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General information
An example of implementation of a fault-tolerant scheme for enabling ESRs operating in wireless-controller mode is shown in Fig. 1:
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Traffic processing is performed by the ESR VRRP MASTER. In case of its failure, the VRRP mastery is taken over by the ESR VRRP BACKUP. Last-mile router redundancy (NAT router in Fig. 1) is not covered in this paper. It can be accomplished in the same way using VRRP or by using a different enabling scheme (this scheme can be found in Configuring ESR in wireless-controller mode with last-mile router redundancy).
When using channel aggregation on ESR you cannot use sub-interfaces on the "port-channel" interface.
Enabling scheme
The enabling scheme will be considered on the example of real addressing shown in Table 1.
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If the "vrrp preempt disable" setting is not used, it is necessary to set "vrrp preempt delay <time in seconds>" to at least 180 seconds in the VRRP configurations on the interfaces. This will prevent the higher priority router from immediately seizing mastery after booting. If this is not done, the router that captured the VRRP mastery after booting will not have time to synchronize the state of the AP tunnels and they will be brought up again as the AP triggers the gre keepalive mechanism. |
ESR configuration
ESR tuning will be considered on an example based on the circuit shown in Fig. 3. The wireless-controller configuration will use the SoftGRE dynamic tunnel configuration profile. Do not forget that a license is required to access the wirless-controller functionality (more details can be found in the Configuring ESR by connecting AP via L3 access network (WiFi L3 diagram)).
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Adding an ESR to the EMS tree
Both ESRs with real interface address 100.123.0.173 and 100.123.0.175 respectively should be added to the EMS tree. The VRRP address 100.123.0.174 will be used as a gateway for routes to the AP management and WiFi user subnets, it will not appear anywhere in the EMS.
Add ESR-VRRP-MASTER - open EMS, stand on the node to which we plan to add ESR and click the "+" button located at the top left of the node tree:
Fig. 4.
In the window that opens, in the field:
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After that it is necessary to stand on the added ESR (if it does not appear in the tree - click the button above "
" ) open the "Access" tab on the right:
Fig. 5.
In the opened window edit the fields:
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Change the radius password that will be used when interacting with the ESR. To do this, open "RADIUS" → "Access Point Management" in the EMS menu. Select the previously added ESR (in case of a large number of devices you can filter by ESR IP address) and click the "Edit" button:
Fig. 6.
In the opened window in the "Key" field set the previously configured for ESR key "testing123" and click "Accept".
Similarly, add ESR-VRRP-BACKUP using its real address 100.123.0.175.
Possible reasons of network failure
Below we will consider possible variants of user traffic passing during normal operation and network failure.
Network operational state
Figure 7 shows the operational state of the network.
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The traffic of WiFi users connected to the SSID the AP encapsulates in the GRE is routed to the ESR VRRP MASTER (which is in the VRRP MASTER state) where the traffic is decapsulated. Then it is routed to the Internet. The ESR VRRP BACKUP does not participate in traffic processing.
Failure of one of the stack switches
Figure 8 shows the failure state of one of the stack switches to which ESR VRRP MASTER and BACKUP are connected:
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In this situation, one of the physical interfaces (gi1/0/1 or gi1/0/2) belonging to the port-channel on both ESRs will be disconnected. But, since the remaining interfaces belonging to the port-channel connected to another stack switch will remain in operation, the circuit will remain fully functional and there will be no changes in traffic flow compared to the state before the failure.
Failure of an ESR in the VRRP master state
Figure 9 shows the failure of an ESR in the VRRP MASTER state:
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In this situation, the VRRP BACKUP ESR, upon detecting the absence of VRRP announcements, will switch to the VRRP MASTER state and begin processing traffic in the same manner as the VRRP ESR MASTER did previously.
Recovery from a failure of an ESR that has a higher VRRP priority
Figure 10 depicts a situation where the ESR VRRP MASTER has returned to operation after a failure:
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After the ESR VRRP MASTER is brought back online and booted, upon detecting VRRP announcements from the ESR VRRP BACKUP, which is in the VRRP master state, due to the "vrrp preempt disable" setting on the VRRP interfaces, will be in the VRRP backup state and will not attempt to perform a master takeover. Traffic will continue to flow in the preempted state.
Appendixes
Different versions of the spanning-tree protocol family may be used on the switches to which the ESR connects. In this case, it is possible that after the ESR is booted, the switch ports to which it is plugged will not immediately switch to the "forwarding" state and allow traffic to pass. The ESR will then have time to switch to the VRRP MASTER state, which will result in the ESR with a higher VRRP priority taking over the mastery after the traffic transfer starts. There are two ways to avoid this:
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