Cisco ASR 9000 Series System Configuration Guide

Behavior of Multicast Offload During Satellite Topology Events
These are the behavior of the Multicast offload during various satellite topology events:
•
Split brain: During a split brain event, the hosts function independently and the system behaves as two
separate chains with IGMP snoop sync broken between satellites and each host offloading to its own
set of satellites. On recovery, there is reconciliation and some reprogramming that can result in traffic
loss as IGMP snoop protocol states sync up.
•
RSP failover: This is expected to be hitless for traffic going over routes that have been offloaded already.
New joins during the failover may experience the same set of events and traffic impact as the split brain
condition.
•
Ring break and Recovery: On ring breakage, any satellites aligned to other host as designated multicast
forwarder has to realign to the remaining host on the ring. This can have a minimal (sub-second for L2
core) traffic impact on a stable system where the routes are already synchronized. On recovery, there is
no revertive switchover and so there must be no traffic impact.
For more information, refer Features of Dual Home Network Architecture section in the Configuring the
Satellite Network Virtualization (nV) System chapter.
Note
Difference between Layer 2 and Layer 3 (over BVI) core support
For the Dual host topology systems connecting to an Layer 3 core through BVI, the current recommendation
is to align all satellites to a single host as Unicast active. This is required as BVI protocol states are kept down
on the standby host to avoid ECMP being triggered from the core for an active/standby Satellite nV system
that could lead to traffic drop of half the packets on the standby host.
Currently, multicast offload which is implemented as an inherently active/active system needs to track the
BVI redundancy states as well to avoid picking the Designated multicast forwarder on the host having the
standby BVI. While this ensures a common feature model with the Layer 2 core, the benefits of an active/active
system, especially, the sub-second switchover convergence is not valid anymore. This can be improved by at
least allowing IGMP snoop sync to continue by configuring an internal querier with system IP address lower
than the BVI IP address and a query max timeout of 1s.
Uplink/Core Isolation Tracker
Under redundancy group configuration, there is an option to configure a backbone interface. Multiple backbone
interfaces can be configured depending on the number of links from the Host to the core.ICCP protocol keeps
a watch over the link states of these interfaces and if all of these backbone interfaces go down, then a core
isolation event is notified to the client.
As part of the multicast offload feature, IGMP Snoop registers as a client to ICCP protocol and propagates
these notifications to the satellite devices. Any satellite which still expect this host to be the designated multicast
forwarder can then switchover to the other host as core connectivity is lost. In order to avoid traffic downtimes
during flaps, this event is triggered only for core link up to down condition and is non-revertive. A satellite
Cisco ASR 9000 Series Aggregation Services Router nV System Configuration Guide, Release 5.3.x
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Configuring Multicast Offload on the Satellite nV System
Behavior of Multicast Offload During Satellite Topology Events

Cisco ASR 9000 Series Specifications

General

Brand	Cisco
Model	ASR 9000 Series
Category	Network Router
Language	English