Virtual Router Redundancy Protocol
Why we need Gateway Redundancy?
LAN users are configured with Default-Gateway IP address, which is the IP of their Local router, which acts as a Proxy, and helps them communicate with remote/distant users. LAN users resolve the MAC address of Gateway router using the Proxy ARP method.If the gateway router fails, the LAN users would be affected. This means that Specifying a default gateway for LAN Users leads to a single point of failure. Instead of making the LAN users responsible for choosing a new gateway, router redundancy protocols enable two or more routers to support a shared MAC address. If the primary router fails, the backup router assumes control of traffic forwarded to that MAC. Virtual Router Redundany Protocol (VRRP) helps us in building Gateway Redundancy.Protocols that provide Gateway redundancy
- HSRP (Hot Standby Routing Protocol)
- VRRP (Virtual Router Redundancy Protocol)
- GLBP (Gateway Load Balancing Protocol)
How VRRP works?
- VRRP is similar to HSRP but is an Open Standard protocol (RFC 2338) developed to provide redundancy to LAN Users.
- Using VRRP, the gateway routers or Multilayer switches(Supported on Cisco 4500 and 6500 only) are configured to behave as a single virtual gateway to the LAN users.
- VRRP works on top of IP (protocol No. 112) i.e. at the Transport layer.
- VRRP gateways/Routers/Multilayer Switches communicate among themselves using Multicast IP 224.0.0.18 (Reserved for VRRP).
- VRRP provides one redundant gateway address from a group of gateway routers. The active router is called the Master router, while all others are in the Backup state. The Master router is the one with the highest router priority in the VRRP group. There is no 3rd state in VRRP besides Master and Backup. All other routers besides Master in a will remain in Backup state.
- VRRP group numbers range from 0 to 255. Router priorities range from 1 to 254 (254 is the highest; 100 is the default).
- The Virtual MAC address in VRRP is like 0000.5e00.01xx, where xx is group number in Hex.
- VRRP Hello/Advertisement interval is 1sec(default).
- Backup routers can optionally learn the advertisement interval from the master router.
- Hold down interval is 3 x Advertisement (or Hello) interval plus Skew time. Formula to calculate Skew time is (256–priority) / 256.
- Preemption is enabled by default in VRRP. The router with the highest priority preempts to become master.
- There is no mechanism for tracking interfaces in VRRP to allow more capable routers to take over the Master role.
- But, VRRP can track IP SLA object groups.
VRRP Sample Lab and Commands - Configuring VRRP
R1 is Master (Priority 120), R2 and R3 are Backup routers (Priority 110 and 100 respectively).
VRRP Tracking using IP SLA
Below is the configuration from Gateway Router R1. Interface F0/1 is tracked using an IP SLA and Track commands.Router(config-ip-sla)#ip sla 1
Router(config-ip-sla)#icmp-echo 10.10.17.7 source-ip 10.10.17.1
Router(config-ip-sla)#frequency 5
Router(config)#ip sla schedule 1 life forever start-time now
Router(Config)#track 10 rtr 1 reachability
Router(Config-if)# vrrp 20 track 10 decrement 30
Router R1 logs of role change as Track 10 goes down and back to up
*Mar 1 00:31:00.891: %TRACKING-5-STATE: 10 rtr 1 reachability Up->Down
*Mar 1 00:31:21.683: %VRRP-6-STATECHANGE: Fa0/0 Grp 20 state Master -> Backup
*Mar 1 00:33:45.891: %TRACKING-5-STATE: 10 rtr 1 reachability Down->Up
*Mar 1 00:34:06.655: %VRRP-6-STATECHANGE: Fa0/0 Grp 20 state Backup -> Master
R1#show track 10
Track 10
Response Time Reporter 1 reachability
Reachability is Up
5 changes, last change 00:02:09
Latest operation return code: OK
Latest RTT (millisecs) 104
Tracked by:
VRRP FastEthernet0/0 20
Outputs for VRRP related Commands
R1#show run int f0/0interface FastEthernet0/0
ip address 10.10.123.11 255.255.255.0
vrrp 20 ip 10.10.123.1 >>> Group number 20, Virtual IP 10.10.123.1
vrrp 20 preempt delay minimum 20 >>> Delays preemption by minimum 20 seconds
vrrp 20 priority 120 >>> Router with Highest Priority becomes "Master"
vrrp 20 authentication password >>> Only authorized routers involve in VRRP election
vrrp 20 track 10 decrement 30 >>> VRRP Tracking using IP SLA. Config shown below.
end
R1#sh vrrp
FastEthernet0/0 - Group 20
State is Master
Virtual IP address is 10.10.123.1
Virtual MAC address is 0000.5e00.0114
Advertisement interval is 1.000 sec
Preemption enabled, delay min 20 secs
Priority is 120
Track object 10 state Up decrement 30
Authentication text "password"
Master Router is 10.10.123.11 (local), priority is 120
Master Advertisement interval is 1.000 sec
Master Down interval is 3.531 sec
R2#show vrrp
FastEthernet0/0 - Group 20
State is Backup
Virtual IP address is 10.10.123.1
Virtual MAC address is 0000.5e00.0114
Advertisement interval is 1.000 sec
Preemption enabled, delay min 20 secs
Priority is 110
Authentication text "password"
Master Router is 10.10.123.11, priority is 120
Master Advertisement interval is 1.000 sec
Master Down interval is 3.570 sec (expires in 3.498 sec)
R3#show vrrp
FastEthernet0/0 - Group 20
State is Backup
Virtual IP address is 10.10.123.1
Virtual MAC address is 0000.5e00.0114
Advertisement interval is 1.000 sec
Preemption enabled, delay min 20 secs
Priority is 100
Authentication text "password"
Master Router is 10.10.123.11, priority is 120
Master Advertisement interval is 1.000 sec
Master Down interval is 3.609 sec (expires in 2.957 sec)
R1#show vrrp brief
Interface Grp Pri Time Own Pre State Master addr Group addr
Fa0/0 20 120 3531 Y Master 10.10.123.11 10.10.123.1
Output from Host4. See the VRRP MAC address learned for the Virtual IP 10.10.123.1
Host4#show arpProtocol Address Age (min) Hardware Addr Type Interface
Internet 10.10.123.1 0 0000.5e00.0114 ARPA FastEthernet0/0
Internet 10.10.123.4 - ca04.16b0.0000 ARPA FastEthernet0/0
Output from Switch
Switch#sh mac vlan 1 | i 0000.5e00.0114 >> VRRP Virtual MAC0000.5e00.0114 Dynamic 1 FastEthernet1/2 >> Only R1 (Master) replies to ARP Queries.
VRRP - Few More Commands
Command to change the timers on the Master as it is the router that advertises hellos:Router(config-if)# vrrp 20 timers advertise 5
Command to change the timers on the Backup routers because they hear the hellos from the Master:
Router(config-if)# vrrp 20 timers learn
