In the world of I.T, redundancy is regarded the most important element when implementing and designing a system and/ or Infrastructure. First Hop Redundancy Protocols are commonly used to create redundancy in networking to allow a router or a layer 3 switch act as a backup gateway on a LAN segment to automatically take over if the primary gateway fails. Without this redundancy, if this router or router interface (that serves as a default gateway) fails, the hosts configured with that default gateway will loose connection from outside networks.
End devices are typically configured with a single IP address for a default gateway. This address does not change when the network topology changes. Instead of configuring a PC with a backup default gateway an FHRP can be used to provide this redundancy seamlessly without the individual configurations on each host.
HSRP / VRRP / GLBP are all examples of first hop redundancy protocols which we will cover briefly in the blog. The are all very similar in operation with slight differences, so it is important to note the slight differences in operation / configuration of each, along with knowing that FHRP itself is not a protocol but rather describes a specific protocol in use!
HOT STANDBY ROUTER PROTOCOL (HSRP) the first FHRP we will dicuss is the first ever created and it is also Cisco proprietary, which means, it will can only used with Cisco devices. HSRP provides hosts a transparent failover mechanism connecting to a primary gateway. This provides users at the access layer with uninterrupted service to the network if the primary gateway becomes inaccessible. To configure HSRP you enable it on a particular interface and this interface is part of a “standby” group. Besides the physical IP address of the defined interface, there’s a virtual IP address in the same subnet. The idea behind this is to perform, parallel to this, a similar configuration in an interface belonging to another router. This will effectively generate redundancy where two different interfaces from two different devices will share the same virtual IP address. This virtual IP address will then act as the host’s gateway.
- # int vlan (vlan #)
- #Stanby (vlan #) ip x.x.x.x
- #standby (vlan #) priority 100 (default – 100)
- #stanfby (vlan #) preempt
HSRP interface tracking –
- #stanby 10 track fa0/3 (decrement) default/blank is 10
- Cisco Proprietary
- States – Standby and Active
- Highest priority becomes Active
- Timers hello 3 secs hold 10 secs- timers do not need to match
- Always watch preemption. If Status does not change as expected, preemption may not be enabled.
THE VIRTUAL ROUTER REDUNDANCY PROTOCOL (VRRP) VRRP stands for Virtual router redundancy protocol. VRRP is not Cisco propriety and is supported today by vendors all across the world. Network performance is improved by dynamically assigning one of the virtual routers as a default gateway using a VRRP election algorithm. This algorithm increases reliability and provides a fail-over mechanism and support for static routing. The VRRP assigns one of the routers as the master router, which manages the forwarding and routing of traffic toward all the virtual IPs associated with these routers. It also dynamically switches over to another router if the master is unavailable.
The VRRP does have its limitations. Its scope can extend only to a single subnet. The VRRP cannot affect the routing table in any way and it does not promote the IP routes either, Cisco has been trying to patent this particular protocol for some time now.
- #vrrp (vlan #) ip x.x.x.x
- #vrrp (vlan #) priority 100 (default 100)
VRRP interface tracking
- Create track object – #track (#) interface (interface number) line protocol
- #vrrp (vlan #) track 1 decrement (default, blank is 10)
- Not Cisco propriety, industry standard
- States – Master and Backup
- Highest priority becomes Master
- Preemtion enabled by default
GATEWAY LOAD BALANCING PROTOCOL (GLBP): – HSRP and VRRP have some great features but load balancing with these protocols is more of a work around then a native behaviour. The Gateway Load Balancing Protocol is a Cisco proprietary protocol designed to overcome the limitation of the existing redundant router protocols. GLGP uses the same concepts as with HSRP/VRRP, but the terminology is different and the behaviour is much more dynamic. Instead of having just one active router performing forwarding for the virtual router address, all routers in the group can participate and offer load balancing by forwarding a portion of the overall traffic. The advantage of that is, one of the clients has to be pointed toward a specific gateway address, and they can all have the same default set to the virtual IP address. The load balancing is provided completely through the use of virtual router MAC addresses in ARP replies returned to the client send an ARP reply with the virtual MAC address of a selected router in the group. The result is that all clients use the same gateway address but have different MAC addresses for it.
- #int (interface number)
- #glbp (#) ip x.x.x.x
- #glbp (#) preempt
- #glbp (#) priority 100
- Cisco Propriety
- Active Virtual Gateway and Active Virtual Forwarder
- Each member of a GLBP group is an active forwarder
- Timers hello 3 secs hold 10 secs-