GLBP (Gateway Load Balancing Protocol) is a Cisco proprietary
protocol that attempts to overcome the limitations of existing redundant router
protocols by adding basic load balancing functionality. GLBP is a virtual
gateway protocol similar to HSRP and VRRP.
However, unlike its little brothers, GLBP is capable of using
multiple physical gateways at the same time. As we know, a single HSRP or VRRP
group represents one virtual gateway, with single virtual IP and MAC addresses.
Only one physical gateway in a standby/redundancy group is responsible for
packet forwarding, others remain inactive in standby/backup state. If you have
R1, R2, R3 sharing the segment 174.X.123.0/24 with the physical IP addresses
174.X.123.1, 174.X.123.2 and 174.X.123.3 you may configure them to represent
one single virtual gateway with an IP address 174.X.123.254. The physical
gateway priority settings will determine which physical gateway takes the role
of the active packet forwarder. The hosts on the segment will set their default
gateway to 174.X.123.254, staying isolated of the physical gateway failures.
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| GLBP Terminology |
GLBP brings this idea to new level, by allowing multiple
physical gateways to participate in packet forwarding simultaneously. Consider
this example, Imagine you need the hosts on the segments to fully utilize all
existing physical gateways, yet provide recovery from a gateway failure. For
instance, you may want 50% of outgoing packets to be sent up to R1, 30% to R2
and 20% to R3. At the same time, you want to ensure, that hosts using either of
the gateways will automatically switch to another if their gateway fails. On
top of that, all hosts in the segment should reference to the virtual gateway
using the same IP address 174.X.123.254. This is a complicated task, which has
being addressed by GLBP protocol design.
By default GLBP load balances in Round-Robin fashion.
GLBP elects one AVG (Active Virtual Gateway) for each group.
Other group members act as backup in case of AVG failure. In case there are
more than two members, the second best AVG is placed in the Standby state and
all other members are placed in the Listening state. This is monitored using
hello and holdtime timers, which are 3 and 10 seconds by default. The elected
AVG then assigns a virtual MAC address to each member of the GLBP group,
including itself, thus enabling AVFs (Active Virtual Forwarders). Each AVF
assumes responsibility for forwarding packets sent to its virtual MAC address.
There could be up to four AVFs at the same time.
By default, GLBP routers use the local multicast address
224.0.0.102 to send hello packets to their peers every 3 seconds over UDP 3222
(source and destination).
Cisco implemented IPv6 support for GLBP in IOS release
12.2(33)SXI.
GLBP (Gateway Load Balancing Protocol) Points to Remember:
1. Cisco Proprietary (2005)
2. It uses UDP Port 3222
3. It sends Multicast Hello 224.0.0.102
4. Default Priority 100
5. Default weight 100
6. Default Preempt disable
7. Default decrement in weight using track = 10
8. GLBP Load Balancing Algorithm
(i) Round Robin
(ii) Weighted
(iii) Host Dependent
9. Default load balancing algorithm – Round robin.
10. Hello – 3 sec
11. Hold – 10 sec
12. There is no default in built track command
13. In GLBP we configure external track.
14. It supports authentication MD-5 & Plain Txt.
GLBP Roles
- AVG (Active Virtual Gateway)
- AVF (Active Virtual Forwarder)
AVG– A router which gives the reply of ARP request of
clients for gateway based on load balancing algorithm. Default algorithm is
Round Robin. AVG is also responsible to assign MAC to AVF.
AVF– In GLBP all router act as an AVF. AVF would be
responsible for forward the data.
GLBP MAC
0007. B4xx.xxyy
Xxxx – Group ID
YY – Forwarder ID
Max Group No - 0 to 1023
Important Points
- IP uses Protocol Numbers. (1 to 255)
- TCP/UDP uses Port Numbers. (0 t0 65535)
Load balancing algorithm
GLBP load sharing is done in one of three ways:
- Round-robin load-balancing Algorithm: Each router MAC is used sequentially to respond to ARP requests. This is the default load balancing mode in GLBP and is suitable for any number of end hosts.
- Weighted load-balancing Algorithm: Traffic is balanced proportional to a configured weight. Each GLBP router in the group will advertise its weighting and assignment; the AVG will act based on that value. For example, if there are two routers in a group and R1 has double the forwarding capacity of router B, the weighting value of router A should be configured to be double the amount of R2.
- Host-dependent load-balancing Algorithm: A given host always uses the same router.
GLBP over HSRP & VRRP
The main disadvantage of HSRP and VRRP is that only one
gateway is elected to be the active gateway and used to forward traffic whilst
the rest are unused until the active one fails. Gateway Load Balancing Protocol
(GLBP) is a Cisco proprietary protocol and performs the similar function to
HSRP and VRRP but it supports load balancing among members in a GLBP group
.
Prerequisites for Gateway Load Balancing Protocol
Before configuring the GLBP feature, ensure that the routers
can support multiple MAC addresses on the physical interfaces. For each GLBP
forwarder to be configured, an additional MAC address is used.
Supported Platforms
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GLBP Active Virtual Gateway
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 AVG 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 AVG is responsible for answering Address Resolution
Protocol (ARP) requests for the virtual IP address. Load sharing is achieved by
the AVG replying to the ARP requests with different virtual MAC addresses.
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| Router A is the AVG for a GLBP group, and is responsible for the virtual IP address 10.21.8.10. 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 IP address of 10.21.8.10 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. |
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.
GLBP Virtual MAC Address Assignment
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 Virtual Forwarder Redundancy
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 redirecting hosts to
the virtual forwarder, although the virtual forwarder will continue to forward
packets that were sent to the old virtual forwarder MAC address.
The secondary holdtime is the
interval during which the virtual forwarder is valid. When the secondary
holdtime 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
GLBP gateway priority determines
the role that each GLBP gateway plays and what happens 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.
GLBP Gateway Weighting and Tracking
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 determines 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, and when it 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.
GLBP Benefits
- Load Sharing
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.
- Multiple Virtual 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.
- Preemption
The redundancy scheme of GLBP
enables you to preempt an active virtual gateway with a higher priority backup
virtual gateway that has become available. Forwarder preemption works in a
similar way, except that forwarder preemption uses weighting instead of priority
and is enabled by default.
- Authentication
You can use a simple text password
authentication scheme between GLBP group members to detect configuration
errors. A router within a GLBP group with a different authentication string
than other routers will be ignored by other group members.
Customizing GLBP
Customizing the behavior of GLBP
is optional. Be aware that as soon as you enable a GLBP group, that group is
operating. It is possible that if you first enable a GLBP group before
customizing GLBP, the router could take over control of the group and become
the AVG before you have finished customizing the feature. Therefore, if you
plan to customize GLBP, it is a good idea to do so before enabling GLBP.
SUMMARY STEPS
- enable
- configure terminal
- interface type number
- ip address ip-address mask [secondary]
- glbp group authentication text string
- glbp group forwarder preempt [delay minimum seconds]
- glbp group load-balancing [host-dependent | round-robin | weighted]
- glbp group preempt [delay minimum seconds]
- glbp group priority level
- glbp group timers [msec] hellotime [msec] holdtime
- glbp group timers redirect redirect timeout
- exit
Configuring GLBP Weighting Values and Object Tracking
GLBP weighting is used to determine
whether a router can act as a virtual forwarder. Initial weighting values can
be set and optional thresholds specified. Interface states can be tracked and a
decrement value set to reduce the weighting value if the interface goes down.
When the GLBP router weighting drops below a specified value, the router will
no longer be an active virtual forwarder. When the weighting rises above a
specified value, the router can resume its role as an active virtual forwarder.
SUMMARY STEPS
- enable
- configure terminal
- track object-number interface type number {line-protocol | ip routing}
- interface type number
- glbp group weighting maximum [lower lower] [upper upper]
- glbp group weighting track object-number [decrement value]
- exit
Enabling and Verifying GLBP
This task explains how to enable
GLBP on an interface and verify its configuration and operation. GLBP is
designed to be easy to configure. Each gateway in a GLBP group must be
configured with the same group number, and at least one gateway in the GLBP
group must be configured with the virtual IP address to be used by the group.
All other required parameters can be learned.
Prerequisites
If VLANs are in use on an
interface, the GLBP group number must be different for each VLAN.
SUMMARY STEPS
- enable
- configure terminal
- interface type number
- ip address ip-address mask [secondary]
- glbp group ip [ip-address [secondary]]
- exit
- show glbp [interface-type interface-number] [group] [state] [brief]
GLBP Authentication
GLBP has
three authentication types:
- No authentication
- MD5 authentication
- Plain text authentication
MD5 is the
most security method so far. With this method, the same keys are configured on
both ends. One end will send the encrypted key (called hash, using MD5) to the
other. At the other side, the same key is also encrypted and compared with the
receiving encrypted key. If the two encrypted keys are the same then
authentication is approved. The advantage of this method is only the encrypted
key is sent through the link. 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.
Example for GLBP
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| GLBP Topology |
R1 (config) #int fa0/0
R1 (config-if) #ip add
192.168.101.2 255.255.255.0
R1 (config-if) #no shut
R1 (config-if) #int s0/0
R1 (config-if) #ip add
192.168.1.1 255.255.255.0
R1 (config-if) #no shut
R1 (config-if) #int s0/1
R1 (config-if) #ip add
192.168.2.1 255.255.255.0
R1 (config-if) #no shut
R3 (config) #int fa0/0
R3 (config-if) #ip add
192.168.102.1 255.255.255.0
R3 (config-if) #no shut
R3 (config-if) #int S0/0
R3 (config-if) #ip add
192.168.1.2 255.255.255.0
R3 (config-if) #no shut
R3 (config-if) #int S0/1
R3 (config-if) #ip add
192.168.2.2 255.255.255.0
R3 (config-if) #no shut
R3 (config-if) #int S0/2
R3 (config-if) #ip add
192.168.3.1 255.255.255.0
R3 (config-if) #no shut
R2 (config) #int fa0/0
R2 (config-if) #ip add
192.168.101.3 255.255.255.0
R2 (config-if) #no shut
R2 (config) #int S0/0
R2 (config-if) #ip add
192.168.3.2 255.255.255.0
R2 (config-if) #no shut
R1 (config) #router ei 100
R1 (config-router) # network
0.0.0.0
R1 (config-router) # no auto summary
R2 (config) #router ei 100
R2 (config-router) # network
0.0.0.0
R2 (config-router) # no auto summary
R3 (config) #router ei 100
R3 (config-router) # network
0.0.0.0
R3 (config-router) # no auto summary
R1 (config) #int fa0/0
R1 (config-if) #glbp?
0 to 1023
R1 (config-if) #glbp 1 ip 192.168.101.1
R2 (config) #int fa0/0
R2 (config-if) #glbp?
0 to 1023
R2 (config-if) #glbp 1 ip 192.168.101.1
Comp# ip add 192.168.101.10
Comp# default gateway
192.168.101.1
Comp# tracert –d
192.168.102.1 Via 192.168.101.2
Comp# Arp –d
Comp# tracert –d
192.168.102.1 Via 192.168.101.3
Comp# Arp –d
Comp# tracert –d
192.168.102.1 Via 192.168.101.2
Comp# Arp –d
Comp# tracert –d 192.168.102.1 Via 192.168.101.3
Here we
can see one Arp request will forward to R1 and second will forward to R2.
R2# sh glbp
Hello – 3 sec, Hold – 10 sec
By default preemption is disabled
Default load balancing is Round Robin
R1 (config) #int fa0/0
R1 (config-if) glbp 1 load –
balancing?
R1 (config-if) glbp 1 load – balancing weighted
R2 (config) #int fa0/0
R2 (config-if) glbp 1 load –
balancing?
R2 (config-if) glbp 1 load – balancing weighted
By default weighted is 100
Right now on both the router weight is equal
so it will perform load balancing Round Robin.
Comp# Arp –d
Comp# tracert –d
192.168.102.1 Via 192.168.101.2
Comp# Arp –d
Comp# tracert –d 192.168.102.1 Via 192.168.101.3
Now we will increase the weight of R1
R1 (config) #int fa0/0
R1 (config-if) #glbp 1
weighting?
R1 (config-if) #glbp 1 weighting 200
Comp# Arp –d
Comp# tracert –d
192.168.102.1 Via 192.168.101.3
Comp# Arp –d
Comp# tracert –d
192.168.102.1 Via 192.168.101.2
Comp# Arp –d
Comp# tracert –d 192.168.102.1 Via 192.168.101.2
Here we can see two packets are going via R1
and one packet are going via R2
R1 (config) #int s0/0
R1 (config-if) #shut
Comp# tracert –d 192.168.102.1 Via 192.168.168.102.1
Here we can see requests first come on R1 and
then move to R2.
Now we will use track Command
R1 (config) #track 1 int
s0/0 line-protocol
R1 (config-track) #track 2
int s0/1 line protocol
R1 (config) #int fa0/0
R1 (config-if) #glbp 1
weighting track 1 decrement 100
R1 (config-if) #glbp 1
weighting track 2 decrement 100
R1#sh glbp
R1 (config) #int s0/0
R1 (config-if) #shut
R1 #sh glbp
R1 (config) #int s0/0
R1 (config-if) #no shut
R1#sh glbp
R1 (config) #int fa0/0
R1 (config-if) #glbp 1 load-balancing host-dependent
R2 (config) #int fa0/0
R2 (config-if) #glbp 1 load-balancing host-dependent
Comp# Arp –d
Comp# tracert –d 192.168.102.1 Via 192.168.101.3
All packets will go via 192.168.101.3
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