3.3 Fundamental routing concepts

i infrequently use this command… because i’m a dumbass…

R3#sh ip route
Routing entry for
Known via “ospf 1”, distance 110, metric 65, type intra area
Last update from on Serial2/0, 00:05:24 ago
Routing Descriptor Blocks:
*, from, 00:06:09 ago, via Serial2/1
Route metric is 65, traffic share count is 1, from, 00:05:24 ago, via Serial2/0
Route metric is 65, traffic share count is 1

it tells so much about the topology…  there are two serial links to get to router 4, whose lo0 is…  the AD is the default and it is in area 0… it’s metric is 65…  why? because 1544, the default bandwidth of the serial interface translates to a cost of 64 for ospf… because the reference bandwidth 100,000 (unless you’ve changed the reference bandwidth) divided by the default bandwidth of serial 1544 (unless you’ve changed that too) is 64… or more exactly, 64.766839378… the cost of a hop (the loopback) is factored in to make it 65…

as for the rdb (routing descriptor blocks)

here’s cisco: http://www.cisco.com/en/US/tech/tk365/technologies_tech_note09186a0080094820.shtml

You can usually use the show ip route command to find equal cost routes. For example, below is the show ip route command output to a particular subnet that has multiple routes. Notice there are two routing descriptor blocks. Each block is one route. There is also an asterisk (*) next to one of the block entries. This corresponds to the active route that is used for new traffic. The term ‘new traffic’ corresponds to a single packet or an entire flow to a destination, depending on the type of switching configured.

  • For process-switching—load balancing is on a per-packet basis and the asterisk (*) points to the interface over which the next packet is sent.
  • For fast-switching—load balancing is on a per-destination basis and the asterisk (*) points to the interface over which the next destination-based flow is sent.

The position of the asterisk (*) keeps rotating among the equal cost paths each time a packet/flow is served

R3#sh ip cef
Prefix               Next Hop             Interface            no route            drop           receive              FastEthernet0/1              FastEthernet0/1           receive              Loopback1             Serial2/0