From:
Date: Thu Jul 22 1999 - 20:49:04 GMT-3
The route has to be in the IGP. BGP will not advertise a network until all
of
the routers in an IBGP know about the route. If static routes are in both
routers tables then the route will be advertised in BGP. Remember the main
difference between BGP and Interior protocols. IGP's network statements
describe
the nets that will participate in routing. BGP advertises what networks can
be
reached. It's like the Chicken and the Egg problem. The BGP routers within
an
AS have to all know how to reach a network BEFORE they will advertise the
network.
Here is an example from Cisco CCIE Fundamentals: Network Design and Case
Studies:
AS 100 Router E
AS 100 Router A====(Network 2.0.0.0)===Router B AS 100
AS 300 Router C (network 170.10.0.0) Router D AS 400 (network
175.10.0.0)
Routers A, B and E are in AS 100 2.0.0.0. (IBGP neighbors)
Router C is AS 300 170.10.0.0
Router D is AS 400 175.10.0.0
Router A and Router C are EBGP neighbors
Router B and Router D are EBGP neighbors
" If Router B advertises to AS 400 (RD) that it can reach 170.10.0.0 before
Router E learns about the network via IBGP, traffic coming from Router D to
Router B with a destination of 170.10.0.0 will flow to Router E and be
dropped."
This is because Router E advertises network 170.10.0.0, but when traffic
comes
into Router E it doesn't know to forward the traffic to Router A. Therefore
Router E, A and B have to agree on how to forward traffic destined for a
network
before the traffic reaches any of the routers and needs to be forwarded to
that
network. Synchronization forces the routers to plan ahead.
Think of it this way. If you saw an advertisement for 1999 BMW's for $200
dollars, you wouldn't jump in your car and drive until you knew what car
dealer
to go to.
Jason Aarons wrote:
> If I didn't want to do the "no sync", is there other option to add static
> routes ? To me it seems crazy that the Internal routing protocol must
know
> about the external network before the External routing protocol will talk
to
> the External routing protocol neighbor.
>
> I was reading more about this...but haven't yet fully grasped it.
>
> ----Original Message Follows----
> From: Bill Carter <bcarter@family-net.net>
> To: Jason Aarons <jaarons@hotmail.com>
> Subject: Re: BGP config
> Date: Thu, 22 Jul 1999 20:48:19 -0500
>
> You need to add no sync to BGP 300. The 16.0.0.0 route is not in the
route
> tables for the 2 routers. They will not accept routes not known by
internal
> routing protocols. Try debug ip bgp events and debug ip bgp updates.
>
> Jason Aarons wrote:
>
> > Do anyone have some BGP "show ip bgp neighbor" or "show ip bgp
summary"
> > outputs between two ASs that are talking ?
> >
> > I have two routers connected but am not sure what the "show ip bgp
> summary"
> > should show ? I can ping both ways, etc....
> >
> > !
> > interface Loopback0
> > ip address 16.0.0.1 255.0.0.0
> > !
> > interface Serial1
> > ip address 172.16.1.2 255.255.255.252
> > clockrate 148000
> > !
> > router bgp 300
> > network 16.0.0.0
> > neighbor 10.0.0.1 remote-as 100
> > !
> > ip classless
> > ip route 10.0.0.0 255.0.0.0 Serial1
> > !
> > END
> > iowa#ping 10.0.0.1
> >
> > Type escape sequence to abort.
> > Sending 5, 100-byte ICMP Echos to 10.0.0.1, timeout is 2 seconds:
> > !!!!!
> > Success rate is 100 percent (5/5), round-trip min/avg/max = 12/14/16
ms
> > iowa#
> >
> > iowa#show ip bgp neigh
> > BGP neighbor is 10.0.0.1, remote AS 100, external link
> > Index 1, Offset 0, Mask 0x2
> > BGP version 4, remote router ID 0.0.0.0
> > BGP state = Idle, table version = 0
> > Last read 00:00:15, hold time is 180, keepalive interval is 60
seconds
> > Minimum time between advertisement runs is 30 seconds
> > Received 0 messages, 0 notifications, 0 in queue
> > Sent 0 messages, 0 notifications, 0 in queue
> > Connections established 0; dropped 0
> > Last reset 00:00:16, due to : User reset request
> > No. of prefix received 0
> > External BGP neighbor not directly connected.
> > No active TCP connection
> > iowa#show ip bgp summary
> > BGP table version is 2, main routing table version 2
> > 1 network entries (1/3 paths) using 208 bytes of memory
> > 1 BGP path attribute entries using 96 bytes of memory
> > 0 BGP route-map cache entries using 0 bytes of memory
> > 0 BGP filter-list cache entries using 0 bytes of memory
> >
> > Neighbor V AS MsgRcvd MsgSent TblVer InQ OutQ Up/Down
> > State/PfxRcd
> >
> > 10.0.0.1 4 100 0 0 0 0 0 never
Idle
> > iowa#
> >
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