Forwarder election process in GLBP

[Hemant Kumar]

Can somone please explain how the AVF are elected in GLBP? I mean if we have 5 routers in the network then which router will be in backup mode and how the active forwarders are elected?

[Reza Sharifi]

GLBP is different from HSRP and VRRP. There is no active and stand-by.

GLBP  differs from Cisco Hot Standby Redundancy Protocol (HSRP) and IETF RFC  3768 Virtual Router Redundancy Protocol (VRRP) in that it has the  ability to load balance over multiple gateways. Like HSRP and VRRP an  election occurs, but rather than a single active router winning the  election GLBP elects an Active Virtual Gateway (AVG). The job of the AVG  is to assign virtual MAC addresses to each of the other GLBP routers  and to assign each network host to one of the GLBP routers.

The routers that receive this MAC address assignment are known as Active Virtual Forwarders (AVF).

more info here:

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

HTH

[Hemant Kumar]

Thanks for your input. I was reading few documents about the GLBP but none of them explains how the selection of AVF happens.

Lets say we configure glbp on five routers connected to one switch. As we know we can have only 4 forwarders but which four will forward the traffic and on what basis they will get selected.

I hope now I am clear about the question.

[Reza Sharifi]

what basis they will get selected.

It is per host in a round-robin scheme

HTH

[Hemant Kumar]

Can you please exlpain more abut how out of five router which four will be slected as forwarder?

[Abzal]

Hi,

GLBP group members elect one Active Virtual Gateway (AVG). Other members will be backup to AVG(known as AVF, active virtual forwarder). Then AVG assign one virtual MAC address to each GLBP member. Also AVG is responsible to answering to ARP requests. By default load balancing method is Round-Robin. With this method when ARP request comes asking MAC address of default gateway AVG answers with MAC address one of AVFs. On Second request it answers with next AVFs MAC address and so on.
More info you can find here
http://www.cisco.com/en/US/prod/collateral/iosswrel/ps6537/ps6554/ps6600/product_data_sheet0900aecd803a546c.html


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[Peter Paluch]

Hello Hemant, Reza and Abzal,

What Hemant is asking about is how exactly out of 5 or more routers on a common segment, exactly 4 are chosen to be AVFs. And this is indeed a very good question for which I have no answer currently. The debug glbp event and debug glbp packet shows that AVFs get assigned some kind of priority that does not seem to correspond either to the configurable GLBP priority (used to elect the AVG) or the weight (used in load balancing and optionally in AVF preemption), and is thus not to be directly influenced. This priority appears to be either 167 for primary AVFs (the AVFs that have originally been assigned their vMAC by the AVG) or 135 for secondary AVFs (the AVFs that have taken on additional vMAC for a former AVF that has since failed). However, why these numbers are 167 and 135, whether they can be influenced by configuration and how these numbers are originally assigned is a mystery to me.

I wish that a GLBP developer read this thread and provided his/her valuable insight!

Best regards,

Peter

[Hemant Kumar]

Thanks Peter. I hope I will get my answer soon.

[FlorianCokl]

Hi Hemant Kumar,

I've tried it with a simulation and used a sniffer to verify.

Enable preemption and set priorities like

• R1 priority 104
• R2 priority 103
• R3 priority 102
• R4 priority 101
• R5 default 100

If you don't assign priority then the highest IP for the segment wins the AVG election. In this case R1 wins AVG election. However, enable preemption to have it deterministic instead of who comes online first, dies first, last .......

When it comes to AVF - it follows these same priorities i.e.

R1 will become active forwarder 1 (the first virtual MAC-address) - also the AVG

R2 will become active forwarder 2 (the second virtual MAC-address)

R3 will become active forwarder 3 (the third virtual MAC-address)

R4 will become active forwarder 4 (the fourth virtual MAC-address)

The sniffer told me that the AVG priorities are exactly as I've set them via CLI - AVF priority in the hello from R1 through R4 is 167 - except for R5 since is listening only for all 4 virtual forwarder addresses - wherever that comes from? I haven't found a way to change the AVF priority.

Although there is a command glbp 1 forwarder preempt which supposingly enables preemption i.e. overthrow the current forwarder, I hadn't configured it - yet - the forwarder gets overthrown anyway after 30 seconds?

Kind Regards