Fabric Routing Functionality

The VRRP backup router enabled with fabric routing mode is called a VRRP FREB router (Fabric Routing Enabled Backup router). The VRRP FREB router is responsible for the following:
  • Performing routing for the packets destined for subnets other than the received interface‘s subnet. These packets should also have a destination MAC matching Virtual MAC. These packets will be routed by hardware.
  • Forwarding the packets having a Destination IP matching Virtual IP, towards VRRP Master. These packets will have Destination MAC matching Virtual MAC. These packets will be forwarded by hardware.
  • Responding for unicast ARP requests with target IP matching Virtual IP.
  • Responding for unicast Neighbor Solicitation requests with target IP matching Virtual IP.
  • Does not respond to broadcast ARP / multicast Neighbor solicitations targeted for Virtual IP.
  • Does not respond for ICMP requests destined for Virtual IP.
  • Does not generate gratuitous ARP/ Neighbor Advertisements.
  • Does not advertise Router Advertisement Prefixes in this state.

VRRP Router Accepts Packets Destined to Virtual IP

Only the master serves as protocol servers like NTP, telnet, SSH, etc and accepts connections destined to the virtual IP. By doing so, hosts always connect to the same virtual router by using the virtual IP, at any given point of time. This ensures that the host is getting a consistent response from the protocol server. This arrangement allows the host to reach the NTP server using the same IP i.e. virtual IP, even if VRRP mastership moves to a different router. A network monitoring tool is another example, which can use virtual IP to collect data about VRRP domain, by connecting to current VRRP Master. It is not recommended to change the configurations of the switch, when a management session is connected using virtual IP. When VRRP FREB router sits in between the host and VRRP Master router, FREB router does hardware forwarding of these packets from host towards VRRP Master, at Layer 3.

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VRRP Fabric Routing
../Graphics/VRRP_Fabric_Routing.png
Note

Note

A caveat is that TTL/hop count is decremented for the packets destined for virtual IP, when forwarded by FREB. This may be a problem to run any protocol that expects TTL not to be decremented, between host and Master.

Hosts can generate unicast ARP to validate a ARP cache entry. Similarly, unicast Neighbor Solicitation is generated to perform Neighbor Unreachability Detection for a neighbor. These requests are periodic. The unicast ARP/NS requests will be responded by FREB, if it receives the request. A downside of allowing VRRP Master to respond these requests is that it may take considerable CPU cycles when large numbers of hosts are present in VRRP domain.