Q1: I have read that in layer 2, if any of the links of a portchannel goes down,the cost of the portchannel is recalculated. Does that apply to L3 portchannel also?
If so, assume I am running OSPF in a port channel environment, and one of the links of the portchannel goes down, will the SPF be recomputed?
Q2: Why at all the cost recalculation take place in layer 2 portchannel when the link goes down? I mean, STP treats the entire port channel as a single link, so why the reconvergence happens?
You didn't tag this with a vendor, but you use the Cisco term, "portchannel," so I assume you mean this as a Cisco question.
Q1: I have read that in layer 2, if any of the links of a portchannel goes down,the cost of the portchannel is recalculated. Does that apply to L3 portchannel also?
Yes, the link gets a new layer-3 cost if the routing protocol dynamically calculates this. For instance RIP would not change the metric, but OSPF would change the metric.
If so, assume I am running OSPF in a port channel environment, and one of the links of the portchannel goes down, will the SPF be recomputed?
Yes, OSPF will change the metric and the routers in an area would all rerun the SPF algorithm.
Q2: Why at all the cost recalculation take place in layer 2 portchannel when the link goes down? I mean, STP treats the entire port channel as a single link, so why the reconvergence happens?
It does this because that link may no longer be the preferred link; the bandwidth of the link has changed.
