10-06-2008 11:18 AM - edited 03-03-2019 11:48 PM
I have never seen BGP set up this way so I was hoping someone could at least break it down for me. I would appreciate it if someone could break it down kind of line-by-line and then explain how it all works together.
router bgp xxxxx
no synchronization
bgp log-neighbor-changes
network 100.150.0.0
network 120.220.0.0
timers bgp 20 30
neighbor 46.30.3.11 remote-as 5633
neighbor 46.30.3.11 soft-reconfiguration inbound
neighbor 46.30.3.11 prefix-list ProviderA out
neighbor 46.30.3.11 route-map localpref in
neighbor 126.66.211.117 remote-as 56371
neighbor 126.66.211.117 soft-reconfiguration inbound
neighbor 126.66.211.117 route-map VProvider-only in
neighbor 126.66.211.117 route-map VProvider-prepend out
no auto-summary
ip as-path access-list 20 permit ^56371$
!
!
ip prefix-list level3 seq 5 permit 100.150.0.0/16
ip prefix-list level3 seq 10 permit 120.220.0.0/16
access-list 10 permit any
access-list 30 permit 100.150.0.0 0.0.255.255
access-list 30 permit 120.220.0.0 0.0.255.255
route-map VProvider-only permit 10
match as-path 20
set local-preference 200
!
route-map VProvider-prepend permit 10
match ip address 30
set as-path prepend xxxxx xxxxx xxxxx
!
route-map ospfdefault permit 10
set metric 10
set metric-type type-1
!
route-map localpref permit 10
match ip address 10
set local-preference 200
Solved! Go to Solution.
10-06-2008 01:49 PM
no synchronization
Disables synchronization, default behavior in newer IOS version.
bgp log-neighbor-changes
Log BGP changes, self explanatory
network 100.150.0.0
network 120.220.0.0
Advertises these networks, self explanatory.
timers bgp 20 30
BGP timers are changed from the default 60 180
neighbor 46.30.3.11 remote-as 5633
Peers with 46.30.3.11 which is holding AS 5633
neighbor 46.30.3.11 soft-reconfiguration inbound
Soft reconfiguration enables you to generate inbound updates from a neighbor, change and activate BGP policies without clearing the BGP session
neighbor 46.30.3.11 prefix-list ProviderA out
I don't see the prefix-list in the portion of the config that was posted.
neighbor 46.30.3.11 route-map localpref in
Let's see the route-map
route-map localpref permit 10
match ip address 10
set local-preference 200
Based on the route-map, any BGP route entering from that neighbor gets its local-preference modified to 200.
neighbor 126.66.211.117 remote-as 56371
neighbor 126.66.211.117 soft-reconfiguration inbound
Same old as above....
neighbor 126.66.211.117 route-map VProvider-only in
Let's see the route-map
route-map VProvider-only permit 10
match as-path 20
set local-preference 200
Any BGP route that originated from 56371 has no other transit AS after 56371, gets a local-pref of 200.
neighbor 126.66.211.117 route-map VProvider-prepend out
Let's see the route-map...
route-map VProvider-prepend permit 10
match ip address 30
set as-path prepend xxxxx xxxxx xxxxx
These 2 subnets
access-list 30 permit 100.150.0.0 0.0.255.255
access-list 30 permit 120.220.0.0 0.0.255.255
are advertised with a longer AS_PATH to neighbor 126.66.211.117.
HTH,
__
Edison.
10-06-2008 01:47 PM
Hello Aaron,
your router in AS xxxx will advertise nets 100.150/16 and 120.220/16 if they are present in its routing table by any mean/source : static routes, dynamic routing protocol like OSPF.
Your router has two external eBGP neighbors.
For The first neighbor providerA the settings are:
the prefixes 100.150/16 and 120.220/16 if present are advertised any other prefix present in the BGP table is not sent to it
all prefixes received by providerA are accepted and their local preference attribute is increased to 200 (default 100 highest is best)
For eBGP neighbor V-provider settings are:
of all prefixes that V-provider sends to local router accepts in BGP table those that have an AS path attribute that contain only AS 56371 = prefixes that are locally generated in V-provider all other prefixes received, if any, are filtered. Local preference is raised to 200 for accepted prefixes.
To V-provider the same two prefixes 100.150/16 and 120.220/16 are sent by local router but their BGP AS path attribute is modified to look like longer (more AS hops) by prepending (the local router AS number xxxx will appear multiple times as seen by V-provider router it should appear 3+1 times)
Here the strategy is that provider-A is primary probably providing a full routing table.
V-provider is a backup provider that is probably providing a default route and a few local routes (or only default route it depends on V-provider settings)
The use of prepending is an effort to influence return path to local AS via primary provider (preferred because not prepended).
Hope to help
Giuseppe
10-06-2008 01:49 PM
no synchronization
Disables synchronization, default behavior in newer IOS version.
bgp log-neighbor-changes
Log BGP changes, self explanatory
network 100.150.0.0
network 120.220.0.0
Advertises these networks, self explanatory.
timers bgp 20 30
BGP timers are changed from the default 60 180
neighbor 46.30.3.11 remote-as 5633
Peers with 46.30.3.11 which is holding AS 5633
neighbor 46.30.3.11 soft-reconfiguration inbound
Soft reconfiguration enables you to generate inbound updates from a neighbor, change and activate BGP policies without clearing the BGP session
neighbor 46.30.3.11 prefix-list ProviderA out
I don't see the prefix-list in the portion of the config that was posted.
neighbor 46.30.3.11 route-map localpref in
Let's see the route-map
route-map localpref permit 10
match ip address 10
set local-preference 200
Based on the route-map, any BGP route entering from that neighbor gets its local-preference modified to 200.
neighbor 126.66.211.117 remote-as 56371
neighbor 126.66.211.117 soft-reconfiguration inbound
Same old as above....
neighbor 126.66.211.117 route-map VProvider-only in
Let's see the route-map
route-map VProvider-only permit 10
match as-path 20
set local-preference 200
Any BGP route that originated from 56371 has no other transit AS after 56371, gets a local-pref of 200.
neighbor 126.66.211.117 route-map VProvider-prepend out
Let's see the route-map...
route-map VProvider-prepend permit 10
match ip address 30
set as-path prepend xxxxx xxxxx xxxxx
These 2 subnets
access-list 30 permit 100.150.0.0 0.0.255.255
access-list 30 permit 120.220.0.0 0.0.255.255
are advertised with a longer AS_PATH to neighbor 126.66.211.117.
HTH,
__
Edison.
10-07-2008 04:49 AM
One excellent and concise introduction to BGP can be found here, it covers most of your configuration:
http://www.cisco.com/en/US/tech/tk365/technologies_tech_note09186a00800c95bb.shtml
HTH, Thomas
10-07-2008 05:06 AM
Thank you!
Discover and save your favorite ideas. Come back to expert answers, step-by-step guides, recent topics, and more.
New here? Get started with these tips. How to use Community New member guide