bgp ibgp/ebgp & as_path attribute - query

mikey250
mikey250 used Ask the Experts™
on
hi ive configured a small test network representing both 'ibgp & ebgp with router 1 connected to both router 2 & 3.

1st task:

note: router 1 & 2 have configured both 'bgp & ospf representing igp'

- router 1 is connected via a serial cable to router 3
- router 1 is also connected via a fastethernet cable to router 2
- router 1 also has a single loopback

- router 2 is connected to router 1
- router 2 also has a single loopback

- router 3 is also configured with just 'bgp & not ospf' as it is external
- router 3 has also a fastethernet connection to a lan

ive configured on router 1 & router 3 for example:

router 1

router bgp 200
network 172.16.1.0 mask 255.255.255.0

router 3

router bgp 100
network 192.168.1.32 mask 255.255.255.224

2nd task:

ive configured a small network for using the 'as_path attribute' - but instead of configuring something simular to above 'mask' command ive done the following:

router 1

router bgp 300
no synchronization
neighbor 192.168.1.5 remote-as 100
neighbor 172.16.1.18 remote-as 65000
network 202.2.2.0

router 2

router bgp 100
no synchronization
neighbor 192.168.1.6 remote-as 300
network 201.1.1.0

router 3

router bgp 65000
no synchronization
neighbor 172.24.1.17 remote-as 300
network 203.3.3.0

question.  i assume it is something to do with how ibgp & ebgp distinguish itself with 'bgp' as why step 2 does not use the 'mask' command ?
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Commented:
Hi there,

The difference between iBGP and eBGP, in terms of configuration, is the AS number. The AS number of the router itself is indeed the "router bgp 100", for example, for AS number 100. Then when you configure a bgp 'neighbor', if it has a different AS number it is a eBGP session, and if it is the same number it is iBGP.

The "mask" part of the network statement is need only when you want to use a none "classful" mask. For example:

network 192.168.1.32 mask 255.255.255.224

If you had not included the mask, the router would have assumed the mask was 255.255.255.0.

Author

Commented:
hi pergr, apologies for taking so long to return back to this question.

i had previously configured the following for 'bgp'

- sanjose3 serial0 & 1 connected to isp1a & isp12

note: all 3 routers have separate 'as' numbers

- sanjose3 - loopbacks were set as 'network' statements:
network 192.168.0.0
network 192.168.1.0

- isp1a - loopback was set as 'network 12.0.1.0 mask 255.255.255.0

- isp2 - loopback was set as 'network 172.16.1.0 mask 255.255.255.0

"so when you say the below im not getting my head around it although i understand what you have said, unless it refers to not using loopbacks or something: uuuuum

The "mask" part of the network statement is need only when you want to use a none "classful" mask. For example:

network 192.168.1.32 mask 255.255.255.224

If you had not included the mask, the router would have assumed the mask was 255.255.255.0."

Author

Commented:
hi pergr, i have used a 'classful' address.!!!

question 1.

i responded on my last 'thread' about 'mask', but my query is ive used a 'classful mask', although you say 'mask' is supposed to be used as per below:

The "mask" part of the network statement is need only when you want to use a none "classful" mask. For example:

network 192.168.1.32 mask 255.255.255.224 - ?
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Commented:
Yes.

If you do not enter the mask, it is by default taken as the classful mask.

Author

Commented:
hi,

oh ok so if 'no mask' command was added it would by default be taken as a classful mask .!!

so by myself using a 'classful address - 255.255.255.0 - ie adding this command statement are you saying it still reacts in the same manner & so it does need to be a 'classless address' to get the correct routing response ?

Commented:
Yes, for 192...., if you do not include a mask, it takes it as if you had entered a mask 255.255.255.0.

Author

Commented:
ok i will need to make a note of this!!

Author

Commented:
sound advice!!

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