Cisco Application Control Engine (ACE) Troubleshooting Guide -- Troubleshooting Layer 4 Load Balancing
This article describes how to troubleshoot Layer 4 (L4) load balancing on the ACE.
Overview of ACE L4 Load Balancing
Load balancing at L4 involves selecting a server in a server farm to service a client request based on the VIP address and protocol in the request. You configure a class map to classify (match) interesting traffic arriving at the ACE and associate the class map with a policy map to perform an action on the traffic based on the classification. With L4 load balancing, the ACE selects a server based on the first packet it receives in a particular flow. See the "Overview of ACE Connection Handling" section in the Troubleshooting Connectivity article.
For detailed information about ACE load balancing, see the Cisco Application Control Engine Module Server Load Balancing Configuration Guide.
Classifying L4 Traffic for Server Load Balancing
You classify inbound network traffic destined to or passing through the ACE based on a series of flow match criteria specified by a class map. Each class map defines a traffic classification, which is network traffic that is of interest to you. A policy map defines a series of actions (functions) that you want applied to a set of classified inbound or outbound traffic.
ACE L3 and L4 traffic policies support the following server load-balancing (SLB) traffic attributes:
- Source or destination IP address
- Source or destination port
- Virtual IP (VIP) address
- IP protocol
The three major steps in the traffic classification process are as follows:
- Create a class map using the class-map command and the associated match commands, which comprise a set of match criteria related to Layer 3 and Layer 4 traffic classifications or Layer 7 protocol classifications.
- Create a policy map using the policy-map command, which refers to the class maps and identifies a series of actions to perform based on the traffic match criteria.
- Activate the policy map by associating it with a specific VLAN interface or globally with all VLAN interfaces using the service-policy command to filter the traffic received by the ACE.
Figure 1 provides a basic overview of the process required to build and apply the Layer 3, Layer 4, and Layer 7 policies that the ACE uses for SLB. The figure also shows how you associate the various components of the SLB policy configuration with each other.
- Figure 1. SLB Flow Diagram
Example of a Layer 4 Load-Balancing Configuration
The following example shows a L4 load-balancing configuration:
access-list ACL1 line 10 extended permit ip any any rserver host SERVER1 ip address 192.168.252.245 inservice rserver host SERVER2 ip address 192.168.252.246 inservice rserver host SERVER3 ip address 192.168.252.247 inservice rserver host SERVER4 ip address 192.168.252.248 inservice rserver host SERVER5 ip address 192.168.252.249 inservice rserver host SERVER6 ip address 192.168.252.250 inservice serverfarm host SFARM1 probe TCP_PROBE predictor roundrobin rserver SERVER1 weight 10 inservice rserver SERVER2 weight 20 inservice rserver SERVER3 weight 30 inservice serverfarm host SFARM2 probe TCP_PROBE predictor roundrobin rserver SERVER4 weight 10 inservice rserver SERVER5 weight 20 inservice rserver SERVER6 weight 30 inservice class-map match-all L4WEB_CLASS 2 match virtual-address 192.168.120.112 tcp eq www policy-map type loadbalance first-match LB_WEB_POLICY class class-default serverfarm SFARM1 backup SFARM2 policy-map multi-match L4WEB_POLICY class L4WEB_CLASS loadbalance vip inservice loadbalance policy LB_WEB_POLICY loadbalance vip icmp-reply active nat dynamic 1 VLAN 120 interface vlan 100 description Upstream VLAN_100 - Clients and VIPs ip address 192.168.120.1 255.255.255.0 fragment chain 20 fragment min-mtu 68 access-group input ACL1 nat-pool 1 192.168.120.70 192.168.120.70 netmask 255.255.255.0 pat service-policy input L4WEB_POLICY no shutdown ip route 10.1.0.0 255.255.255.0 192.168.120.254
Troubleshooting L4 Load Balancing on the ACE
To troubleshoot L4 load-balancing issues, follow these steps:
1. Ensure that your load-balancing configuration is correct and that the following conditions exist:
- Real servers have valid IP addresses and are in service.
- Servers are associated with server farms of the same type.
- A load-balancing policy exists with an associated server farm and is associated with a L4 multimatch policy.
- An L4 class map contains a valid match virtual-address command and is associated with the L4 multimatch policy map.
- The L4 policy is applied to the appropriate active interface using a service policy.
- A static route is configured for the server network.
Use the following show commands:
- show running-config rserver
- show running-config serverfarm
- show running-config policy-map
- show running-config class-map
- show running-config interface
- show ip route
2. Check the ACE connectivity. See the "Troubleshooting Connectivity" section.
3. Verify that the L4 VIP class map is referenced in a L4 policy by entering the following command. Also, check the following fields:
- VIP address and port
- VIP state
- Hit count
- Dropped connections
ACE_module5/Admin# show service-policy L4WEB_POLICY detail Status : ACTIVE Description: - ----------------------------------------- Interface: vlan 100 service-policy: L4WEB_POLICY <------- L4 multimatch policy map class: L4WEB_CLASS <------- L4 VIP class map VIP Address: Protocol: Port: 192.168.120.112 tcp eq 80 <------- VIP address, protocol, and port loadbalance: L7 loadbalance policy: LB_WEB_POLICY VIP Route Metric : 77 VIP Route Advertise : DISABLED VIP ICMP Reply : ENABLED VIP State: INSERVICE <------- VIP state should be INSERVICE curr conns : 0 , hit count : 56 dropped conns : 14 <------- Number of attempted connections to this VIP that the ACE discarded client pkt count : 6297 , client byte count: 1047583 server pkt count : 1238 , server byte count: 1325495 L7 Loadbalance policy : LB_WEB_POLICY class/match : class-default LB action : serverfarm: SFARM1 hit count : 0 <-------|-- Check these counters to see if they are increasing dropped conns : 0 <-------|
The dropped conns counter under a VIP in the output of the show service policy detail command is incremented whenever the ACE discards a connection request destined to that VIP. There are several reasons why the ACE discards such connection requests. For example:
- If all the real servers in the server farm associated with the VIP go down, then the VIP will go down. So, all the incoming connections to that VIP are discarded.
- If the URL in a connection request to the VIP is unknown, then the connection request is discarded.
- If the server to which the ACE load balances the connection does not respond to the request, then, after the maximum number of retries, the ACE discards the connection.
The dropped conns counter is cumulative and the value may comprise entries from any of the following show command counters:
- show stats loadbalance
- - Total Layer4 rejections
- - Total Layer7 rejections
- - Total Layer4 LB policy misses
- - Total Layer7 LB policy misses
- - Total times rserver was unavailable
- show stats connection
- - Total Connections Timed-out
- - Total Connections Failed
- The failures counter of the show serverfarm serverfarm_name command
- The Total drop decisions counter of the show stats inspect command
4. Verify that the L4 policy is applied as a service policy to an active interface by entering the following command:
ACE_module5/Admin# show running-config interface Generating configuration.... interface vlan 100 ip address 192.168.120.1 255.255.255.0 access-group input ACL1 access-group output anyone service-policy input L4WEB_POLICY no shutdown . . .
5. Check the total conn-dropcount field for the primary server farm in the output of the following command. Also, check the IP address, state, and the connection statistics for each real server that is configured in the server farm.
ACE_module5/Admin# show serverfarm SFARM1 detail serverfarm : SFARM1, type: HOST total rservers : 3 active rservers: 3 description : - state : ACTIVE <------- Current state of the server farm predictor : ROUNDROBIN <------- Load-balancing method weight : - autoadjust : MAXLOAD failaction : - back-inservice : 40 partial-threshold : 40 num times failover : 0 num times back inservice : 0 total conn-dropcount : 0 <------- Total number of connection attempts to this server farm that the ACE discarded --------------------------------- ----------connections----------- real weight state current total failures ---+---------------------+------+------------+----------+----------+--------- rserver: SERVER1 192.168.252.245:0 10 INSERVICE 0 0 0 <------- Real server IP address, state, and connection statistics max-conns : 4000000 , out-of-rotation count : 0 min-conns : 4000000 conn-rate-limit : - , out-of-rotation count : - bandwidth-rate-limit : - , out-of-rotation count : - retcode out-of-rotation count : - load value : 0 rserver: SERVER2 192.168.252.246:0 20 INSERVICE 0 0 0 max-conns : 4000000 , out-of-rotation count : 0 min-conns : 4000000 conn-rate-limit : - , out-of-rotation count : - bandwidth-rate-limit : - , out-of-rotation count : - retcode out-of-rotation count : - load value : 0 rserver: SERVER3 192.168.252.247:0 30 INSERVICE 0 0 0 max-conns : 4000000 , out-of-rotation count : 0 min-conns : 4000000 conn-rate-limit : - , out-of-rotation count : - bandwidth-rate-limit : - , out-of-rotation count : - retcode out-of-rotation count : - load value : 0
6. Check the L4 load-balance statistics by entering the following command:
ACE_module5/Admin# show stats loadbalance +------------------------------------------+ +------- Loadbalance statistics -----------+ +------------------------------------------+ Total version mismatch : 0 Total Layer4 decisions : 0 Total Layer4 rejections : 0 Total Layer7 decisions : 0 Total Layer7 rejections : 0 Total Layer4 LB policy misses : 0 Total Layer7 LB policy misses : 0 Total times rserver was unavailable : 0 Total ACL denied : 0 Total IDMap Lookup Failures : 0