1. BGP routing black hole problem
Causes of BGP routing black holes
The control plane is reachable, but the data plane is unreachable; routing entries are transmitted normally through unicast between BGP neighbors; routers that are not running the BGP protocol can be skipped; the route is shown to be reachable, but data plane access traffic must pass through these routers that are not running the BGP protocol. The router is actually unable to pass through;
Indirectly connected neighbor establishment reaches the control plane route entry and can be passed, and the recursive calculation route is reachable;
However, actual data plane traffic cannot pass through a router that does not run the BGP protocol, and ultimately ends up with no return.
Routing black hole solution
1. Fully connected physical and logical topology - direct physical link connection, or VPN (not achievable in actual projects)
2. Neighbor relationships are fully connected – all devices in the network run BGP (the cost is too high)
3. BGP redistributes to IGP (LAB) (experimental scenario)
4. MPLS multi-protocol label switching (recommended practice)
2. BGP’s anti-ring mechanism
1. EBGP split horizon: solving EBGP loops
[1] Relies on an attribute in the BGP routing entry for loop prevention; AS-PASH path attribute;
[2] In the process of transmitting routing entries, the BGP protocol will record the numbers of all passing ASs;
[3] EBGP split horizon: If there is a local AS number in the received routing entry, the entry will be rejected;
2. IBGP split horizon: a mechanism to solve IBGP ring routing
[1] Routing entries learned locally from an IBGP neighbor must not be passed to other local IBGP neighbors;
【2】AS-BY-AS will not modify any attributes by default during the transfer of an AS internal entry;
[3] Since BGP can establish neighbor relationships indirectly, within an AS, you can establish BGP neighbor relationships with multiple routers running the BGP protocol to stabilize the relationship network; therefore, devices running the BGP protocol within an AS, EBGP neighbors normally exist (all are connected to other ASs at the same time)
[4] Under the restriction of IBGP split horizon, although IBGP loops are avoided, it also makes it necessary to establish IBGP neighbor relationships between two in order to transmit routing entries within the AS. The neighbor relationship increases exponentially and the configuration amount is huge. ;
[5] Later, you can rely on the mechanism of breaking the horizontal split to solve the problem---federation, route reflector