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Afroz Ahmad

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Archive for June 2nd, 2011

BGP Default Route Origination

We can originate default route to specific BGP neighbor through

route bgp [AS No.]
neighbor [ip address] default-originate

Note:-By doing this the Default route is originated to the BGP peer along with the other routes in the BGP table.If you want to advertise only Default-route then do backup your configuration with a prefix-list or access-list or route-map.

>>router bgp [AS No.]
neighbor [ip address of neighbor] prefix-list ONLY_DEFAULT_ORIGINATE  out

Your Prefix-list could look like this.
>>ip prefix-list ONLY_DEFAULT_ORIGINATE seq 5 permit

Here in the above prefix-list we are only permitting Default-route by “” if we have taken “ le 32” then all the routes including Default route will advertise to the neighbor.Do watch for the granular things while doing configurations in BGP.

Important thing:-One very important thing to remember to check the routing table of the IBGP neighbor where we are advertising the Default route,whether the route that we have redistributed into IBGP neighbor is installed into routing table or not.check with the command:-
>>sh ip route | inc
if the Neighbor routing table is already learing the default route from other IGP then do deny this route by access-list,suppose we have originated earlier a default route via ospf in the routing domain , it will look like this.

R1#sh ip route | include
Gateway of last resort is X.X.X.X to network
O*E2 [110/1] via X.X.X.X, 00:33:11, FastEthernet1/12

Here we are already learing the default route from OSPF as the OSPF has lower Admin Distance(110) than IBGP(200) the default route via OSPF is installed in the routing table.
We can deny it in OSPF by.
>>router ospf 1
distribute-list PREFER_DEFAULT_VIA_BGP in

>>ip access-list standard PREFER_DEFAULT_VIA_BGP
 permit any




Today while i was doing lab i was stuck in the scenario where i have to do unequal cost load balancing in the eigrp domain.After many hours i finally got the desired result , thought of sharing this , kindly tell me if i am wrong anywhere.

One of the most important factors in choosing EIGRP as an IGP in the Network is the ability of  doing unequal cost load balancing.To enable this feature into Eigrp we need to issue “Variance” command under the Eigrp process.In order for path to be considered for unequal cost load balancing it must be a feasible successor with a metric less than or equal to the successor’s metric times the Variance.

Feasible Distance= Local Distance + Advertised Distance

A path whose advertised distance is lower than the feasible distance of the successor is deemed as feasible successor

Note:-Only routes that are feasible successors can be used for unequal cost load

The formula for Eigrp Metric calculation:-

Metric = [k1 * bandwidth + (k2 * bandwidth)/(256 – load) + k3 * delay] *[k5/(reliability + k4)]

The “k” values are derived from the metric weights command, where K1 and
K3 are 1 by default and all other values are 0. This essentially means that only
bandwidth and delay are taken into account. “Bandwidth” is the inverse
bandwidth in Kbps times 107 (107/BWKbps). “Delay” is delay in tens of microseconds (DLYusec/10). These values are added together and then scaled
by a factor of 256. The composite metric is therefore represented by default as:

Metric = (107/BWKbps + DLYusec/10) * 256

Note:-Try to use Delay instead of Bandwidth because by changing bandwidth and QoS is running in the network , it will effect the traffic drastically in the network.

In order to achieve the any desired ratio suppose we want to distribute traffic in a ratio of 3:1 among the successors and feasible successors, and we are using delay we can set our equation like BW1 and Delay1 for the one path and BW2 and Delay 2 for other path.Now the equation will look like this.

3*[107/BW1 in Kbps + Delay1 in usec/10) * 256]=(107/BW2 in Kbps + Delay2 in usec/10) * 256

Here in the above formula watchout for BW1 ,Delay1,BW2 and Delay2.

By taking BW1 and BW2 as it is from the interface , we can find the end result something like :- Delay1=something times Delay2 ,now we can put the value of Delay 2 to find out Delay 1.

Lastly we can apply this Delay to the interface and check and verify our configuration by :-

sh ip route [destination network address] | include frame|share

Check the ratio in the traffic share count to see the exact result between both the paths.Also dont forget to include the Variance command  in the  Eigrp process.

sh ip eigrp topology [destination network address]