[AusNOG] GLBP Forwarder Weighting Question

[Nathan Nogic]

Hi guys,

 

Breaking with recent tradition, I thought I'd throw a network question out
to the group to see if anyone has had a similar experience or can point me
at some documentation other than the cisco articles on how GLBP AVFs should
work.

 

The long and short of it is that the AVFs seem to be ignoring the weighting
when deciding which connection to send outbound traffic through. I know we
could drop one of the AVFs by decrementing its value below the lower
threshold but that would mean removing some of the load balancing and
availability options.

 

The setup is a GLBP group of two routers each with their own IP transit
connection. One router & link has a weighting of 10 and the other
router/link has a weighting of 200 with the load balancing methodology set
to weighted. In theory, the router with the weighting of 200 should be
taking 95 odd % of the traffic for that subnet.

 

The config is as follows:

 

*         Downstream devices are configured to hit the VIP not the
individual router IPs

*         The ARP tables on the router with the weighting of 200 shows that
it is picking up most of the IP addresses in the subnet as expected as it is
also the AVG. 

*         The router with the weighting of 200 is shown as the Active AVF
and the router with the weighting of 10 is in listen state (aware that a
listen AVF will still forward traffic)

*         The router with the weighting of 10 has an ARP table that does not
show any IPs from the subnet

*         Forwarder pre-emption is enabled

*         Bouncing the router with the weighting of 10 has the traffic
redirect through the other link, but once the AVF comes up (even in listen
state) it then starts transmitting traffic again.

*         Netflow reporting shows that the router with the weighting of 10
is transmitting a lot of data that should be going through the other router.

 

My question is whether it's just luck that the small amount of IP addresses
allocated to the AVF with the weighting of 10 happen to have noticeable
traffic or if there is some other behaviour that would explain why an AVF
that should be getting virtually no traffic seems to be sending out a lot of
traffic for that subnet?

 

The other working theory is that it's ARP / client device affinity rather
than a GLBP issue.

 

Happy to get thoughts by direct email rather than to the entire list.

 

Thanks


Nathan

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