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ISE and Load Balancing

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Cisco Employee

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

Edit section

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:
Edit section

  • 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)?

Edit section

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)

Source_is-ACE.png

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)
LB - Cert_FQDN.png

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
smp Enthusiast
Enthusiast

laszlo, your attachments only show screenshots of the :1813 (RADIUS Accounting) VIP. Do you also have a :1812 (Authentication/Authorization) VIP?

Beginner

Hello guys,

i have a question regarding this.

My ISE deployment is:

1Adm/mnt Node and 1 PSN on Site A

1Adm/mnt Node and 1 PSN on Site B

So in this scenario putting the Loadbalancer as default gateway is not possible.

But i need the Loadbalancer to have redundancy for the guest portal. I am just using ISE with my WLC, so in that case it wouldn´t be a problem to use SNAT untill i have a second PSN on each Site right ?

My general question is is it not recommened to use SNAT because all NADs on ISE will be the Loadbalancer or is there a function issue when using SNAT ?

Many thanks

/Philip

Beginner

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.

Answered the question by myself... Cannot delete a post here don´t know why

Beginner

Aaron/All,

You mention the NAS IP field in ISE.  Have you ever seen this field populated with 0.0.0.0?  Any idea what might cause that?  

Thank you!

Tim

Enthusiast

I am having a hard time finding a configuration guide using Citrix Load Balancers.  Followed the link but not seeing it.

Beginner

Hi Aaron,

Do we need to add switches, WLC, ASA in ISE as network devices while using F5 load balancer or just the VIP of F5??. Just for RADIUS authentication.

Thanks

Deepu

Contributor

Deepu,

The goal of this configuration would be to add the switches / WLCs / ASAs as NADs so you can better provide privileges throughout your organization based on who should have access to what. If you did a SNAT deployment, the authenticating device would look like the LB to ISE and you would need to discriminate on other radius attributes to try to write policy.

This setup is all about preserving the originating NAD IP address so that ISE is able to use that additional data (i.e. as a member of a device group).

-J. Sloan

Hello Team,

Hoping someone would pleas help me here :

I have ISE running in the production network , I have 4 PSNs, 2 MNT and 2 admin nodes.

Now we are in the process of deploying guest ( hotspot ).

Please note all 4 PSNs are located behing the f5 Load balancer.

Can you please advise on below :-

1- on Pre-auth ACL that we need to configure for hotspot can we have just access opened for F5 VIP not to PSNs ? 

2- we also need to install public CA certificate to avoid any certificate error when guest will redirect to the AUP portal, for this can we have one certificate having SAN fields as 4 PSNS node and 2 admin nodes ? while generating CSR i need to select certificate is for portal ??

Thanks in advance for your help