ICND2 200-105

ICND2 200-105

Gateway Load Balancing Protocol

In the previous lesson we showed you a load balancing mechanism and I'll still argue that that's the best way to go. Gateway Load Balancing Protocol - GLBP is the third and final First Hop Redundancy Protocol - FHRP. First Hop Redundancy Protocol, that's a gateway protocol. GLBP is far more complicated. It's mission in life though is to dynamically distribute the load within a group. So what we showed you is a per group load balancing. You could only get any kind of load balancing if you add multiple groups. Gateway Load Balancing Protocol on the other hand, allows you to load balance within one group. How does this happen? Well we still have a virtual MAC, short for Media Access Control address, a virtual IP address, and we point our clients to that. But there is far more intelligence baked into this technology, where when a client ARPs, short for Address Resolution Protocol, for it's default gateway, one client could get one response, a different client would get a different response, because all of the routers are forwarding. We call them active virtual forwarders, and there is a virtual MAC address that is tied to each of them. And there is a router that is responsible to say, hey here is your virtual MAC address, next one, here's a different virtual MAC address.

GLBP Group

You still have fail over by the way, you still have fail over, but we get load balancing baked into this technology. I'll tell you the reality though and the reality is this. We're still far more likely to use HSRP, short for Hot Standby Router Protocol, and here is why? HSRP is going to give us what we need, and that's predictability. GLBP is a workable technology. It's the most advanced, but it can generate some traffic flow that is downright undesirable. Because what can happen is traffic can go up to the distribution layer and then get forwarded across the distribution layer link to the other router. And it could have done this because spanning tree led us one way and we need to go another. It seems pretty benign, right? Well for large volumes of traffic, it results in some pretty inefficient and potentially problematic traffic flow. Nevertheless, GLBP is a very nice technology on paper but I would encourage you, regardless of what your skill level is, to use HSRP.

Here is a sample output, similar to what we saw for HSRP.

R1(config)#interface ethernet 0/1
R1(config-if)#ip address
R1(config-if)#glbp 1 ip

R2(config)#interface ethernet 0/1
R2(config-if)#ip address
R2(config-if)#glbp 1 ip

R1#show glbp
Ethernet0/0 - Group 1
  State is Active
    1 state change, last state change 00:03:08
  Virtual IP address is
  Hello time 3 sec, hold time 10 sec
    Next hello sent in 2.080 secs
  Redirect time 600 sec, forwarder timeout 14400 sec
  Preemption enabled, min delay 0 sec
  Active is local
  Standby is, priority 100 (expires in 8.096 sec)
  Priority 100 (default)
  Weighting 100 (default 100), thresholds: lower 1, upper 100
  Load balancing: round-robin
Group members:
    aabb.cc00.0300 ( local
    aabb.cc00.0400 (
  There are 2 forwarders (1 active)
Forwarder 1
    State is Active
      1 state change, last state change 00:02:57
    MAC address is 0007.b400.0101 (default)
    Owner ID is aabb.cc00.0300
    Redirection enabled
    Preemption enabled, min delay 30 sec
    Active is local, weighting 100
    Client selection count: 1
Forwarder 2
    State is Listen
    MAC address is 0007.b400.0102 (learnt)
    Owner ID is aabb.cc00.0400
    Redirection enabled, 598.112 sec remaining (maximum 600 sec)
    Time to live: 14398.112 sec (maximum 14400 sec)
    Preemption enabled, min delay 30 sec
    Active is (primary), weighting 100 (expires in 10.080 sec)

Now by the way, any time HSRP comes up in the IOS it's going to say standby. Why? Because they didn't have the foresight to call it HSRP. So the configuration commands are standby, the show commands are standby and that's really annoying. Here instead of standby, we see GLBP. Back in the day when HSRP was created, they only had one. So they didn't actually bake that into the names of all the commands. I've digressed. OK. So we can see we're active, virtual IP address but the reality is there is far more to it than what we can even get into in this discussion. So I would say, keep this at a high level for now and if you have a choice, HSRP is your best answer for First Hop Redundancy Protocol - FHRP needs.