NetworkTalk & BGP

B.3.c) Explanations and remarks about the Ibgp sessions between the routers RT-A and RT-B of ISP-A

B.3.c) Explanations and remarks about the Ibgp sessions between the routers RT-A and RT-B of ISP-A

1. In order to make evident the case, we will focus our attention on Zone-G inbound/outbound traffic. According to our traffic engineering desires, Zone-G inbound/outbound traffic leaves-out through ISP-B link, as we can see in Figure B.8.

2. Zone-G outbound/upload traffic streams through ISP-B, since all routes received from ISP-B’s BGP table, are the best (this BGP table was untagged; LocalPref is set at 100).

For instance:

RT-B#sh ip bgp 8.8.8.8
BGP routing table entry for 8.8.8.0/24, version 261842458
Paths: (3 available, best #1, table default)
Advertised to update-groups:
2
300 15169
20.20.20.85 from 20.20.20.85 (20.20.20.85)
Origin IGP, metric 40, localpref 100, valid, external, best
Community: 300:2 300:100 300:2065
400 15169
30.30.30.127 from 30.30.30.127 (30.30.30.127)
Origin IGP, metric 100, localpref 90, valid, external
Community: 400:30
500 15169
11.11.226.26 (metric 10) from 11.11.226.26 (10.10.193.62)
Origin IGP, metric 1, localpref 90, valid, internal
Community: 500:666 500:212

Now if ISP-B link is cut, outbound/upload traffic will pass through ISP-C link.

Routes from ISP-C ebgp neighbor and RT-A Ibgp neighbor are both tagged with LocalPref 90 in our network.

We have here equivalent routes, which are routes that have equal BGP path attributes used in the BGP route selection rules (local-preference).

Still with the same attributes, a router always prefers routes from Ebgp neighbor instead of an Ibgp neighbor.

This is due of the fact that a router realizes it is better to make the IP-packets leave sooner through the Ebgp speaker right away. So the update that was received from the Ebgp session is preferred over the update that was received from the Ibgp session.

RT-B#sh ip bgp 8.8.8.8
BGP routing table entry for 8.8.8.0/24, version 265622392
Paths: (2 available, best #1, table default)
Advertised to update-groups:
2
400 15169
30.30.30.127 from 30.30.30.127 (30.30.30.127)
Origin IGP, metric 100, localpref 90, valid, external, best
Community: 400:30
500 15169
11.11.226.26 (metric 10) from 11.11.226.26 (10.10.193.62)
Origin IGP, metric 1, localpref 90, valid, internal
Community: 500:666 500:212

If ISP-B and ISP-C links are cut down, since there is not any other Ebgp speaker which sends the full internet routes table, best routes will be the one coming from the Ibgp neighbor RT-A. Upload of Zone-G will pass through the Ibgp neighbor.

This is the result of the route-map “IBGP-Neighbor-In” that we defined above in RT-B.

router bgp 1000
bgp log-neighbor-changes
neighbor ibgp_client peer-group
neighbor ibgp_client remote-as 1000

neighbor 11.11.226.26 peer-group ibgp_client
neighbor ibgp_client next-hop-self
neighbor ibgp_client route-map IBGP-Neighbor-IN in
neighbor ibgp_client route-map IBGP-Neighbor-OUT out
route-map IBGP-Neighbor-IN permit 10
set local-preference 90


RT-B#sh ip bgp 8.8.8.8
BGP routing table entry for 8.8.8.0/24, version 265622392
Paths: (1 available, best #1, table default)
Advertised to update-groups:
500 15169
11.11.226.26 (metric 10) from 11.11.226.26 (10.10.193.62)
Origin IGP, metric 1, localpref 90, valid, internal, best
Community: 500:666 500:212

3. We still stay in the situation in where ISP-B is cut.

Inbound/download traffic of Zone-G will come from ISP-D-Link1 Ebgp neighbor through the router RT-B, which is the Ibgp neighbor. This has been defined in our traffic engineering requirements.

We remember that communities tag attached to Zone_G:

Table B-9

Let’s focus on the Ibgp session. It is important to notice that RT-A receives routes from Zone-G though the Ibgp session (next-hop 11.11.226.27).

So IP-packets coming from ISP-D-Link1 in RT-A, are sent to RT-B.

For instance, for the prefix 11.11.232.0/24 belonging to Zone-G, we have:

RT-A#sh ip bgp 11.11.232.0/24
BGP routing table entry for 11.11.232.0/24, version 4813058
Paths: (1 available, best #1, table default)
Advertised to update-groups:
261
Local
11.11.226.27 (metric 10) from 11.11.226.27 (11.11.226.250)
Origin IGP, metric 0, localpref 90, valid, internal, best
RT-B#sh ip bgp 11.11.232.0
BGP routing table entry for 11.11.232.0/24, version 19706
Paths: (1 available, best #1, table default)
Advertised to update-groups:
2 269 323 332
Local
11.11.224.3 from 0.0.0.0 (11.11.226.250)
Origin IGP, metric 0, localpref 100, weight 32768, valid, sourced, local, best

This is the result of the route-map “IBGP-Neighbor-OUT” defined in RT-B, which advertise the FIB to RT-A, including Zone-G networks..

router bgp 1000
bgp log-neighbor-changes
neighbor ibgp_client peer-group
neighbor ibgp_client remote-as 1000
neighbor ibgp_client description “Peering with iBGP Core Routers”
neighbor ibgp_client password 7 “…”
neighbor ibgp_client update-source Loopback1
neighbor ibgp_client version 4
neighbor 11.11.226.26 peer-group ibgp_client
neighbor ibgp_client send-community both
neighbor ibgp_client next-hop-self
neighbor ibgp_client route-map IBGP-Neighbor-IN in
neighbor ibgp_client route-map IBGP-Neighbor-OUT out

With the following route-map we advertise the FIB of RT-B to RT-A, including Zone-G prefixes list defined as network entries in router bgp 1000 session.

route-map IBGP-Neighbor-OUT permit 20

See below Figure B.9: Ingress/egress traffic of Zone-G if ISP-B Ebgp speaker is down.

 

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