If an IS-IS domain exists out of multiple areas, the operator must redistribute SRv6
locators between areas for inter-area SRv6-based transport services. Each SRv6
locator is associated with an applied topology algorithm as follows:
- algorithm 0 for the base SPF topology
- algorithm in the range of 128 to 255 for flexible algorithms
Scaling is impacted when all existing SRv6 locators are redistributed between all
existing areas. IS-IS SRv6 locator summarization reduces the size of the LSDB and
the IPv6 routing table and increases network stability.
SRv6 locators can be summarized when they are redistributed from one area into
another area. This helps to reduce the number of SRv6 locators existing in each
area. A summary SRv6 algorithm-aware locator address is achieved with the following
syntax:
summary-address {ip-prefix/ip-prefix-length | ip-prefix netmask} [level] [tag tag] [algorithm <algo-id>]
no summary-address {ip-prefix/ip-prefix-length | ip-prefix netmask}
The following is a description of the configuration parameters:
-
ip-prefix/ip-prefix-length | ip-prefix netmask
contains the summary locator address of the route.
-
level configures the level where the summary route is
applied.
-
tag assigns a route tag to the summary address.
-
algo-id identifies the flex-algorithm topology for the
summary address.
The following considerations apply:
- When an algorithm-aware summary address is generated, all
matching algorithm-aware member prefixes are suppressed.
- When the algorithm is not explicitly configured, the summary
address is only generated for algorithm 0 in an IP Reachability TLV,
and all more specific prefixes are suppressed from both IP reach
TLV and SRv6 locator TLV for algorithm 0.
- When the algorithm is configured in the range 128 to 255 or 0,
the summary address is generated only for member prefixes
belonging to the configured algorithm (including explicit
algorithm 0 configurations). Only member prefixes within the
configured algorithm are summarized and suppressed, while member
prefixes that belong to a different algorithm are not summarized
and not suppressed.
The following apply when the summary address is configured, and RTM has matching
member prefixes:
- For algorithm 0 (with "algorithm 0" not explicitly configured) the summary
is inserted as IS-IS IP Reachability TLV and all more specific prefixes from both
IP Reachability TLV and SRv6 locator TLV are suppressed.
- For algorithm 0 (with "algorithm 0" explicitly configured) the summary is
inserted in an IS-IS IP Reachability TLV and an SRv6 locator TLV, and all more
specific prefixes from both IP Reachability TLV and SRv6 locator TLV are
suppressed.
- For algorithms in the range of 128 to 255, the summary is inserted as an
SRv6 locator TLV and all more specific locator prefixes in the SRv6 locator
TLV are suppressed.
- The summary-address SRv6 locator does not contain any END
SIDs of member SRv6 locator prefixes in the SRv6 locator TLV.
The following apply when using administrative tags for SRv6 locator summaries:
- When SRv6 locators are summarized from one IS-IS level into another IS-IS
level, special care must be taken to avoid re-redistributing them back into
the original IS-IS level and potentially causing routing loops. Routing
filters must be used to prevent such routing loops.
- Existing filters can use 32-bit administrative tags to match upon routes and
avoid routing loops. This route tag can be set using the
summary-address command when originating an
algorithm-aware SRv6 summary locator.
- A routing policy with a match tag will support matching to both classic IPv6
prefix tags and SRv6 locator tags.