Routing

Here we will discuss important questions based on routing concepts.

Routing Concepts

• In Classful addressing, Routing Table has 3 columns While in Classless addressing, at least 4 Columns are needed:- CIDR Mask, Network Address, Next hop address, Interface.

• There is no MTU for Serial Interfaces

• Applying Route Maps:-

 int fe 0/0
 ip policy route map                 #for transit traffic
 ip local policy route map           #for local generated traffic (e.g self telnet,ssh)

• Redistribution configuration is done in Destination protocol. To Redistribute OSPF routes to BGP, Configure BGP.

• The longest prefix match always wins among the routes actually installed in the routing table, while the routing protocol with the lowest administrative distance always wins when installing routes into the routing table:

• The main considerations while building the routing table are:

Administrative distance - This is the measure of trustworthiness of the source of the route. If a router learns about a destination from more than one routing protocol, administrative distance is compared and the preference is given to the routes with lower administrative distance.
Metrics - This is a measure used by the routing protocol to calculate the best path to a given destination, if it learns multiple paths to the same destination. Each routing protocol uses a different metric.
Prefix length

• To determine the best route to a destination, a router considers three elements (in this order):

 
1st: Prefix-Length - Choose the route that has the longest prefix match.
2nd: Administrative Distance(between separate routing protocols) - Choose the route that has the lowest administrative distance. Administrative distance is a value a router uses to trust a route, if the route has multiple exits to the destined network.
3rd: Metric(within a routing protocol) - Choose the route that has the lowest metric.

• AD of common routing protocols (lowest wins):

Protocol	AD
Connected	0
Static	1
EIGRP Summary	5
External BGP	20
Internal EIGRP	90
OSPF	110
RIP	120
External EIGRP	170
Internal BGP	200
Unknown	255

• If there is a more specific route available, How can you make the L3 device use the other route?

Ans) By using Static ARP entry.

References

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