Saturday, January 3, 2009

Understanding EIGRP from the bottom up Part I I

EIGRP Metrics:
Routers 'discuss' their topology tables. So unlike other protocols, EIGRP has a table with all the routes to a destination.

show ip eigrp topology all-links
!--is the command that shows this table - all links is optional
This topology table is the one holding all the info to make decisions, the outcome is usually a distance and vector to each destination.

Minimum bandwidth and the total delay is the information used to compute the metric. The values are automatically picked from configured values on your interfaces. Generally bandwidth will be more critical for lower bandwidth interfaces while delay is more key where high speed interfaces are used.

Formula to get :
the bandwidth:
bandwidth = 10 000 000 / bandwidth * 256
The delay
delay = delay * 256

The values got for bandwidth and delay are then used in the metric computation. I won't go into detail, please check this cisco link or read a book.

the default behaviour is to just calculate as follows:
metric = bandwidth + delay

(remember to round off your figures after each calculation, I can't remember why but i think floating point math on cisco was the reason)

so if you have the following scenario:

bw 56k
/Delay 2000---Router4---| |
/ | |
Router1== |---------Router2------|
\ |bw10000 |
\__bw 128 |delay100 |Destination Network
Delay 1000-Router3---| |bw 10000
|delay 100

metric = bandwidth + delay
minimum bandwidth = 56k
Total Delay = 100 + 100 + 100 + 2000
= 2200

[(10 000 000/56) + 2200] x 256 = (178571 +2200) x 256
= 180771 x 256
= 46277376

10 000 000/56 is actually = 178571.42857142857142857142857143 but we round it off...
If you calculate the metric through router 3, you find the metric is 20307200

so router one will use the path via Router3 to get to the destination network.
  • The bandwidth is calculated from the configured interface through which the desired network is visible ie where the update is coming in through.
  • The delay is cumulative; ie each router adds a delay and sends it backwards...Im not sure how to explain this if you can't see it....just count the delay from the destination backwards

Feasible Distance (FD)
  • This is the best metric/best path to the destination network - includes the metric to the neighbor advertising the network - from our diagram thats Router2.
reported Distance (RD)
  • This is the total metric as advertised by an upstream router/neighbor. from the ascii diagram above, that will be the distance advertised by Router4 or Router3 (obviously one is going to be used as a FD
feasible Successor (FS)
  • This is the path whose reported distance is less than the feasible distance. This is usually installed in the topology table as a backup.
The reported distance is always calculated from the router advertising the route to the network. so the reported distance from router4 is the metric to get to the destination network from Router4 and the same goes from the reported network from Router3.

If the link between Router1 and Router3 goes down, the convergence is almost instant, users will probably not even notice it since the feasible successor/backup route is immediately picked.

Checking loops:
The FD,FS and RD concepts are used to breack loops using the logic that the reported distance CANNOT be higher than the feasible distance. A route/path with a higher Rd than the FD won't show up on the topology table.

Other loop 'taking care of mechanisms'

Split Horizon - A route WILL NEVER be advertised through the interface it was learnt from.

Poison reverse - after learning of a route through a certain interface, any advertisements back the same interface are sent us unreachable for the said route/network.

  • If a FD sends an update, queries regarding the same network are not sent to it.
  • Stuck in active (SIA) occurs if a query takes too lon gto be answered by a neighbor....
Redistribution: Ahh this is when I started having fun....I think I'll deal/write about redistribution after going throroughly through OSPF

now lets see if my trusty old motor bike survived the holidays:-).......happy new year....

Want to read some more.....go here

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