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GLBP weight options

Posted on 2010-01-12
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Last Modified: 2012-05-08
Hey guys, I need to configure two cisco routers that currently have GLBP configured on them as I have pasted below to stop forwarding packets to the other router unless one of the trackers goes down. How can I do this? My understanding of weights option is that it forwards packets to other routers in the GLBP group though I dont know if its forwarding enough packets to share the traffic equally for both routers or what. What exactly does "glbp 1 weighting 100 lower 95" do exactly and how can I configure it only to forward traffic to the other members of the group ONLY if the ip SLA's are not met?



Router 1

 interface Vlan1
 ip address 10.183.0.121 255.255.255.128
 ip nat inside
 ip virtual-reassembly
 ip route-cache flow
 glbp 1 ip 10.183.0.126
 glbp 1 preempt delay minimum 30
 glbp 1 weighting 100 lower 95
 glbp 1 load-balancing host-dependent
 glbp 1 weighting track 3 decrement 10
 glbp 1 forwarder preempt delay minimum 0


track 1 rtr 1 reachability

track 2 rtr 2 reachability

track 3 list boolean or
 object 1
 object 2



ip sla 1
 icmp-echo 4.2.2.2 source-interface Vlan1
 frequency 10
ip sla schedule 1 life forever start-time now
ip sla 2
 icmp-echo 4.2.2.3 source-interface Vlan1
 frequency 10
ip sla schedule 2 life forever start-time now


Router 2

Interface vlan1
 interface Vlan1
 ip address 10.150.7.122 255.255.255.128
 ip nat inside
 ip virtual-reassembly
 ip route-cache flow
 glbp 1 ip 10.183.0.126
 glbp 1 priority 95
 glbp 1 preempt delay minimum 30
 glbp 1 weighting 100 lower 95
 glbp 1 load-balancing host-dependent
 glbp 1 weighting track 3 decrement 10
 glbp 1 forwarder preempt delay minimum 0



track 1 rtr 1 reachability

track 2 rtr 2 reachability

track 3 list boolean or
 object 1
 object 2

ip sla 1
 icmp-echo 4.2.2.2 source-interface Vlan1
 frequency 10
ip sla schedule 1 life forever start-time now
ip sla 2
 icmp-echo 4.2.2.3
 frequency 10
ip sla schedule 2 life forever start-time now
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Question by:Network_Padawan
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7 Comments
 
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Expert Comment

by:Don Johnston
ID: 26303169
>I need to configure two cisco routers that currently have GLBP configured on them as I have pasted below to stop forwarding packets to the other router unless one of the trackers goes down. How can I do this?

SLA and GLBP are two different things.

SLA is used to remove a route from the routing table while GLBP provides next hop redundancy.

I think some confusion is occurring since both protocol offer "tracking". But they have two different purposes.

The only thing you can track with GLBP is whether an interface is UP or DOWN. If a path is unusable yet the interface is still up, the host using that router will still forward packets to that router (the router will then forward the packets to a router that still has a route to the destination).

>My understanding of weights option is that it forwards packets to other routers in the GLBP group though I dont know if its forwarding enough packets to share the traffic equally for both routers or what.

The weighting is used to determine IF a router will be an AVF (Active Virtual Forwarder). It is also used to determine how many hosts get that router as it's next hop address. In default configs, the AVF's are designated in a round-robin fashion. If you have two routers then every other host will get router A and the rest get router B. By tweaking the weights, you can get two hosts using router A for every one host that get router B.

A point to note: There is no "load balancing" here. If all the hosts that get router A as their next hop generate 98% of the outbound traffic then 98% of the outbound traffic will be forwarded by router A.

>What exactly does "glbp 1 weighting 100 lower 95" do exactly

It sets the weight to 100 with a minimum value.

>how can I configure it only to forward traffic to the other members of the group ONLY if the ip SLA's are not met?

You can't. SLA and GLBP are not... connected (or related). There's no way for SLA to decrement the GLBP weighting.
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Accepted Solution

by:
mikebernhardt earned 2000 total points
ID: 26305877
Not true, Don. SLA tracking can be used in HSRP and GLBP to decrement priority or weight based on SLA results. It's in his existing config: glbp 1 weighting track 3 decrement 10

So, what your config is doing is, the router is configured to a weight of 100. If the weight drops below 95, it will no longer forward act as a gateway for that LAN. The object tracking will decrement the weight value by 10, so that you put it at 90 and the router will stop forwarding traffic out.

You have specified host-dependent as the load balancing protocol. If you want the best load-sharing, use round-robin which is the default.

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LVL 28

Expert Comment

by:mikebernhardt
ID: 26305908
Everything you could want to know about GLBP is here:
http://www.cisco.com/en/US/docs/ios/12_2t/12_2t15/feature/guide/ft_glbp.html
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LVL 28

Expert Comment

by:mikebernhardt
ID: 26305956
Oh, another clarification for you: GLBP does not cause a router to forward traffic to another one. It cause a router to respond or not respond to ARPs for the default gateway, and to receive traffic for a MAC address. So I can't tell you whether your object tracking is correct. I can tell you that if either of those pings fails, the GLBP weight will drop below the threshold of 95 and the other router will pick up its MAC address, thereby taking over it's gateway function on that LAN.
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Author Comment

by:Network_Padawan
ID: 26309516
That's fantastic thansk Mike.

>>>You have specified host-dependent as the load balancing protocol. If you want the best load-sharing, use round-robin which is the default.

We tried round-robin and found problems with this.

Quote from Cisco:

Host dependent load balancing will need to be used when using stateful Network Address Translation (NAT) because it requires each host to be returned the same virtual MAC address each time it sends an ARP request for the virtual IP address
 
What this means is that Host-dependent ensures a host uses one and only one gateway for its traffic. In case of round-robin/weighted, a host sends some packets through 1st gateway and some packets of the same session to the 2nd gateway and this doesnt work well with NAT.
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LVL 28

Expert Comment

by:mikebernhardt
ID: 26316288
That makes sense. So for load balancing, you will have to just have to hope that they balance out evenly.
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Author Closing Comment

by:Network_Padawan
ID: 31676426
Perfect. Thanks.
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