Руководство По Проектированию для Cisco Cisco Nexus 5010 Switch

 

 

Design Guide 

© 2010 Cisco and/or its affiliates. All rights reserved. This document is Cisco Public Information. 

Page 28 of 38 

 

Multicast traffic is copied over the peer link to help ensure that orphan ports get the multicast stream and to help with 

failure scenarios, such as the loss of Link 3 (L3) in Figure 19. This happens regardless of the presence of receivers 

on the vPC peer. 

Because of this it is important to properly size the peer link to prevent the peer link from becoming the bottleneck in 

the infrastructure. 

Thus, as a best practice for vPC designs, you should be sure to provision the peer link with sufficient links according 

to the bandwidth needs of your multicast traffic. Remember that all multicast traffic traverses the peer link. 

At the time of this writing, vPC works with Protocol Independent Multicast Any Source Multicast (PIM-ASM) but not 

with Bidirectional (Bidir-PIM) or PIM Source-Specific Multicast (PIM-SSM). 

In PIM-Sparse Mode the PIM Designated Router (DR) encapsulates the traffic from a given source and unicasts it to 

the rendezvous point. Conversely, traffic from a source is drawn toward the PIM designated router for forwarding on a 

VLAN. 

In vPC environments, both aggregation-layer devices operate as PIM designated routers. This behavior allows a 

multicast source to send traffic and have the traffic hashed to either vPC peer, which will then simply forward the 

traffic to the rendezvous point. 

When a receiver is located in a vPC VLAN, the IGMP reports are synchronized, and Layer 3 forwarding entries (*, G) 

are created on both vPC peers. Both vPC peers send PIM (*, G) joins to the upstream rendezvous point. As a result, 

both vPC peer switches draw traffic, causing temporary duplicates. 

After a multicast source starts sending traffic, only one vPC peer becomes the forwarder for a given source and 

sends (S, G) joins. The choice of the forwarder is based on the distance to the source (if the distances are identical, 

the vPC primary is chosen) and converges on the designated data forwarder for these VLANs on a per-stream basis, 

to prevent duplicates. 

In summary, with the dual-designated-router approach, both vPC peers have IGMP routes, but only one of the peers 

has the Outoing Inteface List for (S, G). 

As with Layer 2 traffic, multicast traffic received from the core is copied to the peer link to reach potential orphan 

ports. 

This section describes the expected behavior of a vPC design for various link failures. 

If one vPC member port goes down - for instance, if a link from a NIC goes down - the member is removed from the 

PortChannel without bringing down the vPC entirely. Conversely, the switch on which the remaining port is located 

will allow frames to be sent from the peer link to the vPC orphan port (ports; recall the vPC duplicate avoidance 

technique). The Layer 2 forwarding table for the switch that detected the failure is also updated to point the MAC 

addresses that were associated with the vPC port to the peer link. 

If both the peer link and the peer-keepalive link are disconnected, the Cisco Nexus switch does not bring down the 

vPC, because each Cisco Nexus switch cannot discriminate between a vPC device reload and a combined peer-link 

and peer-keepalive-link failure.