Glbp Or Gateway Load Balancing Protocol

Published Date: 02 Nov 2017

Active Virtual Gateway

Active Virtual Forwarder

Active virtual gateway takes care of all virtual MAC assignments in the network. In case of its failure, active virtual forwarder with second highest priority takes its place.

Quick Facts:

GLBP uses 224.0.0.102 UDP/3222

GLBP default Hello interval is 3 seconds

GLBP has virtual MAC 0007.b4xx.xxyy with xx being group number and yy being forwarder number

GLBP allows up to 4 virtual MAC address per group

Elected Active Virtual Gateway (AVG) assigns/revokes virtual MAC to/from group members

Each gateway is an Active Virtual Forwarder (AVF) and answers own ARP request

GLBP Configurations:

We will use GNS3 to implement GLBP. We are using two hosts with same default gateway. Our configuration check list is:

Basic GLBP Configuration

GLBP priority and preempt

GLBP MD5 authentication

GLBP Load Balancing Method

AVG and AVF Failover

GLBP Packet Analysis

We are using the following GNS3 topology:

We are using R1 and R2 to simulate our hosts in GNS3. The configuration on both these routers is:

R1(HostA)

R1#show running-config interface fastEthernet 0/0

Building configuration...

Current configuration : 96 bytes

!

interface FastEthernet0/0

ip address 192.168.1.1 255.255.255.0

duplex auto

speed auto

end

and a static route is define to point it to our default-gateway: 192.168.1.10

R1#show ip route static

S* 0.0.0.0/0 [1/0] via 192.168.1.10

Same sort of configuration is done on R2(HostB) too:

R2#show running-config interface fastEthernet 0/0

Building configuration...

Current configuration : 96 bytes

!

interface FastEthernet0/0

ip address 192.168.1.2 255.255.255.0

duplex auto

speed auto

end

R2#show ip route static

S* 0.0.0.0/0 [1/0] via 192.168.1.10

R3 and R4 are our Gateways on which we will load balance the traffic and create redundancy using GLBP. Please note EIGRP is configured as routing protocol with AS # 4 on R3-R4-R5. In the next step we will enable GLBP on Fast Ethernet 0/0 interface of R3 and R4. The configuration done on each router interface is as follow:

R3#show running-config interface fastEthernet 0/0

Building configuration...

Current configuration : 283 bytes

!

interface FastEthernet0/0

mac-address 0033.3333.3333 ---- > mac address changed for simplicity

ip address 192.168.1.3 255.255.255.0

duplex auto

speed auto

glbp 4 ip 192.168.1.10

glbp 4 priority 120

glbp 4 preempt

glbp 4 weighting 6

glbp 4 load-balancing weighted

glbp 4 authentication md5 key-string shah123

end

The routing configuration on R3 is as follow:

R3#show ip route eigrp

D 10.0.0.0/8 [90/409600] via 192.168.3.5, 00:09:27, FastEthernet0/1

D 192.168.2.0/24 [90/307200] via 192.168.3.5, 00:09:27, FastEthernet0/1

[90/307200] via 192.168.1.4, 00:09:27, FastEthernet0/0

R3#show ip route

Output ommited

Gateway of last resort is not set

D 10.0.0.0/8 [90/409600] via 192.168.3.5, 00:09:35, FastEthernet0/1

C 192.168.1.0/24 is directly connected, FastEthernet0/0

D 192.168.2.0/24 [90/307200] via 192.168.3.5, 00:09:35, FastEthernet0/1

[90/307200] via 192.168.1.4, 00:09:35, FastEthernet0/0

C 192.168.3.0/24 is directly connected, FastEthernet0/1

As you can see GLBP group number 4 is configured on R3 with virtual gateway IP : 192.168.1.10. The priority is set to 120, as we want to make this router AVG ( Active Virtual Gateway), authentication and load balancing also adjusted. We can create various type of load balancing but here we are using weighting. The configurations on R4 are almost same, but we have given a little bit low priority number to this gateway as we would like to make it GLBP Virtual Forwarder. The configurations are as follow:

R4#show running-config interface fastEthernet 0/0

Building configuration...

Current configuration : 283 bytes

!

interface FastEthernet0/0

mac-address 0044.4444.4444 ---- > the mac address changed for simplicity

ip address 192.168.1.4 255.255.255.0

duplex auto

speed auto

glbp 4 ip 192.168.1.10

glbp 4 priority 110

glbp 4 preempt

glbp 4 weighting 7

glbp 4 load-balancing weighted

glbp 4 authentication md5 key-string shah123

end

R4#show ip route eigrp

D 10.0.0.0/8 [90/409600] via 192.168.2.5, 00:16:45, FastEthernet0/1

D 192.168.3.0/24 [90/307200] via 192.168.2.5, 00:16:45, FastEthernet0/1

[90/307200] via 192.168.1.3, 00:16:45, FastEthernet0/0

R4#show ip route

Output omitted.

Gateway of last resort is not set

D 10.0.0.0/8 [90/409600] via 192.168.2.5, 00:17:10, FastEthernet0/1

C 192.168.1.0/24 is directly connected, FastEthernet0/0

C 192.168.2.0/24 is directly connected, FastEthernet0/1

D 192.168.3.0/24 [90/307200] via 192.168.2.5, 00:17:10, FastEthernet0/1

[90/307200] via 192.168.1.3, 00:17:10, FastEthernet0/0

The configuration on our last router, R5, on which we will create a loop back 5 interface to test ping it from R1 and R2 to check our GLBP load balancing and redundancy, is:

R5#show ip route

Gateway of last resort is not set

10.0.0.0/8 is variably subnetted, 2 subnets, 2 masks

C 10.0.0.0/24 is directly connected, Loopback5

D 10.0.0.0/8 is a summary, 00:31:59, Null0

D 192.168.1.0/24 [90/307200] via 192.168.3.3, 00:19:12, FastEthernet0/0

[90/307200] via 192.168.2.4, 00:19:12, FastEthernet0/1

C 192.168.2.0/24 is directly connected, FastEthernet0/1

C 192.168.3.0/24 is directly connected, FastEthernet0/0

And we are done! Now we are good to go, we can check the GLBP and verify it via the following commands:

From the above output we can see that our Virtual gateway is 192.168.1.10, our GLBP group is 4 ( it’s not some constant value, you can choose any value you want), we have following MACs in our GLBP 4 group:

0033.3333.3333- > the mac address manually assigned for simplicity, it denotes R3 Interface Fast Eth 0/0

0044.4444.4444- > the mac address manually assigned for simplicity, it denotes R4 Interface Fast Eth 0/0

The virtual MAC addresses are as follow

For Frowarder1 R3: 0007.b400.0401

Forwarder Number

Group Number

Same is true for Forwarder 2 R4: 0007.b400.0402

And the output of the same command on R3 is as:

Redirection enabled, 599.216 sec remaining (maximum 600 sec)

Time to live: 14399.212 sec (maximum 14400 sec)

Preemption enabled, min delay 30 sec

Active is 192.168.1.4 (primary), weighting 7 (expires in 9.208 sec)

Arp replies sent: 2

To see our GLBP in action, we issue a ping from Host A (R1) to 10.0.0.4 loopback interface on R5, arp debugging has been enabled on Host A to check GLBP in action. Here is the output:

*Mar 1 00:47:57.511: IP ARP: creating incomplete entry for IP address: 192.168.1.10 interface FastEthernet0/0

*Mar 1 00:47:57.515: IP ARP: sent req src 192.168.1.1 c001.17a4.0000,

dst 192.168.1.10 0000.0000.0000 FastEthernet0/0

*Mar 1 00:47:57.547: IP ARP: rcvd rep src 192.168.1.10 0007.b400.0402, dst 192.168.1.1 FastEthernet0/0.

*Mar 1 00:47:59.591: IP ARP: rcvd req src 192.168.1.4 0044.4444.4444, dst 192.168.1.1 FastEthernet0/0

*Mar 1 00:47:59.595: IP ARP: creating entry for IP address: 192.168.1.4, hw: 0044.4444.4444

*Mar 1 00:47:59.599: IP ARP: sent rep src 192.168.1.1 c001.17a4.0000,

dst 192.168.1.4 0044.4444.4444 FastEthernet0/0.!!!

Success rate is 60 percent (3/5), round-trip min/avg/max = 28/42/56 ms

R1#show ip arp

Protocol Address Age (min) Hardware Addr Type Interface

Internet 192.168.1.10 36 0007.b400.0402 ARPA FastEthernet0/0

R1#traceroute 10.0.0.4

Type escape sequence to abort.

Tracing the route to 10.0.0.4

1 192.168.1.4 28 msec 36 msec 20 msec ---- > R4 Fast Ethernet 0/0 interface for outgoing packet

2 192.168.2.5 40 msec * 40 msec

That is great! Our new gateway has been resolved by host A while communicating with R5 loopback interface (10.0.0.4). Okay we have confirmed that our GLBP is working great virtual MAC and IP assignment is working perfectly. Now if we want to check whether redundancy is working or not, we can do the following, we will disable the Fast Ethernet 0/0 interface on R4, and check if R3 is taking its place or not:

R1#ping 10.0.0.4 repeat 2000

Type escape sequence to abort.

Sending 2000, 100-byte ICMP Echos to 10.0.0.4, timeout is 2 seconds:

!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!

!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!

!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!

!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!.....!!!!!!!!

!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!

!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!.

A successful ping indeed!

During the above highlighted instance Interface Fast Ethernet 0/0 was shut down on R4 and the traffic was shifted after a minor glitch to R3, as can be seen from below output:

R1#show ip arp

Protocol Address Age (min) Hardware Addr Type Interface

Internet 192.168.1.10 0 0007.b400.0401 ARPA FastEthernet0/0

Internet 192.168.1.1 - c001.17a4.0000 ARPA FastEthernet0/0

Internet 192.168.1.3 0 0033.3333.3333 ARPA FastEthernet0/0

As you can the virtual MAC address corresponding to Virtual GW (192.168.1.10) changed from 0007.b400.0402 ---- > 0007.b400.0401!! Isn’t it great? 

In short GLBP is a very good redundancy and load balancing protocol. AVG is responsible for keeping any eye on all Virtual forwarders and assigning virtual MACs according to network requirements. Active Virtual Gateway redundancy is managed by GLBP priority value and Active virtual forwarders are controlled via weight value in the configurations.

Okay, one more thing to test, what if our Active Virtual Gateway fails? Let’s shut down Fast Ethernet 0/0 interface of Router R3. There was no outage; Host A was able to communicate with a minor outage with our test loop back interface on R5. What happened on R3 can be shown by below output:

The interface changed in around 12 second, in the same instant our Virtual forwarder came into action on R4 and took the charge. Okay we have re-enabled the interface back and all is back to normal. One more thing, two helpful commands in GLBP debugging are debug glbp event, debug glbp packets

R3#debug glbp packets

GLBP Packets debugging is on

R3#

*Mar 1 00:16:13.007: GLBP: Fa0/0 Grp 4 Hello out 192.168.1.3 VG Active pri 120 vIP 192.168.1.10 hello 3000, hold 10000 VF 1 Active pri 167 vMAC 0007.b400.0401

R3#

*Mar 1 00:16:14.027: GLBP: Fa0/0 Grp 4 Hello in 192.168.1.4 VG Standby pri 110 vIP 192.168.1.10 hello 3000, hold 10000 VF 2 Active pri 167 vMAC 0007.b400.0402

To check debug event in action, just disable fast Ethernet 0/0 on R4:

*Mar 1 00:20:30.191: GLBP: Fa0/0 API Software interface going down

*Mar 1 00:20:30.199: GLBP: Fa0/0 API Software interface going down

*Mar 1 00:20:30.199: GLBP: Fa0/0 Interface down

*Mar 1 00:20:30.203: GLBP: Fa0/0 4.1 Listen: e/Forwarder disabled

*Mar 1 00:20:30.203: GLBP: Fa0/0 4.1 Listen -> Init

*Mar 1 00:20:30.207: GLBP: Fa0/0 4.2 Active: e/Forwarder disabled

*Mar 1 00:20:30.207: GLBP: Fa0/0 4.2 Active -> Init

*Mar 1 00:20:30.211: %GLBP-6-FWDSTATECHANGE: FastEthernet0/0 Grp 4 Fwd 2 state Active -> Init

*Mar 1 00:20:30.219: GLBP: Fa0/0 API MAC address update

*Mar 1 00:20:30.223: GLBP: Fa0/0 4 Standby: e/GLBP disabled

*Mar 1 00:20:30.227: GLBP: Fa0/0 4 Active router IP is unknown, was 192.168.1.3

*Mar 1 00:20:30.227: GLBP: Fa0/0 4 Standby router is unknown, was local

*Mar 1 00:20:30.231: GLBP: Fa0/0 4 Standby -> Init

*Mar 1 00:20:30.239: GLBP: Fa0/0 API Add active GLBP addresses to ARP table

*Mar 1 00:20:30.243: GLBP: Fa0/0 API Add active GLBP addresses to ARP table

*Mar 1 00:20:30.267: %DUAL-5-NBRCHANGE: IP-EIGRP(0) 4: Neighbor 192.168.1.3 (FastEthernet0/0) is down: interface down

*Mar 1 00:20:30.279: GLBP: Fa0/0 API Add active GLBP addresses to ARP table