How to choose metric while summarizing

[mirror512]

I am prepairing for CCNP . and i have seen many questions where the question asks for if i have three routes to summarize then which metirc is chosen  lowest or highest.

IN some question i have seen the highest metric is chosen and in some question lowest metric is chosen.

What should i do

[Peter Paluch]

Hello,

In these cases of doubt, the most reliable way of finding out is to simply test it yourself. Or ask here on CSC

I can confirm that in IOS, the summary route retakes the lowest metric from among the summarized routes. I have tested it on dynamips for RIP, IS-IS, OSPF and EIGRP. In this aspect, the IOS behavior does not always align to RFCs - in particular, RFC 2328 for OSPF requires that when performing summarization, the summary route shall take the highest metric, not the lowest - but with summarization, doing either is equally bad.

So to sum it up - it is the metric of the lowest component route that is being retaken by the summary route.

Best regards,

Peter