IT Certification and Training Blog

VRRP, HSRP and the Master Router

Posted by Kelson Lawrence on Feb 8, 2011 10:22:00 AM

By Brian Scheibe

Does VRRP choose a Master (Active) router in the same way as HSRP?

Well that depends. Is preempt enabled and are priorities set on the routers in the VRRP group? Which router has the highest IP address? Which router owns the IP address for the VRRP group? All of these matter when determining which device will become the Master VRRP router.

One of the differences between VRRP and HSRP is that preempt is enabled by default in VRRP. Logic would seem to indicate that by configuring Router1 with the maximum configurable priority value of 254, Router1 would become the master router. This is not necessarily true. Here are some of the rules:

• The maximum VRRP priority value is 255.
• The maximum VRRP priority value that you can manually configure on the router is 254.
• Only one router can have a VRRP priority setting of 255.
• The router that “owns the IP address” for the group automatically gets a VRRP priority setting of 255.

Based on the rules above, if you configure the IP address assigned to Router2’s VLAN 1 interface (the interface on which the VRRP group is configured) to the VRRP group. Router2, not Router1, will always be the master router when Router2 is functional and preempt is enabled.

Also, since we know that Router2 will always be the master router when it is functional in the network, we have to wonder what happens if it goes down and we bring it back up. The answer is that it becomes the master router again. This is not always true with HSRP. Remember that the default preempt setting for HSRP is disabled. The only way the former HSRP master will resume the master role is if preempt has been enabled and the current HSRP priority value is higher than the HSRP priority values on all other routers.

There are several reasons you would want to know how the master VRRP router is chosen. For example, How long does it take your network to converge. If you add a router to the VRRP group that will become the master you need to consider how long it will take for the router to learn routes. While the router is learning it will drop packets for destinations that are not in its routing table. Knowing this will enable you to configure the router to prevent it from becoming the master or to set a delay in the VRRP configuration that allows the router time to learn routes prior to assuming the role of master. Another reason might be that you want the fastest, or newest, or most reliable device to be the master router.

Although VRRP and HSRP provide redundancy, it is not safe to assume that they choose a master in the same way.

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References:
IETF RFC 5798
http://tools.ietf.org/html/rfc5798

Cisco: Virtual Router Redundancy Protocol: Feature Overview: VRRP Router Priority
http://www.cisco.com/en/US/docs/ios/12_0st/12_0st18/feature/guide/st_vrrpx.html#wp1035078

Tags: Brian Scheibe, networking, VRRP, HSRP, Master Router