Multicasting

How can you tell whether packets are coming in via SPT or RP by observing multicast routing table. It creates entries for both S,G and *,G. The lab manual says that you can tell by looking at flags but doesn’t explain what each flag stands for.

Find more posts tagged with

Save $250 on 2025 certification boot camps from Infosec!

Book now with code EOY2025

Button

Comments

miller811

singh8281 wrote:

How can you tell whether packets are coming in via SPT or RP by observing multicast routing table. It creates entries for both S,G and *,G. The lab manual says that you can tell by looking at flags but doesn’t explain what each flag stands for.

(*, G) entry: Timers, the RP address for the group, and the flags for the group (S is sparse) are listed.
The incoming interface is the interface toward the RP—if it is Null, the router itself is the RP. The Reverse Path Forwarding (RPF) neighbor is the next-hop address toward the RP.
If it is 0.0.0.0, the router is the RP for the group.
The outgoing interface list (OIL) lists the outgoing interfaces, along with modes and timers.

(S, G) entry: Timers and flags for the entry are listed (T indicates that it is on the SPT; A indicates that it is to be advertised by Multicast Source Discovery Protocol [MSDP]).
The incoming interface is the interface toward the source S.
The RPF neighbor is the next-hop address toward the source. If it is 0.0.0.0, the source is directly attached.
The OIL lists the outgoing interfaces, in addition to modes and timers.

singh8281

Thanks Miller but more specifically, I wanted to know about the flags.

marlon23

SAN-43-17-9#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, z - MDT-data group sender,
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

singh8281

Thanks Marlon, It's strange to have missed that that under that command.

marlon23

Now it is challenge to understand what they all mean in some scenarios. The good connection of flags with theory is in CCIE Exam Certification Guide from Odom. However sometimes you get such a combination of flags, that it starts to make sense after losing half of your hair (little misconfiguration and.. uff)

singh8281

True, these bunch of alphabets drive you nuts sometimes.

Should add the bottles of Minoxidil to the lab rack as well.