Cisco Support Community
cancel
Showing results for 
Search instead for 
Did you mean: 
Announcements

Welcome to Cisco Support Community. We would love to have your feedback.

For an introduction to the new site, click here. If you'd prefer to explore, try our test area to get started. And see here for current known issues.

New Member

Loadsharing with glbp

Hi,

Attached is a diagram, would it be fine if glbp is run across the devices R1 & R2 at location B.R1 & R2 are 2 different service providers link which end up on the L3 switch.Would this work for outwards loadbalancing.All hosts are behind the switch?

Kindly suggest the problems with this setup also ..as it seems glbp has issues with address resolution.

Thanks.

1 ACCEPTED SOLUTION

Accepted Solutions
Super Bronze

Re: Loadsharing with glbp

Victor, you're tecnically correct, since GLBP replies to host ARPs with its virtual gateway MACs, just one host MAC would not preclude load balancing.

For instance, if our single host was dealing with GLBP configured for round-robin load balancing and it ARPed before sending each IP packet, we would effectively have achieved load balancing (similar to CEF's packet-by-packet).

Practically, though, since hosts don't usually ARP for every IP packet, we're not going to see concurrent load balancing when there is only one host MAC.

Also from a practical level, there's reasons why IP doesn't normally ARP per packet. Further, although IP itself doesn't require flow packet sequencing, it's something we try to maintain.

So, for practical reason, there's a problem when GLBP only "sees" one host MAC although you're correct that it's more to do with GLBP only seeing ARPs from the one host. Because, in a round-robin GLBP configuration, when the host's ARP time-outs, GLBP will move the host to the other gateway, but during the ARP cache time period, only one GLBP gateway will be used.

Since GLBP does respond to ARPs, all flows from one host will go to the same gateway. Further, ARP times can run into minutes. Given these facts, many routing protocols might effectively load-balance better than GLBP. Given the L3 switch, in this instance, this is one reason why I recommended routing vs. using GLBP.

17 REPLIES
New Member

Re: Loadsharing with glbp

Figure attached..missed out.

Blue

Re: Loadsharing with glbp

You can do this, yes.

Your local hosts would point to the GLBP VIP that both R1 and R2 share.

The problem arises when one of the routers loses its connection to the service provider network. You need to leverage the weighting and object tracking features to make sure that a router is no longer advertised as an AVF once it loses its connection to the WAN.

http://www.cisco.com/en/US/docs/ios/12_2t/12_2t15/feature/guide/ft_glbp.html#wp1046944

HTH

Victor

Super Bronze

Re: Loadsharing with glbp

Are R1 and R2 visible as gateways to LAN hosts, or is the L3 switch routing between them and the LAN?

If the L2 switch is only supporting L2 to R1 and R2, GLBP should work, but if you have a L3 switch, it would likely work better to route from it to both R1 and R2.

Blue

Re: Loadsharing with glbp

Good catch, Joseph.

I looked at the drawing too quickly and didnt realize that it was a L3 switch between the LAN and the WAN routers.

Rated.

Victor

New Member

Re: Loadsharing with glbp

Ok...L3 is between the hosts & both routers..L3 is planned to act as g.w for hosts & this switch will den point to vip of glbp for both routers...or simply the interfaces of L3 getting the upstream routers links would be enabled with ospf to achieve outbound loadsharing.Which one should be better ..merits & demerits?

also will glbp have arp issues in this setup?

Thanks.

Super Bronze

Re: Loadsharing with glbp

You do not want to use GLBP between routers (and/or L3 switches running L3). Reason being, GLBP will usually only "see" one source MAC (i.e. there would be no load balancing).

You can use MHSRP and static routing, or dynamic routing. Either works, which to choose depends on other factors. (Normally, I learn toward dynamic routing.)

New Member

Re: Loadsharing with glbp

You mean the mac seen will be only the one from the switch until the timeout occurs..so it wont loadshare with this setup?Pls throw some more light to get me understand this.

& how if i enable ospf on 2 interfaces of L3 as routed ones and so will it loadshare?

Thanks.

Super Bronze

Re: Loadsharing with glbp

If you statically route from an L3 switch to GLBP addresses, GLBP will only see the source MAC for the L3 switch. I.e. as far as GLBP is concerned, all flows are from one host, so all traffic will use just one gateway.

OSPF load share should (usually) be automatic once the L3 switch sets up an OSPF adjacency with both routers assuming cost to destination is the same via both routers.

You configure OSPF to match the network or networks shared between the L3 switch and the routers. Depending on the L3 switch, the L3 switch ports that connect to the routers might be routed ports or switch VLAN ports. If VLAN ports, they could be VLANs unique to each router, used by both routers, used by both routers and other hosts, or other mixture of hosts and routers.

PS:

You might want to avoid having other hosts share your external router subnets, but if they do, you'll also want to avoid the L3 switch allowing redirects.

New Member

Re: Loadsharing with glbp

That educated me!so as the switch would arp here..it wont be ideally useful.

So what scenario does glbp fit nicely in doing its task correctly..

Thanks.

Blue

Re: Loadsharing with glbp

GLBP is typically used as a first-hop router redundancy protocol, like VRRP or HSRP, only GLBP leverages load balancing to take advantage of the available bandwidth.

In a LAN environment, there will be many different users who will ARP for the default gateway's MAC address. The AVG (Active Virtual Gateway) router that controls the forwarding group will return to each user one of the AVF (Active Virtual Forwarder) MAC addresses. Hence, the load balancing.

HTH

Victor

Hall of Fame Super Blue

Re: Loadsharing with glbp

"So what scenario does glbp fit nicely in doing its task correctly"

Lets say you have an access-layer switch (AS1) that is connected to 2 distribution switches (DS1 & DS2).

The inter-vlan routing for the clients connected to AS1 is handled by DS1 & DS2. AS1 is connected to DS1 with a L2 trunk and AS1 is connected to DS2 with a L2 trunk.

DS1 & DS2 are connected together via a L3 connection. Because there is no L2 loop AS1 can use both connections to forward traffic from clients ie. either AS1 -> DS1 or AS1 -> DS2.

GLBP works well here as some clients will use one link because DS1 is their default-gateway and other clients will use the other link because DS2 is their default-gateway.

Note that if the link between DS1 & DS2 is a L2 trunk GLBP still works but not as well because one of the uplinks from AS1 will blocked so even though the clients would still use both DS1 & DS2 as default-gateways all traffic would go up one link and then some would have to go across the DS1 -> DS2 link.

You could manipulate the STP costs so the L2 trunk between DS1 & DS2 is blocking but thsi may affect other traffic flows in DS1/DS2.

Jon

Blue

Re: Loadsharing with glbp

Jon:

Good morning/afternoon ( for you):

Not to be nit-picky, but just to be crystal clear for the OPs sake. When you say:

"GLBP works well here as some clients will use one link because DS1 is their default-gateway and other clients will use the other link because DS2 is their default-gateway."

It may be worth mentioning that all hosts will have the same default gateway IP address configured, and that will be the VIP for the GLBP group. What will be different in each host is the IP-to-MAC mapping. The AVG will assign each user an AVF in the group to use to forward their traffic to.

I am 110% confident than you know this, it is just for the OP's sake that I am getting anal. I thought it was worth the effort...

Thanks

Victor

Hall of Fame Super Blue

Re: Loadsharing with glbp

Victor

Morning for you.

"It may be worth mentioning that all hosts will have the same default gateway IP address configured, and that will be the VIP for the GLBP group"

Not nit-picky at all my friend. Well worth pointing out as the other way of load-balancing ie. MHSRP does indeed use different default-gateways on the clients.

Jon

Blue

Re: Loadsharing with glbp

Jon, I really like this explanation because of the way you tied GLBP with STP. I think you made an important contribution to the discussion that was missing.

Rated.

Victor

Blue

Re: Loadsharing with glbp

"If you statically route from an L3 switch to GLBP addresses, GLBP will only see the source MAC for the L3 switch."

Joseph:

Your point is well taken, but the inability to load balance in this case is not so much a function of GLBP "seeing" one address more than it is a result of the fact that, since there is only one host, it will not continuously send out new ARP requests for the gateway. It will do it once and cache it.

What GLBP "sees" are new ARP requests when they come in and it is that they will not be seeing if there is only one host on the LAN.

HTH

Victor

Super Bronze

Re: Loadsharing with glbp

Victor, you're tecnically correct, since GLBP replies to host ARPs with its virtual gateway MACs, just one host MAC would not preclude load balancing.

For instance, if our single host was dealing with GLBP configured for round-robin load balancing and it ARPed before sending each IP packet, we would effectively have achieved load balancing (similar to CEF's packet-by-packet).

Practically, though, since hosts don't usually ARP for every IP packet, we're not going to see concurrent load balancing when there is only one host MAC.

Also from a practical level, there's reasons why IP doesn't normally ARP per packet. Further, although IP itself doesn't require flow packet sequencing, it's something we try to maintain.

So, for practical reason, there's a problem when GLBP only "sees" one host MAC although you're correct that it's more to do with GLBP only seeing ARPs from the one host. Because, in a round-robin GLBP configuration, when the host's ARP time-outs, GLBP will move the host to the other gateway, but during the ARP cache time period, only one GLBP gateway will be used.

Since GLBP does respond to ARPs, all flows from one host will go to the same gateway. Further, ARP times can run into minutes. Given these facts, many routing protocols might effectively load-balance better than GLBP. Given the L3 switch, in this instance, this is one reason why I recommended routing vs. using GLBP.

Blue

Re: Loadsharing with glbp

Joseph:

I agree with what you have said in all your previous posts, including the one in which I made a clarification.

I was just splitting hairs, I guess, and trying to expound on one specific concept regarding the reason why only one host will not result in leveraging GLBP's load balancing function.

Thanks

Victor

225
Views
26
Helpful
17
Replies