OTV on ASR fragmentation not working

[Difan Zhao]

I want to implement OTV in my network with the ASR routers. But as usual, I am testing and studying it in my VIRL lab first. First of all, there is no ASR in VIRL. I am using CSR instead. I am assuming that the behavior would be the same between the ASR and CSR.

Here is my config. Everything works except the fragmentation. My topology is super simple:

SW1 --- Gi3 CSR01 Gi2 --- Layer 2 switch --- Gi2 CSR02 Gi3 --- SW2

CSR01

otv site bridge-domain 1
otv fragmentation join-interface GigabitEthernet2
otv site-identifier 1111.1111.1111
otv isis Overlay1
 log-adjacency-changes all
otv isis Site
!
interface Overlay1
 no ip address
 otv join-interface GigabitEthernet2
 otv adjacency-server unicast-only
 service instance 3 ethernet
  encapsulation dot1q 3
  bridge-domain 3
 !
interface GigabitEthernet3
 no ip address
 service instance 1 ethernet
  encapsulation untagged
  bridge-domain 1
 !
 service instance 3 ethernet
  encapsulation dot1q 3
  bridge-domain 3
 !

CSR02

otv site bridge-domain 1
otv fragmentation join-interface GigabitEthernet2
otv site-identifier 2222.2222.2222
otv isis Overlay1
otv isis Site
!
interface Overlay1
 no ip address
 otv join-interface GigabitEthernet2
 otv use-adjacency-server 123.123.123.1 unicast-only
 service instance 3 ethernet
  encapsulation dot1q 3
  bridge-domain 3
 !
interface GigabitEthernet3
 description to iosvl2-2
 service instance 1 ethernet
  encapsulation untagged
  bridge-domain 1
 !
 service instance 3 ethernet
  encapsulation dot1q 3
  bridge-domain 3
 !

The SW1 and SW2 just have a trunk port connected with the CSR routers. I created a vlan3 interface (10.0.3.1 and 10.0.3.2 on the two switches, respectively) for testing. Ping with 1458 works but not 1459. 

iosvl2-1#ping 10.0.3.2 size 1458 re 5
Type escape sequence to abort.
Sending 5, 1458-byte ICMP Echos to 10.0.3.2, timeout is 2 seconds:
!!!!!
Success rate is 100 percent (5/5), round-trip min/avg/max = 6/9/18 ms
iosvl2-1#ping 10.0.3.2 size 1459 re 5
Type escape sequence to abort.
Sending 5, 1459-byte ICMP Echos to 10.0.3.2, timeout is 2 seconds:
.....
Success rate is 0 percent (0/5)

Where did I do it wrong? Is there any debug command that can help me troubleshoot this?

Thanks!

Difan

[Giuseppe Larosa]

Hello Difan,

OTV has been introduced on Nexus NX-OS but it is supported on ASR 1000 too.

As a reference see the following document on ASR 1000 Fuji 16.9.x cfg of OTV

https://www.cisco.com/c/en/us/td/docs/ios-xml/ios/wan_otv/configuration/xe-16-9/wan-otv-xe-16-9-book/wan-otv-confg.html?dtid=osscdc000283

Indeed it is based on IS-IS protocol that thanks to its flexibility it used TLV (Type Length Value) it can be used to advertise MAC addresses and Vlans so it is a good protocol to interconnect few datacenters.

Overlay transport virtualization can use an IP4 unicast routed network or an IPv4 multicast routed network.

In case of use og IPv4 multicast SSM multicast is preferred.

see the following restrictions:

Restrictions for OTV

• Configure the join interface and all Layer 3 interfaces that face the IP core between the OTV edge devices with the highest maximum transmission unit (MTU) size supported by the IP core. OTV sets the Don’t Fragment (DF) bit in the IP header for all OTV control and data packets so that the core cannot fragment these packets.

• Ensure that PIM is not enabled on the join interface; enable only passive PIM on the join interface. Configure SSM for the OTV data group multicast address range by using the ip pim passive command.

To be honest I don't know if CSR in an emulated environment can support OTV.

However. OTV and fragmentation are not compatible also on real devices as explained so your results are to be considered normal behaviour and not the sign of a wrong configuration

Hope to help

Giuseppe

[Difan Zhao]

Thanks Giuseppe!

However, as per this document, the fragmentation is supported on the ASR routers...

https://www.cisco.com/c/en/us/products/collateral/routers/asr-1000-series-aggregation-services-routers/guide-c07-735942.html#_Toc436723625 

"""

Some service providers are unwilling or unable to provide larger MTU values for their WAN circuits. If that is the case, the ASR 1000 router can perform fragmentation of the OTV transported data. The Nexus 7000 switch does not have this capability. Mixed ASR 1000 and Nexus 7000 OTV networks with fragmentation enabled on the ASR 1000 are not supported.

"""