Router Redundancy Protocols
Introduction
There exist different protocols that control connections between computers that are connected through a network. Theses different protocols improve speeds and efficiency in transmission of data between the computers that are interconnected to each other. Some are called router protocols, and some are router redundancy protocols which are either propriety or non-propriety, (patent-free) protocols. Though they are called by different names, their common goal is to boost up efficiency and speed in transmitting and receiving data among computers in a network.
This essay discusses two different router redundancy protocols. It will define these two protocols, describe the general aims of each protocol, their operations, and highlight the similarities and differences between them. The two router redundancy protocols that are discussed in this essay are the Hot Standby Router Protocol (HSRP) and the Virtual Router Redundancy Protocol (VRRP).
HSRP and VRRP
Hot Standby Routing Protocol or HSRP for short is a propriety router redundancy protocol for Cisco which is designed to be used over multi access, multicast or broadcast capable LANs. HSRP’s goal is to allow hosts in network to appear to be using a single router and to continue to maintain network connectivity in the event that the actual first hop router being used fails. The appearance of having a single router done through the participation of multiple routers in a Local Area Network in order to create what is called a virtual router. This virtual router is the key player of the protocol since all traffic is done through this virtual router. Only one among the routers that create the virtual router is ensured to be forwarding packets in behalf of the virtual router and this router is known as the active router. If an event occurs wherein the active router would fail, a standby router is selected to replace the failed active router. The mechanism which selects active and standby routers is provided by the protocol by using the IP addresses of the routers that make up the virtual router. Should the active router fail, its standby router takes over without any major disruption to the connectivity of the host. Once the standby router fails or becomes an active router, the protocol selects another standby router. Only the active and standby routers are allowed to send packets of data which ensures the network of lesser traffic. The protocol prevents problems due to failure of the first hop router when the source host can’t dynamically learn the IP address of the first hop router. On a LAN where multiple HSRPs exist each HSRP forms its own virtual router and each virtual router is assigned its own MAC address and IP address. Each IP address assigned to each virtual router belongs to the primary subnet used in the LAN but differs from the IP addresses of the routers that participate in the protocol and it also differs to the IP addresses of other virtual routers. In this scenario, load splitting is achieved by distributing hosts among the different HSRP groups in the LAN.
The protocol uses port number of 1985 of UDP and sends its packets via multicast address 224.0.0.2. The HSRP protocol multicasts hello messages to all routers that make up the virtual router in order to define priority, and select which routers become active and standby routers. The router with the numerically highest priority number becomes the active router and the router with the next highest priority number becomes the standby router. If routers have the same priority number, the router with a higher IP address is selected.
Regarding the Protocol’s operational parameters, there is certain information that has to be known to all the routers within the HSRP group and these are the Standby Group Number, Virtual MAC Address, Priority, Hello Time, and Hold Time. There is also certain information that must be known to at least one router in the group and this is the Virtual IP Address which is the virtual router’s IP address. A router’s preemption capability may also be configured in any of the routers. If a router’s preemption capability is enabled and it has a higher priority than the active router then it may take over the active router using a Coup message.
Virtual Router Redundancy Protocol or VRRP is a non propriety or patent free router redundancy protocol which works much like HSRP with some differences. This protocol specifies an election protocol that assigns responsibility for a virtual router to one of the member routers of the VRRP protocol on a LAN dynamically. One router controls the IP addresses or addresses associated with the virtual router and it forwards the packets sent to these addresses and this router is called the Master Router. If the master router fails, the protocol employs an election process to select a new Master Router. VSRP has the advantage of a higher availability of the default path without having to configure dynamic routing or router discovery protocols on every end host.
The election process takes place of selecting a new master router takes place once the current master router fails to receive a multicast packet for a period longer than three times. Once this happens, all the back up routers assumes that the master router is dead and the protocol begins the election process by sending multicast packets to all the other routers that form the VSRP and the backup router with the highest priority number becomes the new master router. But if there is a case where two or more back up routers have the same priority number then the router with the highest IP address gets the position of master router. The only exception to this election process is that if a physical router is configured to always overthrow the current master router after it has been added to the virtual router. An example of this is when a physical router is more powerful than other routers in the virtual router or if that router consumes less bandwidth.
In a VSRP protocol all physical routers that make up the virtual router must be within one hop of each other. There is periodical communication within the virtual router and this period is adjusted if needed by changing the advertisement interval times. If the advertisement interval times are shorter then the black hole period, the period when no packets are sent, would be shorter although this would cause more traffic in the network.
In conclusion, the HSRP and VSRP router redundancy protocols are very much alike but they aren’t exactly carbon copies of each other. Each protocol has characteristics and operations that are different from each other. Both protocols are not router protocols since they don’t advertise IP routes or affect the routing table in any way, but HSRP is a patented or proprietary protocol that is licensed to Cisco while VSRP is a non-proprietary protocol and is not under any patent. Both protocols run on LANs particularly the Ethernet LAN. Both make use of virtual routers that are comprised of the physical routers. In HSRP only one router is used to represent the virtual router while in VRRP two routers represent the virtual router. HSRP calls the router that represents the virtual router as the active router which has one standby router to replace it if it fails while VRRP calls its version of the active router as the master router and all other routers in the virtual router act as back up routers. Both protocols employ an election process to select which router replaces the active or master router should the said router fail and this involves the priority numbers and IP addresses of the other routers. In the case of the HSRP the standby router automatically replaces the active router. The VRRP selects the replacement router from among the many back up routers. Both HSRP and VRRP have almost the same characteristics except for a few differences
Reference:
T. Li (Jupiter Networks), B. Cole (Jupiter Networks), P. Morton (Cisco Systems), D. Li (Cisco Systems), March 1998, Cisco Hot Standby Router Protocol (HSRP), Full description of HSRP protocol, viewed on March 2, 2008, <http://tools.ietf.org/html/rfc2281>.
R. Hinden Ed. (Nokia), April 2004, Virtual Router Redundancy Protocol (VRRP), Full description of VRRP protocol, viewed on March 2, 2008, <http://tools.ietf.org/html/rfc3768>.
Credit:ivythesis.typepad.com
0 comments:
Post a Comment