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Myles Bradshaw is a portfolio manager with PIMCO.

The risk of one or more countries leaving the euro - or indeed the euro breaking up into national currencies or new, smaller currency blocks - is no longer something to casually dismiss. The volume of PIMCO client questions on this topic attests to this.

The possibility is also no longer politically unthinkable. French President Nicolas Sarkozy and German Chancellor Angela Merkel contemplated the previously unimaginable last November when they said: "The question is whether Greece remains in the eurozone."

So even if the euro survives this crisis intact, scenario planning is indispensable for investors. Indeed, even if no country actually leaves the euro, investors need to think about the implications, because the market will price in this uncertainty as the euro debt crisis evolves.

As 19th century British Prime Minister Benjamin Disraeli famously said: "I am prepared for the worst but hope for the best."

Here's hoping for the best. But we also want to offer investors a framework for thinking about - and positioning for - "the worst."

First, we must think about how a country may leave the eurozone and therefore be ready to reassess the probability of that risk if some of the anticipated signposts begin to materialize.

Second, by thinking about the economic and market spillovers, we can reassess the correlation of other assets to a euro breakup. And third, by thinking about the implications of possible policy responses, we can be prepared to take advantage of opportunities and work to avoid the negative costs that may arise as a consequence.

Remember, regardless of what happens, it's likely that rising euro-exit risk will be reflected in higher credit premiums for a wider variety of assets, including many eurozone sovereign bonds.

This can be countered by reducing credit risk directly or by owning more nominally "risk-free" assets. But establishing what is a risk-free eurozone asset is far from straightforward. For example, the consequences of a credible eurozone policy response mean the credit quality of German bunds could change for the worse.

Within the eurozone, we believe investors should instead look at alternatives to the government sector, including agency, regional government and covered bonds. These have credit characteristics similar to government bonds but typically offer higher yields.

In addition, we think investors should consider increasing the duration risk in other developed government markets and reduce exposure to the euro itself. Finally, faced with such uncertainty, it makes sense to increase and diversify holdings of cash or near-cash instruments.