VRRP Technology White Paper

Keywords: VRRP, virtual router

Abstract: This manual introduces the principles and applications of VRRP, and the characteristics and networking of the VRRP solution of H3C.

Acronyms:

Acronym

Full spelling

VRRP

Virtual Router Redundancy Protocol

NQA

Network Quality Analyzer

BFD

Bidirectional Forwarding Detection

IRDP

ICMP Router Discovery Protocol

VRID

Virtual Router ID

 



Overview

1.1  Background

With the development of the Internet, users have higher requirements on network reliability. It is very important to keep contact with other parts on a network especially for end users. Generally speaking, a host communicates with the external networks through the default gateway, as shown in Figure 1 .

Figure 1  LAN networking

Normally, a host sends the packets to be sent to the external networks to the gateway, which then sends the packets to the external networks. This enables hosts on a network segment to communicate with the external networks. However, when the gateway fails, all the hosts using the gateway as the default next-hop router fail to communicate with the external networks. A common way to improve system reliability is to use more egress gateways. However, most hosts can only be configured with one default gateway. In case that a default gateway fails, you need to manually configure another default gateway for the hosts that originally use the failed gateway, so that the hosts can continue to communicate with the external networks. Some solutions require hosts to run a dynamic routing protocol such as Routing Information Protocol (RIP), or Open Shortest Path First routing protocol (OSPF) to solve the problem. However, these protocols cannot satisfy users’ needs due to their complicated configuration or low security guarantee.

VRRP is thus addressed to solve the above-mentioned problems. VRRP does not change the original networking, nor does it require any configuration on the hosts. You only need to configure several commands on the related routers to implement backup of the gateway, without bringing any working load for the hosts. Compared with other methods, VRRP can better satisfy the users’ needs.

1.2  Benefits

VRRP is an error-tolerant protocol. With VRRP deployed on a network, if the next-hop router of hosts fails, another router will take it over to ensure continuous and reliable network communication.

VRRP has the following advantages:

l              Simplified network management: Deploying VRRP on multicast and broadcast LANs such as Ethernet, you can ensure that the system can still provide highly reliable default links without changing configurations (such as dynamic routing protocols or route discovery protocols) when a device fails, and prevent network interruption due to a single link failure.

l              High adaptability: A VRRP packet is encapsulated in an IP packet, and supports different kinds of upper layer protocols.

l              Low network overhead: VRRP defines only one packet type, VRRP advertisement, and only the master in a VRRP group can send VRRP advertisements.

Introduction to VRRP

2.1  Concepts

l              Virtual router: It consists of a master and several backups. Every host on the LAN takes the virtual router as the default gateway.

l              VRID: Virtual router identifier. A group of routers with the same VRID form a virtual router.

l              Master: The router that forwards packets in a virtual router.

l              Backup: The router that can take the responsibility of the master when the master fails.

l              Virtual IP address: IP address of the virtual router. A virtual router can have one or multiple IP addresses.

l              IP address owner: The router whose interface IP address is the same as the virtual IP address.

l              Virtual MAC address: A virtual router has one virtual MAC address. The format of a virtual MAC address is 00-00-5E-00-01-{VRID}. Generally, a virtual router responds to an ARP request with its virtual MAC address, and only when special configurations are performed on a virtual router does it respond with the real MAC address of the interface.

l              Priority: VRRP determines the role (master or backup) of each router in a virtual router by priority.

l              Non-preemptive mode: The backup working in non-preemptive mode remains as a backup as long as the master does not fail. The backup will not become the master even if the former is configured with a higher priority.

l              Preemptive mode: The backup working in preemptive mode compares the priority in the packet with that of its own when a backup receives a VRRP advertisement. If its priority is higher than that of the master if preempts as the master; otherwise, it remains a backup.

2.2  Introduction to Virtual Router

VRRP combines a group of routers (including a master and multiple backups) on a LAN into a VRRP group. The VRRP group functions as a virtual router, and is identified by a virtual router ID. A virtual router (VRRP group) has the following features:

l              A virtual router has its own virtual IP address and MAC address. Every host on the LAN takes the IP address of the virtual router as its default gateway and communicates with the external networks through the virtual router.

l              A virtual router consists of multiple physical routers including a master and several backups. When the master works normally, the hosts on the LAN communicate with the external networks through the master; when the master fails, one of the backups becomes the master to forward packets, as shown in Figure 2 .

Figure 2  Network diagram for a virtual router

2.3  VRRP Working Process

VRRP works as the following:

l              The routers in a virtual router elect the master based on their priorities. The master sends a gratuitous ARP packet to notify the devices or hosts connected to it of its virtual MAC address, thus taking on the responsibility to forward packets.

l              The master sends VRRP advertisements periodically to advertise its configuration information (for example, its priority) and working status.

l              If the master fails, the backups in the virtual router elect a new master based on their priorities.

l              When the master in a virtual router is changed, the new master just sends a gratuitous ARP packet carrying the virtual router MAC address and IP address to update the ARP-related information of the hosts or devices connected to it; while the hosts in the network cannot be aware that the master has been changed to another one.

l              If the priority of a backup is higher than that of the master, whether a new master needs to be elected depends on the working mode of the backup (preemptive or non-preemptive).

To sum up, to ensure normal working of the master and backups in a virtual router, VRRP needs to implement the following functions:

l              Master election

l              Master state advertisement

l              Authentication to enhance security

2.3.1  Master Election

VRRP determines the role (master or backup) of each router in a virtual router by priority. A router with a higher priority has more opportunity to become the master.

A router in a virtual router works as a backup after it is created, and it gets the master priority by receiving VRRP advertisements.

l              If the master priority in the VRRP advertisement is higher than the priority of the router, the router remains as a backup.

l              If the master priority in the VRRP advertisement is lower than the priority of the router, when the router works in preemptive mode, it becomes the master to periodically send VRRP packets; when the router works in non-preemptive mode, it remains as a backup.

l              If the router does not receive a VRRP advertisement in a certain period, it becomes the master.

VRRP priority is in the range of 0 to 255. A bigger number means a higher priority. Priorities 1 to 254 are configurable. Priority 0 is reserved for master that releases its master responsibility, and priority 255 for the IP address owner. When a router acts as the IP address owner, its priority is always 255. That is, if there is an IP address owner in a virtual router, it acts as the master as long as it works properly.

2.3.2  Master State Advertisement

The master in a virtual router sends VRRP advertisements periodically to inform the other routers in the virtual router of its configuration information (for example, priority) and working status. The backups judge whether the master works normally according to the advertisements received.

The master can release responsibility of a master by sending a VRRP advertisement with priority being 0 to trigger to trigger an immediate master election among backups. The time used for the election is called Skew time, in seconds, and is calculated as ((256 – Priority)/256).

If the master fails and cannot send VRRP advertisements, a backup cannot know the state of the master immediately; it waits for a period of time, and if it still receives no advertisements from the master, it considers that the master fails and assumes itself as the master. If at this time, multiple backups compete for becoming the master, a master election is triggered. The time interval for the backups to declare master down is called Master_Down_Interval, in seconds, and is calculated as (3 × Advertisement_Interval) + Skew time.

On an unstable network, a backup may fail to receive the packets from the master in Master_Down_Interval due to network congestion, thus causing the members in the virtual router to change their states frequently. This problem can be addressed through setting the VRRP preemption delay timer. With the VRRP preemption delay timer set, if a backup receives no advertisement in Master_Down_Interval and then the preemption delay, it considers that the master fails. In this case, it assumes itself as the master and sends VRRP advertisements.

2.3.3  Authentication Mode

VRRP provides three authentication modes:

l              No authentication: No authentication is performed for any VRRP packet, without security guarantee.

l              Simple text authentication: You can adopt the simple text authentication mode in a network facing possible security problems. A router sending a VRRP packet fills an authentication key into the packet, and the router receiving the packet compares its local authentication key with that of the received packet. If the two authentication keys are the same, the received VRRP packet is considered valid; otherwise, the received packet is considered an invalid one.

l              MD5 authentication: You can adopt MD5 authentication in a network facing severe security problems. The router encrypts a VRRP packet to be sent using the authentication key and MD5 algorithm and saves the encrypted packet in the authentication header. The router receiving the packet uses the authentication key to decrypt the packet and checks whether the validity of the packet.

VRRP in Comware

3.1  Monitoring the Uplinks

VRRP needs other technologies to perfect its functions. For example, if the link on the master to a network is down, the hosts cannot access the network through the master. This problem can be solved by the master’s monitoring of the uplink of a specified interface. If the master detects that the uplink fails, it reduces its priority (making the master priority lower than that of a backup), and sends a VRRP advertisement. Upon receiving the VRRP advertisement, the backup with the highest priority becomes the new master in the skew time, so that the hosts can communicate with the external networks.

VRRP can monitor the state of the uplink interface. If a router interface connected to an uplink is down, the priority of the master can be reduced by a specified value, and the value can be reduced to 1.

VRRP can monitor the state of the remote hosts or the network connected to the uplinks with the NQA technology. For example, you can enable the NQA ICMP-echo function on the master to detect the reachability of the remote hosts; if ICMP-echo probe fails, NQA will notify the master of the probe results, so as to reduce its priority in a virtual router.

VRRP can also monitor the state of the remote hosts or the network connected to the uplinks with the BFD technology. As BFD detection time resolution can be 10 milliseconds, BFD can quickly detect the connectivity of links in networks, so as to implement quick preemption in a virtual router. For example, BFD can be used on the master to monitor the physical state of an uplink device; when the uplink device is down, BFD can quickly detect that, and the priority of the master will be reduced to make a backup preempt as the master in the skew time

3.2  Backup’s Monitoring of the Master State

In normal cases, a backup in a virtual router waits for Master_Down_Interval to become the master after the master fails. During this time, the hosts in the LAN cannot communicate as no master can forward packets for them. To solve the problem, VRRP provides the monitoring function for a backup to monitor the master state, making the backup become the new master immediately after the master fails to maintain network communication.

The BFD technology is adopted for a backup to monitor the master state. With this function enabled on a backup, the backup can automatically become the new master as soon as the master fails, with the Skew Time being reduced to milliseconds.

Application Scenarios

4.1  Master/Backup

In master/backup mode, only one router, the master, provides services. When the master fails, a new master is elected from the original backups to take the responsibility of the master, as shown in Figure 3 .

Figure 3  VRRP in master/backup mode

At the beginning, Device A is the master and therefore can forward packets to the external networks, while Device B and Device C are backups and are thus in the state of listening. If Device A fails, Device B and Device C will elect a new master according to their priorities. The new master takes over the forwarding task to provide services to the hosts on the LAN.

4.2  Load Balancing

You can create more than one virtual router on an interface of a router, allowing the router to be the master of one virtual router but a backup of another at the same time.

In load balancing mode, multiple routers provide services at the same time. This mode requires two or more virtual routers, each of which includes a master and one or more backups. The masters of the virtual routers can be assumed by different routers, as shown in Figure 4 .

Figure 4  VRRP in load balancing mode

In Figure 4 , three virtual routers are present:

l              Virtual router 1: Device A is the master; Device B and Device C are the backups.

l              Virtual router 2: Device B is the master; Device A and Device C are the backups.

l              Virtual router 3: Device C is the master; Device A and Device B are the backups.

For load balancing among Device A, Device B, and Device C, hosts on the LAN need to be configured to use virtual router 1, 2, and 3 as the default gateways respectively. When configuring VRRP priorities, make sure that each router holds such a priority in each virtual router that it will take the expected role in the virtual router.

4.3  Master’s Monitoring of Uplinks Through BFD/NQA

VRRP monitors the uplinks through BFD or NQA to make the master quickly find network faults and reduce its priority, thus ensuring a backup whose uplink is working normally to assume the responsibility of a master.

 

Figure 5  Master monitors the uplinks

As shown in Figure 5 , Device A works as the master at first to forward packets; Device B works as the backup and is in the listening state. Device A uses BFD to monitor the state of the uplink to the Internet. If the uplink of Device A fails, Device A can be aware of the network change in milliseconds; then it reduces its priority by a specified value, and sends a VRRP advertisement to Device B; if the priority of Device B is higher than that contained in the VRRP advertisement, Device B will become the master in Skew Time, and then the new master will forward packets for the hosts in the network.

4.4  Backup’s Monitoring of Master State Using BFD

To ensure transmission stability on a network, BFD can be used on a backup to monitor the master state, thus ensuring that the backup can become the master immediately when the master fails.

Figure 6  Backup monitors the master state

As shown in Figure 6 , Device A works as the master at first to forward packets; Device B works as the backup and is in the listening state. Device B uses BFD to monitor the reachability of the IP address 10.1.1.1 on Device A; if Device A fails, Device B can be aware of the change of Device A through BFD and becomes the new master, and then forwards packets for the hosts in the network.

References

l              RFC 3768:Virtual Router Redundancy Protocol (VRRP)

Copyright ©2008 Hangzhou H3C Technologies Co., Ltd. All rights reserved.

No part of this manual may be reproduced or transmitted in any form or by any means without prior written consent of Hangzhou H3C Technologies Co., Ltd.

The information in this document is subject to change without notice.