Network Working Group W. Zhou
Internet-Draft cisco Systems
Intended status: Informational June 12, 2015
Expires: December 14, 2015

VRRP PIM Interoperability


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

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.

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. 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. 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 switchover, it uses the existing mroute states and the recorded MR GenID in its Hello message. This will avoid 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.

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.

4. DF Election for BiDir Group

Change to DF offer/winner metric is handled similarly to PIM Assert handling with VRRP.

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.

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.

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. Acknowledgments

I would like to give a special thank you and appreciation to Stig Venaas for his ideas and comments in this draft.

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.

Author's Address

Wei Zhou cisco Systems Tasman Drive San Jose, CA 95134 USA EMail: