Re: Ceph with Clos IP fabric

Aaron Bassett <Aaron.Bassett@xxxxxxxxxxxxx> · Sat, 22 Apr 2017 19:13:55 +0000

FWIW, I use a CLOS fabric with layer 3 right down to the hosts and multiple ToRs to enable HA/ECMP to each node. I'm using Cumulus Linux's "redistribute neighbor" feature, which advertises a /32 for any ARP'ed neighbor. I set up the hosts with an IP on each
 physical interface and on an aliased looopback: lo:0. I handle the separate cluster network by adding a vlan to each interface and routing those separately on the ToRs with acls to keep traffic apart. 

Their documentation may help clarify a bit:
https://docs.cumulusnetworks.com/display/DOCS/Redistribute+Neighbor#RedistributeNeighbor-ConfiguringtheHost(s)

Honestly the trickiest part is getting the routing on the hosts right, you essentially set static routes over each link and the kernel takes care of the ECMP.

I understand this is a bit different from your setup, but Ceph has no trouble at all with the IPs on multiple interfaces. 

Aaron 

Date: Sat, 22 Apr 2017 17:37:01 +0000

From: Maxime Guyot <Maxime.Guyot@xxxxxxxxx>

To: Richard Hesse <richard.hesse@xxxxxxxxxx>, Jan Marquardt

<jm@xxxxxxxxxxx>

Cc: "ceph-users@xxxxxxxxxxxxxx" <ceph-users@xxxxxxxxxxxxxx>

Subject: Re:  Ceph with Clos IP fabric

Message-ID: <919C8615-C50B-4611-9B6B-13B4FBF69C61@xxxxxxxxx>

Content-Type: text/plain; charset="utf-8"

Hi,

That only makes sense if you're running multiple ToR switches per rack for the public leaf network. Multiple public ToR switches per rack is not very common; most Clos crossbar networks run a single ToR switch. Several >guides
 on the topic (including Arista & Cisco) suggest that you use something like MLAG in a layer 2 domain between the switches if you need some sort of switch redundancy inside the rack. This increases complexity, and most people decide that it's not worth it and
 instead scale out across racks to gain the redundancy and survivability that multiple ToR offer.

If you use MLAG for L2 redundancy, you?ll still want 2 BGP sessions for L3 redundancy, so why not skipping the MLAG all together and terminating your BGP session on each ToR?

Judging by the routes (169.254.0.1), you are using BGP unnumebered?

It sounds like the ?ip route get? output you get when using dummy0 is caused by a fallback on the default route, supposedly on eth0? Can check the exact routes received on server1 with ?show ip bgp neighbors <neighbor> received-routes? once you enable ?neighbor
 <neighbor> soft-reconfiguration inbound? and what?s installed in the table ?ip route??

Intrigued by this problem, I tried to reproduce it in a lab with virtualbox. I ran into the same problem.

Side note: Configuring the loopback IP on the physical interfaces is workable if you set it on **all** parallel links. Example with server1:

?iface enp3s0f0 inet static

 address 10.10.100.21/32

iface enp3s0f1 inet static

 address 10.10.100.21/32

iface enp4s0f0 inet static

 address 10.10.100.21/32

iface enp4s0f1 inet static

 address 10.10.100.21/32?

This should guarantee that the loopback ip is advertised if one of the 4 links to switch1 and switch2 is up, but I am not sure if that?s workable for ceph?s listening address.

Cheers,

Maxime

From: Richard Hesse <richard.hesse@xxxxxxxxxx>

Date: Thursday 20 April 2017 16:36

To: Maxime Guyot <Maxime.Guyot@xxxxxxxxx>

Cc: Jan Marquardt <jm@xxxxxxxxxxx>, "ceph-users@xxxxxxxxxxxxxx" <ceph-users@xxxxxxxxxxxxxx>

Subject: Re:  Ceph with Clos IP fabric

On Thu, Apr 20, 2017 at 2:13 AM, Maxime Guyot <Maxime.Guyot@xxxxxxxxx<mailto:Maxime.Guyot@xxxxxxxxx>> wrote:

2) Why did you choose to run the ceph nodes on loopback interfaces as opposed to the /24 for the "public" interface?

I can?t speak for this example, but in a clos fabric you generally want to assign the routed IPs on loopback rather than physical interfaces. This way if one of the link goes down (t.ex the public interface), the routed IP is still advertised on the other link(s).

That only makes sense if you're running multiple ToR switches per rack for the public leaf network. Multiple public ToR switches per rack is not very common; most Clos crossbar networks run a single ToR switch. Several guides on the topic (including Arista
 & Cisco) suggest that you use something like MLAG in a layer 2 domain between the switches if you need some sort of switch redundancy inside the rack. This increases complexity, and most people decide that it's not worth it and instead  scale out across racks
 to gain the redundancy and survivability that multiple ToR offer.

On Thu, Apr 20, 2017 at 4:04 AM, Jan Marquardt <jm@xxxxxxxxxxx<mailto:jm@xxxxxxxxxxx>> wrote:

Maxime, thank you for clarifying this. Each server is configured like this:

lo/dummy0: Loopback interface; Holds the ip address used with Ceph,

which is announced by BGP into the fabric.

enp5s0: Management Interface, which is used only for managing the box.

There should not be any Ceph traffic on this one.

enp3s0f0: connected to sw01 and used for BGP

enp3s0f1: connected to sw02 and used for BGP

enp4s0f0: connected to sw01 and used for BGP

enp4s0f1: connected to sw02 and used for BGP

These four interfaces are supposed to transport the Ceph traffic.

See above. Why are you running multiple public ToR switches in this rack? I'd suggest switching them to a single layer 2 domain and participate in the Clos fabric as a single unit, or scale out across racks (preferred). Why bother with multiple switches in
 a rack when you can just use multiple racks? That's the beauty of Clos: just add more spines if you need more leaf to leaf bandwidth.

How many OSD, servers, and racks are planned for this deployment?

-richard

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