> mtu 1476 qdisc noqueue state UNKNOWN
    link/gre 192.168.100.223 peer 192.168.100.138
    inet 10.0.1.1/32 scope global netb

Thanks,
Ben

Signature · 2 lines
Ben Greear <[email protected]>
Candela Technologies Inc  http://www.candelatech.com
author
#1 lore raw
Quoted 5 lines · Ben Greear

I was under the impression that GRE tunnels could support MULTICAST traffic, but the MULTICAST flag is not set. This is making the xorp router crap out because it checks the MULTICAST flag and will not start multicast on the interface if it's not set....

Quoted 7 lines

Is it supposed to be set?

7: gre0: <NOARP,UP,LOWER_UP> mtu 1476 qdisc noqueue state UNKNOWN link/gre 0.0.0.0 brd 0.0.0.0
8: netb: <POINTOPOINT,NOARP,UP,LOWER_UP> mtu 1476 qdisc noqueue state UNKNOWN link/gre 192.168.100.223 peer 192.168.100.138
inet 10.0.1.1/32 scope global netb

Well, ifconfig netb multicast seems to add the flag, so maybe all is as it should be.

Thanks,
Ben

Signature · 2 lines
Ben Greear <[email protected]>
Candela Technologies Inc  http://www.candelatech.com
#2 lore raw
Quoted 8 lines · Ben Greear
Quoted 7 lines · Ben Greear

I was under the impression that GRE tunnels could support MULTICAST traffic, but the MULTICAST flag is not set. This is making the xorp router crap out because it checks the MULTICAST flag and will not start multicast on the interface if it's not set....

Is it supposed to be set?

Quoted 8 lines
Quoted 5 lines

7: gre0: <NOARP,UP,LOWER_UP> mtu 1476 qdisc noqueue state UNKNOWN link/gre 0.0.0.0 brd 0.0.0.0
8: netb: <POINTOPOINT,NOARP,UP,LOWER_UP> mtu 1476 qdisc noqueue state UNKNOWN link/gre 192.168.100.223 peer 192.168.100.138
inet 10.0.1.1/32 scope global netb

Well, ifconfig netb multicast seems to add the flag, so maybe all is as it should be.

I distinctively remember that the MULTICAST flag is supposed to be an input to the kernel, not a feature-indication flag output. I'm quite unsure where I picked this up, but I remember using it in that way that xorp does and then finding out it's wrong.

-David