Network Working Group W. Zhou
Internet-Draft cisco Systems
Intended status: Informational February 22, 2013
Expires: August 26, 2013
VRRP PIM Interoperability
draft-zhou-pim-vrrp-01.txt
Abstract
This document introduces VRRP Aware PIM, a redundancy mechanism for
the Protocol Independent Multicast (PIM) to interoperate with Virtual
Router Redundancy Protocol (VRRP). It allows PIM to track VRRP state
and to preserve multicast traffic upon failover in a redundant
network with virtual routing groups enabled.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Tracking and Failover . . . . . . . . . . . . . . . . . . . . 4
3. PIM Assert Metric Auto-Adjustment . . . . . . . . . . . . . . 5
4. DF Election for BiDir Group . . . . . . . . . . . . . . . . . 6
5. Tracking Multiple VRRP Groups on an Interface . . . . . . . . 7
6. Support of HSRP . . . . . . . . . . . . . . . . . . . . . . . 8
7. Security Considerations . . . . . . . . . . . . . . . . . . . 9
8. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 10
9. Informative References . . . . . . . . . . . . . . . . . . . . 11
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 12
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1. Introduction
Virtual Router Redundancy Protocol (VRRP) [RFC5798] is a redundancy
protocol for establishing a fault-tolerant default gateway. The
protocol establishes a framework between network devices in order to
achieve default gateway failover if the primary gateway becomes
inaccessible .
PIM has no inherent redundancy capabilities and its operation is
completely independent of VRRP group states. As a result, IP
multicast traffic is forwarded not necessarily by the same device as
is elected by VRRP. The VRRP Aware PIM feature provides consistent
IP multicast forwarding in a redundant network with virtual routing
groups enabled.
In a multi-access segment (such as LAN), PIM designated router (DR)
election is unaware of the redundancy configuration, and the elected
DR and VRRP master router (MR) may not be the same router. In order
to ensure that the PIM DR is always able to forward PIM Join/Prune
message towards RP or FHR, the VRRP MR becomes the PIM DR (if there
is only one VRRP group). PIM is responsible for adjusting DR
priority based on the group state. When a failover occurs, multicast
states are created on the new MR elected by the VRRP group and the MR
assumes responsibility for the routing and forwarding of all the
traffic addressed to the VRRP virtual IP address. This ensures the
PIM DR runs on the same gateway as the VRRP MR and maintains mroute
states. It enables multicast traffic to be forwarded through the
VRRP MR, allowing PIM to leverage VRRP redundancy, avoid potential
duplicate traffic, and enable failover, depending on the VRRP states
in the device.
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2. Tracking and Failover
With VRRP Aware PIM enabled, PIM listens to the state change
notifications from VRRP and automatically adjusts the priority of the
PIM DR based on the VRRP state, and ensures VRRP MR (if there is only
one VRRP group) becomes the DR of the LAN. If there are multiple
VRRP groups, the DR is determined by user-configured priority.
PIM triggers communication between upstream and downstream devices
upon failover in order to create mroute states on the new MR.
Depending on the requirements, there are various implementation
options:
o PIM sends additional PIM Hello message using the VRRP virtual IP
addresses as the source address for each active VRRP group when a
device becomes VRRP Active. The PIM Hello will carry a new GenID
in order to trigger other routers to respond to the failover.
When a downstream device receives this PIM Hello, it will add the
virtual address to its PIM neighbor list. The new GenID carried
in the PIM Hello will trigger downstream routers to resend PIM
Join messages towards the virtual address. Upstream routers will
process PIM Join/Prunes (J/P) based on VRRP group state.
o An alternative solution is to have all passive routers maintain
mroute states and record the GenID of current MR. When a passive
router becomes MR upon switchover, it uses the existing mroute
states and the recorded MR GenID in its Hello message. This
solution avoids resending PIM J/P upon switchover and eliminates
the requirement of additional PIM Hello with virtual IP address.
If the J/P destination matches the VRRP group virtual address and if
the destination device is in VRRP active state, the new MR processes
the PIM Join because it is now the acting PIM DR. This allows all
PIM Join/Prunes to reach the VRRP group virtual address and minimizes
changes and configurations at the downstream routers side.
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3. PIM Assert Metric Auto-Adjustment
It is possible that, after VRRP active switched from A to B; A is
still forwarding multicast traffic which will result in duplicate
traffic and PIM Assert mechanism will kick in. PIM Assert with
redundancy is enabled.
o If only one VRRP group, passive routers will send a large penalty
metric preference (PIM_ASSERT_INFINITY - 1) and make MR the Assert
winner.
o If there are multiples VRRP groups configured on an interface,
Assert metric preference will be (PIM_ASSERT_INFINITY - 1) if and
only if all VRRP groups are in passive.
o If there is at least one VRRP group is in Active, then original
Assert metric preference will be used. That is, winner will be
selected between routers using their real Assert metric preference
with at least one active VRRP Group, just like no VRRP is
involved.
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4. DF Election for BiDir Group
Change to DF offer/winner metric is handled similarly to PIM Assert
handling with VRRP.
o If only one VRRP group, passive routers will send a large penalty
metric preference in Offer (PIM_BIDIR_INFINITY_PREF- 1) and make
MR the DF winner.
o If there are multiples VRRP groups configured on an interface,
Offer metric preference will be (PIM_BIDIR_INFINITY_PREF- 1) if
and only if all VRRP groups are in passive.
o If there is at least one VRRP group is in Active, then original
Offer metric preference to RP will be used. That is, winner will
be selected between routers using their real Offer metric with at
least one active VRRP Group, just like no VRRP is involved.
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5. Tracking Multiple VRRP Groups on an Interface
User can configure PIM to track more than one VRRP groups on an
interface. This allows other applications to exploit the PIM/VRRP
interoperability to achieve various goals (e.g., load balancing).
Since each VRRP groups configured on an interface could be in
different states at any moment, the DR priority is adjusted. PIM
Assert metric and PIM Bidir DF metric if and only if all VRRP groups
configured on an interface are in passive (non-Active) states to
ensure that interfaces with all-passive VRRP groups will not win in
DR, Assert and DF election. In other words, DR, Assert, DF winner
will be elected among the interfaces with at least one Active VRRP
group.
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6. Support of HSRP
Although there are differences between VRRP and Hot Standby Router
Protocol (HSRP) [RFC2281] including number of backup (standby)
routers, virtual IP address and timer intervals, the proposed scheme
can also enable HSRP aware PIM with similar switchover and tracking
mechanism described in this draft.
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7. Security Considerations
The proposed tracking mechanism has no negative impact on security.
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8. Acknowledgments
I would like to give a special thank you and appreciation to Stig
Venaas for his ideas and comments in this draft.
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9. Informative References
[RFC2281] Li, T., Cole, B., Morton, P., and D. Li, "Cisco Hot
Standby Router Protocol (HSRP)", RFC 2281, March 1998.
[RFC5798] Nadas, S., "Virtual Router Redundancy Protocol (VRRP)
Version 3 for IPv4 and IPv6", RFC 5798, March 2010.
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Author's Address
Wei Zhou
cisco Systems
Tasman Drive
San Jose, CA 95134
USA
Email: weizho2@cisco.com
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