The Gateway Load Balancing Protocol (GLBP) ensures two or more gateway routers can share a single virtual IP address which is used as the default gateway by hosts in a Local Area Network (LAN). Although a single gateway IP address is configured on the hosts, GLBP uses different virtual MAC addresses to enable load balancing of traffic between each individual router.
With GLBP each participating router is an Active Virtual Forwarder (AVF). This means, the Active Virtual Gateway (AVG) assigns a different virtual MAC address to each AVF. The AVG is the only GLBP router that replies to ARP requests. By sending ARP replies with a different virtual MAC address, GLBP is able to provide load balancing. This is shown in the following packet capture.
The AVG can decide what type of load balancing algorithm to use. By default, the AVG assigns vMAC addresses with the Round Robin method, which means each consecutive ARP reply contains the vMAC address of a different AVF.
For example, if there are two AVFs, the vMAC in the ARP replies alternates between the two available addresses, as seen in the above packet capture. Other load balancing options are host-dependent and weighted (discussed later in this blog post).
R1>
R1>enable
R1#configure terminal
Enter configuration commands, one per line. End with CNTL/Z.
R1(config)#
R1(config)#interface Gi0/0
R1(config-if)#glbp 10 load-balancing ?
host-dependent Load balance equally, source MAC determines forwarder choice
round-robin Load balance equally using each forwarder in turn
weighted Load balance in proportion to forwarder weighting
<cr> <cr>
The following packet capture shows a GLBP Hello message which contains important details about the protocol. It is visible that GLBP uses the multicast IP address 224.0.0.102 and encapsulates control packets with a UDP header using port number 3222. GLBP supports authentication with plain text password or md5 hash, and including authentication with a key-chain. The GLBP AVG default priority is 100. The default Hello and Hold time is 3 seconds and 10 seconds respectively.
GLBP configuration
In the following example scenario, GLBP Group 10 is configured between R1 and R2. The AVG is R1, and it assigns the vMAC address 0007.b400.0a01 to itself, and uses 0007.b400.0a02 for R2. Both R1 and R2 are AVFs. An authentication key chain is configured together with GLBP for added security. The vIP address 10.0.0.1 is used, and it is configured as a static default route on the Host.
Notice in R1's configuration the command glbp 10 priority 200 makes R1 the AVG. A higher priority will be more preferred. By default, each router has an AVG priority of 100 in which case the AVG election process selects the router with the highest interface IP address. This command and election process is independent from the AVF priority or weighting.
Configuration:
R1
R1#show run int Gi0/0 | sec int
interface GigabitEthernet0/0
description ** to Site 1 **
ip address 10.0.0.2 255.255.255.0
glbp 10 ip 10.0.0.1
glbp 10 timers msec 400 msec 800
glbp 10 priority 200
glbp 10 authentication md5 key-chain EXAMPLE-KEYCHAIN
duplex auto
speed auto
media-type rj45
R1#show run | sec ^key chain
key chain EXAMPLE-KEYCHAIN
key 1
key-string P4SSW0RD
R1#show run int Gi0/1 | sec int
interface GigabitEthernet0/1
description ** to internet **
ip address 172.16.1.1 255.255.255.252
duplex auto
speed auto
media-type rj45
R1#show run | sec ^ip route
ip route 0.0.0.0 0.0.0.0 GigabitEthernet0/1 172.16.1.2
R2
R2#show run int Gi0/0 | sec int
interface GigabitEthernet0/0
description ** to Site 1 **
ip address 10.0.0.3 255.255.255.0
glbp 10 ip 10.0.0.1
glbp 10 timers msec 400 msec 800
glbp 10 priority 150
glbp 10 authentication md5 key-chain EXAMPLE-KEYCHAIN
duplex auto
speed auto
media-type rj45
R2#show run | sec ^key chain
key chain EXAMPLE-KEYCHAIN
key 1
key-string P4SSW0RD
R2#show run int Gi0/1 | sec int
interface GigabitEthernet0/1
description ** to internet **
ip address 172.16.2.1 255.255.255.252
duplex auto
speed auto
media-type rj45
R2#show run | sec ^ip route
ip route 0.0.0.0 0.0.0.0 GigabitEthernet0/1 172.16.2.2
Host
Host#show run int Gi0/0 | sec int
interface GigabitEthernet0/0
description ** to R2 and R3 **
ip address 10.0.0.4 255.255.255.0
duplex auto
speed auto
media-type rj45
Host#show run | sec ^ip route
ip route 0.0.0.0 0.0.0.0 GigabitEthernet0/0 10.0.0.1
R1#show glbp brief
Interface Grp Fwd Pri State Address Active router Standby router
Gi0/0 10 - 200 Active 10.0.0.1 local 10.0.0.3 « AVG has priority 200 configured
Gi0/0 10 1 - Active 0007.b400.0a01 local - « Virtual MAC address assigned to AVG
Gi0/0 10 2 - Listen 0007.b400.0a02 10.0.0.3 -
R1#show glbp
GigabitEthernet0/0 - Group 10
State is Active
1 state change, last state change 03:07:25
Virtual IP address is 10.0.0.1« GLBP Virtual IP addressHello time 400 msec, hold time 800 msec« GLBP Hello and Hold time
Next hello sent in 0.160 secs
Redirect time 600 sec, forwarder timeout 14400 sec
Authentication MD5, key-chain "EXAMPLE-KEYCHAIN"« Authentication with key-chain is configured
Preemption disabled
Active is local
Standby is 10.0.0.3, priority 150 (expires in 0.608 sec) « Backup AVG is R2, has priority 150
Priority 200 (configured)
Weighting 100 (default 100), thresholds: lower 1, upper 100
Load balancing: round-robin« Default load balancing method is used
Group members:
5254.0001.1c1c (10.0.0.3) authenticated
254.0006.af56 (10.0.0.2) local « Real (burned in/bia) MAC address of the AVG
There are 2 forwarders (1 active)
Forwarder 1« Details about the AVF 1 (it is actually the AVG)
State is Active
1 state change, last state change 03:07:13
MAC address is 0007.b400.0a01 (default)
Owner ID is 5254.0006.af56
Redirection enabled
Preemption enabled, min delay 30 sec
Active is local, weighting 100
Client selection count: 3
Forwarder 2« Details about AVF 2, this is R2
State is Listen
MAC address is 0007.b400.0a02 (learnt)
Owner ID is 5254.0001.1c1c
Redirection enabled, 599.808 sec remaining (maximum 600 sec)
Time to live: 14399.808 sec (maximum 14400 sec)
Preemption enabled, min delay 30 sec
Active is 10.0.0.3 (primary), weighting 100 (expires in 0.672 sec)
Client selection count: 2
Host#trace 172.16.0.1 probe 1
Type escape sequence to abort.
Tracing the route to 172.16.0.1
VRF info: (vrf in name/id, vrf out name/id)
1 10.0.0.2 3 msec « Host uses R1 as its next-hop
2 172.16.1.2 6 msec
3 172.16.0.1 10 msec
Host#show arp
Protocol Address Age (min) Hardware Addr Type Interface
Internet 10.0.0.1 190 0007.b400.0a01 ARPA GigabitEthernet0/0 « AVG provided vMAC of itself (AVF 1) to the Host
Internet 10.0.0.2 195 5254.0006.af56 ARPA GigabitEthernet0/0
Internet 10.0.0.3 189 5254.0001.1c1c ARPA GigabitEthernet0/0
Internet 10.0.0.4 - 5254.0016.d0fb ARPA GigabitEthernet0/0
GLBP configuration with IPv6
In the following example scenario, GLBP for IPv6 is configured. The AVG is R1 configured with a priority of 120. The GLBP virtual IPv6 address is a link-local address which is autoconfigured based on the GLBP vMAC address 0007.b400.0a00. The vIP address is advertised to the Host using Neighbor Discovery Protocol (NDP) Router Advertisement messages.
The interface-level comand ipv6 nd router-preference High is configured on both routers to set the Router Preference in RA messages, and ensure the GLBP vIP address is installed on the Host. Both routers are active forwarders (AVF). GLBP IPv6 control packets are sent to the multicast address FF02::66. The GLBP vMAC address is provided through NDP Neighbor Advertisement messages (instead of ARP in IPv4).
Configuration:
R1
R1#show run int Gi0/0 | sec int
interface GigabitEthernet0/0
description ** to Site 1 **
no ip address
glbp 10 ipv6 autoconfig
glbp 10 priority 120
glbp 10 authentication md5 key-string P4SSW0RD
duplex auto
speed auto
media-type rj45
ipv6 address FE80::2 link-local
ipv6 address 2001:DB8::2/64
ipv6 nd router-preference High
R1#show run int Gi0/1 | sec int
interface GigabitEthernet0/1
description ** to internet **
no ip address
duplex auto
speed auto
media-type rj45
ipv6 address 2001:DB8:1::1/64
R1#show run | sec ^ipv6 route
ipv6 route ::/0 GigabitEthernet0/1 2001:DB8:1::2
R1#show run | sec ^ipv6 unicast
ipv6 unicast-routing
R2
R2#show run int Gi0/0 | sec int
interface GigabitEthernet0/0
description ** to Site 1 **
no ip address
glbp 10 ipv6 autoconfig
glbp 10 authentication md5 key-string P4SSW0RD
duplex auto
speed auto
media-type rj45
ipv6 address FE80::3 link-local
ipv6 address 2001:DB8::3/64
ipv6 nd router-preference High
R2#show run int Gi0/1 | sec int
interface GigabitEthernet0/1
description ** to internet **
no ip address
duplex auto
speed auto
media-type rj45
ipv6 address 2001:DB8:2::1/64
R2#show run | sec ^ipv6 route
ipv6 route ::/0 GigabitEthernet0/1 2001:DB8:2::2
R2#show run | sec ^ipv6 unicast
ipv6 unicast-routing
Host
Host#show run int Gi0/0 | sec int
interface GigabitEthernet0/0
no ip address
duplex auto
speed auto
ipv6 address autoconfig default
ipv6 enable
R1#show glbp brief
Interface Grp Fwd Pri State Address Active router Standby router
Gi0/0 10 - 120 Active FE80::7:B4FF:FE00:A00« GLBP virtual IPv6 address
local FE80::3
Gi0/0 10 1 - Active 0007.b400.0a01 local -
Gi0/0 10 2 - Listen 0007.b400.0a02 FE80::3 -
R1#show glbp
GigabitEthernet0/0 - Group 10
State is Active
1 state change, last state change 01:41:37
Virtual IP address is FE80::7:B4FF:FE00:A00 (auto-configured) « GLBP virtual IPv6 address is autoconfigured
Hello time 3 sec, hold time 10 sec
Next hello sent in 0.256 secs
Redirect time 600 sec, forwarder timeout 14400 sec
Authentication MD5, key-string « GLBP IPv6 authentication is configured
Preemption disabled
Active is local
Standby is FE80::3, priority 100 (expires in 8.960 sec)
Priority 120 (configured)
Weighting 100 (default 100), thresholds: lower 1, upper 100
Load balancing: round-robin
Group members:« Two AVF routers participate in GLBP Group 10
5254.0000.e516 (FE80::2) local
5254.000f.0541 (FE80::3) authenticated
There are 2 forwarders (1 active)
Forwarder 1
State is Active
1 state change, last state change 01:41:26
MAC address is 0007.b400.0a01 (default) « GLBP vMAC of AVF1
Owner ID is 5254.0000.e516
Redirection enabled
Preemption enabled, min delay 30 sec
Active is local, weighting 100
Client selection count: 4
Forwarder 2
State is Listen
MAC address is 0007.b400.0a02 (learnt) « GLBP vMAC of AVF2
Owner ID is 5254.000f.0541
Redirection enabled, 598.976 sec remaining (maximum 600 sec)
Time to live: 14398.976 sec (maximum 14400 sec)
Preemption enabled, min delay 30 sec
Active is FE80::3 (primary), weighting 100 (expires in 10.560 sec)
Client selection count: 3
Host#show ipv6 route
IPv6 Routing Table - default - 4 entries
Codes: C - Connected, L - Local, S - Static, U - Per-user Static route
B - BGP, HA - Home Agent, MR - Mobile Router, R - RIP
H - NHRP, I1 - ISIS L1, I2 - ISIS L2, IA - ISIS interarea
IS - ISIS summary, D - EIGRP, EX - EIGRP external, NM - NEMO
ND - ND Default, NDp - ND Prefix, DCE - Destination, NDr - Redirect
RL - RPL, O - OSPF Intra, OI - OSPF Inter, OE1 - OSPF ext 1
OE2 - OSPF ext 2, ON1 - OSPF NSSA ext 1, ON2 - OSPF NSSA ext 2
la - LISP alt, lr - LISP site-registrations, ld - LISP dyn-eid
lA - LISP away, a - Application
ND ::/0 [2/0]
via FE80::7:B4FF:FE00:A00, GigabitEthernet0/0 « GLBP virtual IPv6 address learned on the Host
NDp 2001:DB8::/64 [2/0]
via GigabitEthernet0/0, directly connected
L 2001:DB8::5054:FF:FE11:8B1/128 [0/0]
via GigabitEthernet0/0, receive
L FF00::/8 [0/0]
via Null0, receive
Host#show ipv6 route ::/0
Routing entry for ::/0
Known via "ND", distance 2, metric 0 « Neighbor Discovery Protocol advertises the GLBP IPv6 address
Route count is 1/1, share count 0
Routing paths:
FE80::7:B4FF:FE00:A00, GigabitEthernet0/0
From FE80::7:B4FF:FE00:A00
Last updated 01:32:20 ago
Host#trace 2001:DB8:A::1
Type escape sequence to abort.
Tracing the route to 2001:DB8:A::1
1 2001:DB8::3 2 msec 2 msec 1 msec
2 2001:DB8:2::2 3 msec 2 msec 4 msec
3 2001:DB8:A::1 3 msec 2 msec 2 msec
GLBP weighted Load Balancing
In the following example scenario, GLBP weighting is configured so that more hosts prefer the AVF R2 as their default gateway device. Specifically, R2 is assigned a weight of 200, while R1 uses the default GLBP weight 100. This means, the AVG (R1) replies to ARP requests with the vMAC of R2 twice as often as the vMAC of R1. As a result, twice as many hosts prefer to send traffic through R2 as opposed to R1.
Configuration:
R1
R1#show run int Gi0/0 | sec int
interface GigabitEthernet0/0
description ** to Site 1 **
ip address 10.0.0.2 255.255.255.0
glbp 10 ip 10.0.0.1
glbp 10 timers msec 400 msec 800
glbp 10 priority 200
glbp 10 load-balancing weighted
duplex auto
speed auto
media-type rj45
R2
R2#show run int Gi0/0 | sec int
interface GigabitEthernet0/0
description ** to Site 1 **
ip address 10.0.0.3 255.255.255.0
glbp 10 ip 10.0.0.1
glbp 10 timers msec 400 msec 800
glbp 10 priority 150
glbp 10 weighting 200
glbp 10 load-balancing weighted
duplex auto
speed auto
media-type rj45
R1#show glbp
GigabitEthernet0/0 - Group 10
State is Active
1 state change, last state change 05:14:38
Virtual IP address is 10.0.0.1
Hello time 400 msec, hold time 800 msec
Next hello sent in 0.128 secs
Redirect time 600 sec, forwarder timeout 14400 sec
Preemption disabled
Active is local
Standby is 10.0.0.3, priority 150 (expires in 0.576 sec)
Priority 200 (configured)
Weighting 100 (default 100), thresholds: lower 1, upper 100
Load balancing: weighted« Weighted load balancing is used
Group members:
5254.0001.1c1c (10.0.0.3)
5254.0006.af56 (10.0.0.2) local
There are 2 forwarders (1 active)
Forwarder 1
State is Active
1 state change, last state change 05:14:26
MAC address is 0007.b400.0a01 (default)
Owner ID is 5254.0006.af56
Redirection enabled
Preemption enabled, min delay 30 sec
Active is local, weighting 100« Default weight is 100Client selection count: 7« vMAC of R1 was provided to 7 hosts via ARP reply
Forwarder 2
State is Listen
2 state changes, last state change 00:13:52
MAC address is 0007.b400.0a02 (learnt)
Owner ID is 5254.0001.1c1c
Redirection enabled, 599.776 sec remaining (maximum 600 sec)
Time to live: 14399.776 sec (maximum 14400 sec)
Preemption enabled, min delay 30 sec
Active is 10.0.0.3 (primary), weighting 200 (expires in 0.608 sec)« Weight 200 is configuredClient selection count: 12« vMAC of R2 was provided to 12 hosts via ARP reply
GLBP and object tracking
In the following example scenario, R1 and R2 have the default weight 100 assigned and are active forwarders (AVF). The AVG R1 uses the Round Robin load balancing method and alternates between two active vMAC addresses when replying to ARP requests. This means, both routers actively forward traffic and if any of the WAN links goes down a proactive method of AVF switchover needs to take place.
This is ensured with object tracking and IP SLA. Each router monitors its next-hop WAN address with an ICMP-echo IP SLA. If the next-hop is not reachable, the IP SLA and tracked object goes down, and the AVF weight decreases from 100 to 90. Each router gives up its active forwarder role if the weight is below 95, and the other AVF assumes the role for both vMAC addresses with a preempt delay of 1 second.
Configuration:
R1
R1#show run int Gi0/0 | sec int
interface GigabitEthernet0/0
description ** to Site 1 **
ip address 10.0.0.2 255.255.255.0
glbp 10 ip 10.0.0.1
glbp 10 timers msec 400 msec 800
glbp 10 priority 200
glbp 10 weighting 100 lower 95
glbp 10 weighting track 1 decrement 10
glbp 10 forwarder preempt delay minimum 1
duplex auto
speed auto
media-type rj45
R1#show run | sec ^ip sla
ip sla 10
icmp-echo 172.16.1.2
threshold 1000
timeout 1000
frequency 1
ip sla schedule 10 life forever start-time now
R1#show run | sec ^track
track 1 ip sla 10
R2
R2#show run int Gi0/0 | sec int
interface GigabitEthernet0/0
description ** to Site 1 **
ip address 10.0.0.3 255.255.255.0
glbp 10 ip 10.0.0.1
glbp 10 timers msec 400 msec 800
glbp 10 priority 150
glbp 10 weighting 100 lower 95
glbp 10 weighting track 1 decrement 10
glbp 10 forwarder preempt delay minimum 1
duplex auto
speed auto
media-type rj45
R2#show run | sec ^ip sla
ip sla 10
icmp-echo 172.16.2.2
threshold 1000
timeout 1000
frequency 1
ip sla schedule 10 life forever start-time now
R2#show run | sec ^track
track 1 ip sla 10
R1#show glbp brief
Interface Grp Fwd Pri State Address Active router Standby router
Gi0/0 10 - 200 Active 10.0.0.1 local 10.0.0.3
Gi0/0 10 1 - Listen 0007.b400.0a01 10.0.0.3 - « After R1 WAN link failure, it is no longer an AVF
Gi0/0 10 2 - Listen 0007.b400.0a02 10.0.0.3 -
R2#show glbp brief
Interface Grp Fwd Pri State Address Active router Standby router
Gi0/0 10 - 150 Standby 10.0.0.1 10.0.0.2 local
Gi0/0 10 1 - Active 0007.b400.0a01 local - « After R1 WAN link failure, R2 become AVF for both vMAC addresses
Gi0/0 10 2 - Active 0007.b400.0a02 local -
R1#show log | beg Log Buffer
Log Buffer (8192 bytes):
GLBP: Gi0/0 Grp 10 Hello in VG Standby pri 150 vIP 10.0.0.1 hello 400, hold 800 VF 1 Active pri 167 vMAC 0007.b400.0a01
GLBP: Gi0/0 Grp 10 Hello out VG Active pri 200 vIP 10.0.0.1 hello 400, hold 800 VF 2 Active pri 167 vMAC 0007.b400.0a02
GLBP: Gi0/0 Grp 10 Hello in VG Standby pri 150 vIP 10.0.0.1 hello 400, hold 800 VF 1 Active pri 167 vMAC 0007.b400.0a01
GLBP: Gi0/0 Grp 10 Hello out VG Active pri 200 vIP 10.0.0.1 hello 400, hold 800 VF 2 Active pri 167 vMAC 0007.b400.0a02
GLBP: Gi0/0 Grp 10 Hello in VG Standby pri 150 vIP 10.0.0.1 hello 400, hold 800 VF 1 Active pri 167 vMAC 0007.b400.0a01
%TRACK-6-STATE: 1 ip sla 10 state Up -> Down
GLBP: Gi0/0 10 Track 1 object changed, state Up -> Down
GLBP: Gi0/0 10 Weighting 100 -> 90
GLBP: Gi0/0 Grp 10 Hello out VG Active pri 200 vIP 10.0.0.1 hello 400, hold 800 VF 2 Active pri 39 vMAC 0007.b400.0a02
GLBP: Gi0/0 Grp 10 Hello in VG Standby pri 150 vIP 10.0.0.1 hello 400, hold 800 VF 1 Active pri 167 vMAC 0007.b400.0a01
GLBP: Gi0/0 Grp 10 Hello out VG Active pri 200 vIP 10.0.0.1 hello 400, hold 800 VF 2 Active pri 39 vMAC 0007.b400.0a02
GLBP: Gi0/0 Grp 10 Hello in VG Standby pri 150 vIP 10.0.0.1 hello 400, hold 800 VF 1 Active pri 167 vMAC 0007.b400.0a01
GLBP: Gi0/0 Grp 10 Hello out VG Active pri 200 vIP 10.0.0.1 hello 400, hold 800 VF 2 Active pri 39 vMAC 0007.b400.0a02
GLBP: Gi0/0 Grp 10 Hello in VG Standby pri 150 vIP 10.0.0.1 hello 400, hold 800 VF 1 Active pri 167 vMAC 0007.b400.0a01
GLBP: Gi0/0 Grp 10 Hello out VG Active pri 200 vIP 10.0.0.1 hello 400, hold 800 VF 2 Active pri 39 vMAC 0007.b400.0a02
GLBP: Gi0/0 Grp 10 Hello in VF 2 Active pri 135 vMAC 0007.b400.0a02
GLBP: Gi0/0 10.2 Active: i/Hello rcvd from higher pri Active router (135/10.0.0.3)
GLBP: Gi0/0 10.2 Active -> Listen
%GLBP-6-FWDSTATECHANGE: GigabitEthernet0/0 Grp 10 Fwd 2 state Active -> Listen
GLBP: Gi0/0 Grp 10 Hello in VG Standby pri 150 vIP 10.0.0.1 hello 400, hold 800 VF 1 Active pri 167 vMAC 0007.b400.0a01 VF 2 Active pri 135 vMAC 0007.b400.0a02
GLBP: Gi0/0 Grp 10 Hello out VG Active pri 200 vIP 10.0.0.1 hello 400, hold 800 VF 2 Listen pri 39 vMAC 0007.b400.0a02
GLBP: Gi0/0 Grp 10 Hello in VG Standby pri 150 vIP 10.0.0.1 hello 400, hold 800 VF 1 Active pri 167 vMAC 0007.b400.0a01 VF 2 Active pri 135 vMAC 0007.b400.0a02
GLBP: Gi0/0 Grp 10 Hello out VG Active pri 200 vIP 10.0.0.1 hello 400, hold 800 VF 2 Listen pri 39 vMAC 0007.b400.0a02
GLBP: Gi0/0 Grp 10 Hello in VG Standby pri 150 vIP 10.0.0.1 hello 400, hold 800 VF 1 Active pri 167 vMAC 0007.b400.0a01 VF 2 Active pri 135 vMAC 0007.b400.0a02
[...]
In the above GLBP debug logs from R1 it is first visible how the track object goes down and the GLBP weight decreases from 100 to 90. Next, R1 sends GLBP Hello messages with an AVF priority of 39 (and with a weight of 90). Then, the Virtual Forwarder 2 (AVF R2) sends a GLBP Hello with an AVF priority of 135 which overrides the AVF priority of R1 (39). Finally, the VF R2 becomes the AVF for both vMAC addresses.
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glbp-ipv4-avf-failover-hello.pcap — Router 10.0.0.2 is no longer active forwarder (AVF) for this GLBP Group, therefore in the GLBP Hello sent from 10.0.0.3 there are two "Request/Response" sections which show that 10.0.0.3 is the active forwarder for both GLBP virtual MAC addresses. Meanwhile, the router 10.0.0.2 remains the AVG for this GLBP Group, this means it replies to ARP requests from the hosts. Notice also, the Hello sent from the standby AVF 10.0.0.2 contains a "Request/Response" section with the "Priority" set to the value 39, and "VF state: Listen"
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