ASK THE EXPERTS - Information about show ip mfib all

jhcai2005 · ‎10-30-2005

IP Multicast Forwarding Information Base

Entry Flags: C - Directly Connected, S - Signal, IC - Internal Copy

X - Inactive, D - Dormant, * - MFIB special entry

Interface Flags: A - Accept, F - Forward, S - Signal

IC - Internal Copy, NP - Not Platform fast-switched

Packets: Fast/Partial/Slow Bytes: Fast/Partial/Slow

(*, 224.0.0.0/4), flags (S *)

Packets: -/0/0, Bytes: -/0/0

(192.168.100.0/2, 224.0.0.0/4), flags (C *)

Packets: 0/0/1334, Bytes: 0/0/74694

(*, 224.0.0.1), flags (IC *)

Packets: 0/0/0, Bytes: 0/0/0

(*, 224.0.0.2), flags (IC *)

Packets: 0/0/9551785, Bytes: 0/0/458485680

(*, 224.0.0.5), flags (IC *)

Packets: 0/0/0, Bytes: 0/0/0

(*, 224.0.0.6), flags (IC *)

Packets: 0/0/0, Bytes: 0/0/0

(*, 224.0.0.13), flags (IC *)

Packets: 0/0/0, Bytes: 0/0/0

(*, 224.0.0.22), flags (IC *)

Packets: 0/0/0, Bytes: 0/0/0

(*, 224.0.1.40), flags (IC X)

Packets: -/0/0, Bytes: -/0/0

Vlan600 (F S)

(*, 234.74.74.74), flags (X)

Packets: -/0/0, Bytes: -/0/0

Vlan600 (F S)

What does the flags (X) mean? I haven't finded out the explaination in www.cisco.com.

paul.werner · ‎10-30-2005

Recheck your legend/header data for this output:

Entry Flags:

C - Directly Connected

S - Signal

IC - Internal Copy

X - Inactive

D - Dormant, * - MFIB special entry

Interface Flags:

A - Accept

F - Forward

S - Signal

IC - Internal Copy

NP - Not Platform fast-switched

Now if your real question was, "what does inactive mean?" I would say that represents a path that was checked using "Reverse Path Forwarding" and was determined to be a path that is not needed on the multicast distribution tree to ensure all multicasts enabled hosts receive the multicast. In short, nobody is yet listening on VLAN 600 for either 224.0.1.40 (cisco-rp-discovery) or 234.74.74.74 (some type of multicast app you are running) multicast packets.

To check this, when you do a "show ip mroute", does 224.0.1.40 show up in the multicast routing table? If so, what does the entry look like?

pw

jhcai2005 · ‎10-30-2005

c4948-b.bjs#sh ip mroute

IP Multicast Routing Table

Flags: D - Dense, S - Sparse, B - Bidir Group, s - SSM Group, C - Connected,

L - Local, P - Pruned, R - RP-bit set, F - Register flag,

T - SPT-bit set, J - Join SPT, M - MSDP created entry,

X - Proxy Join Timer Running, A - Candidate for MSDP Advertisement,

U - URD, I - Received Source Specific Host Report, Z - Multicast Tunnel

Y - Joined MDT-data group, y - Sending to MDT-data group

Outgoing interface flags: H - Hardware switched, A - Assert winner

Timers: Uptime/Expires

Interface state: Interface, Next-Hop or VCD, State/Mode

(*, 234.74.74.74), 2w6d/00:02:00, RP 0.0.0.0, flags: DC

Incoming interface: Null, RPF nbr 0.0.0.0

Outgoing interface list:

Vlan600, Forward/Dense, 2w6d/00:00:00, H

(*, 224.0.1.40), 2w6d/00:02:05, RP 0.0.0.0, flags: DCL

Incoming interface: Null, RPF nbr 0.0.0.0

Outgoing interface list:

Vlan600, Forward/Dense, 2w6d/00:00:00, H

c4948-b.bjs#sh run int vlan 600

Building configuration...

Current configuration : 216 bytes

!

interface Vlan600

ip address 192.168.100.253 255.255.255.0

no ip redirects

no ip proxy-arp

ip pim dr-priority 10

ip pim dense-mode

standby 4 ip 192.168.100.254

standby 4 priority 110

standby 4 preempt

end

paul.werner · ‎10-31-2005

Okay, since I do not see 224.0.1.39 in your multicast routing table, it validates that you do not have anyone in the rp-announce group, so it stands to reason you would not see any traffice on the RPlistening group. Validate this with a "show ip mroute count" command and see how many packets are sent or received in that group. My guess would be zero, hence inactive. The next better question would be, why you elected to go with dense mode? Do you anticipate having a lot of multicast listeners for the multicasts you send? Any particular reason for not going with sparse-dense mode? Just curious.

pw

jhcai2005 · ‎10-31-2005

c4948-b.bjs#sh ip mroute count

IP Multicast Statistics

4 routes using 2210 bytes of memory

3 groups, 0.33 average sources per group

Forwarding Counts: Pkt Count/Pkts per second/Avg Pkt Size/Kilobits per second

Other counts: Total/RPF failed/Other drops(OIF-null, rate-limit etc)

Group: 239.255.2.2, Source count: 1, Packets forwarded: 0, Packets received: 0

Source: 192.168.100.198/32, Forwarding: 0/0/0/0, Other: 0/0/0

Group: 234.74.74.74, Source count: 0, Packets forwarded: 0, Packets received: 0

Group: 224.0.1.40, Source count: 0, Packets forwarded: 0, Packets received: 0

The configuration of the dense mode is simple. Is it usefull that the hardware forwarding in the inactive entry? Well, how can I use the hardware forwarding?

paul.werner · ‎10-31-2005

You are correct, configuration of dense mode is simpler, hence the reason that many use it first. There are design considerations when employing multicast in a network that should be looked at before optiing for one version over another. Here is an excellent article by Pete Welcher that covers PIM dense mode and what it is all about and the design considerations of using it:

http://www.netcraftsmen.net/welcher/papers/multicast02.html

Likewise, Pete wrote another article about PIM sparse mode and the pros and cons of using it:

http://www.netcraftsmen.net/welcher/papers/multicast03.html

Both are excellent articles that will weigh the relative advantages and disadvantages of using both methods. It's a good read and it may help on what you are trying to do.

pw