Cost of summary route on ABR

wave · February 2012

Hi guys,

Just wanted to double-check. When routes that have different costs are summarized on an ABR the smallest cost of out of the routes being summarized is used as the summary route's metric, correct? ...and that cost/metric is included in the Type 3 LSA right?

Thanks.

wave · February 2012

I think I've stumbled upon the answer to part of my question: OSPF Design Guide - Cisco Systems

"Note about summary address metric calculation: RFC 1583

called for calculating the metric for summary routes based on the minimum metric of the component paths available.

RFC 2178

(now obsoleted by RFC 2328

) changed the specified method for calculating metrics for summary routes so the component of the summary with the maximum (or largest) cost would determine the cost of the summary.

Prior to IOS 12.0, Cisco was compliant with the then-current RFC 1583

. As of IOS 12.0, Cisco changed the behavior of OSPF to be compliant with the new standard, RFC 2328

. This situation created the possibility of sub-optimal routing if all of the ABRs in an area were not upgraded to the new code at the same time. In order to address this potential problem, a command has been added to the OSPF configuration of Cisco IOS that allows you to selectively disable compatibility with RFC 2328

. The new configuration command is under router ospf, and has the following syntax:"

[no] compatible rfc1583

The default setting is compatible with RFC 1583

. This command is available in the following versions of IOS:

12.1(03)DC
12.1(03)DB
12.001(001.003) - 12.1 Mainline
12.1(01.03)T - 12.1 T-Train
12.000(010.004) - 12.0 Mainline
12.1(01.03)E - 12.1 E-Train
12.1(01.03)EC
12.0(10.05)W05(18.00.10)
12.0(10.05)SC

MrBrian · February 2012

Correct. The ABR will create a type 3 summary LSA. By default it will use the best metric of the component subnets.. and those more specific subnets will be suppressed.

Cost of summary route on ABR

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