Network Working Group W. Zhou
Internet-Draft cisco Systems
Intended status: Informational January 21, 2016
Expires: July 24, 2016
VRRP PIM Interoperability
draft-zhou-pim-vrrp-04.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 . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Tracking and Failover . . . . . . . . . . . . . . . . . . . . 3
3. PIM Assert Metric Auto-Adjustment . . . . . . . . . . . . . . 3
4. DF Election for BiDir Group . . . . . . . . . . . . . . . . . 4
5. Tracking Multiple VRRP Groups on an Interface . . . . . . . . 4
6. Support of HSRP . . . . . . . . . . . . . . . . . . . . . . . 5
7. Security Considerations . . . . . . . . . . . . . . . . . . . 5
8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 5
9. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 5
10. Informative References . . . . . . . . . . . . . . . . . . . 5
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 6
1. Introduction
Virtual Router Redundancy Protocol (VRRP) [RFC5798] is a redundancy
protocol for establishing a fault-tolerant default router. The
protocol establishes a framework between network devices in order to
achieve default router failover if the primary router becomes
inaccessible.
PIM [RFC4601] 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
(J/P) 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.
All routers forming a VRRP group need to support this mechanism in
order to achieve correct behavior. Other routers that do not support
the mechanism on the link will not break the functionality of this
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mechanism, as long as their DR priority configurations do not
conflict with the routers in the VRRP group.
2. Tracking and Failover
Without the mechanisms described in this document, PIM daemon will
send PIM J/P with DR's IP address to upstream routers. A GenId in
PIM Hello message is randomly selected when the router boots and
remains the same as long as the router is up. A PIM neighbor reboot
can easily be detected if its GenId is different from before, in this
case PIM J/P and RP-Set information can be redistributed to the new
rebooted neighbor. With VRRP aware PIM mechasim enabled, PIM damon
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.
Upon failover, PIM daemon triggers communication between upstream and
downstream devices in order to create mroute states on the new MR.
PIM daemon 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 Master. The PIM Hello message with new GenID
will trigger other routers to respond to the VRRP failover just the
same way as to neighbor reboot. When a downstream device receives
this PIM Hello, it will add the virtual address to its PIM neighbor
list, and immediately send triggered PIM J/P messages towards the
virtual IP address. Upstream routers will process PIM J/P based on
VRRP group state. If the PIM J/P destination address matches the
VRRP group virtual address and if the destination device is in VRRP
master state, the current VRRP MR processes the PIM J/P message.
This allows all PIM J/P to reach the VRRP group virtual address and
minimizes changes and configurations at the downstream routers side.
Alternatively, implementation can choose to have all passive routers
maintain mroute states and record the GenID of current MR. When a
passive router becomes MR upon failover, it uses the existing mroute
states and the recorded MR GenID in its Hello message. This will
avoid resending PIM J/P messages upon failover and eliminates the
requirement of additional PIM Hello with virtual IP address. There
is no change in on-wire behavior or in the PIM/VRRP message format.
3. PIM Assert Metric Auto-Adjustment
It is possible that, after VRRP master 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.
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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 state.
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.
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.
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 failover and tracking
mechanism described in this draft.
7. Security Considerations
The proposed tracking mechanism does not discuss adding
authentication to the protocols and introduces no new negative impact
or threats on security to PIM in either SSM or ASM mode. Note that
VRRP messages from malicious nodes could cause unexpected behaviors
such as multiple Masters and PIM DRs which are associated with VRRP
specific security issues. Detailed analysis of PIM and VRRP security
is provided in [RFC 5294] and [RFC 5798].
8. IANA Considerations
This document has no IANA actions.
9. Acknowledgments
I would like to give a special thank you and appreciation to Stig
Venaas for his ideas and comments in this draft.
10. Informative References
[RFC2281] Li, T., Cole, B., Morton, P., and D. Li, "Cisco Hot
Standby Router Protocol (HSRP)", RFC 2281,
DOI 10.17487/RFC2281, March 1998,
<http://www.rfc-editor.org/info/rfc2281>.
[RFC4601] Fenner, B., Handley, M., Holbrook, H., and I. Kouvelas,
"Protocol Independent Multicast - Sparse Mode (PIM-SM):
Protocol Specification (Revised)", RFC 4601,
DOI 10.17487/RFC4601, August 2006,
<http://www.rfc-editor.org/info/rfc4601>.
[RFC5798] Nadas, S., Ed., "Virtual Router Redundancy Protocol (VRRP)
Version 3 for IPv4 and IPv6", RFC 5798,
DOI 10.17487/RFC5798, March 2010,
<http://www.rfc-editor.org/info/rfc5798>.
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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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