Improved Convergence With A Vpc Topology And Layer 3 Routing - Cisco Nexus 5000 Series Operation Manual

Improved Convergence with a vPC Topology and Layer 3 Routing

S e n d d o c u m e n t a t i o n c o m m e n t s t o n 5 k d o c f e e d b a c k @ c i s c o . c o m
In Figure
ECMP hashing to choose which router to use as the actual next hop for a given flow. Router R connects
to N5k-1 and N5k-2 via a vPC. This port channel has an IP address on router R, and Router R performs
Layer 3 peering with N5k-1 and N5k-2 over this port channel. It runs the port channel hash algorithm to
choose one physical link to reach the Layer 3 next hop. Because the Layer 3 ECMP and port channel run
independent hash calculations there is a possibility that when the Layer 3 ECMP chooses N5k-1 as the
Layer 3 next hop for a destination address while the port channel hashing chooses the physical link
toward N5k-2. In this scenario,N5k-2 receives packets from R with the N5k-1 MAC as the destination
MAC.
Sending traffic over the peer-link to the correct gateway is acceptable for data forwarding, but it is
suboptimal because it makes traffic cross the peer link when the traffic could be routed directly.
Beginning in Cisco NX-OS Release 5.0(3)N1(1b), you can use the peer-gateway command to allow
Cisco Nexus 5500 Platform switches to perform Layer 3 forwarding if the destination MAC of the
incoming packet is the MAC of its vPC peer switch. The peer-gateway command avoids forwarding
such packets to the vPC peer link.
Note
You must configure the peer-gateway command on both vPC peer switches.
Improved Convergence with a vPC Topology and Layer 3
Routing
Beginning in Cisco NX-OS Release 5.0(3)N1(1b), a delay timer was introduced to avoid the situation
where a vPC member port is brought up before the Layer 3 is converged. For example, when one Cisco
Nexus 5500 Platform switch is reloaded, the switch starts to receive traffic from hosts once the vPC
member ports are up. A delay might occur before the switch establishes a routing protocol adjacency and
learns all routes. During this period of the time, received traffic is dropped due to the lack of a
route-to-destination address.
hole traffic when a Cisco Nexus 5000 Platform switch is configured for Layer 3 with vPC.
Figure 2-4
L3
L2
Cisco Nexus 5000 Series NX-OS Interfaces Operations Guide, Release 5.0(3)N2(1)
2-4
2-3, router R considers N5k-1 and N5k-2 as two Layer 3 ECMP next-hop routers and runs
Figure 2-4
vPC Delay Restore
OSPF
vPC
Primary
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9 3
6
Chapter 2 Cisco Nexus 5500 Platform Layer 3 and vPC Operations
shows an example of where the delay can be used to avoid black
12
9
3
6
vPC
Secondary