ARP on asr RP management port dot1q subif?

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Sep 4th, 2009
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We're toying around with the (probably inadvisable) option of using a subif on the RP management port on an ASR to get a BGP session around our 3rd-party packet filters.

The main interface gi0 locks itself into the Mgmt-intf vrf, but it allows subifs to be defined and all the normal dot1q commands take on the subinterfaces fine. Routes appear as expected, in the global vrf, for the subif.

encap dot1q commands cannot be applied to gi0 directly, but it does show "dot1q vlan 1" as it's encap in show interface -- which is fun because actually it's sending and receiving native packets on the main if, not tagged vlan1 packets, and that personaility of the interface seems to be working fine

despite this.

Other than that, it all looks like it should work, but ARP seems broken somehow. You can see packets (probably ARPs) arriving from the ASR and returning to it, but anything pinged on the subif network remains incomplete in the global ARP table. From the appearance of the mac address tables and arp tables on the rest of the equipment it looks like ARP queries and replies that enter gi0 tagged with vlan ZZZ are being dropped.

Is there something extra to configure on this chassis? Or is this usage of the RP interface not allowed, despite the commands not complaining when applied?

interface GigabitEthernet0

vrf forwarding Mgmt-intf

ip address XXXXXXX

negotiation auto


interface GigabitEthernet0.ZZZ

encapsulation dot1Q ZZZ

ip address YYYYYYY

no cdp enable

arp probe interval 10 count 5


GigabitEthernet0 is up, line protocol is up

Hardware is RP management


Encapsulation 802.1Q Virtual LAN, Vlan ID 1., loopback not set


ARP type: ARPA, ARP Timeout 04:00:00

GigabitEthernet0.ZZZ is up, line protocol is up

Hardware is RP management port,


Encapsulation 802.1Q Virtual LAN, Vlan ID ZZZ.

ARP type: ARPA, ARP Timeout 04:00:00

Hrm, maybe I'll try putting Gi0.ZZZ into a vrf...

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Giuseppe Larosa Sat, 09/05/2009 - 12:28
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Hello Brian,

a collegue of mine had bad experiences trying to use mgmt port on a GSR.

management ports are there only for be used in out of band management on this high end devices.

features can work or not and what is very important the risk is all traffic is process switched impacting on main cpu.

You could face other issues for example problems in routing via the interface.

Hope to help


b.julin Mon, 09/07/2009 - 12:21
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Yeah I figured it was just brokenness.

Though, as far as RP packets -- BGP's going to end up there anyway :-)


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