GLBP and EIGRP

Unanswered Question
Jul 2nd, 2008

We have some large sites which have 2 x routers, both with separate WAN links.

For the most part, one of these WAN links is never used and is a waste of bandwidth.

We're looking at implementing Gateway Load Balancing Protocol (GLBP) on these sites in EMEA and just want to understand how this will work where we have EIGRP running between our LAN switches (Layer 3 cards) and the WAN routers.

Usually, EIGRP will prefer one route in/out of the site but we'd like to take advantage of the unused bandwidth on the second WAN link by configuring GLBP on the LAN interfaces of the WAN routers.

Can anyone advise how we configure EIGRP to use the Virtual address configured for GLBP on the WAN routers (Ethernet interface) so traffic is load balanced across both WAN links.?

Network would look something like this:

* Please Note: All hosts are using a HSRP default g/w on the L3 switches.

L3 switch <-----EIGRP----- Router-1 -------WAN Link-1

L3 switch <-----EIGRP----- Router-2 -------WAN Link-2

Many thanks

John

I have this problem too.
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Jon Marshall Wed, 07/02/2008 - 01:07

John

Routing protocols don't generally use the virtual IP addresses of HSRP/GLBP etc. In your scenario EIGRP will form adjacencies between the physical addresses on both the L3 switches and the WAN routers. HSRP/GLBP are really for end users machines/servers to provide redundancy.

Are your WAN links different bandwidths ?. Ideally what you want is for each L3 switch to see 2 equal cost paths via Router1 and Router2 to any destination subnet.

Is each L3 switch connected to both routers - your diagram suggests not.

So are the same destination subnets reachable via both WAN links and if so are they the same bandwidth. If not you can

1) use the variance command in EIGRP which provide unequal cost load-balancing although it is generally not considered to function that well by people who have used it.

2) Mnaipulate the bandwidth/delay characteristics of the links so each L3 switch see 2 paths to each destination.

3) Do some sort of PBR - ie. http traffic goes out router 2 and the rest out router 1.

Jon

Kevin Dorrell Wed, 07/02/2008 - 01:08

The EIGRP and the GLBP are really two seperate things. The EIGRP does not even need to know anything about the GLBP virtual address.

I am assuming there is EIGRP on your WAN links too. If not, please let me know how the routing is arranged on the WAN links.

The GLBP is just a way to get the hosts to distribute their outgoing traffic between the two routers. The EIGRP is about how the router gets from there to the WAN links.

I am assuming that the EIGRP WAN liks are equal cost. That is all well and good. If that is the case, all you need to do is to implement the GLBP on the LAN side. Some PCs will use Router-1, and Router-1 will forward their traffic on WAN-1. Other PCs will use Router-2, and Router-2 will forward their traffic on WAN-2.

In the other direction, I am assuming that everything is symetrical. That is, the central site EIGRP will have two routes to the remote LAN address: one through WAN-1 and one through WAN-2. The central site will load balance between those. Whichever router gets a packet from central site, it will forward that packet to the LAN.

Kevin Dorrell

Luxembourg

john.pepper Wed, 07/02/2008 - 01:30

Jon / Kevin,

Many thanks for both your replies and useful info.

To answer your questions...

1. All LAN switches are connected to both WAN routers (i.e. routed Vlan). Sorry, it was hard to show this in text format

2. EIGRP is only used between the LAN and Ethernet interface of the WAN routers. BGP is used on the WAN (WAN managed by Verizon).

3.Both WAN links are the same speed - normally 34 or 155mpbs

Currently, EIGRP on the L3 LAN switches tends to prefer one of the WAN routers to remote networks - with an alternative route via the other router should the preferred path fail.

What we would like to achieve is load balancing across the 2 WAN links - but at the same time avoid asymetric routing where a packet goes out via one WAN router and comes back in on the other.

As you suggest, would we be better off configuring GLBP on the LAN side and doing some adjustments to EIGRP to load balance traffic.?

Many thanks for your input.

Cheers

John

francisco_1 Wed, 07/02/2008 - 01:25

John,

with Gateway Load Balancing Protocol GLBP protects data traffic from a failed router or circuit while allowing packet load sharing between a groups of redundant routers but at the same time EIGRP will also do equal cost or unequal load balancing and gives you more flexibilty.

I would just use EIGRP between your L3 switches and your Routers. see attachment for a basic design layout how to connect your switches to routers for redundancy and just enable EIGRP on the connected interfaces. you can also play around with EIGRP metric like bandwidth and delay to take advantage of the unused bandwidth on the second WAN link and have two active paths in the routing table

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john.pepper Wed, 07/02/2008 - 01:38

Thanks for the reply Fraco. The diagram and info helps. Cheers for this.

John

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