BGP knowledges

BGP neighbor states

• idle
• connect
• active
• opensent
• openconfirm
• establisted

BGP message type

• open
• keepalive
• update
• notification

Requirements of forming ebgp neighborship

-AS numbers must match
-Router ID must be different
-if exits md5 auth must pass

Router ID

-configured / highest loopback / higheset other ( up )

MD5 auth

neighbor 192.168.1.1 password almakorte

Preferred option loopback ip source of bgp endpoints

1. configure ip address of loopback interface each router

R1(config)#interface loopback0
R1(config)#ip address 192.168.1.1 255.255.255.255
R2(config)#interface loopback1
R2(config)#ip address 192.168.2.1 255.255.255.255

2. Configure each router to update source
R1(config)#neighbor 192.168.2.1 55000
R1(config)#neighbor 192.168.2.1 update-source loopback0
R1(config)#neighbor 192.168.2.1 ebgp-multihop 2
R2(config)#neighbor 192.168.1.1 55001
R2(config)#neighbor 192.168.1.1 update-source loopback1
R2(config)#neighbor 192.168.1.1 ebgp-multihop 2

Verify ebgp status

show ip bgp summary
memory, and neighbors stats
show ip bgp neighbors
show ip bgp neighbors neighbors neighbor_id
neighbor id, remote as, external/internal link, state of bgp session, uptime, router refresh message counters, hop count, tcp-session ports
show tcp brief

Routing table

show ip bgp

• s -> supressed
• d -> dumped
• h -> history
• * -> valid
• > -> best
• i -> internal
• r -> ribfaulure
• S -> stale

show ip bgp 0.0.0.00.0.0.0
avaidable routers with as_path interal/external, best mark


controlling neighbor status

neighbor neighbor_ip shutdown

BGP path attributes

• next hop
• weight
• local preference
• as_path
• origin
• multi_exit discriminator (MED)

BGP best Path Attributes

0. Next hop is reachable?

---

1. Weight ( bigger is preferred )
2. Local_pref ( bigger is preferred )
3. Locally injected routes
4. As Path Length

---

5. Origin
6. MED ( smaller )
7. Neighbor type ( ebgp is preferred than igbp )
8. Igp metric to next hop

Weight

• It is a cisco attribute and assigned locally to the router.
• 0-65535
• higher weight is preferred

neighbor 192.0.80.2 weight 300

Local pref

• whitch path is preferred to exit the AS
• default=100

set default local preference
neighbor 192.0.80.2 remote-as 30
bgp default local-preference 200
set local preference with route-map

ip as-path 7 permit ^300$

route-map almakorte permit 10
match as-path 65101
set local-preference 400
router-map almakorte permit 20
set local-preference 140

neighbor 192.0.80.2 almakorte in


Metric

• also called Multi Exit Discriminator
• Is a hint to external neighors about the preferred path into an as
• Lower is preferred
• exhanged between ASs

bgp always-compared-med if is have to set and compare between ASs

With route-map
route-map setmetric 10 permit
set metric 120

neighbor 192.0.80.2 route-map setmetric out


Default metric
default-metric 10

Community attribute

• Transitive, optional attribute in the range 0- 4,294,967,200
• Is a way to group destinations in a centrain community and apply routig decisions

set community community-number [additive]
With route-map
ip access-list standard 1
10 permit ip host 192.168.25.2

route-map setcommunity permit 10
math ip address 1
set community 200 additive

router bgp 100
neighbor 192.0.20.1 remote-as 300
neighbor 192.0.20.1 send-community
neighbor 192.0.20.1 route-map setcommunits out

BGP good to know

Path filtering

ip as-path access-list 1 deny ^200$
ip as-path access-list 1 permit .*

neighbor 192.0.20.2 filter-list 1 out

BGP regular expressions

single characters

. any single character
^ matches the beginning or the input string
$ and of an input string
\character match the caracter
- match one of the following characters , { } ^ $ space

single character follows by this

* 0 or more
+ 1 or more
? match the atom or the null string

Expressions:
_100_ -> via AS100
^100$ -> origin
^100 .* come from
^$ form this as

as-path prepend

• Able the manipulate as-path

route-map SETASPATH
set as-path prepend 300 300

peer-groups

• more peers with same policie

neighbor 192.0.80.2 peer-group almakorte
neighbor 192.0.82.2 peer-group almakorte
neighbor almakorte remote-as 300