Solved: trying to understand how this BGP is setup for my network

Aaron Greene · ‎10-06-2008

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

Edison Ortiz · ‎10-06-2008

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.

View solution in original post

Giuseppe Larosa · ‎10-06-2008

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

Edison Ortiz · ‎10-06-2008

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.

tcordier · ‎10-07-2008

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

Aaron Greene · ‎10-07-2008

Thank you!