HSRP – How a Router Gets Elected as an Active Router in HSRP

fhrphsrp

I know the overview of HSRP and that election depends on priority. I also know that only active and standby routers send hello messages every 3 seconds. But when the routers are first configured with HSRP, how do they exchange message and elect the router with highest priority?

Best Answer

Cisco has many documents to explain such things. For example, Hot Standby Router Protocol (HSRP): Frequently Asked Questions:

Q. If there is no priority configured for a standby group, what determines which router is active?

A. The priority field is used to elect the active router and the standby router for the specific group. In the case of an equal priority, the router with the highest IP address for the respective group is elected as active. Furthermore, if there are more than two routers in the group, the second highest IP address determines the standby router and the other router/routers are in the listen state.

Note: If no priority is configured, it uses the default of 100.

Related Solutions

Nat – HSRP and Static NAT behavior

I have found the issue.

The problem was my testing procedure. I simulated the http connection using telnet to http port but I did not generate any traffic(requests). I repeated the tests using GET requests and the nat session table was replicated almost instant.

I post the debug messages.

The NAT replication trigger is the next segment, request in the tcp session.

Debug messages

Telnet from client

telnet 10.31.73.12 3099 
*Nov  7 03:52:17.071: TCBB2701B60 connected to 10.31.73.12.3099
GET / HTTP/1.0
GET / HTTP/1.0
GET / HTTP/1.0

New active

Becomes Active at 03:52:32,34

NATGW2#
*Nov  7 03:52:32.700: %HSRP-5-STATECHANGE: Ethernet0/2 Grp 100 state Standby -> Active
NATGW2#
*Nov  7 03:52:32.701:  IP-ADDR: ipaddr_table_insert_w_tableid() 10.31.71.254, in global table on Ethernet0/2
NATGW2#
*Nov  7 03:52:34.657: %HSRP-5-STATECHANGE: Ethernet0/1 Grp 200 state Standby -> Active

new Active listens for ARP requests for the HSRP IP

*Nov  7 03:52:34.658:  IP-ADDR: ipaddr_table_insert_w_tableid() 192.168.153.253, in global table on Ethernet0/1

The nat session is recreated on the new Active.

*Nov  7 03:52:34.745: NAT: API parameters passed: src_addr:10.31.71.3, src_port:0 dest_addr:10.31.73.12, dest_port:0, proto:6 if_input:Ethernet0/2 pak:B072F0A8 get_translated:1
*Nov  7 03:52:34.745: ipnat_api_translated_address_and_port_common, out->in want IL,OL
*Nov  7 03:52:34.745: NAT: API Translated-Info(1): (src-addr:10.31.71.3, src-port:0) (dest-addr:192.168.153.12, dest-port:0)

High Availability Routers – HSRP, VRRP, GLBP Explained

It's all about the timers. There's an advertise interval and a master down interval (hello and hold time in HSRP and GLBP). The master sends VRRP advertisements according to the advertise interval. This lets the backup know that the master is up. If the backup doesn't receive an advertisement within the master down interval, it will transition to master. The key to keeping connections alive is tuning your intervals down to sub-second failover.

When the backup router takes over, it sends a gratuitous ARP message telling everyone to use its MAC address for the virtual IP.

Best Answer

Related Solutions

Nat – HSRP and Static NAT behavior

High Availability Routers – HSRP, VRRP, GLBP Explained

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