The driving force in implementing First Hop Redundancy Protocols (FHRP) in a Cisco centric High Availability Campus network design has been the reducing cost of Layer 3 devices and more importantly the potential loss of revenue in the event of a network outage to an Enterprise (Pavlik et al, 2014).
To prevent a single point of failure thereby isolating an entire network, multilayer devices need to be employed to offer high availability so that packets can be diverted and forwarded through a stand-in device for a failed Router, with minimal disruption to users and systems.
The FHRP offers a solution for redundancy and is made up of the following group of protocols, HSRP (Hot Standby Router Protocol), VRRP (Virtual Router Redundancy Protocol) and GLBP (Gateway Load Balancing Protocol).
The main factors in selecting GLBP over VRRP has been that it provides load balancing capabilities, better throughput performance and security which VRRP no longer supports as published in RFC 5798 section 9 (Nadas, S., 2010). HSRP and GLBP are Cisco proprietary protocols whereas VRRP is an open standard which allows for different vendors to co-exist on the same network and they all incorporate IPv6 making them resilient in the long term.
The diagram below is a visual representation of how the traffic flows and terms used for both protocols.
HSRP Routers in the same group elect the router with the highest priority value as the Active Router responsible for handling all packets and selecting a single Standby Router which will transition to the active router state should the Active Router fail. The Standby Router will either transition permanently to an Active State or revert back when the active router comes back online. If it remains in the active state a router with the highest priority in the listening state will be promoted to the standby router state.
The HSRP mechanism creates a single virtual Default Gateway IP and Mac address that will be used by all hosts on the downstream subnet, this virtual router acts as the go between for the hosts and the Active Router masking its real IP and Mac address.
Hello messages are sent every 3 seconds to the multicast address of 126.96.36.199 on UDP port 1985 (Li et al, 1998) which is only monitored by the standby router. If no hello messages are received in a 10 second Hold-Time timer window, it will assume the role of the Active Router and a re-election process will be triggered.
A maximum of four routers in the same GLBP group can be in the active state and forwarding traffic, whilst you may have up to 1024 permissible virtual routers within the same group. The Router with the largest priority or the highest IP address will be chosen as the AVG (Active Virtual Group), its main function is to assign each active router a different virtual mac address. Each of these routers are known as the Active Virtual Forwarders (AVF’s), so one router in the group will be both an AVG and an AVF.
The AVG responds to ARP (Address Resolution Protocol) requests and will assign the mac address of one of the AVFs to the hosts based on a round robin algorithm.
The AVG router transmits messages to the multicast address of 188.8.131.52 on UDP port 3222 (Cisco, 2008). The Hello and Hold-Time Timer messages have the same default values and election process as HSRP however all AVF routers are listening unlike in HSRP where only the Standby Router is in the listening state.
In an organization with thousands of hosts, GLBP will distribute the workload to all the active routers on a round robin basis but this can be altered to either being host dependent or weighted traffic, where the packets are divided between the routers as a percentage of the total amount of traffic.
In Cisco proprietary networks you are able to modify the hello and hold down timers from the default in seconds to milliseconds. With using sub-second times you almost ensure that not a single packet is lost when there is an outage (Lammle,T. and Tedder,W., 2014).
Not all Cisco IOS versions support GLBP and unlike HSRP which does support IPsec VPN Tunnels, then from the fact there is no written documentation in the creation of IPsec Tunnels for GLBP and numerous unsolved cases on Cisco’s support forum, we are able to deduce that it doesn’t support GLBP.
The Standby Router’s resources in HSRP are not fully utilized, nor is load balancing supported in the same group. A workaround is to implement VLANs and assign them to different gateways but in the event of an upstream link failure suboptimal routing will result where a packet is routed back to the active forwarding router from the standby router before being routed back out of the network.
A caveat to implementing cisco proprietary protocols, in this case HSRP and GLBP is that they are not vendor neutral. The ever increasing encroachment by other vendors, like Juniper Networks which do not support either HSRP or GLBP, have seen a year-on-year growth (Haranas, M., 2015) in market share, resulting in a dramatic reduction of Cisco’s market share.
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