FireBrick FB6402, Руководство пользователя

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BGP
101
15.2.8. Announcing dead end routes
The top level bgp object includes a dead-end-community attribute which can be set to a tag that is used to mark
routes as a dead end within your network. Any route received on a BGP peer within that config object which
includes the specified community is treated as a dead end route. It is installed in the BGP routes and propagated
as normal but it is internally set as forwarding to nowhere and icmp errors generated (rate limited as usual).
Any route installed as network are announced with this community. Note, this is not set automatically on a
nowhere route, allowing a route to be announced to get to this FireBrick to be propagated via IBGP.
The effect of this is that your network can include one (or more) source of top level network routes which,
within your network, are installed as dead ends at each point. Without this these would be announced to your
internal network so traffic is sent to the originating router and it has to handle all dead end traffic. Using this
system you can ensure dead end traffic is handled at your borders instead.
15.2.9. Bad optional path attributes
The BGP specification is clear that receipt of a path attribute that we understand but is in some way wrong
should cause the BGP session to be shut down. This has a problem if the attribute is one that is not known to
intermediate routers in the internet which means a bad content is propagated to multiple routers on the internet
and they will drop their session. This can cause a major problem in the internet.
To work around this have, by default, ignore-bad-optional-partial set to true. The effect is that if a path attribute
we understand is wrong, and it is optional, and trhe router that sent it to us did not understand or check it (partial
bit is set), we ignore the specific route rather than dropping the whole BGP session.
15.2.10. <network> element
The network element defines a prefix that is to be announced by BGP by default, and tagged with any dead-
end-community, but otherwise treated the same as a nowhere route.
Table 15.3. Network attributes
Attribute Meaning
ip One or more prefixes to be announced
as-path Optional AS path to be used as if we had received this prefix from another AS with this path
localpref Applicable localpref to announce
bgp The bgp mode, one of the well known community tags or true (the default) which is
announced by BGP with no extra tags
15.2.11. <route>, <subnet> and other elements
Subnet and route elements used for normal set-up of internal routing can be announced by BGP using the bgp
attribute with the same values as the well known community tags, please true meaning simply announce with
no tags, and false meaning the same as no-advertise.
Many other objects in the configuration which can cause routes to be inserted have a
bgp
attribute which can
be set to control whether the routes are announced, or not.
15.2.12. Route feasibility testing
The FB6000 has an aggressive route feasibility test that confirms not only routability of each next-hop but also
that it is answering ARP/ND requests. Whenever a next-hop is infeasible then all routes using that next-hop are
removed. When it becomes feasible the routes are re-applied. This goes beyond the normal BGP specification
and minimises any risk of announcing a black hole route.