Notes on Cisco 65xx and IP multicast
Bill Nickless
nickless at mcs.anl.gov
Sun Oct 1 18:31:50 CDT 2000
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If you, or someone you know, is deploying Cisco 65xx switch routers to
support IP multicast for an Access Grid node, here are some pieces of
information that you might find useful. This note does not cover
deployment of PIM Sparse Mode, Multiprotocol BGP, or MSDP, but rather
concentrates on Cisco 65xx specific issues.
RUN IOS 12.1(2)E (OR LATER) ON THE MSFC
=======================================
http://www.cisco.com/cgi-bin/Support/Bugtool/onebug.pl?bugid=CSCdp68820
describes a problem with PIM Sparse Mode on versions of MSFC IOS that are
based on IOS prior to 12.0(10). Essentially what will happen is that a
locally-attached (S,G) will get into a state where it's not sending PIM
Register messages towards your MSDP-speaking Rendezvous Point. When that
happens, your MSDP-speaking RP will not send MSDP Source Active messages
out to your peers. That means your source will not be heard by new
joiners. You can work around this in real time by clearing the (S,G)
[clear ip mroute <group> <source>] but this is obviously not something you
want to be stuck doing regularly.
[Note: this is true of all Cisco IOS releases prior to 12.0(10), not just
those running on the 65xx MSFCs.]
RUN IGMP WITHOUT FAST LEAVE
===========================
This setting on the switch side of the 65xx seems to work best for us:
>stardust> (enable) show multicast protocols status
>IGMP enabled
>IGMP fastleave disabled
>RGMP disabled
>GMRP disabled
http://www.cisco.com/univercd/cc/td/doc/product/lan/cat6000/sw_5_5/cnfg_gd/m
ulti.htm talks about these various services in more detail. In summary,
though, IGMP is the protocol that the Access Grid specified host software
(Windows 9x, Red Hat Linux, and Windows 2000) knows and operates with
best. You have to choose between GMRP and IGMP when you configure the
multicast switching protocols: I strongly recommend IGMP.
In a situation where you have multiple hosts connected to a port on the
65xx, such as all four hosts of an AG node connected by a hub, IGMP fast
leave is probably a bad idea. When an IP multicast tool like vic exits
(say, on the video capture machine) it causes the host to send an IGMP
leave message to the network. If IGMP leaves are being processed by the
65xx, the traffic for that group is immediately stopped. This causes a
loss of traffic to other hosts on that port which still want to receive the
traffic (like the display machine[s]). Thus I strongly recommend
*disabling* IGMP fast leave processing.
MLS IP MULTICAST MAY REQUIRE NO IP UNREACHABLES
===============================================
MLS IP Multicast shortcuts are programmed into the forwarding hardware on
the switch side of the 65xx. This means the traffic never gets up to the
MSFC, so it doesn't require CPU resources to forward. At Argonne we've
seen Access Grid situations where the MSFC doing software switching cannot
keep up with the traffic. When this happens the CPU goes up to 99% and it
starts dropping packets. With MLS IP Multicast the CPU load goes down to
sub-5% given the same traffic load.
There is a problem when you try to do these three things simultaneously on
a Cisco 65xx/MSFC VLAN:
- mls ip multicast
- inbound access list
- ip unreachables
With all three of these features running simultaneously, the MLS IP
Multicast forwarding hardware in the 65xx will be programmed with the
appropriate shortcuts, but packets will simply not be forwarded. You can
run with any two of the three features, but all three together may cause
the 65xx to silently drop the incoming IP multicast packets.
Here at Argonne I run with 'no ip unreachables' in the VLAN configurations,
as the other two features are more important. This may cause you problems
with TCP MTU Discovery if your MSFC is routing between interfaces with
different MTU sizes, such as GRE tunnels or VLANs supporting Gigabit
Ethernet Jumbo Frames. I had to move a couple of GRE tunnels off of my
6509 because of this situation.
===
Bill Nickless http://www.mcs.anl.gov/people/nickless +1 630 252 7390
PGP:0E 0F 16 80 C5 B1 69 52 E1 44 1A A5 0E 1B 74 F7 nickless at mcs.anl.gov
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