[c-nsp] ASR920 Opinions

James Jun james at towardex.com
Tue Dec 19 20:52:41 EST 2017


Hey,

We have about 40 of ASR920's, mostly 24SZ-M and 24SZ-IM variants.  We're running mainly 03.16.04S and 03.16.05S.

We're using ASR920s only for layer-2 transport -- pseudowires and VPLS; For internet customers, we establish an
L2 pseudowire to transport the user from ASR920 to an IP transit hub router (ASR9K or MX480).

For layer-2 services, we use LDP signalled L2CKT and VPLS.  We tried testing layer-3 use case, but last time we 
tested (it was on early SW versions though.. like 3.14.x something), control-plane protection like didn't even 
work as we expected.


My overall experience with ASR920 are as follows.

The Good Stuff:
 - Most layer-2 features behave more or less like a real router (ASR1K) than the traditional Catalyst SW model.
   You get real EFP infrastructure and you can rewrite vlan on ingress on EFPs and such.  Getting these features
   at low price point is very cool.

 - For core-facing L3 interfaces (OSPF, MPLS w/ RSVP-TE, LDP tunneling, BGP), no problems at all.  Works very
   well as expected.

   Overall, ASR920 is like the perfect Metro-E switch but configuration wise, behaves much like a router than a
   switch.  I think this makes 920 much, much more attractive platform when compared to similar MetroE/packet
   backhaul boxes from say.. Ciena, etc.

 - The device appears to perform well as advertised on traffic and packet forwarding rates.


The Bad Stuff:
 - Weird behavior with 10G ports and optics:  Sometimes when upgrading SW, some of the SFPs (e.g. Bidi ones)
   fail to come up.  Bouncing the interface with shut/no shut does nothing; dispatching field service crew to 
   remove and re-insert the optic solves the problem. 

   We also had issues with 10G ports going admin-down upon upgrade as well.  Long story short, OOB is highly
   desirable to have if SW upgrade is required on this platform.

 - Shallow buffers - 12MB for the whole box; and default values are ridiculously small.
   I'm not sure what Cisco was thinking regarding buffers on this box. ASIC speed has nothing to do with
   buffering requirements when you're downstepping from 10G to 1G -- you either have buffers to make up for
   the Tx/Rx rate difference or you tail drop, it's as simple as that.

   We applied 100% shared buffers with policy-map, but we did run into buffer exhaustion when several customers
   are doing heavy inbound traffic.  It's fine for putting in typical end-user / retail users, but for placing
   lot of enterprise 1GE internet customers on the box, I don't know..  We ended up configuring fixed 512KB queue
   on every 1GE port (so we don't really oversubscribe the 12MB buffer space) to absorb up to ~2ms worth of burst,
   but this now brings back lot of tail drops on long distance TCP flows.  So, we're now having upstream IP
   transit routers at head-end sites provide traffic-shaping with very low burst on customer EVCs terminating
   on ASR920s.  It's not ideal, as this means I'll need -SE line card at upstream side to deal with the increased
   queueing requirements, but it is a decent compromise.

 - FAT-PW is not supported on ASR920s, and last I checked, is not even on the roadmap.  ASR90x (902, 903, 907)
   with RSP-3C have FAT-PW support starting with Everest release SW.

   Long story short, ASR920 is meant to aggregate 1GE end-users.  The built-in 4x10GE ports are really meant to
   form uplink ring architecture; not for customer facing ports by design.. I think.  Lack of FAT-PW makes 
   terminating burstable 10G customer interface for IP backhaul extremely undesirable.


In conclusion, given the economical price point of the ASR920, this is really geared to go after competing vendors
for packet networking/MetroE/cell backhaul of 1GE revenue ports.  For the given price point, ASR920 works really 
well and does its job as advertised, so we are very happy with them for our use case.   Anything more advanced,
well I guess that's where the expensive routers come in..

James


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