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Routing
Classless vs
Classfull
Classful
Routing Classless Routing
In classful routing,
VLMS(Variable Length
Subnet Mask) is not
supported.
While in classless routing,
VLMS(Variable Length
Subnet Mask) is
supported.
Classful routing requires
more bandwidth.
While it requires less
bandwidth.
In classful routing, hello
messages are not used.
While in classless routing,
hello messages are used.
Classful routing does not
import subnet mask.
Whereas it imports subnet
mask.
In classful routing,
address is divided into
three parts which are:
Network, Subnet and
Host.
While in classless routing,
address is divided into
two parts which are:
Subnet and Host.
In classful routing, regular
or periodic updates are
used.
Whereas in this, triggered
updates are used.
In classful routing,
CIDR(Classless Inter-
Domain Routing) is not
supported.
While in classless routing,
CIDR(Classless Inter-
Domain Routing) is
supported.
8. In classful routing, subnets
are not displayed in other
While in classless routing,
subnets are displayed in
major subnet. other major subnet..
In classful routing, fault
can be detected easily.
While in classless routing,
fault detection is little
tough
AD value Purpose The administrative distance ( AD ) is used when a router must choose between routes learned using different routing protocols. Each routing protocol has a default AD value AD values of Protocols Connected interface 0 Static route 1 Enhanced Interior Gateway Routing Protocol (EIGRP) summary route 5 External Border Gateway Protocol (BGP) 20 Internal EIGRP 90 IGRP 100 OSPF 110 Intermediate System-to-Intermediate System (IS-IS) 115 Routing Information Protocol (RIP) 120 Exterior Gateway Protocol (EGP) 140 On Demand Routing (ODR) 160 External EIGRP 170 Internal BGP 200 Unknown* 255
Criteria to select the router ID:
1. Manual configuration of the router ID.
2. Highest IP address on a loopback interface.
3. Highest IP address on a non-loopback interface.
https://www.cisco.com/c/en/us/support/docs/ip/open-shortest-path-first-ospf/9237-9.html
Tables in OSPF
● Neighbor table – stores information about OSPF neighbors.
● Topology table – stores the topology structure of a network.
● Routing table – stores the best routes.
How to manipulate router ID
Hello/Dead packets Interval
On Ethernet interfaces you will see a 10 second hello interval and a 40 second dead interval
OSPF Neighborship criteria
● The devices must be in the same area.
● The devices must have the same authentication configuration.
● The devices must be on the same subnet.
● The devices hello and dead intervals must match.
● The devices must have matching stub flags.
DR / BDR Purpose
OSPF Neighbor States
http://www.firewall.cx/networking-topics/routing/ospf-routing-protocol/1142-ospf-adjacency- neighbor-states-forming-process.html
E1 vs E
E1 or External Type Routes –
The cost of E1 routes is the cost of the external metric with the addition of the internal
cost within OSPF to reach that network.
E2 or External Type2 Routes –
The cost of E2 routes will always be the external metric, the metric will takes no notice
of the internal cost to reach that network.
E1 includes – the internal cost to ASBR added to the external
cost,
E2 does not include – internal cost. it is same as external cost
https://ipwithease.com/ospf-external-e1-and-e2-routes/
LSA types
LSA Type 1: OSPF Router LSA.
packets are sent between routers within the same area of origin and do not leave the area
LSA Type 2: OSPF Network LSA.
packets are generated by the Designated Router ( DR ) to describe all routers connected to its segment directly. LSA Type 2 packets are flooded between neighbors in the same area of origin and remain within that area.
LSA Type 3: OSPF Summary LSA.
packets are generated by Area Border Routers ( ABR ) to summarize its directly connected area, and advertise inter-area router information to other areas the ABR is connected to, with the use of a summary prefix
LSA Type 4: OSPF ASBR Summary LSA.
It is generated by the ABR to broadcast the location of the ASBR
LSA Type 5: OSPF ASBR External LSA.
It is generated by an ASBR and is the routing information of non-OSPF devices.
This does not go to Stub, Totally Stubby and Not-so-stubby areas. External routes are
propagated through an OSPF area as a type 5 from an ASBR, or type 7 LSA from an
ASBR in an NSSA
LSA Type 6: OSPF Group Membership LSA.
LSA Type 7: OSPF Not So Stubby Area (NSSA) External LSA
Type7 advertises the External Route in NSSA LSA Types
LSA 3 vs LSA 4
Router(config-router)# area 10 nssa Summary Standard areas can contain LSAs of type 1, 2, 3, 4, and 5, and may contain an ASBR. The backbone is considered a standard area. Stub areas can contain type 1, 2, and 3 LSAs. A default route is substituted for external routes. Totally stubby areas can only contain type 1 and 2 LSAs, and a single type 3 LSA. The type 3 LSA describes a default route, substituted for all external and inter-area routes. Not-so-stubby areas implement stub or totally stubby functionality yet contain an ASBR. Type 7 LSAs generated by the ASBR are converted to type 5 by ABRs to be flooded to the rest of the OSPF domain.
