14th Lecture - Networking

[ding]

emaindi aapesaav???

[k2s]

Attributes that are not well-known are called optional. BGP routers do not have to support an
optional attribute. Optional attributes are either transitive or nontransitive.

The following statements apply to optional attributes:
[list]
[*]BGP routers that implement the optional attribute may propagate it to the other BGP neighbors, based on its meaning.
[*]BGP routers that do not implement an optional transitive attribute should pass it to other
[*]BGP routers untouched and mark the attribute as partial.
[*]BGP routers that do not implement an optional nontransitive attribute must delete the attribute and must not pass it to other BGP routers.
[/list]

[k2s]

The following is a list of the common BGP attributes, by the categories that they belong to:

Well-known mandatory attributes
— Autonomous system (AS) path
— Next-hop
— Origin

Well-known discretionary attributes
— Local preference
— Atomic aggregate

Optional transitive attribute
—
Aggregator

Optional nontransitive attribute
—
Multi-exit discriminator (MED)

[riddle]

Present Sir.. Late ayindhi .. kshaminchandi.

[k2s]

[quote name='shakkku' timestamp='1320957295' post='3082423']
emaindi aapesaav???
[/quote]
[quote name='riddle' timestamp='1320957600' post='3082439']
Present Sir.. Late ayindhi .. kshaminchandi.
[/quote]


just go thru once... ....

[k2s]

[b]The ORIGIN Attribute[/b]

The ORIGIN attribute is a well-known mandatory attribute (Type Code 1) that indicates the
origin of the routing update with respect to the autonomous system that originated it. BGP
considers three types of origins:[list]
[*]IGP— The Network Layer Reachability Information (NLRI) is internal to the originating AS.
[*]EGP— The Network Layer Reachability Information is learned via the Exterior Gateway Protocol (EGP).
[*]INCOMPLETE— The Network Layer Reachability Information is learned by some other means. Ex: route learned by redistribution
[/list]



BGP prefers the path with the lowest origin type,
where IGP is lower than EGP and EGP is lower than INCOMPLETE.

[ding]

[quote name='k2s' timestamp='1320957649' post='3082441']


just go thru once... ....
[/quote]

ok

[k2s]

[b]The AS_PATH attribute[/b] is a well-known mandatory attribute (Type Code 2) that contains a
sequence of autonomous system numbers that represent the path a route has traversed.


Prepending is the act of adding the AS number to the beginning of the list. The final list represents all the AS numbers that a route
has traversed.


Consider this example figure for all attributes :

[img]http://i41.tinypic.com/ea39qf.png[/img]

[k2s]

If u look at Router RTF BGP routing table :

[img]http://i42.tinypic.com/34fk7jc.png[/img]



AS_PATH information at the
end of each line. Network 192.68.11.0/24 is learned via IBGP with AS_PATH 1 and via
EBGP with AS_PATH 2 1. This means that if RTF wanted to reach 192.68.11.0/24 via IBGP,
it could go to AS1, and if RTF wanted to reach 192.68.11.0/24 via EBGP, it would have to go
via AS2 and then AS1. BGP always prefers the shortest path, which is why the path via IBGP
with AS_PATH 1 is preferred. The > at the left indicates that out of the two available paths
that BGP has for 192.68.11.0/24, BGP prefers the second one as being the "best" path.

[k2s]

[b]The NEXT_HOP attribute[/b] is a well-known mandatory attribute (Type Code 3). This varies
slightly when used in the context of an IGP, where the next hop to reach a destination is the IP
address of the connected interface of the router that has announced the route.


The next-hop concept with BGP is slightly more elaborate. It takes one of the following four
forms:
[list]
[*]For EBGP sessions, the next hop is the IP address of the neighbor that announced the route.
[*]For IBGP sessions, for routes originated inside the AS, the next hop is the IP address of the neighbor that announced the route.
[*]For routes injected into the AS via EBGP, the next hop learned from EBGP is carried unaltered into IBGP. The next hop is the IP address of the EBGP neighbor from which the route was learned.
[*]When the route is advertised on a multiaccess medium (such as Ethernet, Frame Relay, and so on), the next hop is usually the IP address of the interface of the router connected to the medium that originated the route.
[/list]

[riddle]

[quote name='k2s' timestamp='1320958369' post='3082488']
If u look at Router RTF BGP routing table :

[img]http://i42.tinypic.com/34fk7jc.png[/img]



AS_PATH information at the
end of each line. Network 192.68.11.0/24 is learned via IBGP with AS_PATH 1 and via
EBGP with AS_PATH 2 1. This means that if RTF wanted to reach 192.68.11.0/24 via IBGP,
it could go to AS1, and if RTF wanted to reach 192.68.11.0/24 via EBGP, it would have to go
via AS2 and then AS1. BGP always prefers the shortest path, which is why the path via IBGP
with AS_PATH 1 is preferred. The > at the left indicates that out of the two available paths
that BGP has for 192.68.11.0/24, BGP prefers the second one as being the "best" path.
[/quote]


naaku image kanapadatla... IBGP.. EBGP and AS Path...

But just doubt for clarification... IBGP - EBGP route with ASPATH same no of ASN's ayithe- Router takes EBGP learned path....

IBGP As paths less compared to EBGP ayithe IBGP learned path takes over kadha... <Thinking>

[ding]

lot of confusion... inkosaari modati nundi follow avaali

[k2s]

Nexthop Attribute Example:

[img]http://i39.tinypic.com/352me54.png[/img]



Care must be taken to make sure that reachability of the BGP NEXT_HOP is provided via
some IGP or static routing. If the IBGP NEXT_HOP cannot be reached, the BGP route is
considered inaccessible.

[k2s]

[quote name='riddle' timestamp='1320958865' post='3082533']


naaku image kanapadatla... IBGP.. EBGP and AS Path...

But just doubt for clarification... IBGP - EBGP route with ASPATH same no of ASN's ayithe- Router takes EBGP learned path....

IBGP As paths less compared to EBGP ayithe IBGP learned path takes over kadha... <Thinking>
[/quote]

Agreed kani. 1st it checks shortest AS_PATH prior to checking eBGP vs iBGP

[k2s]

[quote name='shakkku' timestamp='1320959019' post='3082545']
lot of confusion... inkosaari modati nundi follow avaali
[/quote]

just nenu cheppinadi pallow avvu... from......starting