Arista’s PoE gear is here!

If you’ve been fortunate enough to deploy Arista in the datacenter you’re probably a fan. I certainly am. What’s not to love? They’ve got great hardware, the OS is super consistent, they do great DWDM, and CloudVision (CVP) has gotten to be pretty decent as an automation system and VxLAN controller. (I’ve seen datacenters come up in about 6 minutes or so, once all the patching is done. That’s full config, up and passing traffic. You’re not gonna get much better unless you write your own orchestration system.)

My only complaint with Arista has been a lack of love, on their part, for the campus. I’ve been asking them for about 5 years or so for two things: PoE and 802.1x. Once Arista acquired Mojo Networks it became clear that they wanted to expand into campus networking, so it should shock no one that they have been working on what I think of as a “campus feature set” for a while, and the first products in that line are now out in the open. It’s been quite a while since we’ve had a new contender in the campus switching space and I find it exciting.

One of the reasons I’m excited by this development is the approach that Arista has taken in terms of software. There is no code fork. These switches are running off of the same EOS code base as their datacenter leaf and spine switches. They come with all the datacenter features you love, especially all that VxLAN/EVPN goodness. This is really important because other vendors seem to have forked or re-written their SDN stack for campus and sometimes that leads to growing pains. Here you have a battle-proven SDN stack. I do see applications for SDN in the campus at an architectural level but at an engineering level I haven’t seen an implementation I like yet.

Disclaimer: As a function of $DayJob I am under NDA with Arista, so my comments will solely be based upon what’s been publicly released on their website, which you can find here. (Sorting out what I know that’s public and private is just too much stress.) I am participating in their EFT for these switches and have been playing with mine for about a week. I also am a bit of an Arista fanboi. (I love my Cisco friends too. Everyone play nice.)

Before we dive in to specs a couple of notes:

Just like with all PoE switches you have a choice between max power and redundancy. Based on the folks I’ve spoken to at Arista, the 720XP has available PSUs that can more than drive all ports concurrently, up to about 1800W. This is comparable to what the Catalyst 9300 series from Cisco offers. I find the published spec sheets a bit confusing in that regard and I think the data sheets will get updated in the future to make it clear.

Also, you’re going to see that all of the uplinks on these switches are 25G or 100G. I know that to some network engineers that focus on campus environments these port speeds may be unfamiliar. In today’s campus 10G and 40G ports are common but the 40G port is really just 4 “lanes” of 10G. This is the same thing – 100G is 4 lanes of 25G. This jump from 10 to 25 is based around the design of a 28Gbps SerDes (Serializer/Deserializer) – you just lose a bit of the 28Gbps in overhead, and you end up with 25Gbps of payload. But don’t sweat it, all of the 25G SFP+ ports can take 10G SFPs.

There are 4 switches in the 720XP line – here’s a quick summary.

720XP-24Y6 and 720XP-48Y6 start with 24 or 48 1G PoE ports and 6 25G uplinks. All of the ports provide 802.3at (30W) power.

720XP-24ZY4 has 16 2.5G 30W ports, 8 5G 802.3bt (60W) ports, and 4 25G uplinks. This is what Arista gave me as part of their EFT so my hands on comments with specifically apply to this model.

720XP-48ZC2 is the big daddy with 40 2.5G 30W ports, 8 5G 60W ports, 4 25G uplinks and 2 100G uplinks.

When comparing features and what not I think these switches are best compared to Cisco’s Catalyst 9300 offering. Before we get in to dueling spec sheets, I will note that the Arista switches don’t stack. Arista is staying true to their spine-leaf architecture here (and you don’t stack in spine-leaf). That may seem like a management challenge, but keep in mind that automation scales to solve this problem. (If you’re deploying Catalyst 9000 you’ll be encouraged to deploy DNA Center, and with Arista you’ll be similarly encouraged to deploy CloudVision.) So in the following comparisons we’ll only be looking at single-switch performance.

By the way, these comparisons are based off of spec sheets, not any lab testing I have done. You can find the Arista spec sheets here and the Cisco spec sheets here. Also, Cisco seems to be counting “switching capacity” a bit differently than Arista. With the Arista switches if you add up all the ports the switching capacity matches the port capacities. With Cisco if you add up all the ports then the switching capacity is 2x the port capacity. So let’s just all agree that all switches in this comparison offer “line-rate switching”. The actual switching fabrics inside the boxes exceed the port capabilities.

720XP-24Y6C9300-24P720XP-48Y6C9300-48P
Switching174 Gbps208 Gbps198 Gbps256 Gbps
Forwarding259 Mpps155 Mpps295 Mpps190 Mpps

The 24Y6 and the 48Y6 compare pretty directly to the C9300-24P and C9300-48P. Cisco does offer modular uplinks, but the best option they have is 2x40Gbe. They also have 2×25 Gbe. The Arista version has 6×25 Gbe. (However, given that these are all 24 or 48 port 1G PoE switches, more than 50Gbps of uplink is a bit of overkill.) All of these switches are “non-blocking”, in that there’s both enough backplane capacity to support all ports and enough “northbound” capacity for all ports. Arista, however, provides a pretty solid edge in forwarding.

720XP-24ZY4C9300-24UX720XP-48ZC2C9300-48UXM
Switching180 Gbps640 Gbps560 Gbps580 Gbps
Forwarding268 Mpps476.19 Mpps655Mpps431.54 Mpps

From here on out things get a bit more fuzzy. It’s harder to make “apples to apples” comparisons due to differing port configurations and capacities.

The C9300-24UX has some significant advantages over the 24ZY4. All of the 24UXs ports are 1G/2.5G/5G/10G and 60W capable. The 24ZY4 has a mix of 2.5 and 5G, and a mix of 30W and 60W. However, I will note that while the 24UX has the backplane capacity to switch all ports it lacks the uplink capacity to forward all 24 of it’s copper ports if they’re running at 10G. In fact it’s uplinks are oversubscribed by 3 to 1.

The 48ZC2 and the 48UXM are not the same but they’re a closer comparison. The 48UXM has 36 2.5G ports, and 12 “Multigigabit” (1/2.5/5/10) ports. All ports are 60W capable (but it can only do 30 60W ports). The 48ZC2 has 40 2.5G 30W ports and 8 2.5G/5G 60W ports. Since it doesn’t have as many 60W ports it can always power all ports. (The 48UMX could power the same configuration.) But again we’re limited by the uplink choices. The 48UXM will be trying to shove 210Gbps of traffic from the copper ports through an 80Gbps pipe, or about a 2.625:1 oversubscription. With the 48ZC2 it depends on how you look at those 4×25 ports. It starts with 140Gbps of copper ports. If you use the 4 25G ports as client ports and the 100G as uplinks then you’re 1.2:1 over subscribed. If you only use 2 of the 25G ports for clients and 2 as uplinks (along with the 100G) you’re back to 1:1.

So before folks get out their pitchforks allow me to acknowledge: the campus is not the datacenter. Do you *need* your switches to be non-blocking? Probably not. However, when comparing product lines from two different vendors it’s nice to be able to objectively compare specifications. It may not be important to you that your uplinks aren’t oversubscribed, but it’s interesting that they can be.

One of the things I think about when designing a large-ish campus network is if I want a separate switch fabric for APs or not. On the one hand, life is simpler when you have fewer switches and a unified topology. On the other hand, isolating your APs from the user switch fabric can have it’s advantages. And if you don’t have huge PoE requirements (like, say, you don’t have desk phones) then you may be able to limit your PoE to separate switches.

Right now with PoE switches we’re seeing a move from 802.11at to 802.11bt. How much PoE are we going to actually need going forward? If we look at how the 802.11ax APs are shaping up we can note a few things:

The 8×8:8 APs, with all features on, need 802.3bt power. The Cisco 9117 can run on an 802.3at port, but you lose the USB function. The Aruba AP-555 has to downgrade the 8×8 to 4×4 on single 802.3at power *and* kills the USB (although it does offer you the option to plug both Ethernet ports in to 802.3at power and get full functionality that way). They also both come with 5G ethernet.

The 4×4:4 APs are a bit more forgiving. The Cisco 9120 and the Aruba AP-535 lose the USB port on 802.3at but otherwise survive just fine on 30W. It’s interesting that the AP-535 has a 5G port while the 9120 has a 2.5G port. (Not sure WHY it has a 5G port, I don’t think it needs it.) They both can use 802.3bt and need it for “full functionality”, but if you power them with 802.3at you won’t really be missing anything. The only thing I’ve used the USB for on an AP is to add BLE to APs that don’t have it natively.

Edit: I am somewhat glossing over all the possible AP configurations. As Stephen Cooper pointed out to me, the Cisco APs (and most of them vendors, to be honest) can run on 802.3af power, just with reduced transmit chains. And Scott Lester reminded me, the Aruba AP-555 has Intelligent Power Monitoring to dynamically adjust it’s features based on the provided power. The point of this article isn’t to deep dive on AP power management, but rather to provide a broad overview of what the current landscape looks like when it comes to AP power consumption.

(Yes, Arista C-250, I see you there. But your spec sheet doesn’t help me here because it says it can operate on 802.3at power with “reduced function” without telling me what I’m giving up. You’re an 8×8:8 AP, so I assume that you fall back to 4×4:4 but your spec sheet is missing that data.)

So here’s my take on these switches in light of what the next generation of APs are looking like. Cisco can deliver a lot more 60W ports. If you’re deploying 8×8:8 APs, or using any optional features that require more power (like the USB port) then you need to take that in to account. If you’re deploying mostly 4×4 APs then you’re a bit more free in your choices.

The 24ZY4 could handle a mix of 4×4 and 8×8 APs, as could the 48ZC2. I’m just not sure if I’m going to be mixing and matching APs like that. The 24UX is fine as well, because if you run all the copper ports at 5G you’re much less oversubscribed, and if you’re running them at 2.5G then you’re not even oversubscribed at all. Wi-Fi isn’t likely to run ports at full line rate, so in my mind the 24UX is a really solid choice for “AP switch”. I’d probably choose the 48ZC2 over the 48UXM simply due to the uplink options.

I will note the existence of the C9300-48UN. It’s got 48 ports of 5G/2.5G/1G and they’re all 60W capable, although again you’re still limited to being only able to power up to 30 of them at 60W. If you need 5G then you need 60W, since the only way you truly need 5G is if you have 8×8 APs.

This is Arista’s first set of offerings in the campus switch portfolio. I’m sure it won’t be their last. For now I’m going to play with my EFT sample and let folks know what I find. If you have questions or want me to try something, let me know!

Leave a Reply

Your email address will not be published. Required fields are marked *

This site uses Akismet to reduce spam. Learn how your comment data is processed.