Alcatel Unleashed

Hi, during my recent NRS 1 exam (which I passed), there was a question I wasn't sure how to answer or work out. I cannot remember the exact wording of the question but it went something like this:

Listing 8.1 Routing table output
A:PE1# show router route-table
=========================================================================
Route Table (Router: Base)
=========================================================================
Dest Prefix Type Proto Age Pref
Next Hop[Interface Name] Metric
-------------------------------------------------------------------------
10.1.2.0/24 Local Local 03d23h08m 0
to-p2r1 0
10.1.3.0/24 Local Local 03d23h08m 0
to-p3r1 0
10.1.4.0/24 Local Local 04d00h34m 0
to-p4r1 0
10.2.3.0/24 Remote OSPF 00h41m00s 10
2000
10.1.5.0/24 Local Local 04d00h34m 0
to-p4r1 0

Which egress interface would a packet be sent out of with an ip address
of 10....... (no mask or / included, just an address)

How do you actually work that out? The address i was given seemed to be a host address that could fit into any of the network
addresses in the table hence I was flummoxed? Also, in the example table there were 2 same egress interfaces. Any help trying to
break this down, how you come to the conclusion of where it should be egressing would be appreciated.

Which egress interface would a packet be sent out of with an ip address
of 10....... (no mask or / included, just an address)

may be the question talk about the IP longest prefix match. so what subnet is more specifically with your given IP, then subnet with correlation the egress interface will forwarding the packet to the host IP.

yes, I think you are right however, I don't fully understand longest prefix match...can you explain?

it would be sent out to destination

If you don't understand longest match then you should revisit the topic, this is fundamental to forwarding. If you have kit or VMs then lab it up, look at traceroutes, look at router debugs and see your packets using wireshark.

How it works is the longest prefix length (highest numerically value) is the preferred path, regardless of protocol (local, OSPF, RIP whatever). If you have a question that states which route is used to reach 10.10.10.1 and your options are 10.10.10.0/24 and 10.10.10.0/31 then the /31 network is what is used as the longest match is the highest amount of bits set in the subnet mask (31 being a higher number than 24). If you have a tie between two routes, say two versions of 10.10.10.0/31, one via RIP and one via OSPF then the router will compare the route preference to choose its best path and it will use the OSPF route, as OSPF has a lower numeric preference when compared with RIP.

Check out my awesome book, it covers these topics in detail

page 241 of the Alcatel-Lucent NRS 1 study guide describes longest match.