So, this is my first blog post on here. Hope it goes well.
One of the most commonly asked questions of late is how to properly use a load-balancer with Cisco's Identity Services Engine. Here are some basic guidelines to use when configuring a Load Balancer for the ISE Policy Services Nodes (PSNs).
Understanding terms:
PSN = Policy Services Node. The PSN is the ISE persona that handles all of the radius requests, and make the policy decisions. If you are using profiling, the PSN is also handling the profiling for you.
VIP = Virtual IP Address. This is the IP Address that Load Balancer listens on, and will redirect traffic destined to the VIP to the real IP Addresses of the servers in the Server Farm.
Server Farm = The Grouping of servers that will be load balanced when traffic is destined to the VIP
Endpoint = the actual device accessing the network.
NAD = Network Access Device. The Access-Layer device (switch / wireless controller) that provides and enforces network access to the endpoint.
SNAT = Source Network Address Translation. Function of load balancers to hide the source ip address of the NAD, which allows the load-balancer to run "out of band".
General Guidelines
When using a Load-Balancer (anyone's) you must ensure a few things.
- Each PSN must be reachable by the PAN / MNT directly, without having to go through NAT (Routed mode LB, not NAT). NO NAT. This includes the Accounting messages, not just the Authentication ones.
- Each PSN Must also be reachable directly from the Client's – for redirections / CWA / Posture, etc…
- You may want to "hack" the certs to include the VIP fqdn in the SAN field.
- Perform sticky (aka: persistance) based on Calling-Station-ID and Framed-IP-address
- VIP gets listed as the RADIUS server of each NAD for all 802.1X related AAA.
- Each PSN gets listed individually in the Dynamic-Authorization (CoA). Use the real IP Address of the PSN, not the VIP.
- The LoadBalancer(s) get listed as NADs in ISE so their test authentications may be answered.
- ISE uses the Layer-3 Address to Identity the NAD, not the NAS-IP-Address in the RADIUS packet... This is another reason to avoid SNAT.
Failure Scenarios:
- The VIP is the RADIUS Server, so if the entire VIP is down, then the NAD should fail over to the 2ndary DataCenter VIP (listed as the 2ndary RADIUS server on the NAD).
- Probes on the Load-Balancers should ensure that RADIUS is responding, as well as HTTPS (at minimum). LB Probes should send test RADIUS messages to each server periodically, to ensure that RADIUS is responding, not just look for open UDP ports. Same goes for HTTPS.
- Should use node-groups with the L2-adjacent PSN's behind the VIP. If the session was in process and one of the PSN's in a node-group fails, then another of the node-group members will issue a CoA-reauth; forcing the session to begin again. At this point, the LB should have failed PSN1 due to the probes configured in the LB; and so this new authC request will hit the LB & be directed to a different PSN…
Why can't we use Source NAT (SNAT)?
One of the most common questions when load balancing, is: "Why can't we use SNAT?". Source NAT is a fantastic thing for general Load-Balancing - but not with ISE. The resons listed below pertain to ISE version 1.1.x; and may change with ISE 1.2+
Reason #1: Network Access Device (NAD) will be wrong:
With SNAT, the source Network Access Device will show up in ISE as being the Load-Balancer, NOT the Network Access Device.
(click image to enlarge)
ISE uses sessionized network authentication. This means ISE is tracking the session along with the NAD - so the NAD & ISE stay in sync about the state and location of the endpoint... This session also gives ISE the NAD address to send Change of Authorizations to, as well as the location of the endpoint. We use the source NAD in many different ISE Policies - and if all nodes always appear to be coming from the Load-Balancer, instead of the NAD - how can we know the location of the endpoint?
Location is not nearly as big of a deal as the Change of Authorizations. ISE records the Layer-3 Address of the NAD from the Layer-3 headers. There is a RADIUS field known as NAS-IP-Address; which embeds in the IP Address of the Network Device in the RADIUS Packet. However, ISE does not currently use that field; and therefore the L3 IP Address of the NAD must be correct for Change of Authorization to be sent to the correct device. If the NAD appears as the IP Address of the Load-Balancer, then ISE will send the Change of Authorization to the Load-Balancer - not the switch.
Reason #2: URL Redirection and Web Portals:
Next, ISE 1.1.x only has one interface that can be used for all functions. Yes, we can run RADIUS on any of ISE's four interfaces, but the Gigabit 0/0 interface is the ONLY interface for Management Traffic. Also, the fqdn of the Policy services node is embedded into the certificate for ISE 1.1.x; and that is what gets used for URL Redirection for WebAuth & Device Registration & Supplicant Provisioning, etc...
(click image to enlarge)
So, when the URL Redirection occurs, the endpoints will need to talk to ISE Directly (not the VIP) - and reach the web portals. The Portals can ONLY exist on the Gigabit 0/0 Interface in 1.1.x. (This may change in a future version of ISE).
Reason #3: Routing Tables:
Unless you add a static route to ISE for every NAD Subnet, ISE does not have the ability in 1.1.x to return traffic on a different subnet through a different Gateway, only it's default Gateway. Therefore, the Load-Balancer MUST be the Default-Gateway for the ISE PSN's.
Since the Load-balancer must be the default Gateway, then all Management Traffic is also flowing through the Load-Balancer, unles you physically locate the Policy Administrative Node (PAN) and Monitoring & Troubleshooting Node (MNT) behind the load-balancer as well (just don't include those in the ServerFarm).
I hope that helps.
Aaron
38 Comments
