Avaya 8800 Planning And Engineering, Network Design page 94

Redundant network design
When R2 detects that the RSMLT in R1 transitions to the DOWN state (for example, if R1 itself is
down, or its SMLT links are down, or the IST link is down) R2 takes over IPv6 termination and IPv6
Neighbor Discovery functionality on behalf or R1's IPv6 SMLT interface. Specifically:
• When the above event is detected, R2 transmits an unsolicited IPv6 Neighbor Advertisement
for each IPv6 address configured on R1's SMLT link using R1's MAC address (fe80::1 and
2003::1 in this example).
• R2 also transmits an unsolicited Router Advertisement for each of R1's routing prefixes (unless
R1's prefixes are configured as "not advertised").
• R2 responds to Neighbor Solicitations and (if configuration allows) Router Advertisements on
behalf of R1
• R2 terminates IPv6 traffic (such as pings) destined to R1's SMLT IPv6 addresses
When R1's RSMLT transitions back into the UP state and the HoldDown timer expires it resumes
IPv6 forwarding and R2 ceases to terminate IPv6 traffic on R1's behalf.
Note that IPv6 allows a rich set of configuration options for advertising IPv6 routing prefixes
(equivalent to IPv4 subnets) and configuring hosts on a link. A prefix can be configured to be or not
to be advertised, to carry various flags or lifetime. These parameters affect how hosts can
(auto)configure their IPv6 addresses and select their default routers. Most relevant from the RSMLT
perspective is that an RSMLT node fully impersonates its peer's IPv6 configuration and behavior on
the SMLT link – whatever its configuration happens to be. The above network example illustrates
one of the many possible deployment schemes for IPv6 routers and hosts on a VLAN.
RSMLT provides both router failover and link failover. For example, if the Split MultiLink Trunk link
between R2 and R4 is broken, the traffic fails over to R1 as well.
Router R1 recovery
After R1 reboots after a failure, it becomes active as a VLAN bridge first. Packets destined to R1 are
switched, using the bridging forwarding table, to R2. R1 operates as a VLAN bridge for a period
defined by the hold-down timer.
After the hold-down time expires and the routing tables converge, R1 starts routing packets for itself
and also for R2. Therefore, it does not matter which of the two routers is used as the next hop from
R3 and R4 to reach IPv6 prefix 2003::/64.
When an IPV6 RSMLT peer recovers, the peer installs a temporary default route in the IPv6 routing
table to point all the IPv6 traffic to the IST peer IP address for the hold down time. (This is the same
behavior as in IPv4 RSMLT.)
Coexistence with IPv4 RSMLT
The IPv6 RSMLT feature introduces no changes to the existing IPv4 RSMLT state machine
including RSMLT configuration, definitions of events, logic of state transitions, or timer operations. A
single instance of state and configuration parameter set controls both IPv4 and IPv6 RSMLT logic.
With the introduction of this feature, RSMLT is best thought of as a property of the VLAN layer as
opposed to the IP (v4 or v6) layer above it. RSMLT configuration and states affect IPv4 and IPv6
operation simultaneously.
For a given SMLT VLAN RSMLT is supported for any of the following scenarios:
• IPv4 Only: IPv4 is configured on the VLAN and IPv6 is not. RSMLT operation and logic
remains unchanged from the current implementation.
June 2016 Planning and Engineering — Network Design
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