NetworkTalk & BGP

A.4) Recursive lookup interaction between BGP tables and main routing table

A.4) Recursive lookup interaction between BGP tables and main routing table

1. When BGP updates have propagated through the transit AS to all neighboring autonomous systems, the IP traffic can start to flow.

In theory, the external information that is received by border routers (the one who shares an Ebgp session) could be redistributed into the IGP, to the Ibgp routers within the AS.
However, there is not at the moment, an IGP protocol which can handle the volume of routing information like BGP protocol can. The excess and overload volume of routing information for the Internet, will saturate any, causing the total network to fall down in the AS.
Nowadays, the volume of routing information that is carried by BGP for the Internet passed the limits of what it is possible to carry in any IGP. For instance, we passed from 245k in 2007 to 440k prefixes in 2012.

2. A BGP route is installed in the main IP routing table (RIB) of the router only if the IP-address of the next-hop attribute is reachable according to the information already stored in the routing table.
The installed BGP route contains a reference to that next-hop address. So, the network will be reachable via an IP-address, which may or may not be directly connected.
Because the physical interface is not clearly located, the BGP route is installed in the IP routing table without any information of the outgoing interface.

3. For instance, the following examples show that a public route which originates outside the AS, does not state the outgoing interface, contrary to an interface which manages connectivity an Ebgp neighbor.

RT-B#Show ip route 8.8.8.8
Routing entry for 8.8.8.0/24
Known via “bgp 1000”, distance 20, metric 40
Tag 300, type external
Last update from 20.20.20.85 2d16h ago
Routing Descriptor Blocks:
* 20.20.20.85, from 20.20.20.85, 2d16h ago
Route metric is 40, traffic share count is 1
AS Hops 2
Route tag 300
MPLS label: none

RT-B#Show ip route 20.20.20.85
Routing entry for 20.20.20.84/30
Known via “connected”, distance 0, metric 0 (connected, via interface)
Routing Descriptor Blocks:
* directly connected, via POS1/0/0
Route metric is 0, traffic share count is 1

4. The router must evaluate the recursive criteria to the next-hop at one time, this with the aim of permitting packet forwarding to external destinations. The interval time when the recursive reference is resolved, depends on the IP switching mechanism that is used by the router.

At the end, the router performs the recursive lookup when an IP-packet with a destination address that matches the BGP route should be forwarded. The router determines which outgoing interface should be used and which Layer2 address to assign (if applicable). The router creates a cache entry so that successive IP-packets to the matching destination can be routed using the same outgoing interface and Layer2 address.

See below, Figure A.7: Illustration of recursive lookup mechanism

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