Redundant Routing Protocols

Redundant Routing Protocols

HSRP, GLBP, and VRRP

HSRP (Hot-Standby Routing Protocol) - RFC 2281 - mostly used with Cisco routers - a well-known integrated protocol that is available via specific config commands. HSRP does not inherently support load sharing (there is nothing in the RFC that talks of load sharing). However, Cisco has since come up with "MHSRP" (Multigroup HSRP) so that it can be used for load sharing.

*** also see Using HSRP for Fault-Tolerant Routing (Cisco) Load Sharing with HSRP

GLBP (Gateway Load Balancing Protocol) - no RFC (Cisco proprietary) - used with Cisco Routers - this is an enhancement over HSRP, in that it offers load sharing by default. You can configure GLBP in such a way that traffic from LAN clients can be shared by multiple routers, thereby sharing the traffic load more equitably among available routers. GLBP supports up to 1024 virtual routers (GLBP groups) on each physical interface of a router, and up to 4 virtual forwarders per group

*** also see Cisco GLBP Cisco's Data Sheet on GLBP High Availability in Campus Networks with GLBP

VRRP (Virtual Redundant Routing Protocol) - RFC 3768 - this is typically used with non-Cisco routers (such as Juniper), although the Cisco 3000 uses it - and it is similar to HSRP

NOTE on Load Sharing - GLBP vs HSRP/VRRP - GLBP performs a similar, but not identical, function for the user as the HSRP and VRRP. Both HSRP and VRRP protocols allow multiple routers to participate in a virtual router group configured with a virtual IP address. One member is elected to be the active router to forward packets sent to the virtual IP address for the group. The other routers in the group are redundant until the active router fails. With standard HSRP and VRRP, these standby routers pass no traffic in normal operation - which is wasteful. Therefore the concept cam about for using multiple virtual router groups, which are configured for the same set of routers. But to share the load, the hosts must be configured for different default gateways, which results in an extra administrative burden of going around and configuring every host and creating 2 or more groups of hosts that each use a different default gateway.

GLBP is similar in that it provides load balancing over multiple routers (gateways) - but it can do this using only ONE virtual IP address !!! Underneath that one virtual IP address is multiple virtual MAC addresses, and this is how the load is balanced between the routers. Instead of the hassle of configuring all the hosts with a static Default Gateway, you can lket them use ARP's to find their own. Multiple gateways in a "GLBP redundancy group" respond to client Address Resolution Protocol (ARP) requests in a shared and ordered fashion, each with their own unique virtual MAC addresses. As such, workstation traffic is divided across all possible gateways. Each host is configured with the same virtual IP address, and all routers in the virtual router group participate in forwarding packets.

Cisco's HSRP Detailed

Cisco’s “Hot Standby Routing Protocol” – RFC2281

(VRRP is for non-Cisco routers and is very similar to HSRP)

From RFC2281: Using HSRP, a set of routers work in concert to present the illusion of a single virtual router to the hosts on the LAN. This set is known as an HSRP group or a standby group. A single router elected from the group is responsible for forwarding the packets that hosts send to the virtual router. This router is known as the active router. Another router is elected as the standby router. In the event that the active router fails, the standby assumes the packet forwarding duties of the active router. Although an arbitrary number of routers may run HSRP, only the active router forwards the packets sent to the virtual router.

Two "Real Routers" become one "Virtual Router" with HSRP

HSRP is a Cisco proprietary disaster recovery routing scheme, which has one active router and one or more standby routers (usually just one standby), all on the same LAN segment – and together they all form one, virtual router. HSRP can be specifically on a LAN, or it can be done on a WAN, where the routers each have their own access circuit into the IP cloud. For cost considerations, the active is often configured at a higher BW than the standby’s, but they can be the same data rate. HSRP does not inherently support load sharing, but there are workarounds to configure it to work that way.

HSRP Group (also called "Standby Group") - HSRP is designed so that two or more routers can be grouped together as a single "Virtual Router", by sharing a single virtual IP address and a single virtual MAC address

HSRP isn't a routing protocol !!! It's simply a way for routers on the same multi-access network to present a reliable (due to multiple routers and paths) virtual IP address/es. HSRP has the benefit that it keeps host configuration simple—a commonly used static default is all that's required. It also reacts to failures in a matter of seconds.

Overview of How it Works

The routers share the same IP and MAC addresses, therefore in the event of failure of one router, the hosts on the LAN are able to continue forwarding packets to a consistent IP and MAC address. The process of transferring the routing responsibilities from one device to another is transparent to the user.

The protocol is designed for use over multi-access, multicast or broadcast capable LANs (e.g., Ethernet). HSRP is not intended as a replacement for existing dynamic router discovery mechanisms and those protocols should be used instead whenever possible. A large class of legacy host implementations that do not support dynamic discovery are capable of configuring a default router. HSRP provides failover services to those hosts.

Active vs Stanby Routers - using HSRP, a set of routers work in concert to present the illusion of a single virtual router to the hosts on the LAN. This set is known as an HSRP group or a standby group. A single router elected from the group is responsible for forwarding the packets that hosts send to the virtual router. This router is known as the active router. Another router is elected as the standby router. In the event that the active router fails, the standby assumes the packet forwarding duties of the active router. Although an arbitrary number of routers may run HSRP, only the active router forwards the packets sent to the virtual router.

To minimize network traffic, only the active and the standby routers send periodic HSRP messages once the protocol has completed the election process. If the active router fails, the standby router takes over as the active router. If the standby router fails or becomes the active router, another router is elected as the standby router.

On a particular LAN, multiple hot standby groups may coexist and overlap. Each standby group emulates a single virtual router. For each standby group, a single well-known MAC address is allocated to the group, as well as an IP address. The IP address SHOULD belong to the primary subnet in use on the LAN, but MUST differ from the addresses allocated as interface addresses on all routers and hosts on the LAN, including virtual IP addresses assigned to other HSRP groups.

Although HSRP itself does not support load sharing (see GLBP for that) - if multiple groups are used on a single LAN, load splitting can still be achieved by distributing hosts among different standby groups.

Shared IP and MAC Addresses - besides sharing an IP address, that IP address has a common MAC address that the routers share. For example, you have a workgroup of say, 100 computers. Each one of these machines has been configured with a default gateway, if these machines have used the default gateway or router, they have it's MAC address in their ARP cache. So since the routers in the HSRP group share the same virtual IP address with a corresponding virtual MAC address, when they fail over, the workstations are unaware of the change. What they see, is a "virtual" router.

ARP Issues - if for some reason a host loses the static default gateway and sends out an ARP - the response from the router must tell the host that it can be reached by it's virtual IP address. It is important that the routers respond to any ARP's that may occur from the hosts with their "virtual" IP address - not their actual interface IP address. If a router does respond to an ARP with it's interface IP address, the host will still be able to communicate through that address - but if that router goes down then the host loses connectivity.

HSRP messages between Routers (UDP) - ther are 3 types: Keep-Alives (Hello), Resign, and Coup - the routers in an HSRP group send and receive keep alives using the multicast address of 224.0.0.2 and UDP port 1985. By default the hello interval is 3 seconds. Once 3 hello intervals pass without hearing from the active router, the standby router automatically becomes the active router. Each router is configured with a priority number, the router with the highest priority number in a standby group is the active router, everyone else just relaxes. If the ACtive router must

Example HSRP Networks

Standard Dual-Router, Dual Internet Access Circuit HSRP

In this case, the customer has purchased a primary T3, and a backup T1 to the Internet - each connected to a different router. The idea is that in the event of a failure, at least a minimal amount of traffic will still flow so that critical functions can continue, albeit at a much degraded pace. He needs to configure each router with HSRP so that RouterA is the "Active" router and RouterB is the "Standby" router. The "perimeter network" (the LAN segment) interface of Border RouterA is configured with IP address 10.0.0.253, and Border RouterB is given 10.0.0.254. These are the actual IP addresses assigned to the Ethernet interfaces of each router. Both routers also have a "virtual" IP address 10.0.0.1

HSRP on RouterA (the primary, "active" router) is configured so that it normally also holds the shared virtual interface address (10.0.0.1) on its perimeter network interface. HSRP on Border RouterB is configured to monitor the health of Border RouterA. Internet traffic from the host follows the static default route toward 10.0.0.1 to Border RouterA and exits on the T3 when both border routers are operating.

HSRP with Two Border Routers in Normal Operation

But suppose Border RouterA fails as shown in the next diagram:

HSRP with Failed Primary Border Router

Within seconds of Border RouterA's failure, Border RouterB's perimeter network interface takes over the shared virtual interface address (10.0.0.1). The static default route in the host now points to Border RouterB with no work on the host's part. Its Internet traffic now exits on the T1 via Border RouterB.

Now suppose that the T3 fails but Border RouterA continues to operate. We want Border RouterB to take over the shared virtual address even though Border RouterA is still functioning. This case is handled by configuring Border RouterA to "give up" the address whenever it looses carrier detect on the T3.

HSRP with Failed Primary Internet Connection

This behavior is implemented with a priority system. Border RouterA is configured to lower its priority whenever carrier detect is lost on the T3. Border RouterB seizes control of the shared virtual interface address whenever it notices that its priority is now the highest in the group of routers sharing the address.

NOTE: these examples show 2 routers, but HSRP supports more than two routers. Multiple routers can share a single virtual interface address !!!

At this point, HSRP may sound pretty good (and it is), but there are a few of things you should keep in mind.

HSRP won't help you if an interface fails to pass data but carrier detect doesn't drop. This type of failure can happen if line between you and the central office is good but the DAX at the CO fails.
BGP will eventually notice this kind of failure and reroute your traffic—it just won't happen with the speed of HSRP.
HSRP won't help your hosts pick the "optimal" border router. For example, HSRP between the border routers wouldn't help the host choose Border RouterA for traffic to ISP A's customers. It would, however, allow the host to remain connected to the Internet should either border router fail.
Note that HSRP is available on all Cisco routers, but can have only a single IP address on the lower-end routers (e.g. 1600, 2500, 2600, and 3600 series routers as of this writing). Check Cisco's HSRP docs for details. This is generally not a problem since any of the routers participating in an HSRP group can serve as a gateway to other networks.
HSRP can appear to interfere with outbound load sharing if you're not taking at least customer routes from one of your ISPs.
Finally, remember that HSRP alone isn't sufficient for reliable Internet connectivity. You'll still need to have BGP configured correctly at your border routers and at all your ISPs to retain connectivity in the face of line and/or router failure.

Load Sharing with BGP only, vs BGP and HSRP together

HSRP does not support load sharing all by itself !!! For that you could use Cisco's GLBP (Gateway Load Balancing Protocol). But, you can use a combination of BGP and HSRP to offer load sharing and reliability.

For pure load sharing – BGP all by itself is the way to go. HSRP does a lot for reliability, but it can work against outbound load sharing in some cases because it only allow one interface to act as the “Active router” – which means during normal operation there is no load sharing. The standby router is just that – standby only, and does not pass traffic during normal operation. This can really crimp the BW of a site that has 2 T1s, and more than 1 T1s worth of output bandwidth.

Consider the network below:

Load Sharing with BGP but Without HSRP

Since both ISPs are sending only default routes, each border router will use its Internet connection for all exit traffic it receives. If each host generates about the same amount of outbound traffic, reasonably good outbound load sharing is achieved. (This might be especially desirable if both hosts together generated more traffic than would fit on either Internet connection individually.)

Although the outbound load sharing might be good with this configuration, your outbound traffic might be reaching its destination through some pretty circuitous paths. As a quick reminder, think about what happens to traffic from HostB that destined for a customer of ISPA. It would have to be carried by at least ISPB (and perhaps several other ASes) before reaching ISPA.

If an Internet connection fails in a BGP-only topology (no HSRP) - BGP will lose the default route it had heard through that connection. Exit traffic sent to either router will eventually exit on the remaining (working) Internet connection.

· As a comparison – HSRP also can achieve the same effect, as shown above in the “HSRP with Failed Primary Internet Connection” diagram - although probably not quite as quickly.

If a border routers fails in a BGP-only topology (no HSRP) - any hosts using the failed border router as the destination for a static default route would lose Internet connectivity.

· As a comparison - HSRP dealt handily with this problem (see the “HSRP with Failed Primary Border Router” diagram above).

Protection again BOTH possibilities (i.e. Internet connection failure OR a border router failure) – HSRP will protect against both possible failures. You configure HSRP on both border routers and configured both hosts to use the HSRP virtual interface address for their static default route. This will allow better reliability since either Internet connection or either border router could fail without loss of Internet connectivity.

HOWEVER, HSRP does not support Load Sharing !!! So in the absence of failure, all exit traffic from the AS would go out one Internet connection while the outbound side of the other sat largely idle. This could lead to congestion, especially if the total exit traffic from HostA and HostB exceeded the capacity of either Internet connection. In short, adding just HSRP to a dual-router network gives you reliability – but at the expense of load sharing.

An HSRP configuration where BGP is added to allow Load Sharing

There are two changes that could be made to achieve both reliability and good outbound load sharing:

· The border routers running HSRP could receive customer routes from at least one ISP. But this might require more memory be added to your border routers.

· More than one HSRP virtual interface address can be used, so long as you have high-end Cisco routers. The more expensive Cisco routers can be configured with two virtual interface addresses on the same physical interface. One of these addresses could be configured to favor Border RouterA in the normal case while the other were configured to favor Border RouterB in the normal case. Both would be configured to use the remaining working connection in the event of failure. HostA and HostB would then be configured with static default routes toward different HSRP virtual interface addresses.

NOTE: lower-end Cisco routers support only one HSRP virtual interface address per physical interface. The workaround is to use 2 interfaces.

Configuring HSRP

You only need 2 commands to do it, and 2 additional commands to customize it. What's more, it's configured at the interface that you want to participate in the standby group. First off, on the router that you want to be the active router, go to the interface you want HSRP to run on, think up a group number, all routers participating in this scheme must use the same group number, so you must select an IP address you want the HSRP group to share. For example, suppose we want to configure two routers (we’ll name then simply as “router1” and “router2”).

First we will configure the “Active router”, router1:

router1(config-if)#standby 1 ip address 10.1.1.254
router1(config-if)#standby 1 priority 100

This defines a standby group number of 1, an IP address of 10.1.1.254 that the routers are going to share, now let's configure the standby router, router2:

router2(config-if)#standby 1 ip address 10.1.1.254
router2 (config-if)#standby 1 priority 90

The only thing different on the standby router is the priority. The router with the highest priority becomes the active router.

Preemption

Use the “preempt” option to make sure the Active Router can Re-establish itself - in this configuration, if the active router, with the priority of 100 goes down and comes back up, even though it has the higher priority, it will NOT become the active router again? You can easily change that by adding the keyword preempt to the priority command:

Instead of: router1(config-if)#standby 1 priority 100

Use this command: router1(config-if)#standby 1 priority 100 preempt

Tracking the HSRP routers

*** using both the “track” and “priority” options

If the routers are connected to a WAN link, and the routers are running fine, but the WAN link goes down, you can track that (for this example we will track via the serial port 0), by issuing the same configuration commands, but add the “track” option, and the “priority” option. For example, we add the following command to the Active router, “router1”:

router1(config-if)#standby 1 track s0 priority 11

The reason for adding “priority 11”, is to decrease the router’s priority if it’s WAN interface goes down. In this case, 11 is the number to subtract from the router's own original priority number, which will give it an adjusted priority number if the interface to it's tracking goes down.

So in this case, if the Serial 0 interface of router1 goes down, its priority goes from 100 to 89 which will cause the standby router, whose priority is set to 90 - to become the active router. You might need this to happen if the standby router is configured for DDR.

RFC 2281 - HSRP

Network Working Group T. Li Juniper Networks

Request for Comments: 2281 B. Cole Juniper Networks

Category: Informational P. Morton Cisco Systems

D. Li Cisco Systems

March 1998

Cisco Hot Standby Router Protocol (HSRP)

Status of this Memo

This memo provides information for the Internet community. It does not specify an Internet standard of any kind. Distribution of this memo is unlimited.

IESG Note

This document reflects an existing deployed protocol. The IETF does have a working group which is in the process of producing a standards track protocol to address the same issues.

Abstract

The memo specifies the Hot Standby Router Protocol (HSRP). The goal of the protocol is to allow hosts to appear to use a single router and to maintain connectivity even if the actual first hop router they are using fails. Multiple routers participate in this protocol and in concert create the illusion of a single virtual router. The protocol insures that one and only one of the routers is forwarding packets on behalf of the virtual router. End hosts forward their packets to the virtual router.

The router forwarding packets is known as the active router. A standby router is selected to replace the active router should it fail. The protocol provides a mechanism for determining active and standby routers, using the IP addresses on the participating routers. If an active router fails a standby router can take over without a

major interruption in the host's connectivity. This memo also discusses the ARP, MAC address, and security issues with this protocol.

TABLE OF CONTENTS

1 Introduction .............................................. 2

2 Conditions of Use ......................................... 3

3 Scope ..................................................... 4

3.1 Terminology ............................................... 4

4 Definitions ............................................... 4

5 Protocol .................................................. 4

5.1 Packet formats ............................................ 4

5.2 Operational parameters .................................... 7

5.3 States .................................................... 8

5.4 Timers .................................................... 9

5.5 Events .................................................... 9

5.6 Actions ................................................... 10

5.7 State Transitions.......................................... 11

6 MAC address considerations ................................ 13

6.1 General ................................................... 13

6.2 Address Filter ............................................ 14

6.3 ICMP Redirect ............................................. 14

6.4 Proxy ARP ................................................. 15

7 Security Considerations ................................... 15

8 References ................................................ 15

9 Authors' Addresses ........................................ 16

10 Full Copyright Statement .................................. 17

1. Introduction

The Hot Standby Router Protocol, HSRP, provides a mechanism which is designed to support non-disruptive failover of IP traffic in certain circumstances. In particular, the protocol protects against the failure of the first hop router when the source host cannot learn the IP address of the first hop router dynamically. The protocol is designed for use over multi-access, multicast or broadcast capable LANs (e.g., Ethernet). HSRP is not intended as a replacement for existing dynamic router discovery mechanisms and those protocols should be used instead whenever possible [1]. A large class of legacy host implementations that do not support dynamic discovery are capable of configuring a default router. HSRP provides failover services to those hosts.

All of the routers participating in HSRP are assumed to be running appropriate IP routing protocols and have a consistent set of routes. The discussion of which protocols are appropriate and whether routing is consistent in any given situation is beyond the scope of this specification.

Using HSRP, a set of routers work in concert to present the illusion of a single virtual router to the hosts on the LAN. This set is known as an HSRP group or a standby group. A single router elected from the group is responsible for forwarding the packets that hosts send to the virtual router. This router is known as the active router. Another router is elected as the standby router. In the event that the active router fails, the standby assumes the packet forwarding duties of the active router. Although an arbitrary number of routers may run HSRP, only the active router forwards the packets sent to the virtual router.

If multiple groups are used on a single LAN, load splitting can be achieved by distributing hosts among different standby groups.

The remainder of this specification discusses the operation of a single standby group. In the case of multiple groups, each group operates independently of other groups on the LAN and according to this specification. Note that individual routers may participate in multiple groups. In this case, the router maintains separate state and timers for each group.

2 Conditions of Use

US Patent number 5,473,599 [2], assigned to Cisco Systems, Inc. may be applicable to HSRP. If an implementation requires the use of any claims of patent no. 5,473,599, Cisco will license such claims on reasonable, nondiscriminatory terms for use in practicing the standard. More specifically, such license will be available for a one-time, paid up fee.

3 Scope

This document describes the packets, messages, states, and events used to implement the protocol. It does not discuss network management or internal implementation issues.

3.1 Terminology

The language conventions of RFC 2119 [3] are used in this document.

4 Definitions

Active Router - the router that is currently forwarding packets for the virtual router

Standby Router - the primary backup router

Standby Group - the set of routers participating in HSRP that jointly emulate a virtual router

Hello Time - the interval between successive HSRP Hello messages from a given router

Hold Time - the interval between the receipt of a Hello message and the presumption that the sending router has failed

5 Protocol

Within a standby group, the routers periodically advertise state information using various messages.

5.1 Packet formats

The standby protocol runs on top of UDP, and uses port number 1985. Packets are sent to multicast address 224.0.0.2 with TTL 1.

Routers use their actual IP address as the source address for protocol packets, not the virtual IP address. This is necessary so that the HSRP routers can identify each other.

The format of the data portion of the UDP datagram is:

1 2 3

0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1

+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

| Authentication Data |

+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

| Authentication Data |

+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

| Virtual IP Address |

+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Version: 1 octet

The version of the HSRP messages. This document describes version 0.

Op Code: 1 octet

The Op Code describes the type of message contained in this packet. Possible values are:

0 - Hello

1 - Coup

2 - Resign

Hello messages are sent to indicate that a router is running and is capable of becoming the active or standby router.

Coup messages are sent when a router wishes to become the active router.

Resign messages are sent when a router no longer wishes to be the active router.

State: 1 octet

Internally, each router in the standby group implements a state machine. The State field describes the current state of the router sending the message. Details on the individual states are described below. Possible values are:

0 - Initial

1 - Learn

2 - Listen

4 - Speak

8 - Standby

16 - Active

Hellotime: 1 octet

This field is only meaningful in Hello messages. It contains the approximate period between the Hello messages that the router sends. The time is given in seconds.

If the Hellotime is not configured on a router, then it MAY be

learned from the Hello message from the active router. The

Hellotime SHOULD only be learned if no Hellotime is configured and

the Hello message is authenticated. A router that sends a Hello

message MUST insert the Hellotime that it is using in the

Hellotime field in the Hello message. If the Hellotime is not

learned from a Hello message from the active router and it is not

manually configured, a default value of 3 seconds is RECOMMENDED.

Holdtime: 1 octet

This field is only meaningful in Hello messages. It contains the

amount of time that the current Hello message should be considered

valid. The time is given in seconds.

If a router sends a Hello message, then receivers should consider

that Hello message to be valid for one Holdtime. The Holdtime

SHOULD be at least three times the value of the Hellotime and MUST

be greater than the Hellotime. If the Holdtime is not configured

on a router, then it MAY be learned from the Hello message from

the active router. The Holdtime SHOULD only be learned if the

Hello message is authenticated. A router that sends a Hello

message MUST insert the Holdtime that it is using in the Holdtime

field in the Hello message.

A router which is in active state MUST NOT learn new values for

the Hellotime and the Holdtime from other routers, although it may

continue to use values which it learned from the previous active

router. It MAY also use the Hellotime and Holdtime values learned

through manual configuration. The active router MUST NOT use one

configured time and one learned time. If the Holdtime is not

learned and it is not manually configured, a default value of 10

seconds is RECOMMENDED.

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RFC 2281 Cisco HSRP March 1998

Priority: 1 octet

This field is used to elect the active and standby routers. When

comparing priorities of two different routers, the router with the

numerically higher priority wins. In the case of routers with

equal priority the router with the higher IP address wins.

Group: 1 octet

This field identifies the standby group. For Token Ring, values

between 0 and 2 inclusive are valid. For other media values

between 0 and 255 inclusive are valid.

Authentication Data: 8 octets

This field contains a clear-text 8 character reused password.

If no authentication data is configured, the RECOMMENDED default

value is 0x63 0x69 0x73 0x63 0x6F 0x00 0x00 0x00.

Virtual IP Address: 4 octets

The virtual IP address used by this group.

If the virtual IP address is not configured on a router, then it

MAY be learned from the Hello message from the active router. An

address SHOULD only be learned if no address was configured and

the Hello message is authenticated.

5.2 Operational parameters

The following information MUST be known to each router in the standby

group. The mechanisms used to determine this information are outside

of the scope of this document.

Standby group number

Virtual MAC address

Priority

Authentication Data

Hellotime

Holdtime

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RFC 2281 Cisco HSRP March 1998

The following information MUST be known to at least one router in

each standby group and MAY be known by any of the other routers in

the group.

Virtual IP Address

The following information MAY be configured on any router:

Preemption capability

If a router has higher priority than the active router and

preemption is configured, it MAY take over as the active router

using a Coup message.

5.3 States

Each router in the group participates in the protocol by implementing

a simple state machine. This specification describes the externally

visible behavior of this state machine. Implementations MAY vary

their internal implementations within the functional description of

the state machine.

All routers begin in the Initial state. This section discusses the

intent of each state. For specific details on the actions taken in

each state, please see the state transition table in section 5.7.

1. Initial

This is the starting state and indicates that HSRP is not running.

This state is entered via a configuration change or when an

interface first comes up.

2. Learn

The router has not determined the virtual IP address, and not yet

seen an authenticated Hello message from the active router. In

this state the router is still waiting to hear from the active

router.

3. Listen

The router knows the virtual IP address, but is neither the active

router nor the standby router. It listens for Hello messages from

those routers.

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RFC 2281 Cisco HSRP March 1998

4. Speak

The router sends periodic Hello messages and is actively

participating in the election of the active and/or standby router.

A router cannot enter Speak state unless it has the virtual IP

address.

5. Standby

The router is a candidate to become the next active router and

sends periodic Hello messages. Excluding transient conditions,

there MUST be at most one router in the group in Standby state.

6. Active

The router is currently forwarding packets that are sent to the

group's virtual MAC address. The router sends periodic Hello

messages. Excluding transient conditions, there MUST be at most

one router in Active state in the group.

5.4 Timers

Each router maintains three timers, an Active timer, a Standby timer,

and a Hello timer.

The Active timer is used to monitor the active router. The active

timer is started anytime an authenticated Hello message is seen from

the active router. It is set to expire in the Holdtime seen in the

Hello message.

The Standby timer is used to monitor the standby router The Standby

timer is started anytime an authenticated Hello message is seen from

the standby router. It is set to expire in the Holdtime seen in the

Hello message.

The Hello timer expires once per Hellotime period. If the router is

in Speak, Standby, or Active states, it should generate a Hello

message upon Hello timer expiry. The Hello timer MUST be jittered.

5.5 Events

These are the events in the HSRP finite state machine.

a - HSRP is configured on an enabled interface.

b - HSRP is disabled on an interface or the interface is disabled.

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RFC 2281 Cisco HSRP March 1998

c - Active timer expiry. The Active timer was set to the Holdtime

when the last Hello message was seen from the active router.

d - Standby timer expiry. The Standby timer was set to the

Holdtime when the last Hello message was seen from the standby

router.

e - Hello timer expiry. The periodic timer for sending Hello

messages has expired.

f - Receipt of a Hello message of higher priority from a router in

Speak state.

g - Receipt of a Hello message of higher priority from the active

router.

h - Receipt of a Hello message of lower priority from the active

router.

i - Receipt of a Resign message from the active router.

j - Receipt of a Coup message from a higher priority router.

k - Receipt of a Hello message of higher priority from the standby

router.

l - Receipt of a Hello message of lower priority from the standby

router.

5.6 Actions

This section specifies the actions to be taken as part of the state

machine.

A Start Active Timer

If this action occurred as the result of the receipt of a an

authenticated Hello message from the active router, the Active

timer is set to the Holdtime field in the Hello message.

Otherwise the Active timer is set to the current Holdtime value

in use by this router. The Active timer is then started.

B Start Standby Timer

If this action occurred as the result of the receipt of an

authenticated Hello message from the standby router, the

Standby timer is set to the Holdtime field in the Hello

message. Otherwise the Standby timer is set to the current

hold time value in use by this router. The Standby timer is

then started.

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RFC 2281 Cisco HSRP March 1998

C Stop Active Timer

The Active timer is stopped.

D Stop Standby Timer

The Standby timer is stopped.

E Learn Parameters

This action is taken when an authenticated message is received

from the active router. If the virtual IP address for this

group was not manually configured, the virtual IP address MAY

be learned from the message. The router MAY learn Hellotime

and Holdtime values from the message.

F Send Hello Message

The router sends a Hello message with its current State,

Hellotime and Holdtime.

G Send Coup Message

The router sends a Coup message to inform the active router

that there is a higher priority router available.

H Send Resign Message

The router sends a Resign message to allow another router to

become the active router.

I Send Gratuitous ARP Message

The router broadcasts an ARP response packet advertising the

group's virtual IP address and virtual MAC address. The packet

is sent using the virtual MAC address as the source MAC address

in the link layer header, as well as within the ARP packet.

5.7 State Transitions

This table describes the state transitions of the state machine. For

each event and current state of the router, the router MUST perform

the set of actions specified and transition to the designated state.

If no action is specified, no action should be taken. If no state

change is specified, no state change should be performed.

The notation used in this table has the specified set of actions

listed as letters corresponding to the actions listed in section 5.6.

The next state is listed as a number as specified in section 5.3. A

slash ('/') separates the actions and states. Certain state

transitions have alternatives which depend on external state.

Alternatives are separated by a '|'. See the attached notes for

details on these transitions.

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RFC 2281 Cisco HSRP March 1998

States

+-----+----------+----------+----------+----------+----------+----------+

| | 1 | 2 | 3 | 4 | 5 | 6 |

+-----+----------+----------+----------+----------+----------+----------+

|Event| |

+-----+----------+----------+----------+----------+----------+----------+

| a | AB/2|3+ | | | | | |

+-----+----------+----------+----------+----------+----------+----------+

| b | | CD/1 | CD/1 | CD/1 | CD/1 | CDH/1 |

+-----+----------+----------+----------+----------+----------+----------+

| c | | | AB/4 | | CDFI/6 | |

+-----+----------+----------+----------+----------+----------+----------+

| d | | | B/4 | D/5 | | |

+-----+----------+----------+----------+----------+----------+----------+

| e | | | | F | F | F |

+-----+----------+----------+----------+----------+----------+----------+

| f | | | | B/3 | B/3 | |

+-----+----------+----------+----------+----------+----------+----------+

| g | | EAB/3 | EA | EA | EA | AB/4 |

+-----+----------+----------+----------+----------+----------+----------+

| h | | EAB/3 | A|BGFI/6*| A|BGFI/6*| A|BGFI/6*| G |

+-----+----------+----------+----------+----------+----------+----------+

| i | | | AB/4 | A | CFI/6 | |

+-----+----------+----------+----------+----------+----------+----------+

| j | | | | | | ABH/4 |

+-----+----------+----------+----------+----------+----------+----------+

| k | | | B | B/3 | B/3 | B |

+-----+----------+----------+----------+----------+----------+----------+

| l | | | B/4 | D/5 | | B |

+-----+----------+----------+----------+----------+----------+----------+

Notes

+ If the virtual IP address is configured, set state 3 (Listen) If

the virtual IP address is not configured, set state 2 (Learn). In

either case do actions A and B.

* If the router is configured to preempt do actions B, G, F, and I

and set state to 6 (Active). If the router is not configured to

preempt do actions A with no state change.

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RFC 2281 Cisco HSRP March 1998

6 MAC Address Considerations

6.1 General

Each HSRP group has an associated well known virtual MAC address. On

token ring networks, these addresses are actually functional

addresses. The three addresses 0xC0 0x00 0x00 0x01 0x00 0x00, 0xC0

0x00 0x00 0x02 0x00 0x00, and 0xC0 0x00 0x00 0x04 0x00 0x00

correspond to groups 0, 1, and 2 respectively.

On other media, the virtual MAC addresses are 0x00 0x00 0x0C 0x07

0xAC XX where XX represents the HSRP group number. Routers which

implement HSRP SHOULD use well-known HSRP MAC addresses as the

group's virtual MAC address whenever possible.

The active router MUST accept and forward traffic that is destined

for the group's virtual MAC address. It MUST stop accepting or

forwarding such traffic when the router leaves the Active state.

If and only if the router is in the Active state, the router MUST use

the group's virtual MAC address as the source MAC address for its

Hello messages. This is necessary in order to allow learning bridges

to be able to determine which LAN segment the virtual MAC address

currently belongs to.

For each group, there is one virtual IP address and one virtual MAC

address. This is a desirable situation, since the ARP table entries

in the end stations do not need to change over time as the HSRP

active router moves from one router to another.

Additionally, for HSRP to work in bridging environments, the bridges

must be able to quickly update themselves as the virtual MAC address

"moves". Although learning bridges typically are able to do this,

some have been known to have problems with this. It is RECOMMENDED

that only true learning bridges be used with HSRP.

The movement of the virtual MAC address can cause further undesirable

side effects in environments where additional state is tied to the

MAC address. For example on Token Ring, if Source Route Bridging is

in use, a RIF will be stored with the virtual MAC address in a host's

RIF cache. The RIF indicates the path and final ring used to reach

the MAC address. As routers transition into Active state, they will

not be able to affect the RIF caches on the hosts on the bridged

ring. This may lead to packets being bridged to the ring for the

previous active router.

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RFC 2281 Cisco HSRP March 1998

In such circumstances, a router MAY use its normal MAC addresses as

the virtual MAC address. This method of operation is strongly

discouraged. In this mode, the virtual IP address will map to a

different MAC address over time. This can create problems for end

stations, since ARP tables assume a relatively static mapping between

MAC address and IP address. These ARP tables are normally updated

when the end stations receive the gratuitous ARP responses generated

by a router that enters the active state.

6.2 Address Filter

As noted, routers currently emulating a virtual router adopt their

group's MAC and IP addresses. MAC addresses are typically provided

in an address filter or 'list' of MAC addresses in a router's

interface controller. It is desirable for routers to be able to add

one or more virtual MAC addresses to their controllers' MAC address

filter while maintaining their primary MAC addresses.

Unfortunately, some interface controllers support address filtering

for only one unicast MAC address. Or, in the case of Token Ring, the

functional address which HSRP should use is already in use for some

other protocol. In these cases, such routers can still implement

HSRP, but the protocol must change the interface's primary MAC

address when assuming or relinquishing control as the active router.

This is potentially problematic because some traffic may otherwise

wish to use the router's primary MAC address. However, the problem

MAY be mitigated by having the router send out gratuitous ARP packets

regarding its non-HSRP IP addresses. Through this, other network

entities using IP should update their ARP tables to reflect that the

router is now using a group virtual MAC address rather than its

primary MAC address.

Some protocols may not be able to run simultaneously with the standby

protocol due to the interface primary MAC address change. For

example, DECnet phase IV and HSRP will not be able to run at the same

time on some equipment.

6.3 ICMP Redirect

While running HSRP, it is important to prevent the host from

discovering the primary MAC addresses of the routers in its standby

group. Thus, any protocol that informs a host of a router's primary

address should be disabled. Thus, routers participating in HSRP on

an interface MUST NOT send ICMP redirects on that interface.

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RFC 2281 Cisco HSRP March 1998

6.4 Proxy ARP

Typically, hosts learn the HSRP virtual IP address through the

configuration of their default router. These hosts then send packets

for destinations outside of the LAN to the virtual IP address. In

some environments, hosts may instead make use of proxy ARP in order

to route off of the LAN. In this case, the hosts use the MAC address

that is supplied in proxy ARP responses. HSRP functionality is

maintained if the proxy ARP responses specify the HSRP virtual MAC

address.

If an HSRP router is configured to support proxy ARP with HSRP, then

the router MUST specify the HSRP virtual MAC address in any proxy ARP

responses it generates. These proxy ARP responses MUST not be

suppressed based upon HSRP state. Suppression based upon state could

result in lack of any proxy ARP response being generated, since these

proxy ARP responses may be suppressed due to other reasons, such as

split-horizon rules.

7. Security Considerations

This protocol does not provide security. The authentication field

found within the message is useful for preventing misconfiguration.

The protocol is easily subverted by an active intruder on the LAN.

This can result in a packet black hole and a denial-of-service

attack. It is difficult to subvert the protocol from outside the LAN

as most routers will not forward packets addressed to the all-routers

multicast address (224.0.0.2).

8. References

[1] Deering, S., "ICMP Router Discovery Messages", RFC 1256,

September 1991.

[2] United States Patent. Patent Number : 5,473,599. Standby Router

Protocol. Date of Patent: Dec. 5, 1995.

[3] Bradner, S., "Key words for use in RFCs to Indicate Requirement

Levels", BCP 14, RFC 2119, March 1997.

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RFC 2281 Cisco HSRP March 1998

9. Authors' Addresses

Tony Li

Juniper Networks, Inc.

3260 Jay St.

Santa Clara, CA 95054

Phone: (408) 327-1900

EMail: tli@juniper.net

Bruce Cole

Juniper Networks, Inc.

3260 Jay St.

Santa Clara, CA 95054

Phone: (408) 327-1900

EMail: cole@juniper.net

Phil Morton

Cisco Systems

170 Tasman Dr.

San Jose, CA 95143

Phone: (408) 526-7632

EMail: pmorton@cisco.com

Dawn Li

Cisco Systems

170 Tasman Dr.

San Jose, CA 95143

Phone: (408) 527-2014

EMail: dawnli@cisco.com

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RFC 2281 Cisco HSRP March 1998

10. Full Copyright Statement

This document and translations of it may be copied and furnished to

others, and derivative works that comment on or otherwise explain it

or assist in its implementation may be prepared, copied, published

and distributed, in whole or in part, without restriction of any

kind, provided that the above copyright notice and this paragraph are

included on all such copies and derivative works. However, this

document itself may not be modified in any way, such as by removing

the copyright notice or references to the Internet Society or other

Internet organizations, except as needed for the purpose of

developing Internet standards in which case the procedures for

copyrights defined in the Internet Standards process must be

followed, or as required to translate it into languages other than

English.

The limited permissions granted above are perpetual and will not be

revoked by the Internet Society or its successors or assigns.

This document and the information contained herein is provided on an

"AS IS" basis and THE INTERNET SOCIETY AND THE INTERNET ENGINEERING

TASK FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING

BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION

HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF

MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.

Li, et. al. Informational [Page 17]