Configuring First Hop Redundancy Protocols in IPv6

The Gateway Load Balancing Protocol feature provides automatic router backup for IPv6 hosts configured with a single default gateway on an IEEE 802.3 LAN. Multiple first hop routers on the LAN combine to offer a single virtual first-hop IPv6 router while sharing the IPv6 packet forwarding load. GLBP performs a similar function for the user as HSRP. HSRP allows multiple routers to participate in a virtual router group configured with a virtual IPv6 address. One member is elected to be the active router to forward packets sent to the virtual IPv6 address for the group. The other routers in the group are redundant until the active router fails. These standby routers have unused bandwidth that the protocol is not using. Although multiple virtual router groups can be configured for the same set of routers, the hosts must be configured for different default gateways, which results in an extra administrative burden. The advantage of GLBP is that it additionally provides load balancing over multiple routers (gateways) using a single virtual IPv6 address and multiple virtual MAC addresses. The forwarding load is shared among all routers in a GLBP group rather than being handled by a single router while the other routers stand idle. Each host is configured with the same virtual IPv6 address, and all routers in the virtual router group participate in forwarding packets.

GLBP for IPv6 provides the following benefits:

• Load Sharing
• Multiple Virtual Routers
• Preemption
• Authentication

Members of a GLBP group elect one gateway to be the active virtual gateway (AVG) for that group. Other group members provide backup for the AVG in the event that the AVG becomes unavailable. The function of the AVG is that it assigns a virtual MAC address to each member of the GLBP group. Each gateway assumes responsibility for forwarding packets sent to the virtual MAC address assigned to it by the AVG. These gateways are known as active virtual forwarders (AVFs) for their virtual MAC address.

The IPv6 neighbor discovery process uses Internet Control Message Protocol (ICMP) in IPv6 messages and solicited-node multicast addresses to determine the link-layer address of a neighbor on the same network (local link), verify the reachability of a neighbor, and track neighboring routers. The IPv6 neighbor discovery process uses ICMPv6 messages and solicited-node multicast addresses to determine the link-layer address of a neighbor on the same network (local link), verify the reachability of a neighbor, and track neighboring routers. The AVG is responsible for answering ICMPv6 Neighbor Discovery requests for the virtual IPv6 address. Load sharing is achieved by the AVG replying to the ICMPv6 Neighbor Discovery requests with different virtual MAC addresses.

In the figure below, Router A is the AVG for a GLBP group, and is responsible for the IPv6 link-local address FE80::260:3EFF:FE11:6770. Router A is also an AVF for the virtual MAC address 0007.b400.0101. Router B is a member of the same GLBP group and is designated as the AVF for the virtual MAC address 0007.b400.0102. Client 1 has a default gateway IPv6 address of FE80::260:3EFF:FE11:6770 and a gateway MAC address of 0007.b400.0101. Client 2 shares the same default gateway IP address but receives the gateway MAC address 0007.b400.0102 because Router B is sharing the traffic load with Router A.

Figure 1

GLBP Topology

If Router A becomes unavailable, Client 1 will not lose access to the WAN because Router B will assume responsibility for forwarding packets sent to the virtual MAC address of Router A, and for responding to packets sent to its own virtual MAC address. Router B will also assume the role of the AVG for the entire GLBP group. Communication for the GLBP members continues despite the failure of a router in the GLBP group.

A GLBP group allows up to four virtual MAC addresses per group. The AVG is responsible for assigning the virtual MAC addresses to each member of the group. Other group members request a virtual MAC address after they discover the AVG through hello messages. Gateways are assigned the next MAC address in sequence. A virtual forwarder that is assigned a virtual MAC address by the AVG is known as a primary virtual forwarder. Other members of the GLBP group learn the virtual MAC addresses from hello messages. A virtual forwarder that has learned the virtual MAC address is referred to as a secondary virtual forwarder.

GLBP operates virtual gateway redundancy in the same way as HSRP. One gateway is elected as the AVG, another gateway is elected as the standby virtual gateway, and the remaining gateways are placed in a listen state.

If an AVG fails, the standby virtual gateway will assume responsibility for the virtual IPv6 address. A new standby virtual gateway is then elected from the gateways in the listen state.

GLBP virtual forwarder redundancy is similar to virtual gateway redundancy with an AVF. If the AVF fails, one of the secondary virtual forwarders in the listen state assumes responsibility for the virtual MAC address.

The new AVF is also a primary virtual forwarder for a different forwarder number. GLBP migrates hosts away from the old forwarder number using two timers that start as soon as the gateway changes to the active virtual forwarder state. GLBP uses the hello messages to communicate the current state of the timers.

The redirect time is the interval during which the AVG continues to redirect hosts to the old virtual forwarder MAC address. When the redirect time expires, the AVG stops using the old virtual forwarder MAC address in ICMPv6 ND replies, although the virtual forwarder will continue to forward packets that were sent to the old virtual forwarder MAC address.

The secondary hold time is the interval during which the virtual forwarder is valid. When the secondary hold time expires, the virtual forwarder is removed from all gateways in the GLBP group. The expired virtual forwarder number becomes eligible for reassignment by the AVG.

GLBP gateway priority determines the role that each GLBP gateway plays and the results if the AVG fails.

Priority also determines if a GLBP router functions as a backup virtual gateway and the order of ascendancy to becoming an AVG if the current AVG fails. You can configure the priority of each backup virtual gateway with a value of 1 through 255 using the glbp priority command.

In the figure above, if Router A--the AVG in a LAN topology--fails, an election process takes place to determine which backup virtual gateway should take over. In this example, Router B is the only other member in the group so it will automatically become the new AVG. If another router existed in the same GLBP group with a higher priority, then the router with the higher priority would be elected. If both routers have the same priority, the backup virtual gateway with the higher IPv6 address would be elected to become the active virtual gateway.

By default, the GLBP virtual gateway preemptive scheme is disabled. A backup virtual gateway can become the AVG only if the current AVG fails, regardless of the priorities assigned to the virtual gateways. You can enable the GLBP virtual gateway preemptive scheme using the glbp preempt command. Preemption allows a backup virtual gateway to become the AVG, if the backup virtual gateway is assigned a higher priority than the current AVG.

GLBP uses a weighting scheme to determine the forwarding capacity of each router in the GLBP group. The weighting assigned to a router in the GLBP group can be used to determine whether it will forward packets and, if so, the proportion of hosts in the LAN for which it will forward packets. Thresholds can be set to disable forwarding when the weighting falls below a certain value. When the weighting rises above another threshold, forwarding is automatically reenabled.

The GLBP group weighting can be automatically adjusted by tracking the state of an interface within the router. If a tracked interface goes down, the GLBP group weighting is reduced by a specified value. Different interfaces can be tracked to decrement the GLBP weighting by varying amounts.

By default, the GLBP virtual forwarder preemptive scheme is enabled with a delay of 30 seconds. A backup virtual forwarder can become the AVF if the current AVF weighting falls below the low weighting threshold for 30 seconds. You can disable the GLBP forwarder preemptive scheme using the no glbp forwarder preempt command or change the delay using the glbp forwarder preempt delay minimum command.

The HSRP is an FHRP designed to allow for transparent failover of the first-hop IP router. HSRP provides high network availability by providing first-hop routing redundancy for IP hosts on Ethernet configured with a default gateway IP address. HSRP is used in a group of routers for selecting an active router and a standby router. In a group of router interfaces, the active router is the router of choice for routing packets; the standby router is the router that takes over when the active router fails or when preset conditions are met.

IPv6 hosts learn of available IPv6 routers through IPv6 neighbor discovery RA messages. These are multicast periodically, or may be solicited by hosts. HSRP is designed to provide only a virtual first hop for IPv6 hosts.

An HSRP IPv6 group has a virtual MAC address that is derived from the HSRP group number, and a virtual IPv6 link-local address that is, by default, derived from the HSRP virtual MAC address. Periodic RAs are sent for the HSRP virtual IPv6 link-local address when the HSRP group is active. These RAs stop after a final RA is sent when the group leaves the active state.

Periodic RAs for the interface link-local address stop after a final RA is sent while at least one virtual IPv6 link-local address is configured on the interface. No restrictions occur for the interface IPv6 link-local address other than that mentioned for the RAs. Other protocols continue to receive and send packets to this address.

HSRP uses a priority mechanism to determine which HSRP configured router is to be the default active router. To configure a router as the active router, you assign it a priority that is higher than the priority of all the other HSRP-configured routers. The default priority is 100, so if you configure just one router to have a higher priority, that router will be the default active router.

HSRP IPv6 uses a different virtual MAC address block than does HSRP for IP:

0005.73A0.0000 through 0005.73A0.0FFF (4096 addresses)

Port number 2029 has been assigned to HSRP IPv6.

Note	This feature is supported only in Cisco IOS Release 12.2(33)SXI4.

The HSRP global IPv6 address feature allows users to configure multiple nonlink local addresses as virtual addresses, and it allows for the storage and management of multiple global IPv6 virtual addresses in addition to the existing primary link-local address. If an IPv6 address is used, it must include an IPv6 prefix length. If a link-local address is used, it must not have a prefix.

The figure below depicts a deployment scenario that uses an HSRP IPv6 global virtual interface:

Figure 2

Scenario Using Gan HSRP IPv6 Global Virtual Interface

In the figure above, the provider equipment (PE) routers need to inject a route to reach the customer premises equipment (CPE) from the backbone routers. Because there are two CPEs, HSRP is convenient to use. The static route will be set with a link-local next hop (FE80::1:1:1:CAFE). If this address is injected in the backbone, this route is useless with a link-local next hop, as link-local addresses only have scope within the Layer 2 local LAN space. To address this issue, the next hop of the static route toward the virtual address must be set to a nonlink-local address, so backbone routers can route packets to the PE routers. At the next-hop address resolution, the active HSRP group member will reply to neighbor solicitation (NS) messages sent to the nonlink-local address.

1.    enable

2.    configure terminal

3.    interface type number

4.    ipv6 address ipv6-address / prefix-length | prefix-name ipv6-prefix / prefix-length | autoconfig [default-route]}

5.    glbp group timers [msec] hellotime[msec] holdtime

6.    glbp group timers redirect redirect timeout

7.    glbp group load-balancing [host-dependent| round-robin| weighted]

8.    glbp group priority level

9.    glbp group preempt [delay minimum seconds]

10.    glbp group name redundancy-name

11.    exit

The following sections describe configuration tasks for GLBP authentication. The task you perform depends on whether you want to use text authentication, a simple MD5 key string, or MD5 key chains for authentication.

GLBP MD5 authentication provides greater security than the alternative plain text authentication scheme. MD5 authentication allows each GLBP group member to use a secret key to generate a keyed MD5 hash that is part of the outgoing packet. A keyed hash of an incoming packet is generated and, if the hash within the incoming packet does not match the generated hash, the packet is ignored.

The key for the MD5 hash can either be given directly in the configuration using a key string or supplied indirectly through a key chain.

A router will ignore incoming GLBP packets from routers that do not have the same authentication configuration for a GLBP group. GLBP has three authentication schemes:

• No authentication
• Plain text authentication
• MD5 authentication

GLBP packets will be rejected in any of the following cases:

• The authentication schemes differ on the router and in the incoming packet.
• MD5 digests differ on the router and in the incoming packet.
• Text authentication strings differ on the router and in the incoming packet.

• Configuring GLBP MD5 Authentication Using a Key String
• Configuring GLBP MD5 Authentication Using a Key Chain
• Configuring GLBP Text Authentication

1.    enable

2.    configure terminal

3.    track object-number interface type number {line-protocol | ip routing

4.    interface type number

5.    glbp group weighting maximum lower lower ] [upper upper

6.    glbp group weighting track object-number [decrement value]

7.    glbp group forwarder preempt [delay minimum seconds]

8.    end

9.    show track [object-number| brief] [interface [brief]| ip route [brief] | resolution| timers]

1.    enable

2.    configure terminal

3.    interface type number

4.    ipv6 address ipv6-address / prefix-length | prefix-name ipv6-prefix / prefix-length | autoconfig [default-route]}

5.    glbp group ipv6 [ipv6-address | autoconfig

6.    exit

7.    show glbp [interface-type interface-number] [group] [state] [brief]

1.    enable

2.    configure terminal

3.    no logging console

4.    Use Telnet to access a router port and repeat Steps 1 and 2.

5.    end

6.    terminal monitor

7.    debug condition glbp interface-type interface-number group [forwarder]

8.    terminal no monitor

1.    enable

2.    configure terminal

3.    interface type number

4.    standby version {1| 2

1.    enable

2.    configure terminal

3.    ipv6 unicast-routing

4.    interface type number

5.    standby [group-number] ipv6 {ipv6-global-address | ipv6-address / prefix-length | ipv6-prefix / prefix-length | link-local-address | autoconfig

6.    standby [group-number] preempt [delay minimum seconds | reload seconds | sync seconds}]

7.    standby [group-number] priority priority

8.    exit

9.    show standby [type number [group]] [all | brief

10.    show ipv6 interface [brief] [interface-type interface-number] [prefix