Connected: An Internet Encyclopedia
16.4. Calculating AS external routes
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RFC 1583
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16. Calculation Of The Routing Table
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16.4. Calculating AS external routes
16.4. Calculating AS external routes
AS external routes are calculated by examining AS external link
advertisements. Each of the AS external link advertisements is
considered in turn. Most AS external link advertisements
describe routes to specific IP destinations. An AS external
link advertisement can also describe a default route for the
Autonomous System (Destination ID = DefaultDestination,
network/subnet mask = 0x00000000). For each AS external link
advertisement:
- If the cost specified by the advertisement is LSInfinity, or
if the advertisement's LS age is equal to MaxAge, then
examine the next advertisement.
- If the advertisement was originated by the calculating
router itself, examine the next advertisement.
- Call the destination described by the advertisement N. N's
address is obtained by masking the advertisement's Link
State ID with the network/subnet mask contained in the body
of the advertisement. Look up the routing table entry for
the AS boundary router (ASBR) that originated the
advertisement. If no entry exists for router ASBR (i.e.,
ASBR is unreachable), do nothing with this advertisement and
consider the next in the list.
Else, this advertisement describes an AS external path to
destination N. Examine the forwarding address specified in
the AS external link advertisement. This indicates the IP
address to which packets for the destination should be
forwarded. If the forwarding address is set to 0.0.0.0,
packets should be sent to the ASBR itself. Otherwise, look
up the forwarding address in the routing table.[23] An
intra-area or inter-area path must exist to the forwarding
address. If no such path exists, do nothing with the
advertisement and consider the next in the list.
Call the routing table distance to the forwarding address X
(when the forwarding address is set to 0.0.0.0, this is the
distance to the ASBR itself), and the cost specified in the
advertisement Y. X is in terms of the link state metric,
and Y is a type 1 or 2 external metric.
- Next, look up the routing table entry for the destination N.
If no entry exists for N, install the AS external path to N,
with next hop equal to the list of next hops to the
forwarding address, and advertising router equal to ASBR.
If the external metric type is 1, then the path-type is set
to type 1 external and the cost is equal to X+Y. If the
external metric type is 2, the path-type is set to type 2
external, the link state component of the route's cost is X,
and the type 2 cost is Y.
- Else, if the paths present in the table are not type 1 or
type 2 external paths, do nothing (AS external paths have
the lowest priority).
- Otherwise, compare the cost of this new AS external path to
the ones present in the table. Type 1 external paths are
always shorter than type 2 external paths. Type 1 external
paths are compared by looking at the sum of the distance to
the forwarding address and the advertised type 1 metric
(X+Y). Type 2 external paths are compared by looking at the
advertised type 2 metrics, and then if necessary, the
distance to the forwarding addresses.
If the new path is shorter, it replaces the present paths in
the routing table entry. If the new path is the same cost,
it is added to the routing table entry's list of paths.
Next: 16.5. Incremental updates -- summary link advertisements
Connected: An Internet Encyclopedia
16.4. Calculating AS external routes