Contents

Introduction

Prerequisites

Requirements

Components Used

Configure

Default Behaviour - ASR 1006

Default Behaviour - ISR4321

Reason for the Behavioural Difference

Configuration/Workaround

Verify

Introduction

This document describes the behavioural difference between Aggregation Services Routers 1000 series (ASR1K) and Integrated Services Routers 4000 series (ISR4K) routers in accounting for input and output packets when a sub-interface is configured with encapulation dot1q even though they run the same IOS-XE software.

Contributed by Venkat Ramasamy Kannan, Cisco TAC Engineer.

Prerequisites

Requirements

Cisco recommends that you have knowledge of ASR1K and ISR4K routers along with basic vlan and sub-interface tagging concepts.

Components Used

This document is not restricted to specific software and hardware versions.

However, The information in the document is created using the below hardware.

- ASR 1006 running IOS-XE 3.13.3S

- ISR 4321 running IOS-XE 3.16.0C

The information in this document was created from the devices in a specific lab environment. All of the devices used in this document started with a cleared (default) configuration. If your network is live, make sure that you understand the potential impact of any command.

Configure

Default Behaviour - ASR 1006

Config

!

interface GigabitEthernet0/0/1

no ip address

load-interval 30

negotiation auto

!

interface GigabitEthernet0/0/1.1115

encapsulation dot1Q 1115 native

ip address 192.168.2.1 255.255.255.0

!

Input and Output Counters

R-ASR1006-2#show vlans dot1q 1115

Total statistics for 802.1Q VLAN 1115:

4021 packets, 241260 bytes input <-- counter is very low (and frozen)

10 packets, 420 bytes output <-- counter is very low (and frozen)

0 oversubscription packet drops

However these counters seem to be wrong because this is the only sub-interface under the physical interface, and there is a huge mismatch in the values.

NR-ASR1002-2#sh int gigabitEthernet 0/0/1

GigabitEthernet0/0/1 is up, line protocol is up

<trunc>

2429325386 packets input, 1438158021764 bytes, 0 no buffer

2429263775 packets output, 1438124149520 bytes, 0 underruns

<trunc>

Default Behaviour - ISR4321

Config

!

interface GigabitEthernet0/0/1

no ip address

load-interval 30

negotiation auto

!

interface GigabitEthernet0/0/1.3503

encapsulation dot1Q 1115 native

ip address 57.211.249.6 255.255.255.252

!

Input and Output Counters

ISR4321-1#show vlans dot1q 3503

Total statistics for 802.1Q VLAN 3503:

141584463 packets, 101578951916 bytes input

142482559 packets, 45106997466 bytes output

ISR4321-1#show int gig 0/1

<trunc>

141584583 packets input, 2794771512 bytes, 0 no buffer

142609327 packets output, 2166121790 bytes, 0 underruns

<trunc>

ISR4321-1#

Values between physical interface gig0/1 and the subinterface gig0/1.3503 match.

Reason for the Behavioural Difference

The reson for this behavioural difference between ASR1K and ISR4K even though they run the same IOS-XE is that, when encap dot1q 10 native is configured, the stats are grabbed from the data-plane. However in ASR1K, the stats are grabbed from the Shared Port Adaptor (SPA) hardware and the SPA is not as intelligent as the data-plane processor, so it doesn't know how to count untagged packets against a specific vlan.

Configuration/Workaround

In order to make the ASR1K work like the ISR4K, you should count the packets at the data-plane level like the ISR4k.The way to do that is to configure this on the ASR.

Router(config)# hw-module subslot x/y ethernet vlan unlimited

Warning: This command must be used with caution. Once this is enabled, the SPA stops doing the packet classification. So, if the router is already in an over-subscribed scenario, any packet (high or low priority, because the packets are not classified anymore) could be dropped

Verify

Counters on the ASR after configuration is added

R-ASR1006-2#show int giga 0/1

<trunc>

2429657821 packets input, 1438159132874 bytes, 0 no buffer

2429643228 packets output, 1438125250620 bytes, 0 underruns

<trunc>

R-ASR1006-2# R-ASR1006-2#show vlans dot1q 1115

Total statistics for 802.1Q VLAN 1115:

2429657834 packets, 1438159133962 bytes input

2429643241 packets, 1438125251511 bytes output