Switches - Juniper JUNOS OS 10.3 - SOFTWARE Manual

Complete Software Guide for Junos
Limitations of the Unicast RPF Implementation on EX3200 and EX4200 Switches
1108
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OS for EX Series Ethernet Switches, Release 10.3
An asymmetrically routed interface uses different paths to send and receive packets
between the source and the destination, as shown in Figure 25 on page 1108. This means
that if an interface receives a packet, that interface does not match the forwarding table
entry as the best return path back to the source. If the receiving interface is not the best
return path to the source of a packet, unicast RPF causes the switch to discard the packet
even though it comes from a valid source.
Figure 25: Asymmetrically Routed Interfaces
NOTE: Do not enable unicast RPF on EX3200 and EX4200 switches if any
switch interfaces are asymmetrically routed, because unicast RPF is enabled
globally on all interfaces of those switches. All switch interfaces must be
symmetrically routed for you to enable unicast RPF without the risk of the
switch discarding traffic that you want to forward.
On EX3200 and EX4200 switches, the switch implements unicast RPF on a global basis.
You cannot enable unicast RPF on a per-interface basis. Unicast RPF is globally disabled
by default.
When you enable unicast RPF on any interface, it is automatically enabled on all switch
interfaces, including link aggregation groups (LAGs) and routed VLAN interfaces (RVIs).
When you disable unicast RPF on the interface (or interfaces) on which you enabled
unicast RPF, it is automatically disabled on all switch interfaces.
NOTE: You must explicitly disable unicast RPF on every interface on which
it was explicitly enabled or unicast RPF remains enabled on all switch
interfaces.
The EX3200 and EX4200 switches do not perform unicast RPF filtering on equal-cost
multipath (ECMP) traffic. The unicast RPF check examines only one best return path to
the packet source, but ECMP traffic employs an address block consisting of multiple
paths.
Using unicast RPF to filter ECMP traffic on EX3200 and EX4200 switches can result in
the switch discarding packets that you want to forward because the unicast RPF filter
does not examine the entire ECMP address block.
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