Monday, July 9, 2012

ccnp route ospf concepts

OSPF concept:

First of the link state routing protocol and other is ISIS
Every else other protocol in world is distance vector routing protocol

Maintain Three tables:
the neighbor table
topology table==roadmap of every single network in that area.every single router in that area has that roadmap to reach a particular destination.they not only know what their neighbor knows as in eigrp but have a total roadmap.
routing table== have the best path

Use dijkstra's shortest path first (spf) algorithm

more process intensive.
Since here SPF has so much information to deal with.

Send triggered updates to announce network changes.However, send periodic updates (LS refresh) at long intervals once every 30 mins.

Terms used:
divides networks into diff areas. Every area consist different routers.
All areas must connect to areas 0.
All routers in an area have the same topology table.

To localize updates within an area.

Requires a hierarchical design
designing network in the hierarchy of ip address. each area will be in diff network.

ABR : one interface in each area.This is simply a design criterion.

ASBR: a router that connects outside of its AS.The most common network will be internet,eigrp or bgp.

ABR and ASBR are only two routers in OSPF that has the ability to do summarization.

How ospf neighbor relationship form
neighbors only form within the routers in same area.
-determine your own Router id ie the router name.
-Thr Router id is simply the router's name in the ospf process.
1.Highest active interface ip address when ospf start (loopbacks beat physical interfaces)
The ospf router id only changes when you restart the router or clear the process.

2. add interfaces to the link state database (dictated by the network command)

3.Send a hello message on chosen interfaces(s).======Down state========
-once every 10 seconds on broadcast/P2P netowrks
-once every 30 sec on NBMA networks,

Contains all sorts of information
Router ID
hello and dead timers*
network mask*
area id*
router priority
DR/BDR ip address
authentication password *
* ==neighbor sud have equal value.

4. R2 receives the hello == init state==========
Check hello/dead interval
check netmasks
check area id
check authentication passwords.

5. Send reply hello == two way state=====
Am I listed as a neighbor in your hello packet. If yes, reset dead timer, if no add as a new neighbor.

6. master- slave relationship determined======Exstart state=========
determined by priority, router-id breaks tie. Who will send the data first. Master will send first followed by slave
Master sends database description (DBD ) packet
DBD = cliff notes of link-state database
Slave sends its DBD packet.

7. DBDs are acknowledged and reviewed.== Loading state =======
becoz ospf is reliable.
During loading state very in-depth process goes on
slave request details (Link sate request-- LSR)
Master sends updates (Link state updates - LSU)
Master requests details (LSR)
Slave Sends updates (LSU)

One LSR for every network.

8. neighbors are synchronised . === Full state===

Now database has synchronized so its time to run the SPF algorithm to figure out what to do with all the data.

OSPF's cost:
how it finds the best way to network

cost = 100/(BW in mbps)

Common costs
56k = 1785
64k = 1562
T1(1.544) = 65
E1(2.048) = 48
Ethernet = 10
fast ethernet = 1

Eigrp keeps the backup path but ospf doesn't.
So if a link failure occurs, it has to run the spf algo once again.

Understanding the DR/BDR
in ethernet and NBMA

How ospf propagtes updates?
if any links goes down , it sends an update to all other routers and the process goes on and process bumps up.
solution : in a shared n/w , one DR and other BDR is elected

If network goes down in a shared n, instead of sending update to all connected routers, it will send update to DR and BDR router to multicast address

Now DR shoots the updates to all its neighbors to multicast address

DR/BDR for every shared segments .

But on point to point links it is not true . Only the ip used here is

How election happens?
Based on router priority present in hello packet.By default priority is set to 1.
Breaking the tie is router-id.

Note: In shared network it doesn't matter which router is selceted as DR or BDR but it does in case of FR.

How neighbor look at each other ?
Every router in shared network except DR and BDR should be in 2-way state.The only one which will be in full relationship is routers connected with DR and BDR.

In shared network , we will see max of 2 FULL relationship.

OSPF Packet Types:
database description (DBD)
Link-state request(LSR)
Link-state advertisement (LSA)
Updates about individual routes.
Link-state update (LSU)
Link-state acknoeldgement (LSACK)

one LSU can have more than one LSA's of diff networks.
LSA has more than one update requested by slave.

Ospf has its known transport layer protocol
Ospf doesn't use TCP and UDP. IT has its own ack mechanism. For every single messsages excpet hello it ack.

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