[c-nsp] ARP requests

Eugene Grosbein eugen at grosbein.net
Wed Aug 19 09:02:13 EDT 2020


I've Cisco 7201 router acting as PE that floods the network with ARP requests
and I cannot understand, why?

The router is connected to the network core with an interface GigabitEthernet0/1 in trunk mode:

interface GigabitEthernet0/1
 mtu 1618
 no ip address

And there are multiple client-facing sub-interfaces configured similar to:

interface GigabitEthernet0/1.1993
 encapsulation dot1Q 1993
 ip unnumbered Loopback4

Some sub-interfaces have non-unnumbered IP address assigned. Note ARP-proxying is not disabled.
The router also uses an interface GigabitEthernet0/0 as IP up-link/MPLS:

interface GigabitEthernet0/0
 mtu 1636
 ip address X.X.X.X
 ipv6 address Y:Y:Y:Y::Y/64
 ipv6 enable
 mpls label protocol ldp
 mpls ip

The problem: the router sends lots of untagged ARP broadcasts (requests)
over its GigabitEthernet0/1 to a switch. I've enabled local SPAN mirroring
at the switch to make sure this is true. The requests are from router's MAC address
and some of its IPv4 addresses without 802.1q tags.

Lots of unneeded ARP broadcasts is no good. Why does it do so?
The router has not any interface with "encapsulation dot1Q 1".
It runs c7200p-adventerprisek9-mz.152-4.M6.bin

Switch mirroring configuration is the following:

monitor session 1 source interface Gi2/3 tx
! to the traffic analyzer
monitor session 1 destination interface Po14

It should not strip 801.1q tags while mirroring and I really have tags for other kinds of mirrored packets
seen at my traffic analyzer.

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