Sh GLBP command and Forwarders 1 and 2

[mahesh18]

Hi Everyone,

I have setup GLBP between 2 Routers

R1 is Active and R2 shows standby.

In GLBP they say standby router also forward packets.

R1 config

R1#sh run int ethernet 1/1

interface Ethernet1/1

ip address 192.168.7.2 255.255.255.0

glbp 30 ip 192.168.7.3

glbp 30 preempt delay minimum 60

*************************************************************************************************************************************

R2  config

interface FastEthernet1/1

ip address 192.168.7.1 255.255.255.0

glbp 30 ip 192.168.7.3

glbp 30 preempt delay minimum 120

******************************************************************************************************************************************************

When i run command sh glbp on R2

sh glbp

FastEthernet1/1 - Group 30

  State is Standby

    1 state change, last state change 00:17:28

  Virtual IP address is 192.168.7.3

  Hello time 3 sec, hold time 10 sec

    Next hello sent in 1.412 secs

  Redirect time 600 sec, forwarder time-out 14400 sec

  Preemption enabled, min delay 120 sec

  Active is 192.168.7.2, priority 100 (expires in 9.360 sec)

  Standby is local

  Priority 100 (default)

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

  Load balancing: round-robin

  Group members:??????????????????????????????????????????????????

    0009.b7e7.5611 (192.168.7.2)

    000d.bd3f.6d32 (192.168.7.1) local

  There are 2 forwarders (1 active)????????????????????????????????

  Forwarder 1

    State is Listen

    MAC address is 0007.b400.1e01 (learnt)

    Owner ID is 0009.b7e7.5611

    Time to live: 14399.356 sec (maximum 14400 sec)

    Preemption enabled, min delay 30 sec

    Active is 192.168.7.2 (primary), weighting 100 (expires in 7.020 sec)

  Forwarder 2

    State is Active

      1 state change, last state change 00:17:38

    MAC address is 0007.b400.1e02 (default)

    Owner ID is 000d.bd3f.6d32

    Preemption enabled, min delay 30 sec

    Active is local, weighting 100

I need help to understand the points below

1>Group members---Does it mean that it has 2 members in GLBP and mac address is the BIA of those interfaces.

i checked .5611 is mac address of R1 interface ethe1/1 which is active state.

So it means that in GLBP group memebers are always the mac address of router interfaces ???

2>There are 2 forwarders (1 active)

What does 2 forwarders  mean ?

Does it mean that  it has 2 members and 1 member is Active Router ?

3>How does GLBP decide which forwarder state is  active and listen???

4> Also when i telnet to Virtual IP 192.168.7.3 it always goes to Standby Router R2 need to know why this happens?

Thanks

MAhesh

[John Blakley]

Mahesh,

1>Group members---Does it mean that it has 2 members in GLBP and mac address is the BIA of those interfaces.

The group members are the routers that are in the same glbp group, in your case 30. These are the real mac addresses that are associated with the router's interfaces that have the glbp group configured on them.

2>There are 2 forwarders (1 active)

In GLBP, a router is elected the Active Virtual Gateway (AVG) and it's responsible for assigning a virtual mac address to other members in the group. All members of the group are also seen as active virtual forwarders (AVF) which is configurable.

It means there are 2 AVFs, but 1 is active for this virtual mac address that it's been assigned. If that interface were to go down, you should see the other router be responsible for both virtual mac addresses.

3>How does GLBP decide which forwarder state is  active and listen???

This is all configurable. By default, I believe it's based of IP address in how it assigns a virtual mac address to each router. The highest IP address will become a forwarder, but I also believe there can be a max of 4 AVFs to a group. I'd have to test this. Each router is responsible for the virtual mac that they've been assigned by the AVG. So, each router in the group will show as Active for thei vMAC address and Listen for the other mac addresses. The other vMACs that the router is in Listen state for can go active if it chooses to take over that role from the AVF that failed, and this is configurable with "glbp forwarder preempt" command.

4> Also when i telnet to Virtual IP 192.168.7.3 it always goes to Standby Router R2 need to know why this happens?

When your host ARPs for the mac address that belongs to this IP, the AVG is responsible for assigning you a mac address. In your arp table, you'll notice that your 192.168.7.3 is assigned to 0007.b400.xxxx. This is the mac address that the AVG assigned to you to get to that address. You may need to clear your arp table a few times to get a different mac address, but eventually you'll get one.

HTH,

John

[mahesh18]

Hi John,

Thanks for reply.

For Question No 2.

If we have 2 routers in group then what is elected as AVG  and other we can say is Backup AVG  or AVF.

So this means that in GLBP number of routers in group is equal to number of AVF???

As we can see that AVG can also act as a AVF

For Virttual mac address the format is

0007.b4xx.xxxx.

Need to know how next 6 characters chooses from??

Many thanks

MAhesh

[John Blakley]

Mahesh,

The virtual MAC is always 0007.b400.xxxx. In your case, the mac address for forwarder 2 is:

0007.b400.1e02

1e is the group number in hex. 1e = 30.

The 02 is the forwarder number. For your forwarder 1 your virtual mac is 0007.b400.1e01

For question 2, the AVG is elected by either priority or highest IP address if priorities are equal.

This is why R1 became your AVG:

Active is 192.168.7.2, priority 100 (expires in 9.360 sec)

So this means that in GLBP number of routers in group is equal to number of AVF???

By default, yes up to 4 routers. This is configurable though by changing weights and preferring 1 router over another. Here's a configuration guide:

http://www.cisco.com/en/US/docs/ios/12_2t/12_2t15/feature/guide/ft_glbp.html

[mahesh18]

Hi John,

Thanks for detailed explanation.

One last question if we have 2 routers in GLBP  does this mean that number of routers in group is equal to virtual mac address.??

Here i see that when i use 2 Routers  it has 2 Virtual MAc addresses.

Thanks

MAhesh

[John Blakley · Accepted solution]

You would have a virtual mac address for each router. This is the way that the AVG assigns the mac address to your hosts when they arp for the mac address.

Say that you have 3 hosts and 3 routers. The 3 routers use 192.168.1.1 as the virtual IP address. Suppose the that mac addresses are 0007.b400.0101, 0007.b400.0102, and 0007.b400.0103 (Group 1 with Forwarders 1,2, and 3 respectively.) You have host A, B, and C.

When host A needs to get to the internet, it will arp for it's default gateway of 192.168.1.1. The AVG will answer this request with:

192.168.1.1 0007.b400.0101

Host B does the same and gets back:

192.168.1.1 0007.b400.0102

Host C does the same as well:

192.168.1.1 0007.b400.0103

As you can see, all of them request the mac address for the same IP and they're getting back different mac addresses. Now when host C sends traffic, it will go directly to forwarder 3 in the group until its arp entry times out. When it times out or is cleared, it's quite possible that another mac address could be assigned to host C for the same ip address.

HTH,

John

[mahesh18]

Hi John,

Many thanks for explaining me in detail.

Really appreciated.

Best regards

Mahesh