Using link congestion to control routing updates

Fri Dec 20 15:21:39 UTC 2002

Finally, I found it.  If you diddle the K values for EIGRP, you can make
it consider reliability, load, and delay statistics when populating a
route to the route table.  The default behavior is bandwidth and delay.

---Quote---http://www.cisco.com/warp/customer/103/eigrp1.html 
EIGRP uses these scaled values to determine the total metric to the
network: 

metric = [K1 * bandwidth + (K2 * bandwidth) / (256 - load) + K3 * delay]
* [K5 / (reliability + K4)] 
Note: These K values should be used after careful planning. Mismatched K
values prevent a neighbor relationship from being built, which can cause
your network to fail to converge. 
The default values for K are: 

K1 = 1
K2 = 0
K3 = 1
K4 = 0
K5 = 0

---Quote---http://www.cisco.com/en/US/customer/products/sw/iosswrel/ps18
28/products_command_reference_chapter09186a00800ca5a9.html#xtocid14

COMMAND SUMMARY.
metric weights (Enhanced IGRP)
To allow the tuning of the IGRP or Enhanced IGRP metric calculations,
use the metric weights router configuration command. To reset the values
to their defaults, use the no form of this command.

metric weights tos k1 k2 k3 k4 k5

no metric weights

Syntax Description

tos
 Type of service. Currently, it must always be zero.

k1-k5
 Constants that convert an IGRP or enhanced IGRP metric vector into a
scalar quantity.

---End Quotes---

Note: If you are going to do this and you also work with BGP, you need
to take measures to prevent route-flapping.  One option would be to
redistribute from EIGRP to BGP and use Summary-address / aggregate-only
/ next hop self to force only the summary route to be propagated and
sustained in the event of an IGP route change.  

Here's how that works.

Internet---EdgeRouter---InternalRouters.
Prefix is 192.168.0.0/16

Edgerouter is configured with Summary-address / aggregate-only / next
hop selffor the whole 192.168.0.0/16 netblock.  As long as it has any
route in that netblock then the summary route will be advertised with
edgerouter as the next-hop.  Any Changes in the IGP topology will not be
echoed externally. 

Remember the Carpenters rule, Measure twice and cut once.  For us that's
test twice and then deploy.

-Ejay

-----Original Message-----
From: Anders Lowinger [mailto:anders.lowinger at xelerated.com] 
Sent: Friday, December 20, 2002 7:58 AM
To: Stephen Sprunk
Cc: Ejay Hire; David Scott Olverson; nanog at merit.edu
Subject: Re: Using link congestion to control routing updates

Stephen Sprunk wrote:
> Opposite problem -- he wants to delay routing updates if the link is
full.
> EIGRP by default won't use more than 25/50% (I forget) of link bw, for
> instance, but I'm not aware of any intentional features in other IGPs
to do
> this.

Both OSPF and ISIS in Cisco's have pacing, ie they will not flood
LSAs/LSPs
over a certain configurable limit. Not sure about other platforms..

-A