Does GLBP load-balance by default?

[mprescher]

I am getting conflicting info as to whether GLBP (using a pair of 6509 sup 720's load balances its traffic by default). Anybody know for sure?

Exhibit A:

Output of show glbp...you can see at the end it indicates round-robin load balancing...

GigabitEthernet1/2.99 - Group 3

State is Standby

199 state changes, last state change 9w0d

Virtual IP address is 10.66.33.1

Hello time 250 msec, hold time 750 msec Next hello sent in 0.032 secs

Redirect time 600 sec, forwarder time-out 14400 sec

Authentication MD5, key-string "sssss"

Preemption enabled, min delay 180 sec

Active is x.x.x.3, priority 15 (expires in 0.832 sec)

Standby is local

Priority 10 (configured)

Weighting 100 (default 100), thresholds: lower 1, upper 100

Load balancing: round-robin

Exhibit B:

From a Cisco GLBP doc that indicates that if no load balancing is specified, "all traffic will be directed to the AVG with no occurrence of load balancing."

http://www.cisco.com/en/US/prod/collateral/iosswrel/ps6537/ps6554/ps6600/product_data_sheet0900aecd803a546c.html

[Giuseppe Larosa]

Hello Mike,

the key point is what is configured on current AVG: if there the load-balancig commands is missing it will behave as HSRP

If no load-balance algorithm is specified then GLBP will operate in an identical fashion to HSRP, the AVG will only respond to Address Resolution Protocol (ARP) requests with its own Virtual Forwarder (VF) MAC address, and all traffic will be directed to the AVG with no occurrence of load balancing.

the fact that you could have configured a load balancing method on the standby router has no effect because it is not the owner of the AVG MAC address

You need to configure the load-balancing on the router that is the Active AVG

this allows to have GLBP behaves like HSRP if desired

Hope to help

Giuseppe

[mprescher]

Thanks,

The AVG and stdby devices are configured the same, no specific load balancing commands specified. The configuration only has group ip address, timer, priority,

preempt and auth settings.

The way I read it is the same as you, no load-balancing config? Then it will work like HSRP i.e. no load-balancing at all.

If that is true, then why does the 'show glbp' output say 'Load balancing: round-robin'?

[Giuseppe Larosa]

Hello Mike,

it could be a cosmetic bug just it says a default algorithm is round-robin

It is easy to test:

have a PC or another router on the vlan configured to use the VIP as default gateway.

Configure on it an ip address.

Try to ping something outside

check arp table with

arp -g

note VIP and MAC pair

reconfigure the PC or router interface with another ip address

if really round-robin

VIP and MAC pair are different I did so to test GLBP and I saw the different MAC in different times.

I looked at those tests I did some years ago on c3725/c3745 I hadn't configured any load-balancing command but the behaviour was round-robin as the show command says !

practice is different ...

Hope to help

Giuseppe

[mprescher]

Thanks much. Very helpful.

[mprescher]

post mortem...after some testing, by default round-robin shows up in the show glbp and it is in fact occurring. in addition, if you explicitly enable it, round robin occurs. explicitly disable it (do a no...) round robin still occurs. conclusion: with glbp there appears to be no way to not load balance. should someone come up with some reason to not want load balancing while running some form of recovery for default gateways, you would end up needing to use hsrp. hope this helps somebody else down the road.

[Giuseppe Larosa]

Hello Mike,

thanks for your feedaback on your own tests about this aspect of GLBP

It can be useful to somebody else so it has been rated accordingly.

Best Regards

Giuseppe